Hence, cleavage is the successive mitotic division of egg to form a multicellular blastula. Basically both cleavage and mitosis are identical because these leads to the formation of two cells with diploid number of chromosomes. Cleavage differs from mitosis in the respect that –
There is no growth phase between the successive divisions so cleavage decreases the cell size.
The metabolism becomes fast
There is rapid DNA replication
High consumption of oxygen.
Types of Cleavage:
I. The mode of cleavage is determined by the amount of yolk and its distribution. On this basis cleavage may be holoblastic and meroblastic.
The cleavage in which the segmentation lines pass through the entire egg, dividing it completely. It occurs in alecithal, microlecithal and mesolecithal eggs. E.g. frog, human egg etc. It is of two types: equal holoblastic and unequal holoblastic.
Meroblastic – The lines of segmentation do not completely pass through the egg and remain confined to a part of the egg. Such type of cleavage is found in megalecithal eggs as the yolk provides resistance to the cleavage, E.g. insects, reptiles. Meroblastic cleavage may be discoidal and superficial.
II. Patterns of Cleavage:
Successive cleavage plane cuts straight through the egg e.g. Synapta paracentrotus.
When the three first division planes do not stand at right angles to each other. Ctenophora.
There is a rotational movement of cell parts around the egg axis leading to displacement of mitotic spindle with respect to symmetrically disposed radii. E.g. turbellarians, nematoda, rotifera, annelida, all mollusc except cephalopods.
Bilateral cleavage – Mitotic spindle and cleavage planes remain bilaterally arranged with reference to the plane of symmetry. E.g. tunicates, Amphioxus, amphibia and higher mammals.
III. On the basis of fate of blastomeres, cleavage may be – determinate and indeterminate
Cleavage in the mammalian ovum takes place, about 14 or 15 hours after fertilization, during its passage through the fallopian tube to the uterus. The cleavage in mammalian ova is termed simple holoblastic.
Solid mass of 32 cells formed from zygote after successive mitotic division (after 5th cleavage) is called morula. It takes approximately three days. A morula can be differentiated from blastula in the absence of a cavity. It looks like a mulberry shaped ball of cells. The morula reaches the uterus about 4 to 6 days after fertilization.