Ultrastructural changes in hatching-gland cells of pike embryos (Esox lucius L.) and evidence for their degeneration by apoptosis

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Abstract
Around hatching, when the pike embryo sheds its acellular egg envelope, marked changes occur in the cellular covering of the embryo. This cellular covering consists of a peridermal layer and a mono-layered presumptive epidermis. The periderm begins to disintegrate shortly before hatching and is sloughed off in the first posthatching period. The cellular covering produces hatching enzyme, the protease that partly dissolves the zona radiata interna of the acellular envelope. By means of the peroxidase-anti-peroxidase staining method with antibodies against hatching enzyme the cells producing this enzyme (hatching gland cells, HGCs) could be identified ultrastructurally. They are interspersed as single cells between the periderm and the presumptive epidermis. The secretory cycle of the HGC was studied. Hatching enzyme is released by an exocytotic secretory process in which multiple secretion into a “secretion vacuole” predominates. Exocytosis into surrounding intercellular spaces also occurs. These results show that the HGCs are merocrine glands. The HGC also has some “holocrine nature,” however, in that only a single, massive release of its secretory product occurs. The death of the transitory HGCs in posthatching stages is characterized by condensation of the cell, formation of surface protuberances and splitting up into globular cell fragments. Eventually these fragments are ingested by epidermal cells and digested. These results lead to the conclusion that the pike HGCs degenerate by apoptosis, unlike true holocrine cells.