Relationships between aromatase activity, follicular fluid oestradiol-17 beta and testosterone concentrations, and diameter and atresia of individual ovine follicles.

Aromatase activity was measured in granulosa cells using a 1-h in-vitro assay. This activity correlated with the concentration of oestradiol-17 beta and the ratio of oestradiol-17 beta to testosterone in follicular fluid of individual follicles ranging from 1.5 to 7.0 mm diameter. These data show an 8-10-fold difference in aromatase activity between small and large follicles and that aromatase activity per cell increased in small non-atretic follicles (less than 3.5 mm) whereas it remained relatively constant in large nonatretic follicles (greater than or equal to 3.5 mm). Aromatase activity was much lower in follicles at more advanced stages of atresia. Atresia was assessed using the morphological and the morphometric methods (% of maximum number of granulosa cells/follicle). Although the morphological method of assessment was preferable to the morphometric method, it did not differentiate a decrease in aromatase activity as a very early event in the atretic process. We believe this is due to the inability of these methods to detect follicles in the initial stages of atresia.