Hypothalamo-adenohypophyseal-thyroid interrelationships in the chick embryo. VI. Midgestational adenohypophyseal sensitivity to thyrotrophin-releasing hormone.

The functional abilities of 9.5-, 10.5-, 11.5-, and 12.5-day-old chick embryos to respond to exogenous thyrotrophin-releasing hormone (TRH) were evaluated by means of changes in plasma total thyroxine (T4). T4 concentrations were determined 3.0, 6.0, 12.0, and 24.0 hr after TRH treatment. The data of the present investigation show that chick embryo adenohypophyseal sensitivity to exogenous TRH, as evidenced by changes in plasma T4 levels, increase during the 10.5- to 12.5-day incubation interval; however, the pattern and magnitude of plasma T4 response to administered TRH change markedly between Days 10.5 and 11.5 of development. This modification of the adenohypophyseal response pattern corresponds in embryonic time with the previously reported increases in immunocytochemically demonstrable TSH cells within the adenohypophysis(R. C. Thommes, J. B. Martens, W. E. Hopkins, D. A. Griesbach, D. J. Williams, M. J. Sorrentino, P. Wernke, and J. E. Woods, 1981, In 'Ninth International Symposium of Comparative Endocrinology"; R. C. Thommes, J. B. Martens, W. E. Hopkins, M. J. Sorrentino, J. C. Caliendo, and J. E. Woods, 1983, Gen. Comp. Endocrinol. 51, 434-443).