Intrafollicular pressure promotes partial evacuation of the antrum during hamster ovulation in vitro

Abstract

AbstractThe role of intrafollicular pressure in evacuation of the hamster cumulus from the follicle at ovulation was investigated experimentally in vitro. Previous work has shown that the cumulus is generally evacuated in two stages (Talbot, '82). Phase 1 occurs immediately at rupture and is characterized by rapid (<60 seconds) extrusion of about 20% of the cumulus and usually the oocyte. Phase 2 includes the gradual (2–10 minutes) removal of the remainder of the cumulus and collapse of the follicle wall. Fresh or aged follicles (approximate intrafollicular pressures = 2.1 or 0.0 mmHg, respectively) were poked with “0” gauge insect pins to produce a rupture site in their apices. Of 69 fresh follicles tested, 68 responded with phase 1 extrusion; 57 of the 68 evacuated the oocyte in addition to a significant portion of the cumulus. In contrast, only seven out of 80 aged follicles showed phase 1 extrusion, and only three out of these seven extruded the oocyte. To verify that phase 1 extrusion in fresh follicles was due to intrafollicular pressure and not smooth muscle cell (SMC) contraction, some fresh poked follicles were fixed for microscopy. Twelve of 14 follicles had flat bases or small V‐shaped constriction in their bases, indicating SMC contraction had not occurred. This was further substantiated by examining several follicles with transmission electron microscopy; SMC in the theca externa were relaxed by morphological criteria. To determine if phase 2 extrusion occurred in poked follicles, fresh follicles were poked, cultured in vitro for 3 hours, and examined microscopically. Phase 1 occurred in all follicles after poking. The remainder of the cumulus was eventually evacuated from the antrum, although much more slowly than during normal in vitro ovulation. The follicle wall did not collapse in these trials. We conclude that intrafollicular pressure is responsible for phase 1 extrusion, i.e., rapid passage of part of the cumulus and oocyte through the rupture site, and that intrafollicular pressure will promote phase 1 extrusion in the absence of SMC contraction. The remainder of the cumulus can be evacuated in the absence of IFP and collapse of the follicle wall; perhaps this occurs by hydration of that portion of the cumulus evacuated during phase 1. Hydration would tend to expand the cumulus and pull the remaining cumulus from the antrum. These results establish a role for intrafollicular pressure in hamster ovulation.