Radiosensitivity of the Cortical and Medullary Lymphocytes in the Thymus

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Abstract
Rats were sacrificed 48 hrs after x-irradiation of the thymic region and differential cell-counts were made on sections to determine the lymphocyte/reticulum-cell ratio in cortex and medulla. Assuming the reticulum cells unaffected, the decrease in the ratio compared with controls gave the decrease in lymphocyte population. It is argued that the results mainly represent direct lymphocyte killing. In the cortex, percentage survival was a linear function of long dose. But in the medulla this relationship only held above 100 r; after lower doses the percentage remaining was unexpectedly high, probably because lymphocytes had migrated from cortex to medulla. The cortical lymphocytes were four times more sensitive than the medullary, and the radiosensitivity boundary was shown to be sharp. It is suggested that lymphocytes may be partially protected by reticulum cells, possibly by transfer of ATP.