Rupture of the Ovarian Follicle

Mammalian ovulation is a unique biological phenomenon in that it requires the physical disruption of healthy tissue at the surface of the ovary.

Initially, during the first several hours after a mature ovarian follicle has been stimulated by an ovulatory surge in pituitary gonadotropins, there is no conspicuous change in the appearance of an ovulatory follicle.

However, 4-6 hours into the ovulatory process, a follicle will begin to blush. There is clear evidence that the capillaries in the follicle wall have dilated, and the tissue has become hypcremic. There is negligible other macroscopic, or microscopic, evidence of pending rupture until 1-2 hours before the follicle wall will actually burst. As the time of rupture nears, the apex of a mature follicle protrudes more and more above the surface of the ovary and the follicle wall itself gradually becomes thinner.

Eventually, the apical most portion of the follicle becomes translucent and rapidly balloons above the normal curvature of the follicle wall to form a stigma This nipple-like bleb may not form in all species of mammals, and will not occur if the vascular supply to the ovary has been impaired. However, a follicle will usually rupture within several minutes after the stigma forms. The eventual rupture of a follicle is dependent on adequate degradation of the collagenous connective tissue in the follicle wall and on a modest, but essential, intrafollicular pressure of about 20mm Hg that arises from capillary hydrostatic pressure.

After the follicle wall bursts, the oocyte and surrounding cumulus cells are usually extruded within 1-2 minutes. Ovulation is complete when the egg-bearing cumulus mass is expelled from the ovary.