Formation of the rupture site in preovulatory hamster follicles: Morphological and morphometric analysis of thinning of the granulosa and thecal layers

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Abstract
The wall of hamster ovarian follicles is composed of the following cell layers: granulosa, theca interna, theca externa, and the surface epithelium. To determine the morphological changes occurring in the follicle during formation of the rupture site, we measured: the thickness of each layer, the number of cells in each layer, and size of cells in each layer, at the apex and base of follicles at specific times during the final 13 hr before ovulation. Changes in the wall occur in 3 stages. During stage 1 (T0–T8), which includes the first 8 hr following the surge in luteinizing hormone, the apical and basal walls thin at the same rate and the antrum increases in size. During stage 2 (T9–T12), there is no change in the thickness of the apical and basal follicle wall nor in the size of the antrum. During the third stage (T12–T13), the size of the antrum decreases slightly and there is an increase in the thickness of the basal wall, which is correlated with its constriction caused by the contraction of smooth muscle cells. The apical wall rapidly thins to the point of rupture. The morphology of cells from each layer is described. Theca interna cells form the final tissue preventing escape of the oocyte-cumulus complex. The roles of cell migration, stretching, and death in thinning of the apical wall are described, and the mechanisms involved in follicle rupture are discussed.