Zona pellucida

Millar, S. E., Lader, E., Liang, L-F., and Dean, J. (1991). Oocyte-specific factors bind a conserved upstream sequence required for mouse zona pellucida promoter activity. Mol. Cell. Biol. 11 6197-6204. 

A domain family that is considerably expanded in nematodes, relative to vertebrates, is the zona pellucida (ZP) domain (Bork and Sander, 1992). In database searches this domain was found in C. elegans cuticlin-1 (cut-1), a component of the nematode cuticle (Sebastiano et al., 1991), and 33 other C. elegans proteins (Table II). On the basis of disulfide-linked domains that accompany the ZP domain in these proteins, it is likely that they localize to the worm s extracellular matrix. Indeed, it is possible that most of these proteins are components of the worm cuticle. The cuticle structure is the multilayered elastic exoskeleton that determines the worm s body shape. Although vertebrates lack an equivalent... 

The repulsive, electrostatic forces of sialic acids contribute to the rigidity of the cell surface, as was shown hy an increase in the defor-mability of sarcoma cells after enzymic removal of sialic acid residues.424 Enzymic release of sialic acids from the zona pellucida of rabbit ovum lessens the rigidity of this cell, and spermatozoa can no longer penetrate it.425 Glycoproteins on the surface of sea-urchin eggs... 

Conformational changes of cell-surface components after the removal of sialic acid, leading to loss of the rigidity of rabbit ova, and to inhibition of the passage of spermatozoa through the zona pellucida, have been mentioned in the preceding sub-Section.425... 

At the time of menarche or puberty, there may be only 300 000 -400 000 oocytes in the ovaries. With the onset of puberty, a second layer begins to form from cuboidal granulosa cells these cells arise from a small number of growing follicles that have an enlarged ooplasm and a zona pellucida. 

Fertilization is the fusion of the sperm and ovum. The sperm head binds to the plasma membrane of the egg (oolemma), and the entire spermatozoon enters the cytoplasm of the ovum. Only capacitated spermatozoa with intact acrosomes can enter and pass through the cumulus oophorus. The acrosome is a membrane-bound, cap-like structure covering the anterior portion of the sperm nucleus. The acrosomal reaction is the release of materials that lyse the glycoprotein coat (zona pellucida) surrounding the ovum. This is necessary for fertilization to take place. Before undergoing the acrosomal reaction, sperm go through a type of hypermotility called hyperactivation. The... 

The acrosome reaction is the loss of the acrosomal and plasma membranes in the acrosome region and the release of acrosin, hyaluronidase and other enzymes that disperse the cumulus complex and allow the sperm to penetrate the zona pellucida. After capacitation and the acrosome reaction, sperm penetrate the extracellular cumulus matrix and bind with zona protein 3, a heavily glycosylated protein of the zona pellucida. The first segment of the sperm to make contact with the oolemma is usually the inner acrosomal membrane, followed by the postacrosomal region. The plasma membrane of the sperm attaches to microvilli on the oolemma. Sperm-egg fusion is apparent from reduced movement of the sperm tail (Yanagimachi, 1970,1988 Takano et al.,... 

Heterologous in vitro fertilization or, perhaps more correctly, sperm-oocyte penetration studies have also received attention. The most extensively used bioassay of this type has been the zona-free hamster oocyte test (WHO, 1999), which investigates the ability of human sperm to penetrate and fuse with the oolemma of a hamster oocyte denuded of its zona pellucida. 

Mori, E., Hedrick, J.L., Wardrip, N.J., Mori, T. and Takasaki, S. (1998) Occurrence of reducing terminal N-acetylglucosamine 3-sulfate and fucosylated outer chains in acidic N-glycans of porcine zona pellucida glycoproteins. Glycoconj J, 15,447-456. 

All animal eggs are surrounded by egg envelopes which must be traversed by sperm on their journey to the egg surface. These egg envelopes are known by various terms in different organisms, being called the vitelline layer in sea urchins, the vitelline envelope in amphibians, and the zona pellucida in mammals. The fertilizing sperm must penetrate the egg envelope in order to fuse with the plasma membrane and transfer its haploid set of chromosomes into the cell cytoplasm. 

