Sequential effects of FSH on the first stages of ovarian follicular development in normal and dwarf Snell mice.

: The population of small growing ovarian follicles was divided into 4 classes according to the number of granulosa cells (from 15 to 95) surrounding the oocyte, and a comparison was made of normal and dwarf mice. Follicular cell proliferation was estimated by tritiated thymidine incorporation. In normal mice, most follicles in classes 1 (15 to 35 granulosa cells in their largest cross-section) and 2 (36 to 55 cells) were labelled (86 and 95%, respectively); FSH treatment increased the labelling index (L.I.) in all follicle classes. In dwarf mice, only 38 and 76% of follicles in classes 1 and 2, respectively, were labelled. However, FSH treatment increased the percentage of labelled follicles and the L.I. to levels which were similar to those in the ovaries of untreated, normal animals. FSH stimulation of the percentage of labelled follicles and L.I. was obvious as early as 3 h after injection. There was a major increase of the L.I. 24 h after FSH stimulation, specially in dwarf mice; several hypotheses are proposed to explain this finding. We conclude that FSH is necessary for the development of the population of small growing follicles in the mouse ovary.