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The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. Each cell within the blastula is called a blastomere.
Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.
In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo.
At this stage of development, illustrated in Figure 2 the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta and nourish the embryo.Visit the Virtual Human Embryo project at the Endowment for Human Development site to step through an interactive that shows the stages of embryo development, including micrographs and rotating 3-D images.
Cleavage of Human Zygote (Explained with Diagram)
Cleavage is the repeated mitotic division of zygote to form a solid ball of cells called morula which later changes into a hollow ball of cells called blastula.
Cleavage of human zygote occurs within the fallopian tube. It is holoblastic, i.e., it divides the zygote completely into daughter cells or blastomeres.
The first cleavage takes place about 30 hours after fertilization. It divides zygote longitudinally into two blastomeres (one slightly larger than the other). The second cleavage occurs within forty hours after fertilization. It is at right angles to the plane of the first resulting in four blastomeres. The third cleavage takes place about 72 hours after fertilization. During these early cleavages, the young embryo moves slowly down the fallopian tube towards the uterus (Fig. 3(B).10).
At the end of the fourth day, the embryo reaches the uterus. It looks like a mulberry and is known as morula. This solid ball like morula has thirty two cells. In human zygote the cleavage is radial (blastomeres are arranged in radial plane around the polar axis) and indeterminate type (fate of each blastomere is not predetermined).
During the early cleavage in mammals capacitation occurs. It occurs at 8-cell stage when the loosely attached blastomeres are held tightly due to production of proteins called cohesions on their surface.
Significance of Cleavage:
(i) It converts a unicellular zygote into a multicellular embryo.
(ii) It maintains the cell size and nucleo-cytoplasmic ratio of the species.
(iii) Cleavage produces large member of cells or blastomeres required for the building of offspring’s body.
(iv) During cleavage quick mitotic division of blastomeres occurs following which there is no growth of blastomeres.
(v) Cleavage brings about the distribution of cytoplasm among the blastomeres.
Gastrula is the stage of the embryo after blastula. The gastrula stage is an important stage in embryonic development. During this stage, the blastula is reorganized into gastrula. Gastrulation meaning is that it takes place after the blastulation and gastrulation process, the embryonic layers or the germ layers are formed. These germ layers are further responsible for the formation of the organs.
To thoroughly understand gastrulation meaning we need to understand the process of blastulation and how the embryo is formed and embedded into the uterus. By understanding these, we can definitely know the meaning of what is gastrula.
Embryonic Development and Cleavage
The development of the embryo after the process of fertilization is known as embryonic development. Cleavage, blastulation, implantation, gastrula stage and organogenesis are the processes that take place for the development of gametes.
Cleavage is known as the division of cells when the zygote is formed. It is also called an internal zygote division. After 30 hours of fertilization, the first cleavage is completed. There is a furrow formed that is known as cleavage furrow. It passes from the animal-vegetal axis and also from the centre of the zygote.
Two blastomeres are formed after this first division. This type of cleavage is known as holoblastic cleavage. In 60 hours, the second cleavage is completed. This cleavage is at a right angle to the first one. This cleavage is also meridional in nature. This forms a 3-celled stage.
8 blastomeres are formed in the third cleavage. This cleavage is horizontal in nature. This division is slightly unequal in nature. And then thereafter the rate and pattern of the cleavage are non-specific in nature.
The humans show the slowest cleavage division. There is an asynchronous type of division in humans. When the cleavage divides, the blastomeres are increased in an arithmetic division. The cleavages show mitotic division and the daughter cells that are formed are known as blastomeres. When cleavage takes place then at that time no growth is seen in the blastomeres. Here, the total size and volume of the embryo remain the same. This is because there is no growth phase in the interphase stage.
There is a decrease in the size of blastomeres at the time of cleavage. This is because there is no growth in the blastomeres. At the time of cleavage divisions, the zona pellucida remains intact. At the time of cleavage, there is no increase in the mass of the cytoplasm. But, the DNA content and the chromosomal amount keeps on increasing. On the amount and distribution of yolk, the rate of cleavage depends.
Morula and Blastula
A solid ball of cells is formed as a result of cleavage. This is known as Morula. It is an 8-16 celled structure. The outer cover is formed by the zona pellucida. There is a process of compaction that takes place in the morula. The outer cells of the morula are smaller in size. They are also flat. They are present with tight junctions with the inner cell mass. The inner cell mass has slightly large cells. They are also round in nature with the presence of gap junctions. As to progress for the process of implantation, the morula starts descending towards the uterus. When this process happens, then the corona radiata is detached from the structure.
On one side of the embryonal knob, the inner cell mass starts to lie. When the blastocoel is formed then, the morula is converted to the blastula. In mammals, it is called a blastocyst. This is because it has a different nature of the surface layer and the inner cell mass is eccentric in nature.
As the blastocyst grows, there is an increase in pressure and due to this a small hole is produced in the zona pellucida. Through this hole, the blastocyst squeezes out. So while coming out sometimes this blastocyst can be broken down into two pieces. When such an event happens then, there are two identical blastocysts present and this results in the formation of identical twins. These identical twins are also known as maternal twins or monozygotic twins. The trophoblast cells that are in direct contact with the embryonal knob are known as cells of Rauber. The animal pole is the area of an embryonal knob.
Just opposite the animal pole is the abembryonic pole. The embryonic disc is formed when the embryonal knob starts to show rearrangement. Periclinal division takes place in the cells of the trophoblast layer. Syncytiotrophoblast and cytotrophoblast are the two layers that are formed. The syncytiotrophoblast in the outer layer and cytotrophoblast in the inner layer. Further, these two layers give rise to the chorion, amnion and the foetal part of the placenta.
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Gastrula Stage and Gastrulation
We understood the gastrulation definition in the above paragraph. We will understand the structure of the gastrula and the process of gastrulation briefly.
Primary germ layers are formed in the process of gastrulation by the movement of cells in small masses or sheets. The three primary germ layers are the ectoderm, endoderm and mesoderm. Morphogenetic movements is the name given to movements that happen at the time of gastrulation. These movements then lead to the initiation of morphogenesis. Gastrula is formed as the product of the gastrulation process. The amniotic cavity is the space that appears between the ectoderm and the trophoblast. It is filled with amniotic fluid. Amniogenic cells form the roof of this cavity. These cells are derived from the trophoblast cells.
Formation of Primary Germ Layer
A germinal disc is formed by the rearrangement of the cells of the inner cell mass or the embryonic knob. The germinal disc then further differentiates into two layers that are the epiblast and the hypoblast. The epiblast is the outer layer and the hypoblast is the inner layer. The process of gastrulation begins with the formation of a primitive streak on the surface of epiblast cells. The hypoblast cells are the first cells to move inwards. They help in creating the endoderm layer. After the endoderm, the mesoderm layer is formed by the inward moving of the epiblast cells. The cells that remain in the epiblast from the ectoderm. So, the epiblast is responsible for the formation of all the germ layers in the body.
19.9: Cleavage and Blastula Stage - Biology
The early stages of embryonic development, such as fertilization, cleavage, blastula formation, gastrulation, and neurulation, are crucial for ensuring the fitness of the organism.
Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. The acrosomal reaction causes an egg to prevent additional sperm from penetrating. As the egg completes meiosis II, sperm and egg nuclei fuse.
The formed zygote undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage that produces over 100 cells in the embryo. This process is called the blastula formation . During cleavage, the cells divide without an increase in mass that is, one large single-celled zygote divides into multiple smaller cells. As the blastula forms the blastocyst in the next stage of development, the cells in the blastula arrange themselves into the inner cell mass, and an outer layer.
The typical blastula is a ball of cells. The next stage in embryonic development is the first cell movement and the formation of the primary germ layers. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation . These three germ layers are the endoderm, the ectoderm , and the mesoderm . The ectoderm gives rise to skin and the nervous system the endoderm to the intestinal organs and the mesoderm to the rest of the organs.
Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm. The ectoderm gives rise to the nervous system by folding into a neural tube.
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• Upon fusion of the two plasma membranes, the sperm’s nucleus enters the egg and fuses with the nucleus of the egg.
