The cortical alveolus stage is characterized by the initial appearance of three components: cortical alveoli, the zona pellucida, and lipid (Figure 2.27A) (Begovac and Wallace, 1988; Selman and Wallace, 1986, 1989; Selman, Wallace, and Player, 1991). Cortical alveoli initially appear circumferentially at various depths in the cytoplasm. They are membrane bound, have a homogeneous appearance, and generally stain for both protein and carbohydrate. Their size and structure are variable; some contain a single large granule within a homogeneous matrix while others contain only the homogeneous matrix (Figures 2.27B and 2.28). They appear to be formed by the granular endoplasmic reticulum and Golgi complex and contain a glycoprotein synthesized within the oocyte (Figures 2.5E,F) (Ulrich, 1969; Tesoriero, 1980; Guraya, 1986; Bruslé, 1985; Selman, Wallace, and Barr, 1986; Selman and Wallace, 1986, 1989). These membrane-bound vesicles are closely associated with the Golgi complex.
As the oocytes grow, the cortical alveoli increase in number and their size becomes more heterogeneous as they fill much of the ooplasm. They enlarge, sometimes to 50+ pm in diameter and, by the end of the cortical alveolus stage, almost fill the cytoplasm of the oocyte. In subsequent stages they continue to form but are displaced to the periphery by yolk protein that accumulates centripetally. The cortical alveoli increase in size and number as they move peripherally in the ooplasm (Figure 2.29) and are found at later stages in variable sizes and numbers beneath the oolemma (see review by Selman and Wallace, 1989). The cortical alveoli will later fuse with the oolemma and, at fertilization, release their glycoprotein into the perivitelline space during the "cortical reaction".
Also in the cortical alveolus stage there is a formation and aggregation of small lipid droplets around the nucleus (Figure 2.27A,D). The lipid mass continues to enlarge in a perinuclear position. Later the lipid droplets increase in size. Lysosome-like bodies of uncertain function are present, often containing electron-dense material. They seem to disappear when yolk begins to accumulate.
The presence of "yolk vesicles" is often described in small oocytes; these appear to be identical in structure and composition with the cortical alveoli that are present in eggs. Since yolk vesicles do not contain yolk as a nutrient source it is suggested that this designation be dropped in favour of "nascent cortical alveoli" or simply "cortical alveoli" (Selman, Wallace, and Barr, 1988).
Pinocytotic vesicles appear during the cortical alveolus stage at the bases of the microvilli (Shackley and King, 1977; Begovac and Wallace, 1988). The zona pellucida is first observed as a thin, PAS-positive band between the oocyte and follicular cells; with the electron microscope it appears as homogeneous material between the microvilli of the oocyte. The microvilli lengthen as the zona pellucida thickens. The follicular cells also form microvilli that interdigitate with the oocyte processes. In some fish the follicular cells remain squamous but in the blenny they become cuboidal to columnar. They contain mitochondria and granular endoplasmic reticulum; Golgi complexes become more abundant. At this stage, one of the follicular cells enlarges and is designated the micropylar cell. It later forms the micropyle, a passage for eventual sperm penetration through the zona pellucida.
Cortical alveoli appear to arise in oocytes of the lamprey Lampetra planeri just before the beginning of vitellogenesis (Busson-Mabillot, 1967c); they do not form in oocytes of the dogfish Scoliodon sorrakowah (Guraya, 1982, 1986).
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