My answer to What is the overall main function of the skin?
Answer by Connie b. Dellobuono:
Homeostasis, protection, proliferation as an organ to nourish other cells in our body and to give rise to the differentiating cells that constitute one or more tissues.
The skin epidermis and its appendages provide a protective barrier that is impermeable to harmful microbes and also prevents dehydration. To perform their functions while being confronted with the physicochemical traumas of the environment, these tissues undergo continual rejuvenation through homeostasis, and, in addition, they must be primed to undergo wound repair in response to injury. The skin's elixir for maintaining tissue homeostasis, regenerating hair, and repairing the epidermis after injury is its stem cells, which reside in the adult hair follicle, sebaceous gland, and epidermis. Stem cells have the remarkable capacity to both self-perpetuate and also give rise to the differentiating cells that constitute one or more tissues.
Just how powerful is the stabilization of β-catenin to promoting hair follicle morphogenesis? Studies over the past decade indicate that it is indeed central to the decision-making process. Thus, when excess β-catenin is stabilized in the epidermis of transgenic mice, ectopic hair follicles appear within the interfollicular epidermis, generating super-furry mice (Gat et al., 1998), and, when the Wnt inhibitor Dickoff 1 (Dkk1) is expressed ectopically or when β-catenin is conditionally targeted for ablation, hair follicle morphogenesis is blocked altogether (Huelsken and Birchmeier, 2001; Andl et al., 2002). Recently, Ito et al. (2007) showed that when the skin of mice is severely wounded, endogenous Wnt signaling is elevated in the skin, and this leads to the induction of hair follicle formation from epidermal stem cells. Collectively, these findings underscore the role for stabilized β-catenin in governing the choice of whether to become epidermis or hair follicle.
All three progenitor populations express K5, K14, and p63. Although p63 has also been implicated in differentiation, p63 remains one of the best candidates to date for a gatekeeper of proliferation in epithelial stem cells (Truong et al., 2006; Senoo et al., 2007). Cells within all three progenitor pools also express E-cadherin and display elevated levels of adherens junctions and reduced levels of desmosomes. A reduction in E-cadherin appears to be an essential feature in mobilizing embryonic epidermal cells to invaginate to form a hair follicle (Jamora et al., 2003), and reductions in α-catenin and p120-catenin have also been linked to epithelial cancers, a less organized form of invagination (Scott and Yap, 2006; Reynolds, 2007).
To what extent do the gene expression programs of other progenitor populations overlap with that of follicle stem cells, and do these similarities reflect their self-renewing, undifferentiated features? When do these different proliferative epithelial compartments form during skin development, and to what extent are their differences in gene expression patterns dependent on their distinct in vivo microenvironments?