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Figure 7. (A) Skin of razorback suckers (Xyrauchen texanus) not exposed to UV. Mucous cells (open arrows) line the very outer surface of the epidermis. A major portion of the epidermis is composed of large PAS-negative cells (filled arrows). Bar equals 50 /zm. (B) Skin of razorback sucker after 72 h of UV exposure. The epidermis is thickened to hyperplasia and hypertrophy of the large PAS-negative cells. There are many leucocytes infiltrating the basal portion of the epidermis (filled arrows), melanocyte accumulations below the dermis (open arrows) and inflammation in the hypodermis. The changes observed were reversible and non-lethal. Bar equals 50 /mi. [From Blazer et al. 15.]

Figure 7. (A) Skin of razorback suckers (Xyrauchen texanus) not exposed to UV. Mucous cells (open arrows) line the very outer surface of the epidermis. A major portion of the epidermis is composed of large PAS-negative cells (filled arrows). Bar equals 50 /zm. (B) Skin of razorback sucker after 72 h of UV exposure. The epidermis is thickened to hyperplasia and hypertrophy of the large PAS-negative cells. There are many leucocytes infiltrating the basal portion of the epidermis (filled arrows), melanocyte accumulations below the dermis (open arrows) and inflammation in the hypodermis. The changes observed were reversible and non-lethal. Bar equals 50 /mi. [From Blazer et al. 15.]

cells appeared to be club cells and were larger near the surface of the epidermis and may contain the photoprotective substance that appeared to afford razor-back suckers a great deal of protection from UV.B injury.

The photoprotective substance, or its precursor, may be of dietary origin and trophically accumulated, secreted by the cells of the epidermis, and concentrated in the epidermis and overlying mucus. However, it has not been determined whether this photoprotective substance is trophically accumulated or induced by ambient UV-B, or whether nature has selected for tolerant individuals with large amounts of this photoprotective substance. The photoprotective substance would act as a sunscreen and effectively block UV-B from causing sunburn in fish that had enough of it in the dorsal skin. If the photoprotective substance is trophically accumulated, dietary accumulation of this compound in fish skin could determine the UV-B tolerance of fish to sunburn. This could be very important in hatchery management in formulating diet supplementation to insure the success of fish stocks released in the wild.

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