Metallothionein response in gills of Oreochromis mossambicus exposed to copper in fresh water

Freshwater Oreochromis mossambicus (tilapia) were exposed to 3.2 mu mol/l C u(NO3)(2) in the water for up to 80 days, and copper (Cu) and immunoreactiv e metallothionein (irMT) were localized in the branchial epithelium. Cu was demonstrated in mucous cells (MC), chloride cells (CC), pavement cells (PC ), respiratory cells (RC), and basal layer cells (BLC) via autometallograph y combined with alcian blue staining for MC and Na+-K+-ATPase immunostainin g for CC and, on the basis of their location in the epithelium of PC, RC, a nd BLC. In control fish (water with Cu concentration less than or equal to 90 nmol/l) incidentally irMT was observed in the area where progenitor cell s of the branchial epithelia reside, as demonstrated by proliferating cell nuclear antigen staining. This was also the area where the first increase i rMT expression of the Cu exposure was observed. After 2 days of exposure to Cu, irMT was found in CC and PC. From 5 days on, a pronounced irMT stainin g was observed in BLC of branchial epithelium, which then appeared to migra te and differentiate into mature CC, PC, and RC. We conclude that MT expres sion in mature CC, PC, and RC requires exposure to Cu in a earlier stage of development of these cells. Once expression is initiated in undifferentiat ed cells, MT remains expressed throughout the Life cycle of the cell.