Exogenous fibronectin is not required for organogenesis in vitro

The biological effect of plasma fibronectin on the differentiation of embryonic mouse kidney and tooth was studied in organ cultures. Transferrin (50 micrograms/ml) was a strong mitogen for kidney cells, whereas the addition of soluble fibronectin (50 to 250 micrograms/ml) had no detectable effect on differentiation or proliferation. The same serum-free, transferrin-containing medium did not support tooth differentiation. However, fibronectin was not a necessary serum component because fibronectin-free serum supported tooth development. It was demonstrated with antibodies specific for human fibronectin that the exogenously added human fibronectin at 50 micrograms/ml did not become incorporated to the cultured organs. Only minimal incorporation to the kidney basement membrane area was observed when fibronectin concentration was 250 micrograms/ml. The mesenchymal stroma and the basement membranes of the kidney and tooth rudiments cultured in fibronectin-free media stained intensely with conventional fibronectin antibodies, indicating endogenous production of fibronectin. Outgrowing epithelial cells from isolated kidney tubules produced fibronectin as well as laminin. The results suggest that the fibronectin found in the stroma and basement membranes is an endogenous product of the developing tissues and that plasma fibronectin is not required for in vitro organogenesis. The results also indicate that it is difficult to study the effect of fibronectin on morphogenetic processes because it may not penetrate the organ explants in vitro.