Figure 4.

Hierarchical model for stem cell commitment during development of various ectodermal organs. Skin and oral epithelia arise from the surface ectoderm. Putative multipotent stem cells residing in both epithelia can take different fate decisions, giving rise to diverse appendages. During further development of ectodermal organs, more committed tissue-specific stem cells arise from these putative multipotent stem cells in the skin and oral epithelium. Subsequently, tissue-specific stem cells give rise to progenitors or transient amplifying cells that differentiate into functional cell populations. Abbreviations: Alv, alveolar cell; Am, ameloblast; Com, companion layer; Cor, cortex; Csf, cuticle of the hair shalf; Csh, cuticle of the hair sheath; Duc, ductal cell; EcSCs, ectodermal stem cells; EpSCs, epithelial stem cells; Gr, granular duct cell; He, Henle’s inner root sheath layer; Hu, Huxley’s inner root sheath layer; Int, intercalated duct cell; Med, medulla; Myo, myoepithelial cell; MuA, mucous acinus; Oee, Outer enamel epithelium; ORS, outer root sheath; SeA, serous acinus; Si, stratum intermedium cell; Sr, stellate reticulum cell; Str, striated duct cell.