Mechanisms of hatching in fish: secretion of hatching enzyme and enzymatic choriolysis

SYNOPSIS. Mechanisms of two constituent steps of the hatching process, i.e., secretion of hatching enzyme from the gland cells and enzymatic choriolysis, in the Medaka, Oryzias latipes, are described. The ultrastructural changes of the hatching gland cells occurring at the initiation of electrically induced secretion as well as of natural secretion were the swelling of each gland cell and the separation of joints of the epithelial cells covering the gland cells, followed by a resultant exposure of the apical part of the gland cells. These changes, though their triggering mechanisms are not sufficiently clarified, suggest an intervention of some mechanical stimuli in the initiation of secretion. Decrease in electron density of the secretory granules also occurred immediately prior to the initiation of secretion. The secreted hatching enzyme was found to dissolve the inner layer of chorion by attacking the scleroprotein of the inner layer at some restricted sites and liberating a group of soluble glycoproteins of high molecular weights. This selective digestion appears to be the reason why choriolysis proceeds efficiently during a short period of time at hatching.