Electron Microscopic Studies of the External Gill Epithelium of the Salamander, Hynobius dunni Tago, Followed Throughout Larval Life

The ultrastructure of the external gill epithelium of the salamander, Hynobius dunni Tago, is described with particular emphasis on the development and degeneration of the epithelial cells, and also its phagocytosis by macrophages. Two basic types of surface cells, ciliated and non-ciliated, were observed in the epithelium of the primary gill bar and secondary gill filaments throughout larval life. Based on their cytoplasmic density and organization as observed by transmission electron microscopy, the non-ciliated surface epithelial cells could be distinguished into three subclasses: pavement cells with mucous vacuoles, club-shaped clear cells with numerous vacuoles, and mitochondria-rich cells. Furthermore, two other types of epithelial cells were noted: basal cells and Leydig cells. During metamorphosis, both the ciliated and non-ciliated cells revealed prominent cytoplasmic vacuolation. In addition, ciliated cells lost their cilia. On the other hand, the pavment cells showed increased amounts of tonofilaments in their cytoplasm and progression of cornification. The basal cells did not degenerate, but their basement membranes became considerably thickened and undulating. Degenerating cells were neither contiguous at the epithelial surface nor sloughed, but were drawn inwards within the epithelium. However, phagocytizing epithelial cells were not observed. The intercellular spaces in the epithelium widened as metamorphosis progressed. Phagocytic macrophages with a strongly positive acid phosphatase or nonspecific esterase reaction in their heterophagic vacuoles were often seen in the intercellular spaces and in the subepithelial connective tissue. It is thought therefore that such macrophages may participate in the removal of degenerated cells, and the packing of cell debris in the macrophages results in external gill reduction in tadpoles.