The ovulated eggs of Xenopus laevis released in the body cavity, referred to as coelomic eggs, are surrounded by coelomic envelope (CE), and are not fertilizable despite completion of meiotic maturation. To become fertilizable, they need to pass through the anteriormost portion (~5%) of the oviduct, the pars recta, and the rest portion (~95%) of the oviduct, the pars convoluta, to obtain jelly layers as seen in uterine eggs. When coelomic eggs enter the pars recta, the component gp43 (Xenopus homolog of mammalian ZP3) of CE is digested to gp41 by the action of a GSR-specific serine protease, oviductin, to give rise to the ultrastructure typical to that of the vitelline envelope (VE). This CE to VE conversion is accompanied by a remarkable increase of the sperm-binding sites exposed on the envelope surface. The envelope surface is further decorated with the acrosome reaction-inducing substance (ARISX) secreted from the posterior part of the pars recta. Thus, the oviduct of amphibians, in association with its well-known function of supplying jelly envelopes, highlights the role prerequisite for making eggs accessible to fertilizing sperm by regulating the egg envelope in multiple ways.
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