Effect of X-Radiation on DNA Metabolism in Various Tissues of the Rat: VI. Correlative Morphologic and Biochemical Changes during the Regeneration of the Thymus

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Abstract
A sequential pattern of morphologic and synthetic (incorporation of labeled precursors of DNA) events was demonstrated. These occurred first in the reticulum cells, then in the monocytoid reticular lymphoblasts, and finally in an emerging population of young lymphocytes. The primitive reticulum cells reproduce with the available preformed Tdr (from the injured lymphocytes) and that these cells probably contain Tdr kinase but lack dCMP deaminase and TMP synthetase. The breakdown or death of lymphocytes may provide the Tdr to the reticulum cell which is capable of phagocytosis, and thus serve as the "trigger" to synthesis of protein in the reticulum cell and then replication of that cell. It is postulated that the "monocytoid lymphoblasts" are capable of de novo synthesis and thus contain dCMP deaminase and TMP synthetase. Finally, mature lymphocytes may demonstrate little or no synthesis because of lack of a rate-limiting enzyme such as TMP kinase or DNA-primer formation.