The Initiation of Follicle and Oocyte Growth in the Mouse Ovary

The morphological and functional events which surround the initiation of oocyte and follicle growth in the mouse ovary are described. Oocytes of the nongrowing pool were active in RNA synthesis, having relatively low but significant levels of endogenous RNA polymerase activity. Nongrowing oocytes were surrounded by 2–8 follicle cells in the widest cross section. The initiation of oocyte growth was detected by an increase in oocyte nucleolar RNA polymerase activity at 9 follicle cells followed by an increase in oocyte area at 10 follicle cells. Subsequently, increases in nucleolar area, nuclear area and nucleoplasmic RNA synthesis occurred in that sequence. As the number of follicle cells surrounding nongrowing oocytes increased, there was a transition from flattened (squamous) to cuboidal morphology. At 2–4 cells, 84% of oocytes surveyed were surrounded by a completely squamous follicular epithelium. At 5 follicle cells a transition in follicular morphology occurred: only 8% of oocytes had a completely squamous epithelium whereas all remaining oocytes were surrounded by a follicle envelope containing 1 or more cuboidal cells. Thereafter, the number of squamous follicle cells decreased and the number of cuboidal follicle cells increased. At 9 follicle cells, the point of initiation of oocyte growth, 96% of the follicular epithelium was cuboidal and 68% of oocytes surveyed were surrounded by a totally cuboidal epithelium. Mitotic figures were found only in the follicular envelope of oocytes surrounded by 10 or more follicle cells in the widest cross section. In the total oocyte population, 81% were found to reside in the nongrowing pool and 52% were surrounded by 4 cells or fewer. Oocytes surrounded by 9 cells, the point at which oocyte growth is initiated, comprised only 2% of the total oocyte population.