Modifications ultrastructurales des cellules germinales femelles au cours de la prophase méiotique chez le rat : étude particulière des membranes après cryofracture

Abstract

Ultrastructural changes in the nuclear and cytoplasmic elements in the germ cells of female rats were followed before meiotic prophase (15.50 days post-coïtum and 17.25 days post-coïtum) and during it (17.75 days post-coïtum to birth). We observed: modifications in the nuclear envelope which was thick during the oogonial stage, becoming thinner when the chromosomes entered preleptotene stage. The thinning of the envelope was due to the disappearance of the chromatin material lining it; variations in the number and distribution of germ cell nuclear pores according to stage; the pores were first scattered in small clusters of 6 to 8 over the entire nuclear membrane. From the preleptotene to zygotene stage, these clusters enriched in pores to form large areas. Finally, in the pachytene and diplotene stages, clusters of more than 100 pores were seen; nucleolar fragmentation from the preleptotene stage, followed by the formation of a new active nucleole in the diplotene; polarization of the mitochondria in the oldest oogonia just before the beginning of meiotic prophase. This polarization disappeared after the onset of the meiotic processes, then appeared again near the developing Golgi apparatus at the end of the pachytene stage; the formation of large gap junctions and numerous bands of tight junctions between the somatic cells; these formations contrasted with small gap junctions, and the tight junctions became scarce just before the meiotic process began. These observations, as well as those concerning nuclear pore distribution were made using the cryofracture technique.