Abstract
All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.
Highlights
Introduction published maps and institutional affilExtracellular matrix (ECM) that surrounds most animal cells can affect cellular adhesion and migration, cell-to-cell communication, as well as gene expression, differentiation, and morphogenesis [1]
CANNTG, an E box domain located ≈200 bp upstream of the transcription start site of both mouse egg ZP (mZP) and human egg ZP (hZP) genes, is identical to the consensus sequence required for binding of proteins that belong to the basic helix-loop-helix family of DNA binding-proteins [42,43]
(ii) The zona pellucida (ZP) is composed of either three or four proteins, ZP1-4, each with a unique polypeptide chain that is heterogeneously glycosylated with both N- and O-linked oligosaccharides
Summary
The ovary was recognized as an anatomical entity by Herophilus in ~300 BC and was described in some detail by Soranus in ~50 AD [12]. Recognized in ~1670 that eggs came from the ovary, but concluded incorrectly that the entire follicle, egg plus surrounding follicle cells, was an egg. Cruickshank (1745–1800) in ~1795, it remained for Karl Ernst von Baer Von Baer was the first to use the term ZP when describing human eggs in. 1827 [13] and by the 1840s the term ZP had gained widespread use among embryologists
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