Cutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails. Look up Epidermis in Wiktionary, the free dictionary. ...

During embryogenesis, the epidermis splits into two layers: the periderm (which is lost) and the basal layer. The basal layer is a stem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through an autocrine signal, TGF-a, and through paracrine signal FGF7 aka keratinocyte growth factor (KGF) produced by the dermis below the basal cells. In mice, over-expression of these factors leads to an overproduction of granule cells and thick skin. Stratum germinativum or stratum basale is the layer of keratinocytes that lies at the base of the epidermis immediately above the dermis. ... Mouse embryonic stem cells with fluorescent marker. ... Autocrine signalling is a form of signalling in which the target cell is the secretory cell itself. ... Transforming growth factor (TGF) is one of many characterized growth factors that exist in nature. ... Paracrine signalling is a form of signalling in which the target cell is close to the signal releasing cell, and the signal chemical is broken down too quickly to be carried to other parts of the body. ...

Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. This reaction-diffusion system combines an activator, Sonic hedgehog, with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern. Sonic hedgehog-expressing epidermal cells induce the condensation of cells in the mesoderm. The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm. Sonic hedgehog homolog (SHH) is one of five proteins in the vertebrate hedgehog (HH) family thus far described, the others being desert hedgehog (DHH), Indian hedgehog (IHH), echidna hedgehog (EHH) and tiggywinkle hedgehog (TwHH). ...

It is believed that the mesoderm defines the pattern. The epidermis instructs the mesodermal cells to condense and then the mesoderm instructs the epidermis of what structure to make through a series of reciprocal inductions. Transplantation experiments involving frog and newt epidermis indicated that the mesodermal signals are conserved between species but the epidermal response is species-specific meaning that the mesoderm instructs the epidermis of its position and the epidermis uses this information to make a specific structure.