STUDIES IN-VIVO AND IN-VITRO ON THE INITIATION OF FOLLICLE GROWTH IN THE BOVINE OVARY

Histological sections prepared from cortical parts of 25 bovine ovarie s were used to study initiation of follicle growth in vivo. Small foll icles were measured and characterized. Initiation of follicle growth c onsisted of two distinct consecutive phases. The first phase was chara cterized by transformation of granulosa cells from a flattened to a cu boidal shape and by their proliferation. In the second phase an increa se in the number of granulosa cells was accompanied by a rapid increas e in the size of the oocyte. Oocytes commenced growth when there were at least 40 granulosa cells in the largest cross-section (fourth gener ation of follicle cells). The oocyte diameter increased from 29.74 +/- 0.30 mu m (mean +/- SEM) in primordial follicles to 92.90 +/- 4.50 mu m in small antral follicles. The zona pellucida first appeared as an island of periodic acid-Schiff positive material in small preantral fo llicles, but formed a complete ring around the oocyte when the late pr eantral stage was reached. Organ culture of ovarian cortical explants was used to study initiation of follicle growth in vitro. Within 2 day s of culture most of the primordial follicles entered the growth phase : granulosa cells changed from a flattened to a cuboidal shape and ent ered S-phase as demonstrated by autoradiography after [H-3]thymidine i ncorporation. On day 2, 48.6% of follicles were labelled compared with 3% on day 0. Follicle growth started in the absence of gonadotrophins , in the serum-free medium, confirming the notion that gonadotrophins are not essential for this process. The culture system used here will be helpful in the study of the involvement of putative factor(s) in th e initiation of follicle growth in large domestic animals.