• Both the sperm and the egg each contain one half the normal number of chromosomes, so when they fuse, the resulting zygote is a diploid organism with a complete set of chromosomes.
• Gastrulation takes place after cleavage and the formation of the blastula.
• The ectoderm gives rise to skin and the nervous system the endoderm to the intestinal organs and the mesoderm to the rest of the organs.
• Neurulation is the formation of the neural tube from the ectoderm, which forms into a neural tube.
blastula : a 6-32-celled hollow structure that is formed after a zygote undergoes cell division
inner cell mass : a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals
gastrulation : the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells
neurulation : The process that forms the vertebrate nervous system in embryos.
notochord : Composed of cells derived from the mesoderm, this provides signals to the surrounding tissue during development.
Cleavage and Blastula Stage
The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage. Cleavage is illustrated in (see the figurebelow). After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).
During cleavage, the zygote rapidly divides into multiple cells without increasing in size.
Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula, shown in the figure below. During cleavage, the cells divide without an increase in mass that is, one large single-celled zygote divides into multiple smaller cells. Each cell within the blastula is called a blastomere.
The cells rearrange themselves to form a hollow ball with a fluid-filled or yolk-filled cavity called the blastula. (credit a: modification of work by Gray’s Anatomy credit b: modification of work by Pearson Scott Foresman, donated to the Wikimedia Foundation)
Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.
In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo. At this stage of development, illustrated in the figure below, the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta and nourish the embryo.
The rearrangement of the cells in the mammalian blastula to two layers—the inner cell mass and the trophoblast—results in the formation of the blastocyst.
Visit the Virtual Human Embryo project at the Endowment for Human Development site to step through an interactive that shows the stages of embryo development, including micrographs and rotating 3-D images.
19.9: Cleavage and Blastula Stage - Biology
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In human embryonic development as the fertilized zygote travels down the fallopian tube into the uterus the process of cleavage, rapid mitotic cell division that does not result in growth begins and is followed by blastulation the first stage of cellular differentiation.
At the onset of cleavage the zygote first divides within a protective membrane called the zona pellucida to form two smaller daughter cells called blastomeres. Which then go through another round of mitosis resulting in four new blastomeres that are smaller than their parent cells.
Beginning with the eight blastomere stage, compaction starts to occur. Blastomeres tightly push against each other making tight making tight junctions and becoming almost indistinguishable from one another. At the 16 blastomere stage the embryo becomes a morula and the cells continue to divide and form an outer and an inner cell layer along with an inner fluid filled cavity.
This stage of development the blastocyst stage in humans is marked by the development of the trophoblast which will will become the placenta and the inner cell mass which are the embryonic stem cells that will continue developing into the embryo. Finally, the zona pellucida will dissolve to allow the blastocyst to implant into the uterus wall and begin the gastrulation stage.
25.4: Cleavage and Blastulation
After a large-single-celled zygote is produced via fertilization, the process of cleavage occurs while zygotes travel through the uterine tube. Cleavage is a mitotic cell division that does not result in growth. With each round of successive cell division, daughter cells get increasingly smaller.
Zygotic Genome Activation
At the beginning of embryogenesis, maternal mRNAs control development. However, by the eight-cell stage of cleavage, embryonic genes become activated in a process called zygotic genome activation (ZGA). As a result, maternal mRNAs get degraded, and ZGA causes a transition from maternal to zygotic genetic control of developing an embryo. Although maternal mRNAs get degraded, previously translated proteins may remain in the embryo through later stages of development.
Cleavage patterns vary between organisms depending on the presence and distribution of egg yolk amongst other factors. For example, mammals have a holoblastic rotational cleavage pattern. They are holoblastic because they have sparse, but evenly distributed yolk and therefore end up with a cleavage furrow that extends through the entire embryo as opposed to being meroblastic where the cleavage furrow does not extend through the yolk-dense portion of the cytoplasm.
At the onset of cleavage, rotational cleavage begins when the zygote first divides to form two smaller daughter cells called blastomeres. During this first cleavage event, division occurs in an austral fashion. The two daughter blastomeres then go through mitosis to each form two new blastomeres. During this second cleavage event, one daughter blastomere cleaves in an austral manner while the second cleaves equatorially. This pattern continues so that the resultant blastomeres end up being smaller than their respective parent cells.
At the eight-blastomere stage, compaction starts to occur&mdashblastomeres tightly push against each other and appear to be one cell where individual cells are indistinguishable from one another. To stabilize the tightly packed blastomeres, tight junctions are formed among the exterior blastomeres while the interior blastomeres form gap junctions that allow the movement of ions and small molecules in between cells. E-cadherin, a calcium-dependent adhesion molecule, helps to further adhere blastomeres to each other.
Once there are approximately thirty-two blastomeres, the zygote becomes a morula. Morula formation marks the end of cleavage. The morula then becomes a blastula that goes through further differentiation during the subsequent stages of development.
Ajduk, Anna, and Magdalena Zernicka-Goetz. &ldquoPolarity and Cell Division Orientation in the Cleavage Embryo: From Worm to Human.&rdquo Molecular Human Reproduction 22, no. 10 (October 2016): 691&ndash703. [Source]
Cockburn, Katie, and Janet Rossant. &ldquoMaking the Blastocyst: Lessons from the Mouse.&rdquo The Journal of Clinical Investigation 120, no. 4 (April 2010): 995&ndash1003. [Source]
De Vries, Wilhelmine N., Alexei V. Evsikov, Bryce E. Haac, Karen S. Fancher, Andrea E. Holbrook, Rolf Kemler, Davor Solter, and Barbara B. Knowles. &ldquoMaternal Beta-Catenin and E-Cadherin in Mouse Development.&rdquo Development (Cambridge, England) 131, no. 18 (September 2004): 4435&ndash45. [Source]
19.9: Cleavage and Blastula Stage - Biology
The product of fertilization is a one-cell embryo with a diploid complement of chromosomes. Over the next few days, the mammalian embryo undergoes a series of cell divisions, ultimately leading to formation of a hollow sphere of cells known as a blastocyst. At some point between fertilization and blastocyst formation, the embryo moves out of the oviduct, into the lumen of the uterus.
The images below demonstrate major transitions in structure during early embryogenesis in cattle. Note that in all of the the early stages, the embryo is encased in its zona pellucida. Embryos from other mammals have a very similar appearance, and the general sequence of stages is seen in all mammals.
Unfertilized oocytes typically fill the entire space inside the zona pellucida, but after fertilization, the one-cell embryo usually is somewhat retracted from the zona pellucida surrounding it. Although not visible in this image, one or two polar bodies are often visible in the perivitelline space , the area between the embryo and the zona pellucida.
The one cell embryo undergoes a series of cleavage divisions , progressing through 2-cell, 4-cell, 8-cell and 16 cell stages. A four cell embryo is shown here. The cells in cleavage stage embryos are known as blastomeres . Note that the blastomeres in this embryo, and the eight-cell embryo below, are distinctly round.
Early on, cleavage divisions occur quite synchronously. In other words, both blastomeres in a two-cell undergo mitosis and cytokinesis almost simultaneously. For this reason, recovered embryos are most commonly observed at the two, four or, and seen here, eight-cell stage. Embryos with an odd number of cells (e.g. 3, 5, 7) are less commonly observed, simply because those states last for a relatively short time.
Soon after development of the 8-cell or 16-cell embryo (depending on the species), the blastomeres begin to form tight junctions with one another, leading to deformation of their round shape and formation of a mulberry-shaped mass of cells called a morula . This change in shape of the embryo is called compaction . It is difficult to count the cells in a morula the embryo shown here probably has between 20 and 30 cells.
Formation of junctional complexes between blastomeres gives the embryo and outside and an inside. The outer cells of the embryo also begin to express a variety of membrane transport molecules, including sodium pumps. One result of these changes is an accumulation of fluid inside the embryo, which signals formation of the blastocyst . An early blastocyst, containing a small amount of blastocoelic fluid, is shown to the right.
