Ultrastructural evidence for apoptosis of pavement cells, chloride cells, and hatching gland cells in the developing branchial area of the trout Salmo trutta

Death by apoptosis of branchial epithelial cells was studied in brown trout embryos by means of transmission electron microscopy. Superficial pavement cells are sloughed off for the renewal of the epithelium after an apoptotic degeneration with shrinkage of the cytoplasm and loss of desmosomal contacts. Chloride cells appear as immature, mature and degenerating cells. Degenerating chloride cells, which are separated from the ambient water by pavement cells, show condensation of the cytoplasm and structural alterations in the tubular system and the mitochondria. Hatching gland cells degenerate either into apoptotic bodies or into cellular debris, depending on the functional stage of the cell. There was no phagocytosis by macrophages or adjacent cells of the degenerating chloride and hatching gland cells, but an infiltration of leucocytes was always observed in the epithelium undergoing cellular degeneration. In some instances, secondary necrosis of apoptotic hatching gland cells was observed. Apoptosis occurs in the three types of cells since early stages of development. However, a massive wave of cellular death occurred in pavement and hatching gland cells during the hatching stage and in the chloride cells during post-hatching stages.