Intercellular communication via connexin43 gap junctions is required for ovarian folliculogenesis in the mouse

The ovarian follicle in mammals is a functional syncytium, with the oocyte being coupled with the surrounding cumulus granulosa cells, and the cumulus cells being coupled with each other and with the mural granulosa cells, vi a gap junctions. The gap junctions coupling granulosa cells in mature folli cles contain several different connexins (gap junction channel proteins), i ncluding connexins 32, 43, and 45. Connexin43 immunoreactivity can be detec ted from the onset of folliculogenesis just after birth and persists throug h ovulation. III order to assess the importance of connexin43 gap junctions for postnatal folliculogenesis, we grafted ovaries from late gestation mou se fetuses or newborn pups lacking connexin43 (Gja1(-)/Gja1(-)) into the ki dney capsules of adult females and allowed them to develop for up to 3 week s (this was necessitated by the neonatal lethality caused by the mutation). By the end of the graft period, tertiary (antral) follicles had developed in grafted normal (wild-type or heterozygote) ovaries. Most follicles in Gj a1(-)/Gja1(-) ovaries, however, failed to become multilaminar, with the sev erity of the effect depending on strain background. Dye transfer experiment s indicated that intercellular coupling between granulosa cells is reduced, but not abolished, in the absence of connexin43, consistent with the prese nce of additional connexins. These results suggest that coupling between gr anulosa cells mediated specifically by connexin43 channels is required for continued follicular growth. Measurements of oocyte diameters revealed that oocyte growth in mutant follicles is retarded, but not arrested, despite t he arrest of folliculogenesis. The mutant follicles are morphologically abn ormal: the zona pellucida is poorly developed, the cytoplasm of both granul osa cells and oocytes is vacuolated, and cortical granules are absent from the oocytes. Correspondingly, the mutant oocytes obtained from 3-week graft s failed to undergo meiotic maturation and could not be fertilized, althoug h half of the wild-type oocytes from 3-week grafted ovaries could be fertil ized. We conclude that connexin43-containing gap junction channels are requ ired for expansion of the granulosa cell population during the early stages of follicular development and that failure of the granulosa cell layers to develop properly has severe consequences for the oocyte. (C) 2001 Academic Press.