As the blastocyst continues to accumulate blastocoelic fluid, it expands to form - you guessed it - an expanded blastocyst . The blastocyst stage is also a landmark in that this is the first time that two distinctive tissues are present. A blastocyst is composed of a hollow sphere of trophoblast cells , inside of which is a small cluster of cells called the inner cell mass . Trophoblast goes on to contribute to fetal membrane systems, while the inner cell mass is destined largely to become the embryo and fetus. In the expanded blastocyst shown here, the inner cell mass is the dense-looking area at the botton of the embryo.
Eventually, the stretched zona pellucida develops a crack and the blastocyst escapes by a process called hatching . This leaves an empty zona pellucida and a zona-free or hatched blastocyst lying in the lumen of the uterus. Depending on the species, the blastocyst then undergoes implantation or elongates rapidly to fill the uterine lumen.
As mentioned, the developmental progression depicted above for bovine embryos is essentially identical to what all mammalian embryos go through, including humans. For example, the image to the left shows an expanded blastocyst from a dog. This embryo was stained to accentuate the trophoblast and inner cell mass.
The length of time required for preimplantation development varies somewhat, but not drastically, among species. The zona-intact bovine blastocysts shown above were collected 5-6 days after fertilization. The same stages would be seen in mice at about 3.5 days after fertilization.
In addition to the morphological changes in the embryo described here, preimplantation development is associated with that might be called an awakening of the embryonic genome. There is, for instance, little transcription in the embryos of most species prior to the 8 cell stage, but as embryos develops into morulae, then blastocysts, a large number of genes become transcritionally active and the total level of transcription increases dramatically.
Cleavage: Meaning, Planes and Types | Embryology
In this article we will discuss about:- 1. Meaning of Cleavage 2. Planes of Cleavage 3. Types 4. Effects of Yolk 5. Mechanism 6. Chemical Changes 7. Different Chordates 8. Importance in Embryonic Pattern.
- Meaning of Cleavage
- Planes of Cleavage
- Types of Cleavage
- Effects of Yolk in Cleavage
- Mechanism of Cleavage
- Chemical Changes during Cleavage
- Cleavage in Different Chordates
- Importance of Cleavage in Embryonic Pattern
Fertilization results into the formation of zygote. The process of segmentation (cleavage) immediately follows fertiliza­tion or any other process which activates the egg. Cleavage consists of division of the zygote into a large number of cel­lular entities. The cells which are produc­ed during segmentation are called blastomeres.
At first, the cells remain closely asso­ciated, but later on they form the lining of a hollow sphere called blastula. The blastula contains a cavity named blastocoel and its outer covering is designated as blastoderm. The formation of blastula culminates the cleavage period.
The process of segmenta­tion prepares the groundwork for the future design of the embryo by producing ade­quate number of cells. The cleavage also establishes the fundamental conditions for the initiation of next developmental stage —Gastrulation.
2. Planes of Cleavage:
During early cleavage, distinct geome­trical relationships exist between the blastomeres, i.e., each plane of cell-division bears a definite relationship with each other.
The planes of division are:
a. Meridional plane of cleavage:
When a furrow bisect both the poles of the egg passing through the median axis or centre of egg it is called meridional plane of cleavage. The median axis runs between the centre of animal pole and vegetal pole.
b. Vertical plane of cleavage:
When a fur­row passes in any direction (does not pass through the median axis) from the animal pole towards the opposite pole.
c. Equatorial plane of cleavage:
This type of cleavage plane divides the egg halfway between the animal and vegetal poles and the line of division runs at right angle to the median axis.
d. Latitudinal plane of cleavage:
This is al­most similar to the equatorial plane of cleavage, but the furrow runs through the cytoplasm on either side of the equatorial plane.
3. Types of Cleavage:
Considerable amount of reorganisation occurs during the period of cleavage and the types of cleavage depend largely upon the cytoplasmic contents.
Different types of cleavage encountered in different eggs are catalogued below:
a. Holoblastic op total cleavage:
When the cleavage furrows divide the entire egg.
When the cleavage furrow cuts the egg into two equal cells. It may be radially symmetrical, bilaterally, symme­trical, spirally symmetrical or irregular.
When the resultant blastomeres become unequal ir size.
b. Meroblastic cleavage:
When segmentation takes place only in a small portion of the egg resulting in the formation of blasto­derm, it is called meroblastic cleavage. Usually the blastoderm is present in the animal pole and the vegetal pole becomes laden with yolk which remains in an tihcleaved state, i.e., the plane of division does not reach the periphery of blasto­derm or blastodisc.
c. Transitional cleavage:
In many eggs, the cleavage is atypical which is neither typi­cally holoblastic nor meroblastic, but assumes a transitional stage between the two.
4. Effects of Yolk in Cleavage:
The fertilized egg in most cases contains yolk, which are inert bodies. During divi­sion these bodies exert mechanical in­fluences. In the egg of Amphioxus, the yolk is thin and remains uniformly distributed. Therefore the division is complete and early divisions occur at a very quicker rate.
The amphibian egg contains yolk which is localised at the vegetal pole. Here division initiates from the animal pole and extends towards the vegetal pole, where the progress of cleavage slows down con­siderably.
Consequently, the animal pole divides faster than the vegetal pole. The eggs of reptiles and birds are fully laden with large masses of yolk, thus restricting the cytoplasm and nucleus on the peri­phery as a circular disc on the animal pole. Here the lines of cleavage divide only the small animal pole region. Such effects of yolk on cleavage pattern influence the pattern of further development.
5. Mechanism of Cleavage:
The incidence of cleavage provides unique opportunity to study the mecha­nism of cell division and specially the role of different cell organelles during division.
Opinions differ regarding the accu­mulation of force for the initiation of cleavage and following factors are believed to be responsible for controlling the clea­vages:
(a) Localised expansion of cortex.
(b) Increased stiffness of the cortical cyto­plasm.
(c) Increase of tangential force activity in the cortex.
(d) Contractile nature of the regions near the cortex and
(e) Formation of new cell membrane from the subcortical cytoplasm.
Though the abovementioned factors are not clearly understood, it is evident that three structures present within the cell: Cortical layer, Spindle structures and Chromosomes play the important part.
The energy which is required during the process is supplied by the metabolic activity of the developing egg. Besides the factors involved in segmentation, there are cleavage laws which govern the behaviour of the cells during cleavage.
The blastomeres tend to divide into identical daughter cells and a cleavage furrow tends to cut the previous cell at right angles.
The position of nucleus is vital and it tends to lie at the centre of the protoplasmic content of the cell. The nucleus exerts influence on cleavage. The long axis of mitotic spindle usually coin­cides with the long axis of the protoplasmic content. During cleavage the long axis of the protoplasm has the tendency to cut transversely.
The rate of cleavage is inversely proportional to the amount of yolk material present in the egg.
6. Chemical Changes during Cleavage:
Significant chemical changes go on in the fertilized egg during cleavage.
Increase of nuclear material:
During cleavage a steady increase in nuclear material (predominantly DNA) is obser­ved. Cytoplasm of the egg is the source of such nuclear material. Cytoplasmic DNA contained in mitochondria and yolk platelets are available.
During cleavage mes­senger RNA (mRNA) and transfer RNA (tRNA) are synthesised during cleavage, especially in late stages.
Synthesis of proteins:
Throughout the period of cleavage there is steady and spectacular increase in protein synthesis.
7. Cleavage in Different Chordates:
The pattern of cleavage differs in diff­erent animals. The following account will give an idea of the process of cleavage in different chordates.
The cleavage in Amphioxus is typically holoblastic (Fig. 5.10). The first cleavage is meridional. The second cleavage is also meridional but at right angle to the first one. Four equal blastomeres are produced. The third cleavage is latitu­dinal and occurs slightly above the equato­rial plane resulting in the production of eight blastomeres—four are smaller called the micromeres and four are larger known as the macromeres.
The micromeres are situated towards the animal pole and the macromeres towards the vegetal pole. The fourth cleavage is meridional which involves all the eight cells resulting in the forma­tion of eight micromeres and eight macro­meres. The fifth cleavage planes are latitudinal.
