Biosynthesis and expression of zona pellucida glycoproteins in mammals

The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte a nd the early embryo that exerts several important functions during fertiliz ation and early embryonic development. The ZP of most mammalian species is composed of three glycoproteins (ZPA, ZPB, ZPC), products of the gene famil ies ZPA, ZPB and ZPC that have been found to be highly homologous within ma mmalian species. Most data on the structure and function of the ZP are obta ined from studies in mouse. New data from pig and other domestic animals, h owever, indicate that the mouse model does not hold for all other species. Whereas in the mouse ZPB is the primary sperm receptor, in the pig ZPA has been shown to possess receptor activity. Contrary to the mouse, where the g rowing oocyte is the only source of zona glycoproteins, in domestic animals these proteins are expressed in both the oocyte and granulosa cells in a s tage-specific pattern and may play also a role in granulosa cell differenti ation. In several mammalian species, the epithelial secretory cells of the oviduct synthesize and secrete specific glycoproteins (oviductins) that bec ome closely associated with the ZP of the ovulated oocyte. Once bound to th e ZP, oviductin molecules could act as a protective layer around the oocyte and early embryo by virtue of their densely glycosylated mucin-type domain s. Copyright (C) 2001 S. Karger AG, Basel.