Vertebrate Embryo: Development of the Skin and Its Appendages

Abstract Vertebrate skin consists of epidermis and dermis, both of which are characterised as highly heterogeneous suborgan/tissues containing complex structures with diverse functions. Skin epidermis starts to stratify during embryogenesis. It plays pivotal barrier functions in protecting the organism from water loss and environmental insults during postnatal life. The dermis is a connective tissue that is separated with the epidermis by a deposition of extracellular matrix called the basement membrane. During skin development, many skin appendages are also induced. Those include epidermal appendages such as feather follicles, hair follicles, sebaceous glands and sweat glands and dermal appendages such as the arrector pili muscle. More and more stem cell populations are characterised in these appendages recently. In addition, considerable progress has been made in identifying molecules and pathways that regulate development and regeneration of skin and its appendages. Key Concepts During chick skin development, feathered areas are formed in the skin with high‐cell density of dermal cells and naked areas are formed in low‐cell density regions. Feather and hair development begins from thickening of epidermis and condensation of dermal cells, which is associated with the interaction among morphogens such as FGFs and BMPs. The mechanism by which feather buds are arranged in a periodic pattern on the skin and branching formation within each bud might be explained by reaction–diffusion model, one of the mathematical models. Feather follicles are differentiated from feather buds with the invaginated epidermis and the feathers have an ability of regeneration throughout lifetime owing to feather stem cells. Development and regeneration of hair follicle involve reciprocal epidermal and dermal cell interactions. Sequential activation of molecules including Wnt‐Eda‐Shh et al. is required for hair follicle development. Secondary hair germ cells first response, followed by bulge stem cell activation during hair regeneration. Regenerative hair cycling is regulated by epigenetic, microenvironment and macroenvironment factors.