title: Endogenous thyroid hormones modulate pituitary somatotroph differentiation during chicken embryonic development.
abstract: Growth hormone cell differentiation normally occurs between day 14 and day 16 of chicken embryonic development. We reported previously that corticosterone (CORT) could induce somatotroph differentiation in vitro and in vivo and that thyroid hormones could act in combination with CORT to further augment the abundance of somatotrophs in vitro. The objective of the present study was to test our hypothesis that endogenous thyroid hormones regulate the abundance of somatotrophs during chicken embryonic development. Plasma samples were collected on embryonic day (e) 9-14. We found that plasma CORT and thyroid hormone levels increased progressively in mid-embryogenesis to e 13 or e 14, immediately before normal somatotroph differentiation. Administration of thyroxine (T4) and triiodothyronine (T3) into the albumen of fertile eggs on e 11 increased somatotroph proportions prematurely on e 13 in the developing chick embryos in vivo. Furthermore, administration of methimazole, the thyroid hormone synthesis inhibitor, on e 9 inhibited somatotroph differentiation in vivo, as assessed on e 14; this suppression was completely reversed by T3 replacement on e 11. Since we reported that T3 alone was ineffective in vitro, we interpret these findings to indicate that the effects of treatments in vivo were due to interactions with endogenous glucocorticoids. These results indicate that treatment with exogenous thyroid hormones can modulate somatotroph abundance and that endogenous thyroid hormone synthesis likely contributes to normal somatotroph differentiation.
authors.affiliation: Department of Animal and Avian Sciences, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: L
authors.initials: L
authors.lastname: Liu
authors.forename: T E
authors.initials: TE
authors.lastname: Porter
mesh_terms: D000818:Animals; D013956:Antithyroid Agents / Q000494:pharmacology; D002452:Cell Count; D002454:Cell Differentiation / Q000187:drug effects; D002642:Chick Embryo; D003345:Corticosterone / Q000097:blood; D004622:Embryo, Mammalian / Q000502:physiology*; D004625:Embryo, Nonmammalian; D005865:Gestational Age; D013006:Growth Hormone / Q000378:metabolism*; D008713:Methimazole / Q000494:pharmacology; D010902:Pituitary Gland / Q000196:embryology* / Q000378:metabolism*; D013963:Thyroid Hormones / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology; D014284:Triiodothyronine / Q000097:blood / Q000494:pharmacology
chemical_list: D013956:Antithyroid Agents; D013963:Thyroid Hormones; D014284:Triiodothyronine; D008713:Methimazole; D013006:Growth Hormone; D013974:Thyroxine; D003345:Corticosterone