Transforming growth factor-β in the early mouse embryo: Implications for the regulation of muscle formation and implantation

In a search for functions of transforming growth factor‐β during early embryonic development we used two different experimental approaches. In the first we made use of embryonic stem (ES) cells. ES cells in culture differentiate to derivatives of all three germ layers and mimic some aspects of organogenesis when grown as aggregates in suspension to form embryoid bodies. Differentiation procedes further when the embryold bodies attach to suitable substrates. Muscle and neuronal cells are among the most readily identified cell types then formed. We examined the effect of all‐trans retinoic acid (RA) and members of the transforming growth factor‐β family(TGF‐βl, TGF‐β2) under these conditions in an assay where single aggregates formed in hanging microdrops in medium supplemented with serum depleted of lipophilic substances which would include retinoids. Endoderm‐like cells formed under all conditions tested. RA at concentrations of 108 M and 107 M induced the formation of neurons but in the absence of RA or at concentrations up to 10−9 M, neurons were not observed. Instead, beating muscle formed in about one‐third of the plated aggregates; this was greatly reduced when RA concentrations increased above 10−9 M. Immunofluorescent staining for muscle specific myosin showed that two muscle cell types could be distinguished: elongated, non‐contractile myoblasts and mononucleate flat cells. The mononucleate flat cells appeared to correspond with rhythmically contracting muscle. The number of non‐contractile myoblasts increased 3‐fold over controls in the presence of 10−9 M RA. TGF‐βs increased the number of contractile and non‐contractile muscle cells by a factor 3 to 7 over controls, depending on the TGF‐β isoform added and the muscle cell type formed. TGF‐β2 also invariably increased the rate at which contracting muscle cells were first observed in replated aggregates. The stimulatory effect of TGF‐βs on the formation of mononucleate flat cells was completely abrogated by RA at 10−9 M while the number of myoblasts under similar conditions was unchanged. These data suggest that a complex interplay between retinoids and TGF‐β isoforms may be involved in regulation of differentiation in early myogenesis. In the second approach, neutralizing polyclonal rabbit antibodies specific for TGF‐β2 were injected into the cavity of mouse blastocysts 3.5 days post coitum (pc). After 1 day in culture, embryos were transferred to pseudopregnant females. The number of decidua, emb