Physiology of Cleavage
The cleavage is a phase of intensified chemical activity and a great amount of turn over of molecules occurs during its metabolism. The metabolism of cleavage can be studied under following two headings:
A. Catabolic activities of cleavage- The cleavage requires large amount of chemical energy in the form of ATP molecules for its completion. For example, in each mitosis of cleavage, the microtubular framework of centrioles and mitotic spindle requires many ATP molecules to spend in poleward movements of chromosomes. These ATP molecules are manufactured in ooplasm and mitochondria during enzymatic anaerobic (glycolysis) and aerobic (Kreb’s cycle) oxidation of yolk, glycogen and other energy yielding chemical molecules of egg. The increased oxidation of a cleaving egg is correlated with increased input of oxygen molecules by the latter.
Besides, these energy-yielding reactions, the catabolic activities provide to the dividing egg a continuous supply of ray materials (such as deoxyribonueleotides, ribonucleotides, purines, pyrimidines, amino acids, ribose and deoxyribose sugars, other carbohydrates and lipids, etc.) for the biosynthesis of macromolecules of DNA, RNA, proteins, lipids and polysaccharides. These macromolecules are required by the cleaving egg for the mitosis and cytokinesis, because they are the main component molecules of chromosomes, microtubules, centrioles and different cellular membranes.
B. Anabolic activities of cleavage- Though cleavage involves quantitative growth of cleaving egg as a whole, but, it does include great qualitative growth of nuclear material at the expense of cytoplasm. The qualitative growth during cleavage involves the bio synthesis of following kinds of macromolecules:
i) Bio synthesis of DNA- During cleavage, the number of nuclei is doubled with every new division of the blastomeres and this increase is accompanied by an increase of DNA amount. The amount of DNA, however remains constant to per molecules. In the S phase of interphase of each mitosis of cleavage, the DNA duplication occurs in the presence of an enzyme, called DNA polymerase. The precursor substances, such as, deoxyadenylate, deoxyguanylate, deoxycytidilate, and thymidilate, all of which are deoxytribonucleotides, move from the the cytoplasm to the egg nucleus and assemble into new polynucleotide strands of DNA under joint guidance of polynucleotide strands of DNA molecule and DNA polymerase enzyme. According to certain workers, the free cytoplasmic DNA molecules which have been reported to occur either freely in the ooplasm of Amphibia (Brachet and Quertier, 1963), and insects (Muckenthaler and Mahowald, 1966), or in the mitochondria (Tyler, 1967) or yolk platelets (Brachet, 1968), are also incorporated during nuclear DNA duplication. However, it is still not clear that either they are used as such or are broken down into simpler units before their incorporation into nuclear chromosomal DNA.
ii) RNA synthesis- During early cleavage, because the interphase is of very short duration, so, the chromosomal DNA remains hurriedly busy in its own duplication and exists in a highly coiled and thus, in an inactive state. The result is that no or little transcription of any kind of RNA occurs during the period of early cleavage. For example, in frog eggs, ribosomal RNA (rRNA) apparently is not produced at all until after the completion of cleavage. As the nucleolus is the site of synthesis and maturation of rRNA, this organoid is completely lacking in the eggs of these animals during cleavage (Brown, 1965). The nucleolus reappears in the nuclei of the blastomeres, at the onset of gastrulation, simultaneously with the resumption of ribosomal RNA synthesis (Gurdon and Brown, 1965)
Messenger RNA (mRNA) and transfer RNA (tRNA), however, are synthesized during cleavage or at least in the later stages of cleavage (Tyler and Tyler, 1966; Gurdon, 1969). However, the synthesis of mRNA and tRNA molecules is not necessary for cleavage, because, fertilized eggs treated with actinomycin D which inhibit or suppress the DNA dependent RNA synthesis or granscription, they continue to divide in a normal fashion. The newly synthesized mRNA molecules are not translated in protein synthesis during cleavage and they are used during later developmental stages. Any protein synthesis, if, occurs during cleavage, it employs the mRNA molecules which are produced during oogenesis and these proteins, thus have the characteristics of the maternal parent only.
Mammalian embryos differ strikingly from most other embryos (e.g., those of amphibians, teleost fishes, echinoderms, insects) not only in requiring an external source for embryogenesis, but also in the precocious onset of RNA synthesis (viz., transcription). In them transcription of different types of rRNAs is started from the 2-cell stage of the embryo. Synthesis of mRNA has not been conclusively demonstrated in early embryos (Austin and Short, 1972).
iii) Protein synthesis- The cleaving egg requires different species of structural, enzymatic and regulator proteins for its diverse mode of functions. For example, the structural proteins are required as building blocks for different cellular membranes, egg cortex, microtubules, ribosomes, etc. The enzymatic proteins are needed at different steps of cleavage for controlling the metabolism of cleavage as a whole. The regulator proteins (such as histone and non-histone proteins) remain associated with DNA molecule and are responsible for different behaviour of the latter during embryonic development. All these proteins are synthesized under the guidance of polyribosomes during the early cleavage. In mammals the rate of protein synthesis increases dramatically from the morula to the blastula stage possibly coinciding with the first major use of RNA templates from the embryo’s own genome (Austin and Short, 1972)
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