Lectin histochemistry of rainbow trout (Oncorhynchus mykiss) gill and skin PATRICIA BURKHARDT-HOLM Interdisciplinary Centre for General Ecology, University of Berne, Falkenplatz 12, and Fish Disease Laboratory, Department of Animal Pathology, University of Berne, Laenggassstr. 122, CH-3012 Berne, Switzerland Received 22 August 1997 and in revised form 23 October 1997 Summary In order to characterize the glycoconjugate residues in skin and gills of the adult rainbow trout, the binding pattern of ®ve biotinylated lectins with different carbohydrate speci®cities was examined. In the skin, mucous cells revealed binding sites for PNA and SBA; ®lament-containing cells were additionally labelled with Con A. However, the basal cell layer showed no reaction. In the gill, subpopulations of mucous cells reacted with Con A, PNA, SBA and UEA-I. This broader spectrum of glycoconjugates in gill mucous cells compared with the epidermal mucous cells could point to the additional function of gill mucus in ion and osmoregulation. Lectin binding sites were less common in the respiratory epithelial cells of the secondary lamellae than in those of the primary lamellae. Chloride cells revealed mannose, galactose and fucose residues. Immature chloride cells, as indicated by a comparison with Na  =K  ATPase immunolabelling, reacted with Con A; subpopulations of them reacted with PNA, SBA and UEA-I. The results form the basis for further investigations in which these cell populations can be analysed under different environmental conditions. Introduction Skin and gills of ®sh are organs exposed directly to the water and, because of their direct contact with the environment, their structure and function has been investigated in several studies (Hoar & Randall, 1984; Whitear, 1986). Both organs secrete mucus, which protects the animal but is also involved in respiration, ion and osmoregulation and is an important factor in disease resistance (Shephard, 1994). Mucins, the main constituents of mucus, are high molecular weight glycoproteins. Some 50% of their dry weight can consist of carbohydrate chains (Strous & Dekker, 1992). The glycoconjugates can be characterized by different histochemical techniques including lectin histochemistry. Mucous cells and the composition of the mucus they produce are in¯uenced by endogenous factors (sex, developmental stage) and exogenous factors, such as stress, acid and infections (Zuchelkowski et al., 1981, 1985; Blackstock & Pickering, 1982; Pottinger & Pickering, 1985; Zaccone et al., 1985a; Ferguson et al., 1992). After exposure to an acidic environment, modi®cations in the carbohydrate content are more obvious when de®ned by lectin histochemistry than those detected by classical histochemistry (Genten & Danguy, 1990a). Thus, lectin histochemistry is a valuable tool in the detection of environmentally caused alterations of mucosubstances. In particular, the production of sialic acid residues has been used as an index for environmental stress (Arillo et al., 1979). To enable this hypothesis to be tested further, the ®rst step, and the purpose of this study, is to characterize the lectin-binding pattern of a ®sh species often used as an indicator species. One such species is the rainbow trout (Oncorhynchus mykiss), an economic- ally important and scienti®cally well-investigated ®sh species. However, the skin and gills of the adult rainbow trout have not yet been characterized with respect to their lectin-binding properties, although classical histochemical stains have been applied in earlier studies for the identi®cation of the mucous glyco- proteins (Fletcher et al., 1976; Ferguson et al., 1992). In this study, the lectin-binding pattern of these organs were compared because it is assumed that differences might be related to the speci®c function of the organ and its cell types. Con A, PNA, SBA, UEA-I and WGA were selected because they bind to Histochemical Journal 29, 893±899 (1997) 0018±2214 # 1997 Chapman & Hall