Myogenesis in chick embryo somites in vitro.

Evidence from a number of studies (e.g. Holtzer, 1964; Coon & Cahn, 1966; Konigsberg, 1963) has suggested that the differentiation of some types of cells in vitro may be influenced by environmental factors. Culture conditions can permit or prevent the phenotypic expression of characteristics already determined in the cells. The length of time involved in the stabilization of differentiation appears to vary widely according to the conditions of culture. Coon (1966), using chondrocytes, and Simpson & Cox (1967), using lizard tail regenerate, among other examples, have demonstated that determination could be maintained over many mitotic divisions in a latent form, before being expressed eventually under the stabilizing conditions. A range of environmental factors has been shown to be effective during stabilization, including protein constituents of the medium (Coon, 1966; Cahn & Cahn, 1966), conditioned medium (Konigsberg, 1963), cell density (Abbott & Holtzer, 1966; Umansky, 1966) and tissue mass (Grobstein, 1964; Wessells & Cohen, 1967). Little is known, however, of how these factors act on the mechanism of expression within the developing cells. A basic question concerns the specificity of the stabilization. Will conditions which are found to be favourable to the differentiation of one cell type, necessarily enhance differentiation equally in all tissues, or has the environment some selective action ? Umansky (1966) showed that during the growth of dispersed limb bud cells, at some initial cell population densities cartilage differentiation was favoured, while at others muscle was also able to develop. Although he interprets this as a determination of developmental fate by density-dependent aggregation, it could equally be seen as a selection of pre-existing populations of cells, and undoubtedly demonstrates that an environmental factor influences different tissues in different ways. In order to study this question in more detail, the differentiation of muscle in chick embryo somite explants was compared over a range of conditions with the incidence of cartilage differentiation as previously reported (Ellison,