thymosin

Thymosin

A polypeptide hormone synthesized and secreted by the endodermally derived reticular cells of the thymus gland. Thymosin exerts its actions in several loci: (1) in the thymus gland, either on precursor stem cells derived from fetal liver or from bone marrow, or on immature thymocytes, and (2) in peripheral sites, on either thymic-derived lymphoid cells or on precursor stem cells. The precursor stem cells, which are immunologically incompetent whether in the thymus or in peripheral sites, have been designated as predetermined T cells or T₀ cells, and mature through stages termed T₁ and T₂, each reflecting varying degrees of immunological competence. Thymosin promotes or accelerates the maturation of T₀ cells to T₁ cells as well as to the final stage of a T₂. In addition to this maturation influence, the hormone also increases the number of total lymphoid cells by accelerating the rate of proliferation of both immature and mature lymphocytes. See Immunity

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

thymosin

[′thī·mə·sən]

(immunology)

Any of a group of hormones secreted by the thymus gland that stimulate lymphocyte production within the thymus and confer on lymphocytes elsewhere in the body the capacity to respond to antigenic stimulation.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.