INITIATION OF OOCYTE GROWTH IN THE MOUSE OVARY

In prepuberal mouse ovaries oocytes in the non-growing pool show constant but low levels of DNA-dependent RNA polymerase activity. The first event in the initiation of oocyte growth is a marked stimulation of RNA polymerase activity in the oocyte nucleolus. This is followed by increases in oocyte area, oocyte nucleolar area, oocyte nuclear area and nucleoplasmic RNA synthesis in that sequence. Non-growing oocytes may be surrounded by 2-8 follicle cells in the widest cross section. The progressive accumulation of follicle cells is marked by a transition from squamous to cuboidal morphology. The initiation of oocyte growth occurs at 9 follicle cells when the bulk (96 p. 100) of the follicular epithelium has become cuboidal. The transition from a squamous to cuboidal follicular epithelium is initiated at 5 follicle cells. The contribution of non-growing oocytes to the total oocyte pool declines from 15 p. 100 at 5 follicle cells to 2 p. 100 at 9 follicle cells. Mitotic division in follicular cells is only evident in follicles with more than 9 cells in the widest cross section. These results suggest that the accumulation of follicle cells becomes increasingly rapid as the oocyte approaches the initiation of growth. The data define the primary event in the initiation of oocyte growth as a stimulation of nucleolar RNA synthesis and demonstrate that oocyte growth is only initiated in follicles with an almost completely cuboidal follicular epithelium. We propose that the stimulation of nucleolar RNA synthesis as the first event in the initiation of oocyte growth is in response to an inductive stimulus from the surrounding follicular cells.