Question

(a) How is the position of the blastomeres controlled during cleavage?

(b) How is the size of the blastomeres controlled during cleavage?

(c) How does the cell-cycle of cleavage-stage embryos differ from the normal mitotic cycle?

Answer 1

a)Blastomeres are type of cells produced by Cleavage of Zygote after fertilization. In most mammals, the rate of cell division and the position of the blastomeres with respect to one another is completely under the control of the proteins and mRNAs stored in the oocyte by the mother.

b)Blastomeres are same size as the Zygote. Blastomeres are stage specific cell size.The relative blastomere size in the embryo is dependent on both the cleavage stage and the regularity of each cleavage division .The blastomeres of 2-, 4- and 8-cell embryos should be equal .Rather than unequal in size . In contrast, blastomeres of embryos with cell numbers other than 2, 4 and 8 should have different sizes as there is an asynchrony in the division of one or more blastomeres.So 2,4,and 8 cell embryos are cell specific while other numbers are not cell specific.

c)Cytokinesis and Karyokinedis work together to form a cleavage cycle.While karyokinesis can occur in the absence of cytokinesis.Mitotic cycle does not depend on cytokinesis.Cleavage stage embryos range from the 2-cell stage to the compacted morula composed of 8–16 cells.whereas in normal mitotic cycle, the mitosis of embryos leads to externalization of parts of the cell cytoplasm, resulting in the presence of anuclear fragments surrounded by a plasma membrane .