Induction of somatotroph differentiation in vivo by corticosterone administration during chicken embryonic development.

Somatotroph differentiation in the embryonic pituitary of avian and mammalian species can be stimulated by glucocorticoids in vitro, and this effect can be augmented by concomitant treatment with growth hormone-releasing hormone (GHRH). Owing to its isolation from maternal influences, the chick embryo is a useful model for studying humoral regulation of pituitary cell differentiation. Somatotroph differentiation in chickens occurs between embryonic day (e-) 14 and e-16, and treatment of e-12 pituitary cells with e-16 serum or corticosterone induces growth hormone (GH) cell differentiation within 2 d in culture. The objective of the present study was to determine whether direct administration of embryonic serum and corticosterone to developing chick embryos was effective in vivo in inducing somatotroph differentiation prematurely. The albumen of fertile eggs was injected on e-11 with 300 approximately microL of 0.9% saline or 150 microL of serum from e-12 or e-16 chick embryos diluted 1:1 with saline. The embryos were allowed to develop until e-14, when pituitaries were dispersed and the resulting pituitary cells were subjected to reverse hemolytic plaque assays (RHPA) and immunocytochemistry to detect GH-secreting and GH-containing cells, respectively. Injection of e-16 serum increased (p < 0.01) GH-secreting and GH-containing cells to 11.5 +/- 1.0% and 1 7.4 +/- 3.3% of all pituitary cells, compared to 5.0 +/- 0.3% and 5.5 +/- 0.9% for saline-injected controls, respectively. Day 12 serum increased GH-containing cells to 9.8 +/- 0.9%, without changing percentages of GH-secreting cells. In experiment 2, saline, e-16 serum, and corticosterone were injected on e-11, and pituitary cells were subjected to GH RHPA on e-14. GH secretors were increased by e-16 serum and corticosterone. In experiment 3, we tested whether GHRH would magnify the effect of corticosterone, as we had seen in extended 6-d cultures previously. Saline, corticosterone, and corticosterone plus GHRH were injected on e-11, and pituitary cells were subjected to GH RHPA on e-18. Treatment with corticosterone alone and combined with GHRH increased the percentage of GH-secreting cells. However, combined treatment with corticosterone and GHRH was not more effective than corticosterone alone. The present findings demonstrate that glucocorticoid administration can stimulate somatotroph differentiation in living vertebrate embryos isolated from maternal interactions.