Running Title: HGF is a FLC terminal differentiation factor

The hepatocyte growth factor (HGF) is a pleiotropic cytokine and a well-known regulator of mouse embryonic organogenesis. In previous papers, we have shown the expression pattern of HGF and its receptor, C-MET, during the different stages of testis prenatal development. We demonstrated that C-MET is expressed in Fetal Leydig Cells (FLCs) and that HGF stimulates testosterone secretion in organ culture of late fetal testes. In the present paper, we analysed the proliferation rate, apoptotic index, and differentiation of FLCs in testicular organ culture of 17.5 dpc embryos to clarify the physiological role of HGF in late testis organogenesis. Based on our data, we conclude the following: a) HGF acts as an anti-apoptotic factor that is able to reduce the number of apoptotic FLCs and testicular caspase-3 active fragment; b) HGF does not affect FLC proliferation; c) HGF significantly increases expression of Insulin-like 3 (INSL3), a marker of Leydig cell terminal differentiation, without affecting 3beta-hydroxysteroid dehydrogenase (3betaHSD) expression; d) HGF significantly decreases the expression of nestin, a marker of Leydig cell progenitors; and e) HGF significantly increases the number of fully developed FLCs. Taken together, these observations demonstrate that HGF is able to act in vitro as a survival and differentiation factor in FLC population.