DEVELOPMENT OF MITOCHONDRION-RICH CELLS IN THE YOLK-SAC MEMBRANE OF EMBRYOS AND LARVAE OF TILAPIA, OREOCHROMIS-MOSSAMBICUS, IN FRESH-WATER AND SEAWATER

The development and changes in the activity of mitochondrion-rich (MR) cells in the yolk-sac membrane of tilapia (Oreochromis mossambicus) embryos and larvae were examined in relation to environmental salinity. Size and density of MR cells were monitored until complete yolk resorption in embryos and larvae spawned and hatched in fresh water (FW) and seawater (SW), and in larvae transferred from FW to SW or from SW to FW. MR cells were present in the yolk-sac membrane of embryos 2 days after fertilization and were significantly larger in SW than in FW. In embryos and larvae maintained until complete yolk resorption in FW, the size of MR cells tended to decrease as the larvae grew older, whereas in those maintained in SW, the large size of MR cells was maintained. After direct transfer of newly hatched larvae from FW to SW, the size of MR cells markedly increased, and decreased when newly hatched larvae were transferred from SW to FW. MR cell density did not vary appreciably between larvae in FW and those in SW, and was not significantly affected by transfer of larvae from FW to SW or vice versa. Electron-microscope examination of MR cells in the yolk-sac membrane revealed the presence of numerous mitochondria, well-developed tubular system and apical pits, indicating similarity with MR cells in branchial and opercular epithelia of adult fish. In tilapia embryos, in which the gills, gut, and kidney are not yet fully developed, MR cells in the yolk-sac membrane are potentially important sites for ion exchange in SW. (C) 1994 Wiley-Liss, Inc.