Mesodermal Differentiation of Embryonal Carcinoma Cells in Coculture with Visceral Endoderm Cell Lines

Abstract

In mammalian embryogenes is mesoderm is formed from embryonic ectoderm in an area adjacent to visceral endoderm. We have established an in vitro model for this process using P19 embryonal carcinoma (EC) cells and various endodermal cell lines, which were characterized with specific antibodies. In coculture with END-2 cells, a visceral endoderm cell line derived from P19, the EC cells formed aggregates and differentiated extensively, with mesoderm—derived beating muscle and adipocytes being the cell types most evident. Differentiation was less pronounced when P19 cells were cocultured with EPI-7 and PSA-5E cells which have some but not all of the visceral endoderm characteristics expressed by END-2. Parietal endoderm (PYS-2) and mesoderm (MES-1) cell lines neither permitted aggregation nor differentiation, whereas Dif 5 cells, reported to have both visceral and parietal endoderm characteristics, allowed aggregation but not differentiation. Mesodermal differentiation of P19 EC cells therefore specifically occured in coculture with visceral endoderm-like cells. Aggregation is a necessary but not sufficient step in this process. Differentiation was not observed on fixed or lysed END-2 monolayers to which END-2 conditioned media were added. However, when P19 cells were aggregated over a monolayer of END-2 cells covered with an agar layer to prevent contacts between the two cell types, they formed beating muscle significantly earlier after replating on tissue culture substrate than when aggregated similarly without an underlayer or over MES-1 monolayers. This finding suggested that a visceral endoderm-derived soluble mediator stimulated mesoderm formation. This coculture system, in which no chemical differentiation inducer is needed, provides a valuable in vitro model of mesoderm induction in mammalian embryogenesis.