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. Superficia l pavement cells are sloughed off for the renewal of the epithelium af ter 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 cyt oplasm and structural alterations in the tubular system and the mitoch ondria. Hatching gland cells degenerate either into apoptotic bodies o r into cellular debris, depending on the functional stage of the cell. There was no phagocytosis by macrophages or adjacent cells of the deg enerating chloride and hatching gland cells, but an infiltration of le ucocytes was always observed in the epithelium undergoing cellular deg eneration. In some instances, secondary necrosis of apoptotic hatching gland cells was observed. Apoptosis occurs in the three types of cell s since early stages of development. However, a massive wave of cellul ar death occurred in pavement and hatching gland cells during the hatc hing stage and in the chloride cells during post-hatching stages.