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

The patterns of ovarian follicular development and the steroidogenic properties of individual follicles (greater than or equal to 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 (less than 5 mm diam.) responds to LH and synthesizes androstenedione before the granulosa cells develop an 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 (greater than or equal to 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 (greater than or equal to 2 mm diam.) per cow may respond to LH by synthesizing androgen whereas only 1-3 follicles (greater than 5 mm diam.) have granulosa cells capable of metabolizing androstenedione or testosterone to oestradiol.