Chronology of the appearance of β, A, and α mitochondria‐rich cells in the gill epithelium during ontogenesis of the brown trout ( Salmo trutta )

Three types of mitochondria‐rich (MR) cells, the α, β, and accessory cells, are observed in the gill epithelium of juvenile and adult freshwater teleosts. In addition to numerous mitochondria, their cytoplasm contains a network of membranous tubules, the tubular system, connected to the laterobasal plasma membrane. Because they are believed to play a role in ionic regulation, it is of interest to examine the order of appearance and the ultrastructural characteristics of such cells during the embryogenesis and larval life of the brown trout. Gills of embryos and fry maintained in freshwater were thus removed at different stages and prepared for transmission and scanning electron microscopic examination. One week before hatching, cells resembling the β cells of juvenile and adult teleosts appeared first among the epithelial cells located at the base of the filaments in the gills of the brown trout larva. In addition to their tubular system, they contained numerous and large apical structures seemingly originating from the Golgi apparatus. At approximately hatching time, small pear‐shaped cells were seen to be closely apposed to the lateral side of the β cells; they were usually devoid of apical structures and were considered to be accessory cells. After yolk sac resorption, additional cells, the α cells, were present along the lamellae. In contrast to the β cells, they only exhibited poorly developed apical structures. The possible role of these three types of MR cells in osmoregulation during fish development is discussed. Anat Rec 259:301–311, 2000. © 2000 Wiley‐Liss, Inc.