The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

Abstract

This article reviews thyroid function and its hypothalamic-pituitary-thyroid (HPT) axis control in birds with emphasis on the similarities and differences in thyroid function compared to mammals and other vertebrate classes. Thyroid hormones are important in metabolism and the thermogenesis required for homeothermy in birds, as in mammals, the other homeothermic class of vertebrates. Thyroid hormones play important roles in development and growth in birds, as is the case for all vertebrate classes. The developmental effects of thyroid hormones in birds are presented in the context of differences in precocial and altricial patterns of development and growth with emphasis on oviparous development. The sections on thyroid hormone actions include discussion of effects on the development of a number of tissue types as well as on seasonal organismal processes and interactions of the thyroid axis with reproduction. The current picture of how environmental chemicals may disrupt avian thyroid function is relatively limited and is presented in the context of the assessment endpoints that have been used to date. These endpoints are categorized as thyroid and HPT axis endpoints versus target organ endpoints. The final section discusses two recommended assay protocols, the avian two-generation toxicity assay and the avian one-generation assay, and whether these protocols can evaluate thyroid disruption in birds.

The chicken embryo as a model for developmental endocrinology: Development of the thyrotropic, corticotropic, and somatotropic axes

“…In this period (between days 11 and 13), thyrotrope numbers in the anterior pituitary increase remarkably and circulating thyroxine (T 4 ) levels start to augment gradually (Muchow et al, 2005;McNabb, 2007). However, in 6.5-day-old et al, 1992).…”

“…By the end of the first week of incubation, both the thyroid and the adrenal glands are supposedly functioning (see reviews by McNabb, 2007, and. However, developmental changes in circulating hormone levels are not merely the result of the functional differentiation and growth of their respective production sites, but also of Using a homologous antibody to chicken TSH, Nakamura and colleagues (2004) found the first thyrotropic cells on embryonic day 10, but TSH mRNA -determined by RT-PCR -was expressed somewhat earlier, on day 9.…”

“…By the end of the first week of incubation, both the thyroid and the adrenal glands are supposedly functioning (see reviews by McNabb, 2007, and. However, developmental changes in circulating hormone levels are not merely the result of the functional differentiation and growth of their respective production sites, but also of Page 5 of 37 A c c e p t e d M a n u s c r i p t 5 the maturation of regulatory systems, top-down (the instalment of the hypothalamohypophyseal axis) as well as bottom-up (the development of feedback interactions).…”

“…However, SS-producing terminating on the primary capillary plexus of the median eminence are present on day 6.5 of incubation, but only between days 10 and 12 the definitive hypothalamohypophyseal portal vascular plexus is formed and the previously autonomous hypophyseal secretion becomes controlled by the hypothalamus (Daikoku et al, 1973;Thommes, 1987). In this period (between days 11 and 13), thyrotrope numbers in the anterior pituitary increase remarkably and circulating thyroxine (T 4 ) levels start to augment gradually (Muchow et al, 2005;McNabb, 2007). However, in 6.5-day-old Page 9 of 37 A c c e p t e d M a n u s c r i p t 9 embryos, TRH or TSH treatment is able to increase circulating T 4 levels, suggesting that even at this very early stage the thyrotropes are readily responsive to TRH (i.e.…”

“…It should be noted, however, that the development of some endocrine systems, especially the thyroidal axis, differs considerably between precocial and altricial species. On the one hand, this limits the generalizability of the Page 4 of 37 A c c e p t e d M a n u s c r i p t 4 chicken embryo model to other avian species, including the important order of passerines (see McNabb et al, 1998 andMcNabb, 2007 for excellent reviews on the differences in the development of the thyrotropic axis between precocial and altricial birds). On the other hand, in many respects (see for instance Darras et al, 1999), the endocrine changes that occur during late embryogenesis and hatching in the chicken closely resemble those observed in humans, making the chicken a much better model to study these changes than the most common (altricial) mammalian models such as rat and mouse.…”