Development of neuroendocrine components of the thyroid axis in the direct-developing frog Eleutherodactylus coqui: Formation of the median eminence and onset of pituitary TSH production

• We examine neuroendocrine components of the thyroid axis in direct-developing frogs.
• Hypothalamic and pituitary components form prior to hatching, during embryogenesis.
• Embryonic pituitary activity is comparable to metamorphosing frogs after hatching.
• Neuroendocrine activity occurs before anatomical changes resembling metamorphosis.

Direct-developing frogs lack, wholly or in part, a wide range of larval features found in metamorphosing species and form adult-specific features precociously, during embryogenesis. Most information on thyroid regulation of direct development relies on hormone manipulations; the ontogeny of many thyroid axis components has not been fully described. This analysis examines differentiation of the median eminence of the hypothalamus and production of thyroid-stimulating hormone (TSH) by the pituitary of the direct-developing frog Eleutherodactylus coqui. The median eminence is established two-thirds of the way through embryogenesis. Cells immunoreactive to human TSHβ antibodies are first detected during embryogenesis and quantitative changes in TSHβ-IR cells resemble those in metamorphosing amphibians. Formation of the median eminence of the hypothalamus and TSHβ production by the pituitary precede or coincide with morphological changes during embryogenesis that occur during metamorphosis in biphasic anurans. Thus, while the onset of neuroendocrine regulation has changed during the evolution of direct development, it is likely that these thyroid axis components still mediate the formation of adult features.