Each micromere is divided into an upper and lower micromere and each macromere likewise divides to form an upper and lower macromere. The fifth cleavage planes produce thirty-two blasto­meres. The sixth cleavage planes are nearly meridional involving all the thirty-two cells resulting in sixty-four cells.
At the 64-cell stage a conspicuous space is produced at the centre and this space becomes filled with a fluid. When the eighth cleavage planes take place, the blastula becomes pear- shaped and the blastocoel becomes large.
The egg of frog is telolecithal with a considerable amount of yolk localized towards the vegetal pole. The cleavage is holoblastic in nature, but differs consider­ably from that of Amphioxus because of larger quantity of yolk.
The first cleavage plane is meridional which occurs at about 3-3½ hours after fertilization. But the time depends largely on extrinsic factors. The first cleavage starts at the animal pole and gradually travels towards the vegetal pole. Thus the egg is bisected along the poles. Two blasto­meres of equal size are produced. The second cleavage is almost meridional but oriented at right angles to the first cleavage plane (Fig. 5.11).
The four blastomeres thus produced are not qualitatively identi­cal, because the grey crescent material is present in two of the four blastomeres. Each blastomere contains dark pigment at the animal pole and yellowish yolk to­wards the vegetal pole. The third cleavage is latitudinal and occurs at right angles to previous cleavage planes but passes slightly above the equator.
The furrow produces eight unequal blastomeres, four micro­meres in the animal hemisphere and four macromeres in the vegetal part. The fourth cleavage planes are meridional which involve the micromeres first and pass on slowly towards the yolk-laden macro­meres of the vegetal pole.
In Amphioxus, the cleavages occur in a synchronous fas­hion, while in frog considerable degree of irregularities (asynchronism) appear in later stages. But it is certain that the micromeres always continue to divide at a faster rate than do the macromeres.
At the eight-celled stage, a small space makes its appearance between the four micromeres. As development goes on, this space becomes conspicuous and forms the blastocoel. The floor of the blastocoel is formed of macromeres. The blastocoel (or segmentation cavity) is eccentrically located and becomes displaced towards the animal pole as development proceeds.
Typical meroblastic cleavage occurs in chick, where the segmentation activity is restricted only at the blastodisc or germinal disc (Fig. 5.12). Thus the clea­vage is incomplete.
The first cleavage starts as a meridional furrow near the centre of the blastodisc at about 4½ hours after fertilization when the egg reaches the isthmus of oviduct. This furrow cuts across the blastodisc and passes towards the vegetal pole but does not reach the pole. The second cleavage is also meri­dional, but approximately at right angles to the first one. The third cleavage is vertical.
The fourth cleavage is also vertical but the division is not synchronous. As a conse­quence eight central cells encircled by twelve marginal cells are produced. From this point onward the cleavage becomes irregular and a disc containing smaller cells appears.
This disc remains firmly connected with the underlying yolk. Soon a cleft appears which separates the disc in the middle from the underlying yolk. The new cavity in between is known as sub-germinal space (Fig. 5.13).
Thus at the end of seg­mentation, the disc contains many-layered small cells which are connected with the yolk only at the periphery. This disc is then termed as blastoderm, the cells of which still continue to divide.
The peripheral part which lies in contact with yolk possesses granular cells called area apaca and the inner layer having clear portion is called area pellucida. At one end of area opaca, aggregation of cells takes place. This deno­tes the formation of future posterior side.
The egg of rabbit is small and does not contain any yolk (i.e. alecithal type of egg). The cleavage is holoblastic and nearly equal. Irregularities and a syn­chronism become the rule in the cleavage of rabbit like all other eutherian mammals.
The first cleavage is vertical resulting in the formation of two unequal blasto­meres. The second cleavage is also vertical but runs at right angle to the first. The third cleavage is horizontal but slightly above the equator.
Subsequent divisions are rapid and irregular. The blastomeres thus produced become clustered together to form a solid cellular ball called morula. Two types of cells (small and large) are recognised in the morula.
The large cells lie at the centre. Soon a cavity appears inside the cell mass on one side. The cavity gradually increases which shifts the central cell mass to one side. The stage is called blastocyst stage. The inner cell mass in the centre is attached with the outer cell layer (trophoblast) of the blastocyst.
The cavity is called the blastocoel or sub-germinal cavity (Fig. 5.13A) which is filled with a fluid. The inner cell mass remains attached at the embryonic knob towards the animal pole. From this em­bryonic knob, the embryo arises. The trophoblast which encloses the blastocoel and the embryonic knob participates in the formation of placenta. The tropho­blastic cells overlying the embryonic knob is called cells of Rauber.
8. Importance of Cleavage in Embryonic Pattern:
The cleavage phase of development and blastulation are extremely significant, be­cause the blastoderm is morphologically elaborated in such a way that the impor­tant presumptive organ forming areas of the future embryo are segregated into definite districts of the blastoderm.
Such orientation of the organ forming areas in the blastoderm permits an ordered move­ment of these areas during gastrulation to take up their fateful position. So the period of cleavage and blastulation is regarded as the phase of preparation for future diff­erentiation.
The cells which are produced at the end of segmentation resemble the zygote—but do they possess the same potentiality as the zygote itself. Driesch (1891), in order to get an answer, separated the two blasto­meres at the two-celled stage and found that both the blastomeres developed into complete embryos.
His conclusion was that each blastomere has the full potentiality to be an entire embryo. But in 1900, Roux showed that if one of the blastomeres of the two-celled stage is killed, the remain­ing one produces ‘half embryo’.
He claim­ed that each cleavage results into the segregation of specialization in the blas­tomeres and this is irreversible. This ex­periment demonstrates that an organising or controlling centre is elaborated to con­trol the development process.
The experi­ment of Spemann and others have shown that it is the grey crescent region which plays the vital role in the process of deter­mination and the blastomeres which are formed due to segmentation are neither completely regulative nor irreversibly determined.
Fig. 5.14 shows the import­ance of grey crescent in the development of amphibian embryo. It has been experi­mentally established that the grey crescent in the amphibian blastula transforms into the dorsal lip of the blastopore which acts as an instigator and controller of the gastrulation process.
Cleavage: Definition and Patterns | Fertilization | Embryology
In this article we will discuss about:- 1. Definition of Cleavage 2. Chemical Changes during Cleavage 3. Patterns 4. Nuclei of Cleavage Cells 5. Morula and Blastula.
Definition of Cleavage:
One of the peculiarities of sexual reproduction in animals is that the complex multicellular body of the offspring originates from a single cell—the fertilized egg. It is necessary, therefore, that the single cell be transformed into a multicellular body.
This transformation takes place at the very beginning of development and is attained by means of a number of cell divisions following in rapid succession. This series of cell divisions is known as the process of cleavage.
Cleavage can be characterized as that period of development in which:
1. The unicellular fertilized egg is transformed by consecutive mitotic divisions into a multicellular complex.
3. The general shape of the embryo does not change, except for the formation of a cavity in the interior—the blastocoele.
4. Apart from transformation of cytoplasmic substances into nuclear substance, qualitative changes in the chemical composition of the embryo in cleavage are limited.
5. The constituent parts of the cytoplasm of the egg are not displaced to any great extent and remain on the whole in the same positions as in the egg at the beginning of cleavage.
6. The ratio of nucleus to cytoplasm, very low at the beginning of cleavage, is, at the end, brought to the level found in ordinary somatic cells.
Chemical Changes during Cleavage:
Although there is no growth during the period of cleavage, chemical transforma­tions do occur, and at least some are markedly intensified, as compared with the conditions in the unfertilized egg.
The most obvious change observed during cleavage is a steady increase of nuclear material at the expense of cytoplasm. The number of nuclei is of course doubled with every new division of the blastomeres, and this doubling is accompanied by an increase of nuclear substance, which involves an increase of deoxyribonucleic acid—the amount per nucleus of the latter remaining constant.
The increase in the chromosomal deoxyribonucleic acid, at least during the earlier phases of development, must be at the expense of materials contained in the egg. There are several possible sources of such materials. First, the nucleic acids present in the cytoplasm of the eggs should be mentioned.
In sea urchin eggs there is a large amount of ribonucleic acid in the egg cytoplasm, and this gradually disappears later in develop­ment. When sea urchin embryos are supplied with radioactively tagged uridine, some of it is later incorporated into the DNA.
