Graafian follicle growth and replacement in anovulatory ovaries of ageing C57BL/6J mice.

The growth of preantral follicles and replacement of large Graafian follicles was studied in reproductively senescent C57BL/6J mice immediately after the loss of oestrous cyclicity when the ovary is approaching exhaustion of its follicular reserve. Granulosa cells were labelled in vivo by injection of tritiated thymidine and were evaluated by autoradiography. At 1 h after injection, labelling was confined almost exclusively to the cumulus and neighbouring cells whereas the distribution of labelled granulosa cells in preantral follicles was approximately uniform. The proportion of labelled mural cells in Graafian follicles rose from initially low values to maximal levels 3 days later; this suggested that those follicles that were Graafian at the time of injection had been replaced by recruitment of preantral stages. The rate of growth of preantral follicles was similar in senescent anovulatory mice and in young adult animals. A simple model was constructed to illustrate how persistent vaginal cornification in ageing mice is sustained by a stream of Graafian follicles and why some of the latter did not respond fully to an ovulatory dose of hCG.