There may be changes in the egg envelope on contact with the external environment following spawning. The term "chorion" will be reserved, in this text, to designate the structural envelope that encloses the ovulated egg or developing embryo (Hart, Pietri, and Donovan, 1984).

glycoproteinaceous material is presumably deposited by exocytosis and becomes the developing zona pel-lucida (Figure 2.60) (Tesoriero, 1977b, 1978). No evidence was found in Cynolebias spp. that the follicular cells are engaged in the production of extracellular material at this stage (Wourms, 1976).

The innermost, thickest layer of the primary envelope (Z3) in many teleosts develops an elegant, transitory, fibrillar structure, whose appearance in sections suggests "art nouveau arabesques" (Figure 2.61) (Wourms, 1976). The positioning of the pore canals at the centre of the ellipsoid curves of these arches may provide flexibility to withstand expansion of the egg as well as mechanical shocks (Figure 2.62) (Caporiccio and Connes, 1977).

The differences in electron density of the layers of the zona pellucida are likely due to differences in their composition: the outer, more electron-dense layer in Oryzias latipes is rich in polysaccharides while the inner layer contains a higher proportion of proteins (Tesoriero, 1977a). In the viviparous teleost Xiphophorus helleri, the zona pellucida is homogeneous and shows no layering; it attains a thickness of about 1.0 pm during vitellogenesis, regresses during maturation, and has disappeared before segmentation begins (Azevedo, 1974).

The zona pellucida of the cod is a hydrophobic protein aggregate consisting of three polypeptides, designated, a, p, and y, with apparent molecular weights of 74,54, and 47 kDa (Oppen-Berntsen, Helvik, and Walther, 1990). Two to four major proteins with molecular weights ranging from 45 to 130 kDa have been separated from the zona pellucida of other teleosts (Brivio, Bassi, and Cotelli, 1991; Hyllner et al„ 1991); minor components with molecular weights ranging from 10 to 100 kDa were also present (Brivio, Bassi, and Cotelli, 1991). The presence of envelope proteins in the plasma of females rises dramatically as sexual maturation proceeds (Oppen-Berntsen et al., 1994) and has been induced in several species of teleosts by oestra-diol-17p (Hyllner et al., 1991; Hyllner, Norberg, and Haux, 1994; Larsson, Hyllner, and Haux, 1994; Hyllner, Silversand and Haux, 1994). It has been suggested that these proteins, like vitellogenin, are produced in the liver (Hamazaki, Iuchi, andTamagami, 1985,1987; Hamazaki et al., 1989; Oppen-Berntsen, Gram-Jensen, and Walther, 1992). This suggestion is contrary to the conventional understanding of the formation of the teleostean primary envelope: that the constituents of the inner layer are synthesized by the oocyte itself. It is not known whether these proteins are incorporated into the oocyte and then secreted from it to form the inner layer of the zona pellucida, after receiving some molecular modification, or whether they go directly to the site of formation of the egg envelope without passing through the oocyte. Evidence supportive of the conventional view was found in developing oocytes of the pipefish Syngnathus scovelli where the proteins of the zona pellucida could not be traced to the liver and it was concluded that they originate within the follicle itself (Figure 2.63) (Begovac and Wallace, 1989).

By the time the oocyte transforms into a mature egg, the microvilli and pore canals are often lost (Figure 2.57D). The Z1 layer is retained throughout oocyte development to maturity, although it becomes much reduced as development progresses. The mature egg envelope (chorion) may retain a lamellar appearance. There is no degeneration of the outermost layer of the zona pellucida of the porgy Pagras major prior to maturation, however, nor are the surface pits remaining from the pore canals plugged (Figure 2.64) (Hosokawa, 1983). These pits may also be observed on the surface of the zona pellucida of the flounder and globefish but not on the surface of the medaka, ayu, trout, or salmon.

A simpler, thinner envelope is described in viviparous poeciliids than in oviparous species (Azevedo, 1974), perhaps permitting more intimacy between foetal and maternal circulations. In viviparous Xiphophorus helleri, the zona pellucida develops in much the same way as in oviparous forms but forms only a single layer. It reaches its maximum thickness of 1 pm by the end of vitellogenesis, then becomes thinner and almost completely disappears by the end of maturation (Figure 2.56). Microvilli from the oocyte penetrate the pore canals to form a mesh against the follicular cells. The follicular cells of poeciliids may be unique in lacking processes that penetrate the pore canals. The zona pellucida disappears during the first stages of segmentation.

Four distinct layers have been described in the zona pellucida of the white sturgeon Acipenser transmon-tanus where the innermost layer is closely apposed to the oolemma (Cherr and Clark, 1982). The inner two layers are fibrous with filamentous substructures and are similar to layers of the zona pellucida of teleost oocytes. Numerous filamentous, screwlike projections appear to anchor the second layer to the outer layers (Figure 2.65). Electrophoretic separation (SDS-PAGE) shows a range of proteins and glycoproteins in the envelopes (including thejelly coat) of molecular weights from 21 to ca 300 kDa (Cherr and Clark, 1985). The most prominent band is a 70 kDa protein which resides in the outermost layer of the zona pellucida. After freshwater exposure, this layer contains both the 70 and a 66 kDa glycoprotein, presumably derived by proteolysis of the 70 kDa glycoprotein. This 66 kDa glycoprotein induces a species-specific acrosome reaction in spermatozoa of A. transmontanus but not of A.fulvescens; no inducer resides in thejelly coat.

A zona pellucida has been described in the developing follicle of a viviparous elasmobranch, the yellow spotted stingray Urolophus jamaicensis (Hamlett, Jezior, and Spieler, 1999). It is more delicate than the zona pellucida of teleosts and never becomes as structurally complex or rigid. There is no surface ornamentation and no micropyle. It is most likely a composite layer, being produced cooperatively by both the oocyte and follicular cells. Perforations containing processes from the follicular cells produce the typical striated appearance in sections but, unlike the condition in teleosts, there are no processes extending from the ovum. The processes extending from the follicular cells make contact with the oolemma, indent it, and even penetrate this membrane (Figure 2.66).

The primary envelope of the lamprey follicle is similar to those described for teleosts (Lewis and McMillan, 1965). The oocyte lays down a double, translucent, fibrous zona pellucida whose outer layer is more electron-dense than the inner (Busson-Mabillot, 1967a; Afzelius, Nicander, and Sjodén, 1968). The fibrils embedded within the zona pellucida form parallel sheets but do not attain the flamboyant patterns seen in teleosts. The zona pellucida is perforated by radial pore canals that contain microvilli extending through its entire thickness from the surface of the oocyte; although short microvilli extend toward the zona pellucida from the follicular cells, they do not appear to enter the pore canals. A complete sheath of squamous follicular cells encloses the oocyte of Petromyzon ma-rinus (Yorke and McMillan, 1980) but, in Lampetra planeri, they form a cup that envelops only the basal portion of the oocyte (Busson-Mabillot, 1967b).

Resting oocytes, 1 to 2 mm in diameter, of the hagfish Myxine glutinosa appear to secrete a primary envelope, rich in polysaccharides, similar to the zona pellucida of other fish (Patzner, 1974,1975). It is penetrated by microvilli extending from both the oocyte and the follicular cell. It attains its maximum thickness of about 6 pm in oocytes 5 to 8 mm in length and then becomes thinner. A unique feature of the hagfish follicle is its ability to secrete a "shell" around the oocyte: not merely a simple investment but one with a micropyle for sperm entry and, at each pole, a fan-like array of hooks (Dodd and Dodd, 1985).

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