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The thymus, long considered a useless vestige, has in recent years become significant. It is an exciting renaissance involving cytokinetics, humoral control, immune response, autoimmunity, leukemia, and carcinogenesis. Metcalf, a solid contributor to this knowledge, assembles critical observations from his and other laboratories and constructs a concise account of present knowledge and thinking about this organ. In this account, the unfolding of perceptive hypotheses and ingenious experiments is as fascinating as are the individual items of knowledge, per se.
The thymus consists of a loose framework of epithelial and reticulum cells (arising from local anlage) in which dense aggregates of lymphoid cells (arising largely from other body sites) are found. Intense mitotic activity occurs in lymphocytes adjacent to some framework cells. About 99% of lymphocytes die in situ after an intrathymic lifespan of three or four days. Despite this graveyard function, the thymus appears important in regulation of lymphocyte levels
Best WR. The Thymus. Its Role in Immune Responses, Leukaemia Development and Carcinogenesis, vol 5.. Arch Intern Med. 1968;121(4):386-387. doi:10.1001/archinte.1968.03640040080042