In vitro ovulation of hamster oocytes depends on contraction of follicular smooth muscle cells.

Smooth muscle cells (SMC) of the hamster follicle contract prior to ovulation, and their contraction produces a V-shaped constriction in the base of the follicle. We have determined when basal constrictions first appear in follicles removed from females at various preovulatory times by examining stained, thick sections with a brightfield microscope. The time to ovulation was controlled by injecting the female with human chorionic gonadotropin (hCG). The shape of the follicle was classified as flat, small V, medium V, or large V depending on the size of the basal constriction. Twenty-nine out of 30 follicles removed 12 hours after the hCG injection had flat bases or small V-shaped constrictions. However, by 12.5 and 13.0 hours after hCG injection, 45 and 54%, respectively, of the follicles had medium and large V-shaped constrictions in their bases. Transmission electron microscopy was used to confirm that flat follicles had uncontracted SMC, follicles with small V's had uncontracted or slightly contracted SMC, and follicles with medium and large V's had highly contracted SMC. When 12 hour follicles were removed from females and cultured in vitro, ovulation did not occur. Microscopic examinations showed that SMC had not contracted in these follicles during culture. Some (36.6%) 12.5 hour follicles and most (69.9%) 13 hour follicles ovulated when cultured in vitro. The majority of those which failed to do so, did not have contracted SMCs. We interpret these data to mean that in hamsters follicular SMC begin to contract in some follicles between 12-12.5 hours after the hCG injection. If contraction has begun in vivo, then the ovary may be removed from the female and ovulation will occur during in vitro culture. If contraction has not begun in vivo, it will not do so using the in vitro conditions described in this study and ovulation will not occur. These observations are consistent with the idea that the contraction of follicular SMC is necessary for hamster ovulation, and suggest that either stretch or neurotransmitters may initiate SMC contraction in vivo.