Page 96 - The Toxicology of Fishes

76 The Toxicology of Fishes

the skin surface. To support this function there is a continual upward movement of differentiating goblet
cells that begins at the basil lamina. The mucus produced by goblet cells serves a protective role and
reduces the hydrodynamic resistance to swimming. Chemical irritation or physical stress will stimulate
an increase in mucus secretion and bring about an increase in the number of differentiating goblet cells.
Leydig cells also differentiate from the basal lamina but do not release their contents unless the
epidermis is damaged. An alarm substance pheromone produced by Leydig cells acts to warn other
fish of possible danger.
The dermis is also divided into two distinct zones, the upper stratum spongiosum and the lower stratum
compactum. The stratum spongiosum contains fibroblasts, collagen, scale pockets, and pigment cells.
The stratum compactum is made up of large collagen bundles attached at right angles to the skin surface.
This architecture provides rigidity and allows the fish to swim without wrinkles forming at the skin
surface. The stratum compactum also contains sense organs (touch and taste), nerves, and pigment cells.
The dermal vascular system supplies the two dermal layers and the basal lamina of the epidermis. The
dermis is connected to the underlying muscle by the hypodermis (subcutis), which is largely composed
of connective tissue and fat cells.
Physiological functions of fish skin include respiration, ion exchange, and acid–base regulation. The
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skin of rainbow trout contributes to Ca balance through active transport within the epithelial cells
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(Marshall et al., 1992) and to acid–base balance through Cl /HCO exchange across the extrabranchial
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epithelium (Ishimatsu et al., 1992). The skin of large fish (>100 g) consumes oxygen but in most species
takes up only enough to satisfy the skin oxygen demand (Kirsch and Nonnotte, 1977; Nonnotte, 1981,
1984; Nonnotte and Kirsch, 1978). Based on morphometric and anatomical information, researchers
have long suggested that cutaneous oxygen flux contributes significantly to total respiration in small
larval fishes (McDonald and McMahon, 1977; McElman and Balon, 1980; Oikawa and Itazawa, 1985).
Because the gill epithelium of both small and large fish consists of one or a few cell layers, its thickness
does not change much with fish size. In contrast, skin thickness tends to decrease with decreasing fish
size and in small fish may approach the thickness of the gill epithelium. In larval Chinook salmon
(Oncorhynchus kisutch), as much as 80% of oxygen uptake takes place across the skin (Rombough and
Moroz, 1990).

Dermal Absorption of Xenobiotics
Direct measurements of dermal uptake in rainbow trout and channel catfish were obtained by exposing
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fish confined in Plexiglas chambers to a mixture of three chloroethanes (McKim et al., 1996). Although
the rates at which both species approached steady state were similar to those observed in inhalation
exposures to the same compounds, steady-state blood concentrations were much lower. A kinetic analysis
of these data suggested that dermal uptake flux was about three to five times greater in catfish than in
trout (Nichols et al., 1996). Some of this difference may have been due to differences in chemical
diffusion rates at the experimental temperatures used for catfish (21°C) and trout (12°C); however,
differences in skin anatomy and physiology, including the presence (trout) or absence (catfish) of scales
were also likely factors. In addition, the analysis suggested that for adult trout dermal absorption would
contribute 2 to 4% of initial uptake (dermal plus branchial) in a hypothetical waterborne exposure, but
for catfish it would contribute 7 to 9%. Based on these findings, it appears that dermal uptake is a minor
route of exposure in large fish, except perhaps for species that live in intimate contact with contaminated
sediments.
In contrast, several studies have suggested that dermal uptake contributes substantially to total uptake
of waterborne chemicals by small fish and juveniles of larger species. Tovell et al. (1975) measured
anionic detergent uptake across the skin of small goldfish and found that 20% of total uptake occurred
by this route. Saarikoski et al. (1986) obtained dermal uptake values for guppies exposed to a series of
phenols, anisoles, and carboxylic acids. To distinguish between gill and skin absorption, fish were
positioned into a hole cut in a rubber membrane separating two exposure chambers. Estimates of dermal
absorption ranged from 25 to 40% of total absorption. Japanese medaka exposed to 2,2′,5,5′-tetrachlo-
robiphenyl in water accumulated considerably more chemical than could be explained by inhalation
uptake (Lien and McKim, 1993). Similar results were reported for fathead minnows exposed to three
chloroethanes (Lien et al., 1994). Lien and McKim (1993) suggested that dermal uptake should increase