REVIEW The genetic control and germ cell kinetics of the female and male germ line in mammals including man*

Abstract

The female germ line (germ cell lineage, Keimbahn) is provided with only one proliferation wave, the oogenic, whereas male gametogenesis involves two successive waves: prespermatogenic, which corresponds to the female proliferation wave, and spermatogenesis, which is responsible for the immense number of male gametes produced in mature testes. Both male proliferation systems are linked by the transitional or T prospermatogonia. Using the reverse percentage of labelled metaphases method, it has been shown that the first differences between female and male germ cells can be identified by the end of the first wave, when oogonia and multiplying or M prospermatogonia are proliferating. This prenatal first wave of proliferation of male germ cells was also demonstrated in man and ceases around the 22nd week of pregnancy. Spermatogenesis involves a stock of stem cells (stem spermatogonia), a flexibly reacting pool of undifferentiated spermatogonia and several generations of differentiating spermatogonia, which proliferate almost exponentially. Furthermore, it consists of spermatocytes and haploid spermatids transforming into spermatozoa. The oocytes pass through the preleptotene stage, synthesizing DNA, and thereafter traverse the meiotic prophase up to the diplotene stage. In mammals they act as 'pre-embryos' in a similar but to a lesser degree than oocytes of amphibia and insects. The maternal chromosomes are largely responsible for the development of the embryo, the paternal genome for the development of the extra-embryonic tissue. The synthesis of transgenic animals is a powerful weapon in the armoury of geneticists, as has recently been demonstrated: a 14 kb genomic DNA fragment (Sry) is sufficient to induce testis differentiation and subsequent male development when introduced into chromosomally female mouse embryos.

Expression of P57kip2 in Germ Cells and Leydig Cells in Human Testis

“…Prospermatogonia is divided into mitotically active M prospermatogonia, non-dividing T1 prospermatogonia and T2 prospermatogonia, which had resumed mitotic activity. Subsequent generations of differentiating spermatogonia divide into preleptotene spermatocytes that start premeiotic S phase after a long G 1 phase [6,20]. In the present study, p57 kip2 immunoreactivity was largely found in the nuclei of the early spermatocytes, but not in the spermatogonia, suggesting the specific role for p57 kip2 in early phase in meiosis I in human male germ cells and that differential role of p57 kip2 between mitosis and meiosis.…”

Germ cell kinetics during early ovarian differentiation: An analysis of the oogonial cell cycle and the subsequent changes in oocyte development during the onset of meiosis in the rat

“…He emphasized the common feature of coarse chromatin flakes that gradually disappear in later cellular phases in all three types. Histological and autoradiographic studies on female and male pre-and perinatal germ cells in the rat (Hilscher et al, 1974;Hilscher andHilscher 1978, 1981) confirmed these findings and provided data on their kinetics. Mauléon (1981, 1984a,b) presented a very detailed picture of the structure of oogonia and the subsequent stages of oocytes entering meiotic prophase at the light and electron microscopic level.…”

“…Even between fetal male and female germ cells there can be a difference. As a consequence of a prolonged S-stage, the G2-stage of M-prospermatogonia is 1 hr shorter than that of the corresponding oogonia (Hilscher, 1991).…”

Regulation of meiotic entry and gonadal sex differentiation in the human: normal and disrupted signaling

“…This proliferating subpopulation of oogonia is mainly found in the periphery of the ovaries, whereas the more differentiated oocytes are found deeper within the medulla (22). After GW 20, the majority of germ cells within the human fetal ovaries have initiated meiosis, and the oocytes arrest in diplotene stage of first meiotic division where they will be surrounded by primitive granulosa cells, resulting in the formation of single-layered primordial follicles (21,23,24) (Figure 1). …”