Such incorporation is made possible by the presence in developing sea urchin eggs of an enzyme, ribonucleotide reductase, which converts ribonucleotides into deoxyribonucleotides. DNA may, however, be synthesized in the cleaving egg directly from low molecular weight precursors. This has been proved by supplying such precursors, labeled with radioactive atoms, to cleaving eggs of sea urchins and amphibians.
When cleaving sea urchin eggs were kept in seawater contain­ing 14C-labeled glycine (which may be used for the synthesis of purine groups in the nucleic acid molecule), it was found that the radioactive carbon atoms were incorpo­rated in large amounts into the deoxyribonucleic acid, bypassing the cytoplasmic ribonucleic acid. Also, when 14C-labeled glycine was injected into fertilized frog eggs, some of it was incorporated into deoxyribonucleic acid.
The second important aspect of metabolism during cleavage is the synthesis of ribonucleic acids, which is believed to be very limited, although not absent altogether. In frogs, ribosomal RNA apparently is not produced at all until after completion of cleavage.
As the nucleolus is the site of synthesis of rRNA, this organelle is completely lacking in these animals during cleavage. It reappears in the nuclei at the onset of gastrulation simultaneously with the resumption of ribosomal RNA synthesis.
In the sea urchin there is very little ribosomal RNA produced during cleavage, but in both the amphibians and sea urchins synthesis increases drastically at the onset of gastrulation. Messenger RNA and transfer RNA, on the other hand, are synthesized during cleavage, or at least in the later stages of cleavage.
Synthesis of RNA, however, does not seem to be necessary for cleavage, since eggs which are treated with actinomycin D and in which presumably DNA dependent RNA synthesis is suppressed continue cleaving normally. It is concluded, therefore, that any messenger RNA produced during cleavage remains inactive or “masked,” similar to the messenger RNA in unfertilized eggs.
Fertilization in sea urchins leads to a spectacular increase in protein synthesis, and this is continued throughout the period of cleavage. In other animals, such as amphibians, protein synthesis does not markedly change after fertilization a certain amount of protein synthesis, however, takes place throughout the period of cleavage. The amount of active cytoplasm increases. One indication of this is the steady increase of respiration throughout the period, which is generally attributed to an increase in the amount of active cytoplasm.
Much of the protein newly produced during cleavage is directly involved in the process of cell multiplication. One group of such proteins is the nuclear histones, which are needed for the chromosome replication in the same degree as additional quantities of DNA. In mid-cleavage of sea urchin embryos as much as 50 per cent of the newly synthesized protein is located in the nucleus.
The mRNA for these proteins is transcribed during cleavage and contrary to other mRNA’s, does not become masked, but is immediately used for translation into protein. This exception is probably due to the need for rapid synthesis of large quantities of nuclear histones. Some mRNA for nuclear histones is, however, present in the egg before fertilization.
Another protein synthesized during cleavage is tubulin, the constituent protein of microtubules—the fibers of the achromatic figures appearing during the mitotic divisions of cleavage cells. Tubulin is synthesized on messenger RNA already present in the egg. In the course of cleavage, tubulin is synthesized in increasing quantities, presumably as a result of progressive “unmasking” of the corresponding mRNA.
A third protein synthesized during cleavage is the enzyme ribonucleotide re­ductase, which in sea urchin embryos converts cytoplasmic ribonucleotides into deoxyribonucleotides, and thus provides a source of material for the replication of the chromosomal DNA.
The messenger RNA for ribonucleotide reductase is present in the unfertilized egg, but becomes active (is unmasked) after fertilization. A fourth protein necessary for chromosomal replication, the DNA polymerase, is already present in necessary quan­tities in the egg, and its quantity does not increase during early development.
If cleaving eggs are treated with puromycin, which inhibits RNA dependent protein synthesis, cleavage stops immediately, thus showing that protein synthesis is indispens­able for cleavage to take place. This is in marked contrast to cleavage being able to proceed in the presence of actinomycin D which effectively prevents the production of new RNA and particularly of new messenger RNA.
Possibly the most important of the proteins which have to be synthesized for cleavage to proceed are those which are used in the replication of the chromosomes – the nucleohistones, actually incorporated into the chromosome structure, and the ribonucleotide reductase, without which the cells are unable to use the supplies of RNA in the cytoplasm for replication of the nuclear DNA.
The synthesis of tubulin seems to be less important, as asters may be formed in the cytoplasm in the presence of puromycin, but do not lead to cell division. Presumably there is enough tubulin in the egg for aster formation, without new synthesis.
Patterns of Cleavage:
The way in which the egg is subdivided into the daughter blastomeres is usually very regular. The plane of the first division is, as a rule, vertical it passes through the animal-vegetal axis of the egg. The plane of the second division is also vertical and passes through the animal-vegetal axis, but it is at right angles to the first plane of cleavage.
The result is that the first four blastomeres all lie side by side. The plane of the third division is at right angles of the first two planes and to the animal-vegetal axis of the egg. It is therefore horizontal or parallel to the equator of the egg. Of the eight blastomeres, four lie on top of the other four, the first four comprising the animal hemisphere of the embryo, the second the vegetal hemisphere.
If each of the blastomeres of the upper tier lie over the corresponding blastomeres of the lower tier, the pattern of the blastomeres is radially symmetrical. This is called the radial type of cleavage.
In many animals, however, the upper tier of blastomeres may be shifted with respect to the lower tier, and the radially symmetrical pattern becomes distorted in various degrees. The distortion may sometimes be due to individual variation, but there are certain groups of animals in which distortion always takes place and is the result of a specific structure of the egg.
In the annelids, molluscs, nemerteans, and some of the planarians (the Poly-cladida), all the blastomeres of the upper tier are shifted in the same direction in relation to the blastomeres of the lower tier, so that they come to lie not over the corresponding vegetal blastomeres, but over the junction between each two of the vegetal blastomeres.
This arrangement comes about not as a result of secondary shifting of the blastomeres, but because of oblique positions of the mitotic spindles, so that from the start the two daughter cells do not lie one above the other. The four spindles during the third cleavage are arranged in a sort of spiral. This type of cleavage is therefore called the spiral type of cleavage.
The turn of the spiral as seen from above may be in a clockwise direction or in a counterclockwise direction. In the first case the cleavage is called dextral in the second case it is called sinistral. Since the cleavage planes are at right angles to the spindles, they also deviate from the horizontal position found in radial cleavage, and each cleavage plane is inclined at a certain angle.
The spiral arrangement of the mitotic spindles can be traced even in the first two divisions of the egg the spindles are oblique and not vertical as in radial cleavage. However, the resulting shifts in the position of the blastomeres are not as obvious as after the third cleavage.
During the subsequent cleavages the spindles continue to be oblique, but the direction of spiraling changes in each subsequent division. Dextral spiraling alternates with sinistral, so that the spindle of each subsequent cleavage is approximately at right angles to the previous one.
Note that the type of cleavage of the egg as a whole, whether dextral or sinistral, depends on the direction of spiraling occurring during the third division of the egg.
Peculiarities of the cleavage pattern can also be introduced by differences in the size of the blastomeres. Of the four blastomeres in the four-cell stage of eggs having a spiral type of cleavage, one blastomere is often found to be larger than the other three.
This allows us to distinguish the individual blastomeres. The four first blastomeres are denoted by the letters A, B, C, D, the letters going in a clockwise direction (if the egg is viewed from the animal pole) and the largest blastomere being denoted by the letter D.
In some animals which otherwise have an approximately radial type of cleavage, two of the first four blastomeres may be larger than the other two, thus establishing a plane of bilateral symmetry in the developing embryo. Subsequent cleavages may make the bilateral arrangement of the blastomeres still more obvious (as in tunicates and in nematodes, although in a different way). The resulting type of cleavage is referred to as the bilateral type.
A very special type of cleavage showing bilateral symmetry is found in nematodes. The first division produces two unequal cells – a slightly larger cell designated as cell AB and a smaller cell P1. The two cells divide next in mutually perpendicular planes, so that the blastomeres in the four-cell stage are placed in the form of a letter T.
