Stage III

Stage III oocytes range from 450 to 600 |im in diameter and account for 15% of the population in stages II to VI. Two important processes are initiated during stage III: acquisition of pigmentation and active uptake of yolk (vitellogenesis; see Subheading 4.). Pigmentation is initially evident by the tan or light brown appearance of oocytes, caused by melanin produced from the melanosomes lying beneath the cortical layer of the oocyte. During this stage, oocytes continue to darken uniformly, with no distinction between animal and vegetal poles, until the entire oocyte appears dark brown or black. Other morphologic changes during stage III include development of visible blood vessels along the surface of the oocyte, increased height of the follicle cells, increased numbers and sizes of microvilli, and continued development of the vitelline envelope (16).

Finally, nuclear changes also occur. First, formerly peripheral nucleoli become vacuolated and relocate to the center of the nucleus. Second, to allow for efficient transcription of genes to service the 105- to 106-fold higher volume of cytoplasm compared to somatic cells, the chromosomes relax into a lampbrush configuration. The presence of this conformation coincides with peak RNA synthesis, which begins in late stage II and lasts until very late stage III (17,18). In fact, stage III oocytes have their full complement of poly(A) mRNAs, but only a small fraction is translated (19).

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