Response of Granulosa Cell Gap Junctions to Human Chorionic Gonadotropin (hCG) at Ovulation

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Abstract
Granulosa cell gap junctions in the mammalian follicle are dynamic bimembranous structures which are assembled within apposed plasma membranes of adjacent cells. Once initially formed, these specialized intercellular junctions grow through the insertion of additional elements into the interacting membranes or through the aggregation of smaller patches into large gap junctional plaques. These junctional aggregates may then be removed from the apposed cell surfaces through an endocytotic process which ultimately leads to the digestion of gap junction membrane within the cytoplasm. Several previous investigations suggest that some of these junctional behaviors may be influenced by hormones, and in this study we have examined the response of granulosa cell gap junctions in preovulatory follicles of rats and rabbits to human chorionic-gonadotropin (hCG). Our quantitative ultrastructural studies suggest that an ovulatory dose of hCG brings about a significant net reduction in the amount of gap junction membrane at the cell surface during the period immediately preceding extrusion of the cumulus-oocyte complex from the ruptured follicle. Morphometric analysis and qualitative studies of surface junctions and cytoplasmic vesicles support the view that this reduction occurs through the endocytosis of gap junctions from the cell surface. Furthermore, a scanning study of rabbit follicles also confirmed the thin section studies of previous investigators which demonstrated the disaggregation of granulosa cells at ovulation. These results are discussed with respect to the possible roles gap junctions may play in granulosa cell interactions pertinent to the growth of the oocyte and to the physical release of the oocyte from the follicle as well as its continued meiotic maturation.