Maturation of the murine oocyte-cumulus cell complex: Lectins as probes for understanding its regulation

Resumption of meiotic maturation is known to be triggered in vivo by the endogenous gonadotropin surge just prior to ovulation. This meiotic resumption can also be stimulated in vitro by gonadotropins or growth-promoting factors. However, in the mammalian oocytes, the precise mechanism(s) involved is not fully elucidated. One hypothesis that has been postulated is that meiotic resumption is brought about by a loss of flux of inhibitory influences from the cumulus cells to the oocyte following a breakdown in junctional communication prior to ovulation. Contrary evidence supports the generation of a positive maturation stimulus of somatic cell origin that stimulate maturation of the oocyte. I have in this study used lectins, as probes to further elucidate the regulation of oocyte-cumulus cell complex maturation in vitro. Mitogenic lectins, but not non-mitogenic lectins, stimulated oocyte maturation and cumulus expansion, and the degree of stimulation was associated with their mitogenic potencies. Additionally, none of the lectins had a stimulatory effect on denuded oocytes. This differential effect on denuded and cumulus cell-enclosed oocytes makes lectins useful tools in examining possible mechanisms of ligand-stimulated oocyte maturation. While concanavalin A (Con A), the most potent of the mitogenic lectins tested, efficiently stimulated oocyte maturation, it had only a nominal effect on junctional coupling. Alkanols, agents that are known to uncouple gap junctional communications, inhibited Con A-induced oocyte maturation, but by itself did not stimulate meiotic resumption. The suppression of Con A- and follicle-stimulating hormone (FSH)-stimulated maturation of the oocyte-cumulus cell complex in the absence of glucose from the culture medium or when the medium was supplemented with non-metabolizable analogs of glucose, supports the idea for the apparent requirement of glycolysis in ligand-stimulated maturation of the oocyte-cumulus cell complex. The results of this study support the idea that ligand-stimulated meiotic resumption is brought about by the generation of a positive maturational stimulus of somatic origin that acts on the oocyte to reinitiate meiosis. Furthermore, it demonstrates that lectins can be used as effective research tools for the elucidation of the mechanism(s) of the preovulatory gonadotropin regulation of oocyte-cumulus cell maturation