Cleavage versus typical mitosis
Definition : The term cleavage refers to a series of rapid mitotic division of the zygote following fertilization, forming a many celled blastula. The cleavage follows fertilization and ends with the formation of a characteristic development stage called blastula.
Cleavage versus typical mitosis
The cleavage division are no doubt mitotic as they produce diploid cells, they differ from typical mitosis in a couple of significant points.
(1) Site of occurrence In zygote or parthenogenetic egg In most of somatic cells
(2) Interphase Of shorter period Of longer period
(3) Growth Does not occur Occurs during interphase
(4) Oxygen consumption High as is very rapid process Low as is slow process
(5) Size of daughter cells Decreases Remains same after growth
(6) DNA synthesis Faster Slower
(7) Nuclear-cytoplasmic ratio Increases Remain same
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Planes of cleavage
The cleavage is initiated by the appearance of a constriction or groove called cleavage furrow. The cleavage furrows may divided the egg from different angles or planes. These are four important planes of cleavage. They are as follows.
(a) Meridional plane : When cleavage furrow bisects both the poles of the egg, passing through the animal vegetal axis, the plane of cleavage is called meridional plane.
Example : Ist and IInd cleavage furrow of frog and Ist cleavage furrow of chick.
(b) Vertical plane : When cleavage furrow passes from the animal pole to the vegetal pole, but it does not pass through the median axis of the egg.
Example : IIIrd cleavage furrow of chick.
(c) Equatorial plane : When cleavage furrow bisect the egg at right angles to the median axis and half way between the animal and vegetal poles.
Example : Ist cleavage plane of eggs of higher mammals.
(d) Latitudinal or transverse or horizontal plane : The transverse plane resemble the equatorial plane, but it passes either above (towards the animal pole) or below (towards the vegetal pole) the equator of the egg.
Example : IIIrd cleavage plane of Amphioxus and frog.
Patterns of cleavage
During segmentation, the cleavage furrows are not formed at random but are oriented in a particular manner with reference to the main (animal-vegetal) axis of the egg. The orientation of successive cleavage furrows with respect to each other and to the main axis of the egg is, however, unlike in different species. As such various patterns of cleavage are found among animals. Based upon symmetry, four patterns of cleavage have been recognized. They are as follows
(a) Radial cleavage : In this cleavage pattern, division take place in such a manner that all the blastomeres are placed in a radially symmetrical fashion around the polar axis. When such an egg is viewed from the poles, the blastomeres seem to be arranged in a radially symmetric form.
Example : Sponges, coelenterates, sea urchin, sea cucumber, amphioxus.
In this pattern four blastomeres arise by the usual two meridional cleavages. The third cleavage plane is vertical resulting in the formation of a curved plate of 8 cells arranged in two rows of 4 each. In these rows, the central cells are larger than the end ones.
Example : Ctenophores like Beroe.
The spiral cleavage is diagonal to the polar axis. In this type, the spindles for the third cleavage, instead of being erect, are oriented diagonally so that the resulting upper tier of cells is sidewise. The upper 4 cells are placed over the junction between the four lower cells. The upper smaller cells are called micro and lower larger cells are known as macromeres. The spiral cleavage results due to oblique positions of the mitotic spindles. This type of cleavage is called the spiral type because the four spindle during the third cleavage are arranged in a sort of spiral.
Examples : Eggs of annelids, molluscs, nemerteans and some of the planarians.