Control of thyroid hormones and their involvement in haemoglobin transition during Xenopus and Rana metamorphosis

Abstract

Abstract It has been known for 75 years that thyroid hormones (THs) are involved in amphibian metamorphosis. In the last 25 years several investigators have shown that at or near the time of tail loss there is a sudden rise in circulating TH concentrations that is dependent on the pituitary and hypothalamus. The required pituitary hormone is thyroid stimulating hormone (TSH); however, c:irculating TSH concentrations have yet to be determined during metamorphosis. During the last decade much effort focused on measurements of thyrotropin releasing hormone (TRH) concentrations in the tissues of several anuran tadpoles, but there is no credible evidence showing that TRH can directly initiate metamorphic climax in any species. Recent data suggest that other hypothalamic hormones are involved in the control of TSH concentrations and therefore TH concentrations before climax. During metamorphosis THs are known to control cell death, stem cell stimulation and differentiation of existing cell populations. In order for larval tissues to respond to THs, they must have TH receptors. Several different types of TH receptors exist and all are members of a larger group of nuclear protein receptors that include receptors for THs, steroid hormones and retinoic acid. More recent evidence suggests that larval tissue TH receptor concentrations change during metamorphosis and that their concentrations are highest about the time of tail loss. The transition from larval to adult haemoglobin is used to document both hormonal and environmental factors involved in the control of this metamorphic process.