Involvement of sperm proteolytic enzymes in fertilization processes has a long history. Perhaps the first definitive observation that sperm proteases affected egg envelopes was that of Yamane in 1935. He demonstrated that an extract of rabbit sperm dispersed the cumulus cells and solubilized the zona pellucida (ZP) or egg envelope of the rabbit egg (1.). The presence of proteases in the sperm extract was presumed by analogy with the dispersing action of the trypsin activity in pancreatin. In 1939, Tyler obtained an extract from sperm of the giant keyhole limpet Meeathura crenulata which dissolved the egg envelope without affecting the egg itself. 

No sperm enzymes have yet been described which meet all of these criteria. However, acrosin satisfactorily meets the first three criteria and, with the identification of the zona pellucida glycoprotein which is a substrate for acrosin, it is now possible to approach experimentally the fourth criterion (7). 

Figure 3. A Model for Acrosin s Role in Zona Pellucida Binding and Penetration of Sperm. The model uses the m -acrosin form depicted in Figure 2. Acrosin is bound to the sperm plasma membrane (PM) via its hydrophobic domain and interacts with the egg zona pellucida (ZP) via its protease and carbohydrate binding domains. The ZP is composed of three glycoproteins which have unique polypeptide chains and both shared and unique oligosaccharide moieties. Symbols used - oligosaccharide...

The frequency of the menstrual/estrous cycle varies among species. For example, monoestrous species (e.g., dogs, cats) exhibit 1-2 cycles per year, whereas polyes-trous species (e.g., rodents, primates) exhibit more frequent cycles of shorter duration. Primordial follicles, each consisting of an oocyte surrounded by a single layer of flattened somatic cells known as granulosa cells, are recruited to become primary follicles, a transition marked by increased oocyte size, formation of a glycoprotein matrix (zona pellucida) around the oocyte, and transformation to cuboidal-shaped... 

The blastocyst is a hollow, fluid-filled ball of approximately 1000 cells. The cells that form the outer layer are referred to as trophoblasts and will ultimately develop as extraembryonic tissues (e.g., placenta), while the cells of the inner cell mass are omnipotent (i.e., stem cells) and form the embryo. Depending on the species, the blastocyst arrives at the uterus within 5-10 days of fertilization, whereupon it hatches from the zona pellucida and implants into the uterine wall, which has been preconditioned by ovarian-derived steroid hormones (see Chapter 33). Shortly after implantation, the inner cell mass undergoes gastrulation to form a trilaminar embryo composed of three primary germ layers, the ectoderm, mesoderm, and endoderm. 

Zona pellucida thickness is probably influenced by the preovulatory hormonal environment (LI 1), and its variation is significantly correlated with pregnancy (P2). If zona pellucida thickness variation [(Zmax — Zmean)/Zmean x 100 is below 15%, pregnancy rate is only 4.5%, even if three embryos are transferred. If zona pellucida thickness variation of any of the replaced embryos is 15-20%, pregnancy rate rises to 24.1 %, and if zona pellucida thickness variation is more than 20%, the pregnancy rate is 76.5% (P2). The reason for thickness variation being important is probably the ability to hatch, which is presumed to be larger in embryos with more variation in zona pellucida thickness. 

P2. Palmstierna, M., Murkes, D., Csemiczky, G., Andersson, O., and Wramsby, H., Zona pellucida thickness variation and occurrence of visible mononucleated blastomers in preembryos are associated with a high pregnancy rate in IVF treatment. J. Assist. Reprod. Genet. 15, 70-75 (1998). 

Tsukui S, Noda Y, Fukuda A, Matsumoto H, Tatsumi K, Mori T. Blocking effect of sperm immobilizing antibodies on sperm penetration of human zonae pellucidae. J In Vitro Fert Embryo Transf 1988 5 123-8. 

N-glycans or SA had no effect [138]. A Gal residue at the nonreducing terminus of these O-glycans is essential for binding of sperm to the zona pellucida. 

---