HORMONAL-REGULATION OF SKIN GLAND DEVELOPMENT IN THE TOAD (BUFO-BOREAS) - THE ROLE OF THE THYROID-HORMONES AND CORTICOSTERONE

At metamorphic climax, anurans develop skin glands that migrate from t he epidermis into the dermis. Thyroxine (T-4) stimulates skin gland di fferentiation and migration, and a previous study showed that corticos terone (Coal treatment of larvae is inhibitory. The current study used histological analyses to address the mechanism of Cort's prevention o f skin gland development. Two types of glands were observed in control s at metamorphic climax: The first type resembled granular glands foun d in adults and the second resembled mucous glands. Differential stain ing revealed that the two gland morphologies represented functionally distinct granular and mucous glands. Treatment of larvae from Gosner S tages 35-42 with Cort or the goitrogen, thiourea (Thio), caused a redu ction in the number of mucous (P < 0.05) but not granular glands. The similarity in the effects of Cort and Thio suggested that Cort inhibit ed skin gland development indirectly by down-regulating the hypothalam o-pituitary-thyroid axis. T-4 treatment of larvae reversed the effects of Thio (T-4 + Thio-treated animals); however, animals treated with T -4 + Cort had no skin glands of either type. Triiodothyronine (T-3) tr eatment of larvae resulted in the complete absence of skin glands with a limited number of gland nests (epidermal precursors of dermal skin glands), but stimulated epidermal growth. T-3 + Thio- or T-3 + Cort-tr eated animals also completely lacked skin glands. These data suggest t hat T-3 favors epidermal growth at the cost of skin gland differentiat ion. Furthermore, we suggest that Cort inhibits skin gland development indirectly through its enhancement of T-4 to T-3 conversions, and tha t inhibition of skin glands is caused by an increase in T-3 resulting from Cort treatment. (C) 1995 Academic Press, Inc.