Cichlids of the genus Oreochromis are fish of economic importance in African countries. They tolerate brackish water, however, with great variations between species. In this work, two species, both from the Ivory Coast but of different origins, O. niloticus (field and laboratory strains) and O. aureus (field strain) were compared during osmotic challenges (10, 20 and 30%o salinity) in order to provide physiological support for their specific behaviour when confronted with natural hypertonic environments. Tolerance to salinity was assessed by correlated observations on gill structure, plasma sodium levels and gill Na+/K+ ATPase activity. In fresh water (FW), all fish presented a gill epithelium structure characteristic of FW stenohaline fish: no chloride cells (CC) on the lamellae and few CC on the filaments. An increase in external salinity induced the proliferation of CC on filaments, a feature typical of seawater teleosts. This change in gill structure was accompanied by an increase of gill Na+/K+ ATPase activity. In the most tolerant strains, plasma Na+ did not change, indicating successful ion regulation in the hypertonic media. With regard to potential interest of field strains in fish culture, O. aureus acclimated more easily to brackish water than O. niloticus. Interestingly, O. niloticus, kept for several generations in the laboratory, performed best in our challenge studies. Plasma Na+ levels and gill CC proliferation upon transfer to an isotonic medium may be the parameters of choice when testing these fish for their response to a salinity change.
We have developed the first explant technique that allows the in vitro study of gill physiology and biochemistry in marine species. Gill fragments were cultured at 17 degrees C, in atmospheric PCO2, with nutrient medium (Leibovitz L15), pH 7.8, supplemented with 10% fetal bovine serum and adjusted to the osmolarity of fish plasma (350 mOsm/liter). Coating plates with collagen, gelatin, or polylysin did not improve our results. Decrease in osmotic pressure, removal of bovine serum, or its replacement by fish serum inhibited growth from the explants. Approximately 50% of the explants produced cell growth, and after 4 days of culture a monolayer of contiguous cells was formed. This technique is rapid and does not require the use of enzymes. The cells appeared flat and thin with an epitheloid shape. They looked polygonal with a maximum length of 10 to 50 microm. Evidence that they are unique gill cells is the presence of polymorphic surface crenelations (microplicae), prominent Golgi apparatus, tight junctions and desmosomes. Comparison with in vivo tissue showed them to be epithelial cells having differentiated in a homogeneous population of respiratory-like (pavement) cells. They are polarized with their apical surface facing the culture medium. The development of this culture system represents a new tool for cellular approaches to determine precisely the functions and transport mechanism of gill cells.
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