Rate of Cleavage
The rate of cleavage varies from species to species. For example, in the gold fish, divisions follow each other continuously at regular intervals of 20 minutes. The inter-cleavage interval of frog is one hour and of mouse is 10 to 12 hours. In most of these cases, the cleavage rate remains rapid (synchronous) during completion of blastula stage. The transition from synchronous to asynchronous cell divisions during cleavage and blastulation is thought to be caused due the gradual exhaustion of a substance which occur in the cytoplasm of the fertilized egg (Lovtrup, 1974). As regards the nature of the substance, Mazia (1961) has suggested that it may be the protein of the mitotic apparatus (viz. TP monomers) Lovtrup (1974) has not approved this idea because the total volume of cleaving egg remains unchanged during cleavage and although the number of mitotic apparatus increases, but the space they must is correspondingly reduced.
When, rate of mitosis remains rapid then the inter mitotic or interphase remains very short and except the synthesis of DNA, no synthesis of other kinds of chemical molecules (e.g. RNA molecule) can take place, because nuclei remain synthetically active only during the interphase state. During most of the mitosis, the nuclei remain transcriptionally inactive because the chromatin is more or less condensed.
The rate of cleavage is determined by the cytoplasm rather than by the nucleus, a fact long known but confirmed by the observations that haploid, diploid and triploid frog embryos, all undergo cleavage with the same rhythm. The rate of cleavage is affected by temperature. Slight increase or decrease of temperature of a cleaving egg may increase the time interval of cleavage. Importantly, temperature change affects the rate of respiratory or oxidative metabolism which appears to be base of all cellular activities. Respiration varies with the temperature and so does cleavage rate. In the absence of oxygen or the presence of excess of carbon dioxide, cleavage in inhibited.
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