Cleavage is the division of cells in the early embryo. Cleavage occurs soon after the fertilization or activation of the egg.
The fertilized egg or zygote is a single cell. It must be converted into a multicellular body having sufficient number of cells.
In the zygote this is achieved by a series of rapid mitotic cell divisions until many cells are produced.
The division of zygote by a series of mitotic cell divisions to form a multitude of cells which become the building units of the future organism is called segmentation or cleavage.
The resulting cells of the cleavage division are called blastomeres.
The end product of cleavage is a compact mass of blastomeres called morula which ultimates transforms into a blastula enclosing a cavity called blastocoel.
Characteristics of Cleavage
The cleavage divisions of eggs in all animals have some common characteristics.
- All divisions are mitotic and occur rapidly one after the other. But their rate depends upon the concerned species and on temperature.
- During the cleavage, there is no growth in the resulting blastomeres and the overall size and volume of the embryo remain the same. The zygote is a relatively large cell and as a result of successive divisions, the size of the blastomere is gradually reduced to that of the ordinary body cells. However, in the ordinary mitotic division of body cells, each division is followed by a period of growth of daughter cells before the latter divides again.
- In the zygote, the volume of the cytoplasm is proportionately greater than that of the nucleus, and therefore, the nucleocytoplasmic ratio is very low. During cleavage, the size of the blastomeres decreases progressively but the nuclear size remains the same. As a result, the nucleocytoplasmic ratio increases and finally becomes the same as in the ordinary cell.
- During cleavage, the blastomeres do not move and therefore, the general shape of the embryo does not change except for the formation of a cavity in the interior.
- Early cleavage divisions occur synchronously, i.e. all the blastomeres divide simultaneously. But this synchrony disappears after a few dozen blastomeres have been formed.
- For rapid nuclear division, there is a great increase in the synthesis of DNA, needed for the duplication of chromosomes.
- The consumption of oxygen is greatly increased during cleavage.
Peculiarities of Cell Division in Cleavage
Basically cleavage is a typical mitosis, but both differ in the following aspects.
- Cleavage occurs in early embryos whereas mitosis occurs in late embryos and adult animals.
- The blastomeres do not increase in size before the new division begins. With every division, nearly half-sized blastomeres are produced and thus, the size of blastomeres gradually decreases with the cleavages. In mitosis, the daughter cells first grow in size to attain the parental size, and thereafter further division takes place. The cells, thus, maintain an average size in every type of tissue.
- The interphase or intermitotic interval remains very short in the cleavage. But in normal mitosis, it is quite long.
- The nuclei of early cleavage cells are considerably larger than they are in ordinary somatic cells of the same animal due to the presence of a large amount of nuclear sap.
Patterns Of Cleavage
During cleavage, the division furrows do not occur in a haphazard direction but are oriented regularly concerning the animal vegetal axis or polar cells of the egg.
This orientation of cleavage furrows is not the same in all species. Therefore various patterns of cleavage are found amongst animals.
The following three types are particularly common.
The radial cleavage occurs when the successive cleavage planes cut straight through one another and the resultant blastomeres are placed in a radially symmetrical manner around the animal vegetal axis (sponges, coelenterates, and some echinoderms).
The cleavage pattern arises when the first three-division planes do not stand at right angles to each other.
In this pattern, the radial symmetry is lost and the blastomeres are so arranged that the right and left sides become apparent (vertebrates, cephalopods, few mollusks, and some echinoderms).
The spiral cleavage results due to oblique positions of mitotic spindles in the blastomeres. It is also called oblique cleavage.
In this cleavage, the blastomeres are arranged in a spiral manner around the animal vegetal axis. (flatworms, annelids, and most mollusks).
Types of Cleavage
Cleavage is influenced by the quantity of yolk and its pattern of distribution in the egg.
This is because the yolk is an inert substance and retards the progress of the furrow to divide the cytoplasm.
Accordingly the following types of cleavage are found in animals.
When the furrow divides the egg or blastomeres completely, it is called holoblastic cleavage.
It is (a) equal if the resulting blastomeres are, of equal size as it happens in most microlecithal or isolecithal type of eggs, (b) unequal if the daughter blastomeres are unequal in size.
