THE IN-VITRO PROLIFERATION OF THYMUS EPITHELIAL-CELLS STIMULATED WITHGROWTH-HORMONE AND INSULIN-LIKE GROWTH-FACTOR-I

The experimental system to scrutinize the in vitro proliferation of th ymus epithelial cells (TECs) was established on the basis of enhancing their mitotic index and DNA synthetic activity. The TEC line, TAD3 de rived from lymphocyte-dominant thymoma, was used as the cell material. Growth hormone (GH) induced a significant proliferation of the cultur ed cell line in the preconfluent state. The optimal concentration of a nd the duration of the incubation with GH, in this system, were 50 ng/ ml and 18 hrs., respectively. Furthermore, insulinlike growth factor-I (IGF-I), the mediator of somatotropic action of GH, also enhanced the DNA synthetic activity of the cultured cells in the preconfluent stat e. The authors recently found that the TECs, stimulated with GH, relea sed significantly more IGF-I than the cells without GH. It is possible that there are two systems in the TEC proliferation, namely, direct i nduction by GH itself and indirect induction by IGF-I released from th e GH-stimulated TECs. The data showing that GH could directly or indir ectly induce proliferation of TECs might possibly be related to the fo rmation of epithelial thymic rudiment in the fetal stage.