An ultrastructure study of oogenesis in the sea urchin, Strongylocentrotus purpuratus Public Deposited

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  • Oogenesis in the sea urchin Strongylocentrotus purpuratus was studied at the fine structural level, with special emphasis on the relationship between the two main cell types within the ovary: the egg cells and the nutritive phagocytes, or accessory cells. The nutritive phagocytes are flagellated cells with extensive pseudopods which store nutrients that are supplied to the growing oocytes and which also phagocytize degenerating eggs and cellular debris. Several aldehyde fixatives at various pH were compared with the best overall results obtained from formaldehyde-gluteraldehyde with a phosphate buffer at pH 7.0 and 0.45 M sucrose followed by osmium postfixation. Oogenesis can be divided roughly into several stages, characterized by certain structural features. The oogonial phase eggs are about 5 μm in diameter and occur in tightly packed clumps near the ovarian wall. During synapsis the chromosomes become very dense and synaptenemal complexes are obvious. In the post synaptic phase, the cytoplasm of the oocyte becomes much denser, large invaginations occur in the oolema, and the number of pores in the nuclear membrane increases. As the oocyte enters the premicrovilli growth phase, it separates from the oogonial clump and becomes surrounded by the nutritive phagocytes or their pseudopods. Yolk and cortical granule formation begins early in this period. Before any Golgi complexes are seen, a finely granular, membrane bound yolk is formed. Later, after Golgi complexes are apparent, a second type of yolk, the yolk platelet, is formed which is made up of subunits and becomes much more numerous than the granular yolk. During this phase the oolema forms narrow invaginations and bulbous pseudopods. As the elongated egg reaches a size of roughly 15 x 30 μm, finger-like extentions, for which the microvilli growth phase is named, appear on the surface. Often these microvilli interdigitate with pseudopods of the nutritive phagocytes. Bordered pits indicative of pinocytosis can be seen on the surface of the oocytes, while large membrane-bound vesicles containing amorphous material aggregate in the cytoplasm near the oolema. Golgi complexes appear more compact as the egg attains its final size of 60 - 70 μm in diameter. During the period of transition from oocyte to ovum, the eggs separate from the nutritive phagocytes and their microvilli become very elongated. With the breakdown of the germinal vesicle the microvilli become shorter, but the cortical granules remain randomly scattered in the cytoplasm. The ovum is characterized by the presence of the pronucleus and cortical granules at its surface, the appearance of heavy bodies and yolk nuclei in the cytoplasm, and formation of the vitelline membrane. Two types of ova can be distinguished in the ovary, based on their difference in electron density.
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