ZP3 KO 1996 - Development 122, 2903-2910 (1996) Printed in...

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INTRODUCTION The mammalian zona pellucida is synthesized by oocytes during folliculogenesis, an on-going cyclic process that begins just after birth and continues throughout the reproductive life of females. In mice, resting oocytes (12-15 μ m), with no visible zona pellucida, are enclosed in a squamous epithelium of granulosa cells forming primordial follicles. As cohorts of follicles begin development, the granulosa cells become cuboidal and the zona pellucida becomes evident as extracel- lular patches that coalesce into a uniform matrix surrounding the growing oocyte. After approximately 12 days of growth, the 70-80 μ m oocyte is encased in a 7 μ m thick zona pellucida and surrounded by multiple layers of granulosa cells, together forming a 125 μ m diameter follicle. The layer of granulosa cells closest to the oocyte constitutes the corona radiata. Although the growth of the oocyte slows dramatically over the next 3 days, the granulosa cells continue to proliferate, resulting in a 600 μ m antral follicle containing a fully grown (75-85 μ m) oocyte (Brambell, 1928). The ultimate goal of folliculogenesis is to ovulate a mature egg for fertilization. Prior to ovulation, the cumulus-cell- enclosed oocyte projects into the antrum while remaining attached to the inner wall of the follicle by a cellular stalk. As a consequence of the luteinizing hormone (LH) peak in mid- cycle, the follicle becomes highly vascularized and the antrum is filled with a fluid rich in serum proteins. Oocytes undergo meiotic maturation and complete the first meiotic division, reaching metaphase II. In response to signals from the oocyte, the cumulus cells secrete a viscous extracellular matrix and dis- sociate from one another. However, the cumulus oophorus and oocyte remain a structural unit, the cumulus-oocyte complex, in which the granulosa cells radiate out from the centralized oocyte (Eppig, 1991). At ovulation, this complex is extruded into the oviduct and the egg is available for fertilization. Although tens of millions of sperm are deposited at coitus, fewer than 20 approach the ovulated egg (Braden and Austin, 1954). One sperm must pass through the cumulus oophorus, penetrate the zona pellucida and fuse with the egg for suc- cessful fertilization. The binding of sperm to the zona pellucida induces the acrosome reaction that is required for zona pene- tration (Saling et al., 1979; Florman and Storey, 1982) and sub- sequent fusion of a sperm with the egg’s plasma membrane (Yanagamachi, 1994). After fertilization, the zona is biochem- ically modified to prevent polyspermy (Sato, 1979; Bleil and Wassarman, 1980a; Bleil et al., 1981). Transfer studies in 2903 Development 122, 2903-2910 (1996) Printed in Great Britain © The Company of Biologists Limited 1996 DEV3497 Mammalian oocytes synthesize and secrete a zona pellucida that surrounds the growing oocytes, ovulated eggs and pre- implantation embryos. The extracellular zona matrix is composed of three glycoproteins (ZP1, ZP2, ZP3) that are involved in folliculogenesis, species-specific fertilization, and passage of the early embryo down the oviduct. We have
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ZP3 KO 1996 - Development 122, 2903-2910 (1996) Printed in...

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