In order to identify the cell type responsible for ACTH production in the rat adenohypophysis, the incorporation of thymidine- 3H into the various cell types of the adenohypophysis has been studied using autoradiography. Sections were examined 1 month following adrenalectomy, when ACTH secretion is greatly increased, and 1 month following the daily administration of cortisol, when ACTH secretion is suppressed. Only 1 cell type, restricted to the pars intermedia, responded to altered levels of ACTH secretion with both an increase in label index following adrenalectomy and a decrease in label index following cortisol administration. Histological changes were restricted to the pars intermedia, where numerous dark cells were observed only after adrenalectomy. Following adrenalectomy, increases in weight were restricted to the colloid and the pars intermedia. Since no changes in plasma or hypophysial MSH content were found, the histological and autoradiographic changes do not simply reflect alterations in MSH secretion. It is concluded that there is a functional relationship between the pars intermedia and ACTH secretion.
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