Ovum Maturation

Abstract

Ovum maturation in mammals is accompanied by the resumption of meiosis. This maturation in mammalian oocytes is initiated during prenatal life or shortly after birth. The oocyte reaches the diplotene stage of the prophase just before or immediately after birth. At this stage, by a mechanism yet not fully understood, the meiotic process is arrested (“first meiotic arrest”) with the nucleus (germinal vesicle) in prophase I of meiosis (Guraya 1977 a). Meiotic maturation refers specifically to the process of nuclear progression from the diplotene (dictyate) stage of the first meiotic prophase to metaphase II (Fig. 45). Upon appropriate stimulation from circulating pituitary gonadotrophins (FSH and LH) or, in some species, after removal from Graafian follicles and culture in vitro, dictyate-stage oocytes resume meiosis, resulting in the “final” maturation of the mammalian oocyte (Tsafriri 1978a, 1983, Lindner et al. 1980, Tsafriri et al. 1982a, b). In most mammalian species studied, meiosis is resumed within the mature, pre-ovulatory follicle(s). FSH and LH cause the expansion of the antrum and maturational changes in the theca and granulosa cells, as discussed in Chap. III, although the oocyte nucleus remians morphologically unaffected until the onset of the so-called LH surge. FSH and LH — either directly or indirectly through steroid intermediates (especially oestrogen) — also regulate the proportions of follicles which undergo ovulation or atresia (see Chaps. III and VII) (see also Harman et al. 1975, Hay and Cran 1978, Erickson 1982). But the details of hormonal interaction with the oocyte, which leads to meiosis, remain to be clarified (Tsafriri 1983, Tsafriri et al. 1982a, b). However, at ovulation, a number of oocytes, depending on the species, are released at the metaphase of the second meiotic division (Fig. 45); in a few species of mammals dictyate oocytes are normally ovulated and maturation takes place in the oviduct (Tsafriri 1978 a).