Preparing for Fertilization: Intercellular Signals for Oocyte Maturation
In the hours preceding fertilization, oocytes prepare to begin development in a process known as maturation. This includes progression of the meiotic cell cycle and development of the ability to undergo the release of calcium that activates development at fertilization. In many species, the signal for oocyte maturation acts initially on somatic cells surrounding the oocyte, rather than on the oocyte itself. This chapter concerns the intercellular signaling events by which the maturation-inducing signal travels from the somatic cells to the oocyte. We first discuss how meiotic prophase arrest is maintained in mammalian oocytes, and how luteinizing hormone (LH) action on receptors in the cells of the surrounding follicle causes meiosis to resume. The LH receptors are located exclusively in the outer granulosa cells of the follicle and signal through a Gs-linked receptor to cause a decrease in cyclic guanosine monophosphate (cGMP). The LH-induced signal propagates inwards to the oocyte by way of cGMP diffusion out of the oocyte through gap junctions. We then briefly discuss the similarities and differences in mechanisms controlling oocyte maturation in animals other than mammals, focusing on hydrozoan jellyfish to emphasize the early evolutionary origin of these regulatory processes. G-protein-coupled receptors and cyclic nucleotides are common regulators in many animals. However, the assembly of these components into a regulatory system differs among species.
KeywordsOocyte Ovarian follicle Meiosis Luteinizing hormone Mammals Hydrozoans cAMP cGMP G-protein-coupled receptor Gap junctions
We thank Lisa Mehlmann and Evelyn Houliston for reviewing the manuscript. Work in the authors’ laboratory is supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (R37HD014939 to LAJ).
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