A morphometric analysis of chloride cells in the gills of the teleosts Oreochromis alcalicus and Oreochromis niloticus and a description of presumptive urea-excreting
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The chloride cells of the teleost gills have structurally and functionally been an extremely intriguing group of cells. Since they were first described by Keys & Willmer (1932), the biology of these cells has attracted great attention (see Maetz, 1971; Laurent & Dunel, 1980; Zadunaisky, 1984; Laurent, 1985; Karnaky, 1986, for recent reviews and accounts). While the basic structure of these cells has been adequately described, particularly with the advent of the transmission electron microscope (Kikuchi, 1977; Hossler, Ruby & McLlawin, 1979; Dunel & Laurent, 1980; Laurent & Dunel, 1980; Karnaky, 1986), the chloride cells have recently been found to be ultrastructurally remarkably heterogeneous with various morphologically distinct populations (Pisam, Caroff & Rambourg, 1987; Pisam, Prunet, Boeuf & Rambourg, 1988; Maina, 1990) to which development and specific functions have not yet been ascribed. The functional and morphological responses of the chloride cells to changing milieu (freshwater to seawater) have been evaluated by Sardet, Pisam & Maetz (1979), Philpott (1980), Foskett et al. (1981), Pisam (1981), Chretien & Pisam (1986), Hwang (1987) and Pisam et al. (1987, 1988), while change from seawater to freshwater has been studied by Doyle & Epstein (1972) and Avella, Mansoni & Mayer-Costan (1987). The effects of pH changes (Wendelaar-Bonga, Flik, Balm & Van der Meij, 1990) have also been investigated in an attempt to explain the remarkable osmoregulatory versatility of the teleosts. Oreochromis alcalicus, the subject of this study, is a unique fish which lives in an extremely severe ecosystem, the hyperosmotic and highly alkaline Kenyan volcanic Lake Magadi. The temperature of the water, which comes from numerous peripherally situated hot springs, may be as high as 46 °C (Coe, 1966). The carbonate-bicarbonate concentration is above 200 mEq -1, and osmolality is in excess of 600 mOsm l-l (Johansen, Maloiy & Lykkeboe, 1975; Eddy, Bamford & Maloiy, 1981; Eddy & Maloiy, 1984; Randall et al. 1989; Wood et al. 1989). The structural and functional homeostatic adaptations of Oreochromis alcalicus, particularly with respect to ionic exchange, are undoubtedly of great scientific interest. Some physiological and ecological studies on this species have been carried out by Coe (1966), Leatherland, Hyder & Ensor (1974), Reite, Maloiy & Aasehaug (1974), Johansen et al. (1975), Maloiy, Lykkeboe, Johansen & Bamford (1978), Maetz & de Renzis (1978), Eddy et al. (1981) and more recently by Randall et al. (1989) and Wood et al. (1989), but morphological studies of the gills, and particularly of the chloride cells, a notably important osmoregulatory site, are entirely lacking in this species except for an earlier preliminary study by Maina (1990). In this study, Maina investigated the general gill design together with the topography and the organisation of the chloride cells and their qualitative ultrastructural changes with water dilution. In view of the fact that many teleosts have a very narrow ionic tolerance, the stratagems that 0. alcalicus has evolved are of particular interest. Further, Oreochromis alcalicus has recently been found to be physiologically exceptional among the completely aquatic teleosts in being entirely ureotelic. This is one of the possibly many, yet unknown, unique adaptational features that may be crucial for its survival in the highly buffered water (Randall et al. 1989). Smith (1929) established that teleost fish excrete most of their waste nitrogen across the gills as ammonia, together with small amounts of urea and negligible amounts of other substances. The excretory process and the particular organs/cells which may be specifically associated with urea excretion in the gills of 0. alcalicus have not yet been established. In the present study, an ultrastructural morphometric comparison of the chloride cells of 0. alcalicus and the freshwater 0. niloticus has been made as an attempt to establish qualitatively and quantitatively, those structural adaptational features which may be unique to the gills of 0. alcalicus to enable it to survive in such an exceptionally hostile ecosystem. Presumptive urea-excreting cells on the gills of 0. alcalicus are described. The teleosts of the genus Oreochromis, formerly Tilapia before the most recent systematic review by Trewavas (1983), are prolific tropical fish that are of great significance as a food source.