The histology and ultrastructural changes in the oesophagus and intestines of the Tilapia, oreochromisurolepis (Norman), during seawater adaptation

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University of Dar es Salaam
The oesophagus, anterior and posterior intestine of the tilapia, Oreochromisurolepis adapted to freshwater and seawater were studied by light and electron microscopy. In the freshwater-adapted tilapia oesophagus, longitudinal folds of the mucosae are lined by stratified epithelium composed of mucous and non-mucous cells. The mucous cells are numerous and show substantial extrusion of mucus. The non-mucous cells are found on the outermost and the basal layers of the stratified epithelium and are scattered in the middle portion among the mucous cells. They are bound to one another by desmosomes and many interdigitations of plasma membrane. The luminal surface of the non-mucous ceps bears microridges. In the seawater-adapted tilapia, irregularly arranged folds and the accretion of the oesophagealmucoua increase the surface area of the mucosal epithelium. The stratified epithelium is extensively replaced by a simple columnar pithelium free of mucous cells. These cells are rich in mitochondria and their luminal surface is provided with short microvilli. Other characteristic features of the columnar cells are heavily dilated esicles, intercellular spaces, and high vascularization of the underlying connective tissue. The significance of the ultrastructural changes occurring after seawater adaptation, are discussed in relation to ion and water transport. In the freshwater adapted tilapia anterior intestine, an elaborately folded mucosa together with columnar cells' well developed brush borders increase the surface area of the luminal surface. Regularly arranged and closely spaced microvilli cover the luminal surface of columnar absorptive cells and goblet cells. Lamellar structures which are closely associated with mitochondria, communicate with intercellular spaces. These are ultrastructural characteristics typical of water and ion transporting epithelia. There is no change in surface area increases in seawater- adapted tilapia anterior intestinal mucosa, but the epithelium exhibits ultra structural features typical of solute linked water transport. The posterior intestine has overall less surface area in freshwater-adapted tilapia, but the columnar epithelium shows the typical ultra structural features of pinocytosis. In addition, ultra structural characteristics typical of water transport, and probably that of ion transport also are exhibited in the mucosal epithelium. Seawater adaptation seems to have increased the surface area of the mucosa by amplifying the height of mucosal, folds' in posterior intestine. Ultra structural features typical of pinocytosis and water and ion transport have been tremendously amplified.
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Histology, Fishes, Tissues
Kirimbai, R. W (1984) The histology and ultrastructural changes in the oesophagus and intestines of the Tilapia, oreochromisurolepis (Norman), during seawater adaptation ( )