Hatching in the pike Esox lucius L.: Evidence for a single hatching enzyme and its immunocytochemical localization in specialized hatching gland cells

Antibodies against purified hatching enzyme (HE) from the pike, Esox lucius L., have been used to examine different aspects of the presence of the enzyme in the ontogeny of this teleostean fish. Immunochemical analysis indicates that the two proteolytic enzymes which occur in the hatching medium arise from a single protease, HE itself. The second proteolytic fraction found in gel filtration of hatching medium could be a heterogeneous population of complexes of HE with digestion fragments of its natural substrate, the zona radiata. Immunofluorescence microscopy by means of anti-HE antibodies demonstrates that HE is localized in the so-called hatching gland cells (HGCs). The HGCs in pike appear as oval to round cells 10–15 μm in diameter containing granules of 1.5–2.3 μm. They are found interspersed between the periderm and the presumptive epidermis. The number of HGCs and their granule content increase significantly until the 35-somite stage to reach about 1200 and 30, respectively. From then on these numbers do not change until hatching in the 66-somite stage. The distribution of the HGCs over the embryo also changes, probably since HGC precursors in the yolk sac differentiate to HGCs later than their counterparts in the head region. The immunocytochemical procedure further shows that HE can be detected from the 10-somite stage on. Discrete hatching gland remnant bodies, phagocytized by epidermal cells, are observed in larval stages until 3–7 days after emergence of the embryo.