The establishment of follicular dominance in co-cultured mouse ovarian follicles.

An in vitro model of dominant and subordinate ovarian follicles was developed to allow a closer investigation of the phenomenon of follicular dominance. Preantral mouse ovarian follicles were cultured either alone or in pairs. Pairs of follicles were either in direct contact or in shared medium, but without physical contact. The experiments showed that where contact was allowed to develop between follicles one follicle invariably became dominant, while the other would grow and develop little during the culture period. In contrast, there was no effect of co-culture on follicle development in the absence of contact between the follicles. There was, therefore, no evidence of secretion of a diffusible factor by a dominant follicle that could affect the development of neighbouring follicles. After 6 days of co-culture with contact, histological examination of the subordinate follicle showed that it was healthy, in spite of remarkably little growth during culture. In a further experiment, the subordinate follicle was separated from the dominant one after 2 days of co-culture (when a significant difference in size had already developed), and cultured alone. These 'released' follicles exhibited a spurt of growth during the remaining culture period, attaining a size and appearance indistinguishable from those of controls by the end of culture. This confirms that the dominant follicle, while depressing the growth of its neighbour, is not inducing irreversible atresia in the subordinate follicle in this model. The in vitro model will allow a more detailed study of direct influences of dominant-subordinate follicle interactions, and should increase our knowledge of a poorly understood phenomenon.