Function of TGF-β2 in the growth of chicken primordial germ cells and germinal ridge stroma cells during embryonic development

The development of chicken embryonic gonads is locally regulated by the systematic action of growth factors. Recently, we used suppressive subtraction cloning to identify transforming growth factor β2 (TGF-β2) as a growth factor gene preferentially expressed in chicken embryonic ovaries and testes during the early periods of development (Hattori et al. 2002a. Prominent expression of transforming growth factor β2 gene in the chicken embryonic gonad as revealed by suppressive subtraction cloning. Gen Comp Endocrinol 125:311-316). In the present study, the function of TGF-β2 in chicken embryonic gonads was investigated using a serum-free culture system in the presence of several growth factors, which may behave as mitogenic or survival factors of primordial germ cells (PGCs). Chicken germinal ridges containing PGCs and germinal ridge stroma cells (GRSCs) were collected from six-day embryos. Addition of TGF-β2 caused a dose-dependent inhibition of the number of co-cultured PGCs and GRSCs in the presence of these growth factors. However, there was no obvious difference between embryonic ovaries and testes in the effects of TGF-β2. Immunocytochemical analysis using anti-SSEA-1 antibody revealed that TGF-β2 induced fragmentation of PGCs. Expression of the TGF-β2 gene was estimated in the co-cultured PGCs and GRSCs by semi-quantitative RT-PCR. The mRNA level of TGF-β2 was significantly suppressed in the presence of the growth factors. These results suggest that TGF-β2 is a gonadal regulator preferentially expressed at the early stages of chicken embryonic development and reduces the growth of PGCs and GRSCs by suppressing proliferation. However, expression of TGF-β2 may be controlled by mitogenic or survival factors of PGCs.