Cellular basis for the production and secretion of the hatching enzyme by frog embryos

Stage 17, 19, and 21 embryos of the grass frog, Rana chensinensis , were deprived of their envelopes, cultured for 36 hours, and the proteolytic activity of the culture media was determined. The activity is low during stages 17–19, increases sharply during stages 19–21, and remains at a high level during stages 21, 22. Comparison of the media in which the embryonic halves were cultured revealed that the secretion of the enzyme is dependent on the anterior part of the embryo. Embryonic extracts showed no significant activity at any stages examined. Culture of the epidermal explants from different embryonic regions proved further that the enzyme secretion is restricted in the dorsal frontal portion where a particular type of cells is predominant. In the tailbud embryo, large, flask‐shaped epidermal cells, designated as the hatching gland cells (HGCs), are mostly distributed in the dorsal frontal region, and in small number along the dorsal mid‐line. Electron microscopically, the HGC is characterized by prominent microvilli at the apical cell surface, a number of apically localized, membrane‐limited secretory granules, well‐developed Golgi complexes, as well as an abundance of rough‐surfaced endoplasmic reticula. The differentiation of HGC is first discernible at stage 17, and the cell acquires its histotypic characteristics at stage 18. The cell continues to synthesize and secrete granules during stages 18–21 and undergoes gradual regression after the hatching stage, accompanied by the formation of an elaborate phagolysosome within the cell. The evidence presented strongly indicates that HGC is a specific epidermal cell adapted for synthesizing and secreting the hatching enzyme in the prehatched frog embryos.