The transverse shaft of the T is made up of blastomeres A and B (descendants of the cell AB), and the vertical shaft is made up of the offspring of blastomere P1. The cells are designated as EMSt (abbreviation for endoderm, mesoderm, stomodeum—which shows the destiny of this cell) and as P2. The “T” arrangement is, however, only temporary, the P2 cell soon shifting toward the B cell.
The blastomeres are then arranged in a rhomboid figure. Next, the third division enhances the bilateral symmetry of the embryo, because the blastomeres A and B each divide into a right and left daughter cell, while the other two blastomeres produce a group of four cells lying one behind the other in the median plane. The blastomeres of this group are designated Mst, E, P3, and C, respectively.
The cleavage in nematodes is also an example of determinate or mosaic cleavage in which definite blastomeres give rise to specific parts of the embryo. Thus, blastomeres A, B, and C give rise to the skin of the animal, blastomere E gives rise to the endoderm of the alimentary tract, blastomere MSt gives rise to the mesoderm and the stomodeum, and blastomere P3 eventually produces the reproductive cells.
From the stage of eight cells a slight asymmetry is noticeable between the right and the left halves of the embryo.
The yolk, which is present in the egg at the beginning of cleavage in greater or lesser quantities, exerts a very far-reaching effect on the process of cleavage. Every mitosis involves movements of the cell components—the chromosomes, parts of the cytoplasm constituting the achromatic figure, the mitochondria, and the surface layer of the cell—the activity of which along the equator of the maternal cell leads to the eventual separation of the daughter cells.
During these movements, the yolk granules or platelets behave entirely passively and are passively distributed among the daughter blasto­meres. When the yolk granules or platelets become very abundant, they tend to retard and even to inhibit the process of cleavage.
As a result, the blastomeres which are richer in yolk tend to divide at a slower rate and consequently remain larger than those which have less yolk. The yolk in the uncleaved egg is more concentrated toward the vegetal pole of the egg. It is therefore at the vegetal pole of the egg that cleavage is most retarded by the presence of yolk, and where the blastomeres are of the largest size.
A good example of the effect of the yolk on cleavage is provided by the frog’s egg. The yolk’s influence may be detected even during the first division of the fertilized egg. During the anaphase of the mitotic division, a furrow appears on the surface of the egg which is to separate the two daughter blastomeres from each other.
This furrow, however, does not appear simultaneously all around the circumference of the egg, but at first only at the animal pole of the egg, where there is less yolk. (It has been indicated that the first cleavage plane is vertical and therefore passes through the animal and vegetal poles of the egg.)
Only gradually is the cleavage furrow prolonged along the meridians of the egg, until, cutting through the mass of yolk-laden cytoplasm, it eventually reaches the vegetal pole and thus completes the separation of the first two blastomeres.
The same process is repeated during the second cleavage. During the third cleavage, when the plane of separation of the daughter blastomeres is horizontal, the furrow appears simultaneously over the whole circumference of the egg, for it meets everywhere with an equal resistance from yolk.
A further accumulation of the yolk at the vegetal pole of the egg causes still greater delay in the cell fission at this pole, so that the cleavage becomes inhibited more and more. This can be clearly traced in a series of various ganoid fishes, whose eggs possess an increasing amount of yolk.
In Acipenser the cleavage is complete, as in the amphibians, but the difference between the micromeres of the animal hemisphere and the macro-meres of the vegetal hemisphere is much greater than in amphibians. In Amia cleavage starts at the pole, and the cleavage furrows reach the vegetal pole, but they are so retarded that subsequent divisions begin at the animal pole before the preceding furrows cut through the yolk at the vegetal pole.
In Lepidosteus the cleavage starts at the animal pole as in Amia, but the cleavage furrows never reach the vegetal pole, so that the vegetal hemisphere of the egg remains un-cleaved, resulting in what is called incomplete cleavage. The type of cleavage during which the whole of the egg becomes subdivided into blastomeres is called holoblastic (complete) cleavage.
The type of cleavage in which only a part of the egg is subdivided into blastomeres is called meroblastic (incomplete) cleavage. As the result of meroblastic cleavage the egg is divided into a number of separate blastomeres, and a residue, which is a continuous mass of cytoplasm, usually with some nuclei scattered in it.
Sometimes the terms holoblastic and meroblastic are applied also to the eggs having a particular type of cleavage thus, one finds in the literature the term “holoblastic eggs,” meaning eggs having holoblastic cleavage, and also the term “meroblastic eggs” for eggs having meroblastic cleavage.
In eggs in which the yolk is segregated from the active cytoplasm (elasmobranchs, bony fishes, birds, and reptiles), the cleavage, right from the start, is distinctly recogniza­ble as meroblastic or incomplete. At first, all the cleavage planes are vertical, and all the blastomeres lie in one plane only.
The cleavage furrows separate the daughter blastomeres from each other but not from the yolk, so that the central blastomeres are continuous with the yolk at their lower ends, and the blastomeres lying on the circumference are, in addition, continuous with the un-cleaved cytoplasm at their outer edges. As the nuclei at the edge divide, more and more cells become cut off to join the ones lying in the center, but the new blastomeres are also in continuity with the un-cleaved yolk underneath.
In a later stage of cleavage, the blastomeres of the central area become separated from the underlying yolk in one of two ways – either slits may appear beneath the nucleated parts of the cells, or else the cell divisions may occur with horizontal (tangen­tial) planes of fission. In the latter case one of the daughter cells, the upper one, becomes completely separated from its neighbors, while the lower blastomere retains the connec­tion with the yolk mass.
The marginal cells, which remain continuous with one another around their outer edges, are also continuous with the mass of yolk and hence with the lower cells resulting from tangential divisions. All these blasto­meres eventually lose even those furrows which partially separated them from one another and fuse into a continuous syncytium with numerous nuclei but no indication of individual cells.
Nuclei of Cleavage Cells:
In his “germ plasm theory,” A. Weismann presented a hypothesis to explain both heredity and the ontogenetic development (Metazoa) of organisms. According to Weismann (1904), every distinct part of an organism (animal or plant) is represented in the sex cell by a separate particle – a determinant.
Thus, the sum total of determinants would represent the parts of the adult organism with all their peculiarities. The complete set of determinants is supposedly handed down from generation to generation, which would account for hereditary transmission of characters.
The determinants, according to Weismann, are localized in the chromosomes of the nucleus, just as the genes of modern genetics are. However, there is a difference – the genes are not supposed to represent parts of the organism but rather properties which may sometimes be discernible in all the parts of the body.
During the cleavage of the egg, the various determinants, according to Weismann, become segregated into different cells. The blastomeres would receive only some of the determinants, namely, those which correspond to the fate of each blastomere.
The successive segregation of the determinants would eventually result in each cell’s having determinants of only one kind, and then nothing would be left to the cell but to differentiate in a specific way in accordance with the determinants present. Only the cells having the sex cells among their descendants, Weismann held, would preserve the complete set of determinants, since these would be necessary for directing the de­velopment of the next generation.
Even though Weismann’s conception of the properties of determinants is not tenable from the genetic viewpoint, it is still important to know whether the difference in the fate of the blastomeres and parts of the embryo may be attributed to differences in the nuclei of the cleavage cells.
Our knowledge of the development of complete embryos from one of the two daughter blastomeres of the egg (as a result of either separating the first two blastomeres or killing one of the first two blastomeres) contradicts Weismann’s hypothesis about the segregation of determinants during cleavage.
The evidence be­comes still more conclusive because it has been found that not only are the first two blastomeres capable of developing into complete embryos, but the blastomeres in later cleavage stages sometimes possess the same ability. In the case of sea urchins, one of the first four blastomeres, and even occasionally one of the first eight, may develop into a whole embryo with all the normal parts but reduced in size.
The method of isolating blastomeres, however, does not permit one to test the properties of nuclei of later generations of cells. In the four-cell stage, the quantity of cytoplasm contained in one blastomere may already be too small for development to take place in an approximately normal fashion. This is true in increasing degree as cleavage proceeds and the individual blastomeres become smaller and smaller.
