Endogenous thyroid hormones modulate pituitary somatotroph differentiation during chicken embryonic development L Liu 1 and T E Porter 1,2 1 Department of Animal and Avian Sciences, University of Maryland, College Park, Maryland 20742, USA 2 Molecular and Cell Biology Program, University of Maryland, College Park, Maryland 20742, USA (Requests for offprints should be addressed to Email: TP44@UMAIL.UMD.EDU) 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 e13 or e14, immediately before normal somatotroph differentiation. Administration of thyroxine (T 4 ) and triiodothyronine (T 3 ) into the albumen of fertile eggs on e11 increased somatotroph proportions prematurely on e13 in the developing chick embryos in vivo. Furthermore, admin- istration of methimazole, the thyroid hormone synthesis inhibitor, on e9 inhibited somatotroph differentiation in vivo, as assessed on e14; this suppression was completely reversed by T 3 replacement on e11. Since we reported that T 3 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. Journal of Endocrinology (2004) 180, 45–53 Introduction The anterior pituitary gland differentiates from Rathke’s pouch into five distinct hormone-secreting cell types. The first functionally mature cell type to appear is the corticotroph. Differentiation of the other cell types is not autonomous and is controlled by various hormones and hypothalamic peptides (Dubois & Hemming 1991, Dubois et al. 1997). We are studying the mechanisms regulating differentiation of growth hormone (GH)-secreting cells in the chicken anterior pituitary. Somatotrophs first become a significant population between embryonic day (e) 14 and e16 of chicken development (Porter et al. 1995a). Somatotroph differentiation does not occur in vitro without an extrapituitary signal, and glucocorticoids have been shown to induce somatotroph differentiation in chicken embryos and fetal rats in vitro and in vivo (Hemming et al. 1988, Nogami & Tachibana 1993, Nogami et al. 1995, 1997, Morpurgo et al. 1997, Dean et al. 1999, Bossis & Porter 2000, Porter et al. 2001, Liu et al. 2003). Thyroid hormones play an important role in embryonic and neonatal development in mammals and birds. Studies with cultured fetal rat pituitary glands indicated that thyroid hormones exert their stimulatory action on fetal GH gene expression and GH cell abundance only in the presence of glucocorticoids (Hemming et al. 1988, Nogami et al. 1997). Furthermore, Nogami et al. (1997) found that intraperitoneal injection of thyroxine (T 4 ) into pregnant rats, in the absence of concomitant dexametha- sone treatment, failed to induce either GH cells or the accumulation of GH in the fetal pituitary gland. In contrast, adding methimazole (MMI) to the drinking water of pregnant rats suppressed GH cell number, GH content and GH mRNA level in the fetal pituitaries, and this suppression was reversed by additional intraperitoneal T 4 injections to the pregnant dams (Nogami et al. 1995). However, as treatments were administered to the dams rather than directly to the fetuses in these studies, the effect of fetal–maternal interactions or maternal thyroid hormones could not be ruled out. Furthermore, as GH immunostaining in histology sections of the pituitary glands was evaluated in this study, quantitative deter- mination of the total abundance of fetal somatotrophs was not achieved. We found that thyroid hormones could act in combi- nation with corticosterone (CORT) to further augment the abundance of somatotrophs in cultures of dispersed, chick embryonic pituitary cells, while having little or no 45 Journal of Endocrinology (2004) 180, 45–53 0022–0795/04/0180–045  2004 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology.org