INCORPORATION OF SULFATE BY THE MOUSE THYMUS: ITS RELATION TO SECRETION BY MEDULLARY EPITHELIAL CELLS AND TO THYMIC LYMPHOPOIESIS

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Abstract
The thymus was examined in suckling mice during normal development and the involution and regeneration produced by injection of cortisol, in experiments designed to test the hypothesis that medullary epithelial cells secrete a ymphopoietic hormone responsible for controlling the magnitude of thymic lymphopoiesis. Cellular events were observed by light and electron microscopy. Lymphopoiesis was assessed, after injection of thymidine-3H, by counting the proportion of lymphocytes labeled in radioautographs of thymus. Cortical lymphopoiesis was distributed heterogeneously, being concentrated in the subcapsular region, but medullary lymphopoiesis was statistically homogeneous in distribution and similar in magnitude to the average level of cortical lymphopoiesis in suckling mice. Therefore counts of the labeling index in the medulla were used to estimate the size of the proliferating population of lymphocytes. Epithelial secretory activity was estimated by measuring the incorporation of 36sulfate by the thymus, using gel filtration chromatography to isolate soluble macromolecular 35sulfate—presumed on radioautographic evidence to represent the mucoid epithelial secretory product.