The zona pellucida of teleosts is secreted by exocy-tosis around the bases of the microvilli that extend from the surface of the oocyte (Figure 2.51). These microvilli are entrapped in this developing envelope, along with microvilli extending from the follicular cells so that the primary envelope is perforated by pore canals containing microvilli extending perpen dicular to its surface (Figure 2.52). These perforations, with their enclosed microvilli, account for the striated pattern of the primary envelope, as seen in sections with the light microscope, giving rise to the term "zona radiata" that is sometimes applied to this layer. The microvilli within the pore canals may be closely apposed (Figures 2.53 and 2.54), even coiling around each other (Kjesbu, Kryvi, and Norberg, 1996). Those from the follicular cells may flatten their tips over the oolemma (Figure 2.55), while the oocyte's microvilli reciprocate by touching the surface of the follicular cells. It is estimated that a single oocyte in the cod Gadus morhua bristles with about 1.8 million microvilli, thereby increasing its surface area by a factor of 12.
The zona pellucida transforms during early oocyte development (Figures 2.56 and 2.57) (Gotting, 1965; Anderson, 1967; Azevedo, 1974; Hosokawa, 1983; Cotelli et al., 1988; Begovac and Wallace, 1989). During primary growth, prior to its formation, oocytes extend microvilli toward the investing monolayer of follicular cells (Begovac and Wallace, 1988). Initial formation of the zona pellucida occurs at the earliest cortical alveolus stage, with the deposition of a homogeneous electron-lucent layer, designated Z1 by Anderson (1967), between the microvilli (Figure 2.57A). By mid to late cortical alveolus stage, the architecturally complex Z2 and Z3 layers form, giving the zona pellucida a trilaminar appearance (Figure 2.57B). During vitellogenesis, the zona pellucida undergoes compaction, particularly of the Z3 layer (Figure 2.57C) and several layers of alternating electron-dense and electron-lucent lamellae may become apparent. Active exchange of metabolites takes place between the oocyte and perifollicular space by way of the pore canals (Figure 2.58).
When the zona pellucida first appears, the peripheral ooplasm displays enhanced apparatus for protein synthesis and secretion: free ribosomes, granular endoplasmic reticulum, and well developed Golgi complexes (Anderson, 1967; Wourms, 1976). Electron-dense vesicles, apparently formed by the Golgi complex, arise in the peripheral ooplasm prior to deposition of the electron-dense layer of the zona pellucida and are apparent during all subsequent stages of its deposition (Figure 2.59) (Tesoriero, 1977a); this
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