The in vitro proliferation of thymus epithelial cells stimulated with growth hormone and insulin-like growth factor-I.

The experimental system to scrutinize the in vitro proliferation of thymus epithelial cells (TECs) was established on the basis of enhancing their mitotic index and DNA synthetic activity. The TEC line, TAD3 derived from lymphocyte-dominant thymoma, was used as the cell material. Growth hormone (GH) induced a significant proliferation of the cultured cell line in the preconfluent state. The optimal concentration of and the duration of the incubation with GH, in this system, were 50 ng/ml and 18 hrs., respectively. Furthermore, insulin-like 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 state. The authors recently found that the TECs, stimulated with GH, released significantly more IGF-I than the cells without GH. It is possible that there are two systems in the TEC proliferation, namely, direct induction by GH itself and indirect induction by IGF-I released from the GH-stimulated TECs. The data showing that GH could directly or indirectly induce proliferation of TECs might possibly be related to the formation of epithelial thymic rudiment in the fetal stage.