Functional Morphology of the Zona Pellucida

Abstract

The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte and the early embryo that exerts several important functions during fertilization and early embryonic development. The ZP of most mammalian species is composed of three major glycoproteins that show considerable heterogeneity due to extensive post-translational modifications. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of the ZP reveals three to four glycoproteins which have been nominated ZPI. ZP2, ZP3 and ZP4. As cloning and characterization of the ZP genes of a variety of mammalian species including domestic animals show a high homology, three classes of ZP genes, ZPA, ZPB and ZPC can be discerned. The corresponding proteins were named ZPA, ZPB and ZPC. Whereas in the mouse ZPB is the primary sperm receptor. the situation is more complicated in other species. For instance, in the pig ZPA has been shown to possess receptor activity. Interaction between gametes during fertilization is at least in part regulated by carbohydrate moieties of the ZP and carbohydrate-binding proteins of the sperm surface. In domestic animals zona proteins are expressed in both the oocyte and granulosa cells in a stage-specific pattern and may play a role in granulosa cell differentiation. The role of ZP glycoproteins in immunocontraception is briefly discussed.

Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited

“…[6]. However, in other species, changes may occur in ZP protein gene expression during the course of synthesis and/or according to the site of synthesis, that is corona cells versus oocytes [3]. In pigs, although a contribution to ZP3 by follicular cells has been reported [53], transcripts of ZP1 were found only in growing oocytes [54], and no labeling was observed at the TEM level in follicular cells using three monoclonal antibodies to ZP proteins [55].…”

“…These glycoproteins all belong to the ZP family and generally contain a common domain or "ZP module" [9]. There are three single-copy ZP genes in mammals ZPA, ZPB, ZPC [3] including marsupials [11]. Depending on the species, ZPA in the mouse [40] or ZPB in the pig [3,41] would constitute sperm receptors.…”

“…There are three single-copy ZP genes in mammals ZPA, ZPB, ZPC [3] including marsupials [11]. Depending on the species, ZPA in the mouse [40] or ZPB in the pig [3,41] would constitute sperm receptors. More generally, it appears that all egg envelope glycoproteins of vertebrates belong to the same family [10,12,21,42].…”

“…around 5 µm in the mouse and around 10 µm in the pig [2,3]; the multiple roles of the ZP have been thoroughly reviewed [4,5]. The site of ZP synthesis is the oocyte during its growth phase, at least in the mouse [6][7][8] and/or the follicular cells in other species [3,9].…”

“…The site of ZP synthesis is the oocyte during its growth phase, at least in the mouse [6][7][8] and/or the follicular cells in other species [3,9]. A unifying hypothesis, at least on the composition of these envelopes, has been offered by molecular biology: the constitutive proteins are homologous and all belong to the ZP protein family [3,[10][11][12].…”