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The avian hatching process is considered to comprise a series of complicated phases including the digestion of vitelline membrane and egg white and the breakdown of shell membrane and the eggshell. The present study focuses on the first phase, i.e., the digestion of vitelline membrane in Japanese quail Coturnix japonica. When embryos were subjected to immunocytochemical tests with an anti-Xenopus hatching enzyme antibody, staining was observed on day 0 of incubation in the outermost ectodermal cells of blastoderms, and later in the ectodermal cells of yolk sacs in the area vitellina. It was confirmed by electron microscopy that what had been specifically stained in these (two locations) were secretory granules in the cells. A 57 kDa protein was detected by immunoblot tests of the extracts of yolk sac material from the area vitellina. Ultrastructural disintegration of the vitelline membrane followed the advance of the yolk sac toward the vegetal pole. We propose that the step-by-step digestion of the vitelline membrane from the animal pole side toward the vegetal pole one is accomplished by an enzyme produced by the outermost ectodermal cells which show a temporary secretory activity.

On the Hatching Mechanism of Quail Embryos: Participation of Ectodermal Secretions in the Escape of Embryos from the Vitelline Membrane