MITOCHONDRIA-RICH CELLS IN THE BRANCHIAL EPITHELIUM OF THE TELEOST, OREOCHROMIS-MOSSAMBICUS, ACCLIMATED TO VARIOUS HYPOTONIC ENVIRONMENTS

Branchial mitochondria-rich (MR) cells were examined on the afferent s ide of gill filaments in tilapia (Oreochromis mossambicus) acclimated to different hypotonic environments, local fresh water (LFW), hard fre sh water (HFW) and 5 parts per thousand salt water (SW). Scanning elec tron micrographs (SEM) identified three types of apical surfaces of th e MR cells, wavy convex, shallow basin and deep hole. In spite of the different types of apical surfaces, light microscopic (LM) and transmi ssion electron microscopic (TEM) studies suggested that these cells we re MR cells. The relative abundance of these 3 types of branchial MR c ells varied with external hypotonic milieus. Wavy-convexed MR cells we re dominant in the gills of fish adapted to HFW, whereas shallow-basin ed MR cells were evident in LFW-adapted fish. In SW-adapted fish, most of the MR cells were deep holes. Experiments on adaptation to various hypotonic milieus revealed that the changes of the branchial MR cells were reversible and occurred within 24 hours following transfer. The morphological alterations of the MR cells correlated with ionic change s in different milieus, indicating that these distinct types of MR cel ls may play key roles for osmoregulation in hypotonic media.