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 the epidermis into the dermis. Thyroxine (T4) stimulates skin gland differentiation and migration, and a previous study showed that corticosterone (Cort) treatment of larvae is inhibitory. The current study used histological analyses to address the mechanism of Cort's prevention of skin gland development. Two types of glands were observed in controls at metamorphic climax: The first type resembled granular glands found in adults and the second resembled mucous glands. Differential staining revealed that the two gland morphologies represented functionally distinct granular and mucous glands. Treatment of larvae from Gosner Stages 35-42 with Cort or the goitrogen, thiourea (Thio), caused a reduction in the number of mucous (P < 0.05) but not granular glands. The similarity in the effects of Cort and Thio suggested that Cort inhibited skin gland development indirectly by down-regulating the hypothalamo-pituitary-thyroid axis. T4 treatment of larvae reversed the effects of Thio (T4+ Thio-treated animals); however, animals treated with T4+ Cort had no skin glands of either type. Triiodothyronine (T3) treatment 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. T3+ Thio- or T3+ Cort-treated animals also completely lacked skin glands. These data suggest that T3 favors epidermal growth at the cost of skin gland differentiation. Furthermore, we suggest that Cort inhibits skin gland development indirectly through its enhancement of T4 to T3 conversions, and that inhibition of skin glands is caused by an increase in T3 resulting from Cort treatment.