Abstract
Optimal maturation of oocytes and successful development of preimplantation embryos is essential for reproduction. Mammalian oocytes remain dormant in the diplotene stage of prophase I until the resumption of meiosis characterized by germinal vesicle breakdown (GVBD) following preovulatory gonadotropin stimulation. In response to the preovulatory luteinizing hormone (LH) increase, oocytes undergo GVBD, followed by first polar body extrusion. Although the preovulatory surge of LH is the primary event responsible for the induction of maturation of the oocyte, LH does not act directly on the oocyte due to the absence of functional LH receptors in germ cells. Instead, actions of LH are mediated either by paracrine factors secreted by LH-responsive somatic cells or by the transport of cellular messengers from granulosa/cumulus cells to oocytes through intercellular gap junctions. In addition to the nuclear maturation exemplified by GVBD and extrusion of the first polar body to complete the first meiotic division, oocytes also undergo cytoplasmic maturation characterized by cytoplasmic changes essential for monospermic fertilization, processing of the sperm, and preparation for development to preimplantation embryos. In this review, we summarize our recent works on the identification and characterization of novel LH-inducible ovarian factors for nuclear and cytoplasmic maturation of oocytes.
