Thymus cell maturation. Studies on the origin of cortisone-resistant thymic lymphocytes.

Outer thymic cortical large lymphocytes were labeled by transcapsular administration of tritiated thymidine. By 2-4 days after labeling, small and medium labeled lymphocyte descendants were found throughout the cortex and in the medulla. The labeled cortical lymphocytes undergo pycnosis after parenteral administration of hydrocortisone 24 h previously, but their (labeled) medullary descendants do not. In addition, parenteral administration of hydrocortisone at the time of surface labeling results in the absence of the appearance of labeled medullary descendants.