The autofluorescent cells of the lymphocytic tissues of the rat

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Abstract
The lymphocytic tissues of normal rats were fixed in cold alcohol and embedded in paraffin. Unstained sections were examined under the fluorescent microscope. The sections were stained, thereafter, by various techniques.Peculiar “autofluorescent cells” were found at the cortico‐medullary junction of each thymic lobule, associated with lymphocytic nodules, and, in the plasmocytic medullary cords of the nodes. These cells contain numerous yellow autofluorescent granules and a few blue ones. The amount of yellow granules increases in the lymphocytic organs of pregnant rats. The granules stain positively with Sudan black B and P. A. Schiff techniques, and stain pale green with toluidin blue.Comparison reveals that the “autofluorescent cells” correspond to the “Lipid‐laden foamy cells” described by Loewenthal and Smith and to the “PAS‐positive reticulum cells” of Metcalf. Unlike Loewenthal et al. it is concluded that these cells are not arising from thymic involution. Metcalf's conclusion that these cells control thymic lymphocytopoiesis is analyzed in view of new findings and his conclusion is questioned. The development of the autofluorescent cells is shown to somehow parallel the development of post‐weaning growth and of immunity. The possible function of the autofluorescent cells in both phenomena is discussed.