Some aspects of thecal and granulosa cell function during follicular development in the bovine ovary

Abstract

The patterns of ovarian follicular development and the steroidogenic properties of individual follicles (≥2 mm diam.) were assessed in Angus cows from Day -5 until Day +1 of the oestrous cycle (oestrus = Day 0). Individual follicles were judged to be healthy or atretic using a new classification system incorporating assessments of thecal vascularity and colour, the number of granulosa cells, the presence or absence of debris in follicular fluid and the status of the oocyte. The results suggest that the theca interna of small antral follicles (<5 mm diam.) responds to LH and synthesizes androstenedione before the granulosa cells develop in appreciable ability to metabolize androgen to oestrogen. Regardless of follicle size, the output of thecal androstenedione per unit mass of tissue remained unchanged in healthy but not in atretic follicles. On a per cell basis, aromatase activity increased in granulosa cells from healthy but not from atretic follicles with increasing follicle size. Peak levels of aromatizing activity were consistently observed in dominant oestrogen-enriched follicles on Day 0 although similar activity was also observed in some healthy follicles (≥8 mm diam.) on other days of the cycle. Early atresia in bovine follicles was characterized by an absence or lowering of aromatase activity in granulosa cells which always preceded any reduction in the thecal steroidogenic response to LH. It was estimated that between 20 and 60 antral follicles (≥2 mm diam.) per cow may respond to LH by synthesizing androgen whereas only 1-3 follicles (>5 mm diam.) have granulosa cells capable of metabolizing androstenedione or testosterone to oestradiol.