Micromorphology of the Bovine Ovarian Follicular System

Abstract

The morphology of the bovine ovarian follicular system was studied in 700 dairy cows with known reproductive histories. Vesicular follicles from the earliest stages of antrum formation were evaluated histologically to determine normal characteristics and identify the types of atresia that occur normally in such follicles. Vesiculation begins in secondary follicles of 0.5 mm. diameter with occurrence of vacuoles between the follicular cells. Follicular growth to 1.0 mm. size is rapid; however, development from 1.0 to 12 mm. appears to be continuous and without periods of acceleration. During the 18 to 24 hr. prior to ovulation, rapid expansion of the follicle occurs. The granulosum and theca layers become thinned by stretching and the granulosa cell nuclei are plump, spherical and hyperchromatic. Follicular atresia, which was classified in this study as early, definite and late, may begin at any stage of follicular development. Early atretic changes may appear as (1) loosening and sloughing of granulosa cells lining the antrum; (2) disappearance of membranapropria, and loss of orientation of the basal layer of the granulosum; (3) shortening and rounding of the theca interna cells, and (4) appearance of Call-Exner bodies. Definite atresia is characterized by more advanced stages of regression and may be evident in follicles that are (1) collapsing, (2) contracting,(3) cystic or (4) luteinized-cystic. The morphology of each of these types is illustrated. Late atresia is the final stage of regression common to all types of atretic vesicular follicles. The size is reduced and cell layers are disorganized. The antrum becomes gradually filled with fibrous granulosa remnants and the theca layers are hyalinized. These structures may remain for several cycles before becoming indistinguishable. Degeneration of the oocyte in vesicular follicles may begin at any stage of the regressive process and does not serve as a good criterion of early follicular atresia. Regressive changes in the vascular supply to a follicle are not evident prior to atretic changes in the follicular cells. Copyright © 1968. American Society of Animal Science . Copyright 1968 by American Society of Animal Science