Estrogen-induced transformation of somatotrophs into mammotrophs in the rat

Summary In the normal male rat pituitary tritiated thymidine labeled mainly STH cells (somatotrophs), no labeled prolactin cell was found. Following estradiol treatment for 21 days tritiated thymidine labeled mainly prolactin cells (mammotrophs). To determine the origin of these mammotrophs tritiated thymidine was given before the estradiol treatment started, thus labeling many somatotrophs. After 21 days of estradiol, out of 42 labeled cells, 14 were mammotrophs and 13 were somatotrophs; these results suggest that there might be a true transformation of somatotrophs into mammotrophs under the influence of estradiol or that there exist two types of somatotrophs: 1) a committed somatotroph which is not transformed by estrogen treatment, and 2) an uncommitted mammosomatotroph, which under normal conditions bears the features of a somatotroph, but which transforms into a mammotroph under the influence of estradiol.This work was supported by grants MA-552 and MT-2701 from the Medical Research Council of Canada. The authors wish to thank Dr. G. M. Brown, Clarke Institute of Psychiatry, University of Toronto, for the radioimmunoassays of growth hormone and prolactin. Reagents for both radioimmunoassays were kindly provided by the National Institute of Arthritis and Metabolic Diseases, through the Rat Pituitary Hormone Distribution Program. — We are also thankful to Dr. L. Endrenyi, Department of Pharmacology, University of Toronto, for the statistical analysis of our data.Fellow of the Medical Research Council of Canada.