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The cytokine transforming growth factor beta 1 (TGF-beta 1) inhibits the growth of certain cells and the differentiation of others. A thymus hormone, thymulin, stimulated the proliferation of fetal male germ cells in explants of testes from 13.5 gestation day rat fetuses. The way in which thymulin acts is unknown. Adding TGF-beta 1 to the culture medium blocked the response of the fetal male rat germ cells to thymulin. We suggest that TGF-beta 1 and thymulin may thus influence the same metabolic chain of events.

Embryonic development involves five major cellular events: division, migration, selective cell-cell contact, differentiation and planned cell death. Among these numerous cellular events the growth factors play a major role in regulating cell growth and differentiation by an autocrine or paracrine mechanism. The growth factors may be defined as polypeptides that stimulate cell proliferation directly through binding to specific high affinity cell membrane receptors. The best characterized group of growth factors includes the epidermal growth factor (EGF), platelet-derived growth factor (PDGF), insulin and insulin-like growth factors (IGF) and the transforming growth factors TGF alpha and TGF beta. These factors are chemically characterized by their requirement for intact disulfide bridges for biological activity and have a very broad target cell specificity. A second group of polypeptide growth factors includes fibroblast growth factors (FGF) which bind specifically to heparin and induce angiogenesis both in vivo and in vitro. Their ability to stimulate cell proliferation is mainly restricted to cells from mesodermal and neuroectodermal origin, including endothelial cells. Some very small peptides, called neuropeptide-like growth factors have been recently described. The role of these growth factors during embryogenesis is studied in vivo in mouse embryos or in Xenopus eggs or in vitro in cell cultures such as murine or human teratocarcinoma cells. Some of these growth factors such as PDGF or TGF beta are chemotactic agents, and are able to modify the extracellular matrix improving the cell migrations. For example in the Xenopus, EGF and TGF beta induce the mesoderm from the ectoderm before gastulation.(ABSTRACT TRUNCATED AT 250 WORDS)