| dc.description.abstract | 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. | en |