To further this investigation, a different method has been devised. A most elegant experiment in this field was carried out by Spemann (1928). Spemann constricted fertilized eggs of the newt Triturus (Triton) into two halves with a fine hair, just as they were about to begin to cleave.
The constriction was not carried out completely, so that the two halves were still connected to each other by a narrow bridge of cytoplasm. The nucleus of the fertilized egg lay in one half, and the other half consisted of cytoplasm only. When the egg nucleus began to divide, the cleavage was at first restricted to that half of the egg which contained the nucleus.
This half divided into two, four, eight cells, and so on, while the non-nucleated half remained un-cleaved. At about the stage of 16 blastomeres, one of the daughter nuclei, now much smaller than at the beginning of cleavage, passed through the cytoplasmic bridge into the half of the egg which had hitherto no nucleus.
Forthwith this half also began to cleave. After both halves of the egg were thus supplied with nuclei, Spemann drew the hair loop tighter and completely separated the two halves of the egg from each other. They were then allowed to develop into embryos. In a number of cases two completely normal embryos developed from the two halves of the egg.
Of the two embryos, each started by having one half of the egg cytoplasm, but as to the nucleus they were in very different situations. While one of the embryos possessed 15/16 or even 31/32 of all the nuclear material of the egg, the other received only 1/16 or 1/32 of the nuclear material. The experiment proves conclusively that even in the 32-cell stage every nucleus has a complete set of hereditary factors necessary for the achievement of normal development.
All the nuclei in this stage are completely equiva­lent to one another and to the nucleus of the fertilized egg. The hypothesis of an unequal division of the hereditary substance of the nucleus, of the segregation of determinants or genes to the different cleavage cells, is thus disproved. It is now assumed that every cell of the metazoan body has a complete set of nuclear factors necessary for development (a complete set of genes, in the terminology of modern genetics).
The experiments on the delayed nuclear supply to one half of an amphibian egg have been corroborated by experiments on the eggs of other animals. It will be useful to relate a corresponding experiment carried out on a very different kind of animal, the dragonfly Platycnemis pennipes.
In the dragonfly, cleavage is incom­plete, and only the nucleus divides at first, the cytoplasm remaining un-cleaved. The egg is elongated, and after the first division the daughter nuclei move, one into the anterior half and the other into the posterior half of the egg. When eight nuclei are available, they are spaced along the length of the embryo.
By further divisions nuclei are provided for all cells in the respective regions. In the stage when two nuclei are present, either of them may be destroyed by a short exposure to a narrow beam of ultraviolet light, which does not damage the cytoplasm to any great extent.
The remaining nucleus continues to divide, and its daughter nuclei are distributed to all parts of the egg instead of supplying only one half of it. Completely normal embryos develop, no matter which of the two nuclei is allowed to survive. The two nuclei prove to be completely equivalent for development, although normally they would have supplied different parts of the embryo.
The methods used in the preceding experiments for testing the properties of the cleavage nuclei are of necessity confined to the earlier stages of cleavage. At present, a more universal method is available which allows the investigation to be extended to nuclei of cells of much more advanced embryos, and possibly it may ultimately be applied even to fully differentiated cells of an adult organism.
This is the method used for transplantation of nuclei. The transplantation of nuclei from one cell to another was first carried out successfully on Amoeba, and the method was then applied to test the properties of nuclei in developing frog embryos. The method, as applied to the frog embryo, consists essentially in taking the nucleus of any cell from a developing embryo and injecting it into an enucleated un-cleaved egg.
The egg receiving the nucleus must be specially prepared. The ripe eggs are removed from the oviducts and activated by pricking with a glass needle. The egg nucleus then approaches the surface of the cytoplasm in preparation for the second maturation division and is removed by a second prick with a glass needle at the exact spot where the nucleus is located.
Next, a cell of an advanced embryo is separated from its neighbors and sucked into the tube of an injection pipette. The diameter of the pipette is smaller than that of the cell and as a result the surface of the cell is broken. The contents of the pipette, consisting of the nucleus and the debris of the cytoplasm, are injected deep into the enucleated egg.
When the pipette is withdrawn, the egg cytoplasm tends to escape through the hole in the egg membranes, forming an extraovate protrusion. This must be cut off by a pair of glass needles to prevent further loss of egg substance.
As a result of these procedures, up to 80 per cent of the eggs operated on start cleaving and producing numerous cells, the nuclei of which are derived from the injected nucleus and the cytoplasm of which is from the enucleated egg (the small amount of cytoplasm injected with the nucleus, comprising less than 1/40,000 of the volume of the egg cytoplasm may be ignored). Not all eggs which start cleaving develop normally later, but at least a small proportion do and may proceed through all the stages of embryonic and postembryonic development up to metamor­phosis.
In the late blastula, the stage used for some of these nuclear transplanta­tions, there are 8000 to 16,000 cells, which means that about 13 to 14 divisions (or generations) of the original nucleus of the fertilized egg had been performed without diminishing the power of the nucleus to support every type of differentiation provided for by the specific genotype. Nuclei of cells of an early gastrula show the same properties.
In further experiments the potentialities of nuclei of cells, which are even more advanced in the process of differentiation, were tested by transplanting them into enucleated eggs. Normal tadpoles developing up to and through metamorphosis were obtained by using nuclei from the neural plate of a frog embryo or nuclei from already ciliated cells of the alimentary tract of a swimming tadpole.
Although the cells of the neural plate were already well on the way to becoming cells of the nervous system, and the cells of the gut were already functionally differentiated, their nuclei were still capable of providing the necessary genetic information for the differentiation of all the various tissues and cell types of an adult frog. Fairly normal embryos were also produced by implanting into enucleated eggs the nuclei from a frog adenocarcinoma.
Finally, a modification of the original methods has made it possible to test the potentialties of nuclei of adult animals by transplanting them into enucleated eggs. When nuclei are taken directly from adult tissues and transplanted into eggs, they are not able to support development.
If adult cells are cultured in vitro, however, where they lose part of the properties of differentiated cells and start reproducing mitotically, and if their nuclei are then transplanted into eggs, development may be initiated in a fair proportion of cases.
Even so, the embryos develop abnormally, producing blastulae with mostly partial cleavage. If nuclei of cells from the more healthy parts of the partial embryos are then removed and transplanted into eggs, the development of such second generation embryos proceeds much better swimming larvae and occasionally even larvae which become metamorphosed into froglets develop.
The tissues used success­fully as sources of nuclei are kidney, lung, and skin of adult frogs. It is noteworthy that no differences in the development of tadpoles could be noted when the nuclei were taken from these three different tissues.
It was noted that with progression of development of the cells from which nuclei were taken the proportion of experiments leading to completely normal tadpoles became increasingly reduced, and more and more operated eggs were arrested in their development in various early stages.
On careful investigation it was found that the chromosomes in the nuclei of arrested embryos showed various defects, such as aneuploidy (chromosomes missing) or defects within chromosomes (deletions, translo­cations).
These defects are due to the inability of the chromosomes in nuclei taken from advanced embryos and adult tissues to adapt themselves to the rapid reproductive rhythm of early development. The duplication of the interphase chromosomes is too slow, and many of them do not complete duplication by the time cell division sets in.
The result is, of course, severe damage to and incompleteness of the chromosome set. Whatever the explanation for the defective development, it remains that at least a proportion of nuclei from cells well on the way to differentiation and from those that have become malignant retain their full potentialities for controlling and directing normal development.
Nuclei of differentiating cells are in every respect similar to the nuclei of fertilized eggs. In experiments with frogs, nuclei transplanted into the egg cytoplasm undergo a change which makes them resemble early embryonic nuclei.
The nuclei of newly fertilized eggs, although small in proportion to the egg cytoplasm, are much larger than the nuclei at later stages. Accordingly, transplanted nuclei increase up to 30-fold in volume during the first 40 minutes after transplantation. The functioning of the nuclei also changes drastically.
Nuclei of ad­vanced embryos do not divide rapidly, and accordingly the synthesis of DNA, necessary for the replication of chromosomes, is slow. On the other hand, they synthesize large quantities of ribosomal RNA, and, as this kind of RNA is produced in the nucleolus, they have prominent nucleoli.