This is seen in the case of telolecithal eggs of frog and some fishes in which the third cleavage produces four small and four large blastomeres.
The smaller blastomeres are called micromeres and the larger ones’s macromeres or megameres.
In the polylecithal or macrolecithal eggs with enormous quantities of yolk, the cytoplasm is confined to a small area near the animal pole or around the periphery of the egg.
The division occurs only in the cytoplasm and the yolk is not affected at all. This is called meroblastic cleavage and is of two types :
In the megalecithal eggs, cleavage occurs only in the cytoplasmic cap on the top of the yolk near the animal pole forming a disc of several layers of cells (reptiles, birds, and egg-laying mammals).
In the centrolecithal type of eggs of insects, at first, the nucleus located in the center of yolk divides repeatedly.
The daughter nuclei then migrate to the peripheral cytoplasm which subsequently divides into many cells, each with a nucleus.
Towards the end of the cleavage the rate of cell division slows down and a blastula is formed.
The blastula generally contains a cavity inside it called the blastocoel. The types of blastula vary a great deal amongst animals depending upon the size of the eggs, the amount of and the distribution pattern of yolk, and the pattern of cleavage.
The following different types of blastula have been recognized in different animal groups.
The blastula of echinoderms and Amphioxus is called coeloblastula. It is in the form of a hollow sphere enclosing a blastocoel and is formed of a single layer of cells.
It occurs in the spirally cleaving eggs of annelids, mollusks, and nemertians. There is no blastocoel.
The micromeres accumulate as a cluster of cells over the larger vegetally placed macromeres.
3. Periblastula or superficial blastula:
The superficially cleaving eggs of insects produce a periblastula without a blastocoel as the nuclei collect in the superficial layer.
The blastocoel from the beginning of cleavage is filled with yolk.
It is found in reptiles and birds. The blastula appears at the animal pole as a small multilayered flat disc separated from the yolk by a narrow segmentation or germinal cavity.
Such a blastula is formed of two structurally different blastomeres.
For example, in Sycon (sponge) the anterior, half of the blastula is formed of flagellated cells, while the posterior half is formed of large rounded granular cells.
In amphibians, the blastula contains two types of cells, namely micromeres towards the animal pole and macromeres towards the vegetal pole.
The blastula found in mammals is called the blastocyst. Cleavage is regular and a blastocoel appears inside the dividing cells.
The blastocoel gradually becomes larger. The cluster of cells differentiates into two distinct groups.
They are an epithelium-like layer called trophoblast or nutritive cells surrounding the expanding blastocoelic cavity and an inner cell mass of formative cells of the embryo which are displaced to one pole of the sphere.
This cell mass spreads inside the cavity as a flat disc. This stage of mammalian development is known as a blastocyst.
Significance of Cleavage
Cleavage and blastulation (formation of blastula) have the following significance :
- Cleavage divides the zygote into an array of cells, which become the building block of future organisms.
- The process initiates and prepares for all differentiation or diversification.
- The process also creates cell aggregates which may be subjected to further changes.
- Due to cleavage and blastulation, the major presumptive organ forming areas of the future embryo are segregate into definite parts on the blastula.
- The blastocoel permits the migration and rearrangement of the major presumptive organs in future development.
Summary on Cleavage
The division of zygote by a series of mitotic cell division to form a multitude of cells, which becomes the building units of the future organism is called segmentation or cleavage.
The resulting cells are known as blastomeres. The end product of cleavage is the formation of a compact mass of blastomeres called morula which transforms into a blastula with a central cavity called the blastocoel.
Basically cleavages are a typical mitosis but they differ from one another. In the cleavage, cell division occurs quick succe ssion sulting in the gradual decrease the the cells.
In mitosis, there is an interphase between two successive divisions during which the cell grows to its normal size after which it divides.
Cleavage occurs in early embryos, whereas mitosis occurs in late embryos and adult animals.
The pattern of cleavage may be radial, bilateral and spiral. Cleavage is influenced by the amount and distribution yolk present in the fertilized egg.
The various types of cleavage met with are: holoblastic equal, holoblastic unequal, meroblastic, discoidal and superficial.
Towards the end of cleavage the rate of cell division slows down and a morula is formed. The morula develops into a blastula with a cavity inside it called the blastocoel.