- 1) Types of Cleavage
- 2) The Planes of Cleavage
- 3) Types of Cleavage
- 4) Cleavage on basis of Yolk
- 5) Meroblastic cleavage
- 6) You may also like to learn:
Types of Cleavage
The event of rapid and repeated mitotic division of cells to form numerous nucleated cells is called cleavage.
The Planes of Cleavage
Cleavage in the egg can occur through different planes. These are described below:
When a cleavage furrow passes through the center of the egg or median axis and bisects both animal and vegetal poles and the egg is divided into two equal halves, is called the meridional plane of cleavage. The axis between the center of the animal and the vegetal pole is a median axis.
The line of division is at right angle to the median axis and the cleavage furrow divides the egg in the middle between both animal and vegetal poles.
The cleavage furrow does not pass through the center of the median axis instead located on either side of the median plane and the cells formed are of unequal size.
This plane is very similar to equatorial and the cleavage furrow is present on either side of the equatorial plane in the cytoplasm. It is also known as horizontal or transverse cleavage.
The pole of the egg that has less yolk is termed as animal pole.
The pole of the egg which is rich in yolk is termed a vegetal pole.
Types of Cleavage
The developmental fate of each cell is predetermined at the early embryonic stage. On the isolation of cells, the embryo will fail to fully develop. It is also known as mosaic cleavage and occurs in most of the protostomes. Mollusks, annelids exhibit determinate cleavage.
All cells produced by cleavage in this type are equally totipotent. If cells are isolated they are able to form the full embryo. It occurs in most of the deuterostomes. The developmental fates are not predetermined. For example, echinoderms exhibit indeterminate cleavage.
Cleavage on basis of Yolk
According to yolk following are the cleavage patterns observed:
The type in which the entire cell is divided or bisected by cleavage furrow. It can be:
- Equal: The cleavage furrow results in the formation of blastomeres of equal size in any symmetry such as radial, bilateral, symmetrical, or spiral.
- Unequal: There is the formation of the unequal size of blastomeres.
Holoblastic cleavage can further be divided into four types of cleavage on the basis of symmetry.
- Radial symmetry
The cleavage in which daughter cells are formed exactly on the top of one another. Deuterostomes possess this type where indeterminate cells are formed which can individually develop into a full embryo.
- Spiral cleavage
The cleavage in which daughter cells are not formed exactly on the top of one another. Protostomes possess this type where determinate cells are formed which cannot develop individually.
- Bilateral cleavage
The first cleavage furrow cuts the zygote into right and left halves which are mirror images of each other. The divisions are complete and separate.
- Rotational cleavage
In this type of cleavage, one daughter cell divides meridionally whereas all others divide equatorially. The first division of this is along a meridional axis which cuts the egg into two cells.
This type of cleavage is restricted to the small active area of the cytoplasm where germinal disc lies at the animal pole. The series of incomplete divisions start forming cells at the animal pole.
It can be of two types.
- Discoidal cleavage
It occurs in eggs such as macrolecithal or telolecithal (eggs with yolk at the end) which have a huge amount of yolk. The cleavage starts at the germinal disc at the animal pole whereas yolk remains unsegmented or uncleaved. Birds and reptiles exhibit discoidal meroblastic cleavage.
- Superficial cleavage
This type of cleavage is restricted to peripheral cytoplasm in centrolecithal eggs. The zygote nucleus lies in the center and divides without the division of the cytoplasm. As a result, a large number of nuclei are formed and embedded in the superficial layer of the cytoplasm.
Later, the peripheral cytoplasm divides extending towards the inner side from the surface. Insects and many arthropods possess this type of cleavage.