Nuclei of cleavage cells synthesize DNA rapidly but do not synthesize any ribosomal RNA and therefore do not show any nucleoli. Transplanted nuclei cease to synthesize ribosomal RNA, and their nucleoli disappear. Instead, they start to synthesize DNA rapidly as do normal cleavage nuclei.
Morula and Blastula of Cleavages:
The blastomeres in the early cleavage stages tend to assume a spherical shape like that of the egg before cleavage. Their mutual pressure flattens the surfaces of the blastomeres in contact with one another, but the free surfaces of each blastomere remain spherical, unless these outer surfaces are also compressed by the vitelline membrane. The whole embryo acquires, in this stage, a characteristic appearance resembling a mulberry. Because of this superficial similarity, the embryo in this stage is called a morula (Latin for mulberry).
The arrangement of the blastomeres in the morula stage may vary in the different groups of the animal kingdom. In coelenterates it is often a massive structure, with blastomeres filling all the space that had been occupied by the un-cleaved egg. Some of the blastomeres then lie externally and others in the interior. (Some embryologists apply the name morula to this type of embryo only.)
More often, as the egg undergoes cleavage, the blastomeres become arranged in one layer, so that all the blastomeres participate in the external surface of the embryo. In this case a cavity soon appears which at first may be represented just by narrow crevices between the blastomeres, but which gradually increases as the cleavage goes on. This cavity is called the blastocoele.
As cleavage proceeds, the adhesion of the blastomeres to one another increases, and they arrange themselves into a true epithelium. In cases in which a cavity has been forming in the interior of the embryo, the epithelial layer completely encloses this cavity, and the embryo becomes a hollow sphere, the walls of which consist of an epithelial layer of cells. Such an embryo is called a blastula. The layer of cells is called the blastoderm, and the cavity is the blastocoele.
Right from the beginning of cleavage the blastomeres become progressively joined closer together by several types of intercellular junctions. The first to be formed are the “tight” junctions forming at the outer edges of the adjoining blastomeres, sometimes already in the two-blastomere stage. At the blastula stage these junctions seal off the interior of the embryo from the outside.
In the sea urchin blastula these junctions take the form of “septate” junctions, which appear as a series of bars connecting the membranes of adjoining cells. “Gap” junctions which provide a means of communication between cells of the embryo are absent in the earliest stages, but develop in the morula stage and are widespread in the blastula stage.
In oligolecithal eggs with complete cleavage (echinoderms, Amphioxus), the blas­tomeres at the end of cleavage are not of exactly equal size, the blastomeres near the vegetal pole being slightly larger than those on the animal pole. When the blastula is formed, the cells arrange themselves into a simple columnar epithelium enclosing the blastocoele.
Because the vegetal cells are larger than the animal cells, the blastoderm is not of an equal thickness throughout at the vegetal pole the epithelium is thicker, and at the animal pole it is thinner. Thus, the polarity of the egg persists in the polarity of the blastula.
Animals with a larger amount of yolk, such as the frog, show a difference in the size of the cells of the blastula that may be very considerable, and the blastula still further departs from the simple form of a hollow sphere. The cells here are also arranged in a layer surrounding the cavity in the interior, but the layer is of very uneven thickness.
The layer of cells at the vegetal pole is very much thicker than at the animal pole, and the blastocoele is consequently distinctly eccentric, nearer to the animal pole of the embryo. Furthermore, the blastoderm is no longer a simple columnar epithelium but is two or more cells thick.
The cells in the interior are rather loosely connected to one another, but at the external surface of the blastula the cells adhere to one another very firmly, because of the presence of tight cell junctions joining the surface of adjoining cells in a narrow zone just underneath the surface of the blastoderm.
A process corresponding to blastula formation occurs also in animals whose eggs have incomplete cleavage. In a bony fish or a shark the early blastomeres tend to round themselves off, showing that they are only loosely bound together. Later, the blasto­meres adhere to one another more firmly and thus become converted into an epithelium.
Again, as in amphibians, the superficial cells are firmly joined to one another, forming a continuous “covering layer,” while the cells in the interior may remain loosely connected until a later stage. The epithelium, however, cannot have the form of a sphere.
Since the cleavage is restricted to the cap of cytoplasm on the animal pole of the egg, the blastoderm is developed only in the same polar region. The blastoderm therefore assumes the shape of a disc lying on the animal pole. The disc, which is called the blastodisc, is more or less convex and encloses, between itself and the un-cleaved residue of the egg, a cavity representing the blastocoele.
In centrolecithal eggs having a superficial cleavage (insects), there is no cavity comparable to the blastocoele. Nevertheless, the formation of the epithelium on the surface of the embryo, after the nuclei have migrated to the exterior, can be compared to the formation of the blastula.
The layer of cells thus formed on the surface of the embryo is the blastoderm. Instead of surrounding a cavity, the blastoderm envelops the mass of un-cleaved yolk. We may also compare this stage to an embryo whose blastocoele has been filled with yolk.
Up to the blastula stage, the developing embryo preserves the same general shape as the un-cleaved egg. So far, the results achieved are the subdivision of the single cell into a multiplicity of cells and the formation of the blastocoele. In addition, the sub­stances contained in the egg remain basically in the same position as before.
The yolk remains near the vegetal pole. In pigmented eggs, such as those of amphibians, the pigment remains as before, more or less restricted to the upper hemisphere of the embryo. Only a slight intermingling of cytoplasm seems to be produced by the cleavage furrows cutting through the substance of the egg.
We have pointed out that during cleavage qualitative changes in the chemical composition of the developing embryo are very limited. Few new substances, either chemically defined or microscopically detectable, have been found to appear during cleavage. It is conceivable, however, that the substances present in the egg may be redistributed in some way during cleavage and that such a redistribution may be essentially important for further development.
In this connection we will first examine whether the numerous nuclei produced during the mitotic divisions of the egg are all alike in their properties, or whether any differences may be discovered among them.
2nd Year Biology MCQ's (Chapter 19: Growth and Development)
Q.1. Which meristem occurs at the base of internodes?
Q.2. The progressive changes that occur in zygote to become an adult is called:
Q.3. The egg of chick is laid at which stage:
Q.4. The study of abnormal development is:
Q.5. Which is the feature of the salamander zygote?
Q.6. After the fourth cleavage the no. of blastomeres in chick embryo:
Q.7. Another name for segmentation cavity is:
Q.8. The rearrangement of cells occurs in:
Q.10. Primitive streak is equal to _____ in frog:
Q.11. The somites develops from:
Q.12. In chick cleavage occurs in:
Q.13. During differentiation which cells stimulate ectodermal cells to develop into nervous system:
Q.14. Process of negative physiological changes in body is:
Q.15. Which of the following is correct?
C) Blastula →Gastrula→Morula
D) Morula →Blastula→Gastrula
Q.16. Which chromosomal abnormality leads to tallness, aggressiveness, and antisocial behavior?
Q.17. The goal of gerontology is:
Q.18. Exposure of plant to red light:
A) Increases cell elongation
B) Increases cell division
Q.19. Abnormal development may be due to:
Q.20. Acetabularia can regenerate is:
Q.21. Neural tissue develops from:
Q.22. The process of embryonic induction was studied by:
Q.23. Leaf primordia develops during _____ of differentiation in plans:
Q.24. An inevitable process is:
Q.26. Greater power of regeneration is present in:
Q.27. Cell volume increases 150 folds during:
Q.28. In plants optimum temperature for growth is:
Q.29. Besides IAA which hormone play important role in apical dominance:
Q.30. Notochord is visible in chick embryo of :
Q.31. During early development gene controlled substances are found in:
Q.32. In chick embryo the pattern of cleavage is:
Q.33. The germ layers are formed during:
Q.34. Heart and mesenteries are formed by:
Q.35. According to Spemann, which is primary organizer:
Q.36. Trisomy condition is found in:
Q.37. Gray vegetal cytoplasm give rise to:
Q.38. The marginal area of blastoderm in which cells remain attached with yolk, is called: