Cleavage (embryo)
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization.
Cytokinesis is mediated by the contractile ring made up of polymers of actin protein called microfilaments.
This point in non-mammals is referred to as the midblastula transition and appears to be controlled by the nuclear-cytoplasmic ratio (about 1:6).
Each blastomere produced by early embryonic cleavage does not have the capacity to develop into a complete embryo.
From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.
Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.
Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells.
The nematode C. elegans, a popular developmental model organism, undergoes holoblastic rotational cell cleavage.
Spiral cleavage can vary between species, but generally the first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo.
[7] With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole.
[5] In other words, the orientation of divisions that produces each quartet alternates between being clockwise and counterclockwise with respect to the animal pole.
[8] After the formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo.
In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become the future D blastomere.
[8][9] Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage.
[8] The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called a polar lobe.
[16] Concomitantly, they develop an inside-out polarity that provides distinct characteristics and functions to their cell-cell and cell-medium interfaces.
[19][14] With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable as they become organised into a thin sheet of tightly adhered epithelial cells.
[21] Initially dispersed in hundreds of water pockets throughout the embryo, the fluid collects into a single large cavity, called blastocoel, following a process akin to Ostwald ripening.
[14][22] The trophoblasts will eventually give rise to the embryonic contribution to the placenta called the chorion.
