Amphibian oogenesis is a well-studied process: the oocyte grows by incorporating liver-originated vitellogenin from the blood stream, RNA and other molecules from its own metabolism or from the follicle cells. Simultaneously meiosis starts, reaching metaphase 1. Later on, during hormone-induced maturation, meiosis continues up to metaphase II. In this stage the oocyte is released into the coelomic cavity. Upon the passage of the egg through the oviduct, the vitelline envelope (VE) is modified, becoming sensitive to sperm proteases and penetrable by sperm. Besides, the eggs are covered by a jelly coat, which has a role in gamete recognition, sperm protection, and embryo preservation. Sperm acrosome contains proteases and glycosidases, which apparently participate in sperm penetration through the VE. Gamete recognition has also been studied at the level of sperm-VE. Egg activation and polyspermy prevention were studied in two steps, the electrical, fast and transient, and the slow and permanent ones. The latter involves molecules released by the cortical granules during their exocytosis.
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