Effects of Water-Borne Cadmium on the Skin of the Common Carp

The skin of carp, Cyprinus carpio, was studied at the ultrastructural level after exposure of the fish to low and high concentrations of cadmium in the water (22 and 560 fxg/L, respectively) for different periods. The effects of the low con­ centration of cadmium were similar to those of the high concentration, although they appeared later. The basal lamina and the skin surface became highly undulating. Chloride cells appeared between the pavement cells. Necrotic pavement cells were seen from the first day on, while apoptotic pavement cells appeared after several days. Filament cells contained many electrontransparent and electron-dense secretory vesicles. Mitotic cells were commonly seen, mainly in cells adjacent to club cells or close to the epidermal surface. Mucous cells differentiated close to the skin surface. They became elongated and synthe­ sized highly electron-dense mucosomes. The epidermis became infiltrated by many leucocytes. As the experiment progressed, many leucocytes degenerated, and their remnants were found within macrophages and club cells. Fibroblasts displayed in­ tense synthesis and, in fish from the low cadmium concentra­ tion, deposited a dense network of collagen fibers in the der­ mis. Melanosomes were located in the extensions of melanocytes. In these cells aggregation of melanosomes and apoptotic processes were common. Several of these changes were observed earlier under the impact of stressors other than cadmium. Some changes, such as the appearance of tumorlike bodies at the skin surface, the appearance of Merkel cells throughout the epidermis, and the coupling of leucocytes, may be specific for cadmium. mium causes changes in several blood parameters, such as a decrease of haematocrit and leucocrit values (Tort and Torres 1988), reduction of plasma electrolytes (Pratap et al. 1989), and elevation of plasma cortisol and glucose levels (Pratap and Wendelaar Bonga 1990: Fu ct al. 1990). Long-term exposure may induce vertebral lesions (Bengtsson ct al. 1988) or the formation of tumorlike bodies in the skin (Iger 1992). The gills of fish belong to the most important target tissues of water-borne as well as dietary cadmium (Pratap and Wendelaar Bonga 1993). The reported changes in plasma electrolytes by cadmium exposure are probably caused by malfunctioning of the chloride cells, the cells responsible for active ion exchange across the gills, and by increased permeability to ions of the branchial epithelium (Verbost et al. 1989; Wendelaar Bonga and Lock 1992). Many authors have reported histopathological changes in the branchial epithelium after exposure to cadmium (Oronsaye and Brafield 1984; Karlsson-Norrgren et al. 1985; Mallatt 1985; Pratap and Wendelaar Bonga 1993). In contrast to the gills, the skin outside the branchial area has received little attention, although this tissue is also in intimate contact with external pollutants. Like the gills, the skin may contain chloride cells, albeit in small numbers (Whitear 1986). It is covered by a mucus layer that may reduce the penetration of cadmium into the body (Part and Lock 1983), and possibly may serve as a vehicle for cadmium excretion (Bryan 1979). However, there is no information available on the effects in­ duced, and the responses evoked, in the skin of cadmiumexposed fish. This paper describes ultrastructural changes of the epidermis and the dermis of carp exposed to low or to high cadmium concentrations in the ambient water. Heavy metals, including cadmium, exert a wide range of patho­ logical effects on fish and other aquatic organisms (Dethlefsen and Tiews 1985; Mallatt 1985). Short-term exposure to cadCorrespondence to: S. E. W endelaar Bonga Materials and Methods Fifty-four carp (Cyprinus carpio), both males and females o f 18-25 g body weight, were kept in three groups for an acclimation period o f 3 weeks. The fish were maintained in artificial freshwater (demineralized water with the following additives in mmol/L: 3.8 NaCl; 0 .8 C aC L ; 0 .335 N a H C O ,; 0 .0 6 KC1; 0 .2 M g S 0 4, pH 7.5) at 22°C, under continuous aeration and filtration, and with a daily water replacement o f about 25% . One group served as control. For two groups appropriate Effects of Cd on Carp 343 am ounts o f Cd (from a stock solution o f C d ( N 0 3)2] were added g radu ­ ally to create (in 3 h) concentra tions o f 0 .2 and 5 jxmol/L Cd (22 .4 jxg Cd/L and 560 jxg Cd/L , respectively). The actual cadm ium concen tra ­ tions were measured at least once per day, using atomic absorption spectrophotometry (PU 9200X , Philips), and were adjusted when the deviation was more than 10% from the nominal concentration. For electron microscopy, small pieces (3 x 3 m m ) o f skin o f three fish from the different groups were excised I h, 24 h, and 3, 7, 14, and 2 1 days after reaching the actual Cd concentration. Sam ples were taken from dorsal areas o f the heads o f fish that had been anaesthetized lightly with hypno-ca lm er (Jungle, Texas) . T he tissues were fixed in 3% glutaraldehyde in sodium cacodylate buffer (0.09 M, pH 7.3), washed in buffer, and post-fixed in osmium tetroxide (1%) in the same buffer. After dehydration in ethanol the tissues were embedded in Spurr 's resin. Thin sections were contrasted with uranyl acetate and lead citrate, and were examined in a Jeol 1(H) CX transmission electron microscope.