Skin stem cells: rising to the surface

Abstract

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. In recent years, scientists have begun to uncover the properties of skin stem cells and unravel the mysteries underlying their remarkable capacity to perform these feats. In this paper, I outline the basic lineages of the skin epithelia and review some of the major findings about mammalian skin epithelial stem cells that have emerged in the past five years.

Figure 1.

Epidermal differentiation. The program of epidermal differentiation is shown in this schematic, illustrating the basement membrane at the base, the proliferative basal layer, and the three differentiation stages: spinous layer, granular layer, and outermost stratum corneum. At the right, key molecular markers are shown, which are described in the first section of this paper.

Figure 2.

Models for the generation of a single innermost (basal) layer of cells with proliferative potential and multiple layers of suprabasal cells. Self-renewing stem cells (SCs) exist in the basal layer of the epidermis. Symmetrical divisions produce two stem cells, a process which can serve to replenish vacancies in the basal layer. In the three-step model, a transit-amplifying (TA) intermediate arises, which has been postulated to divide four to five times before delaminating (straight arrows) and entering into a terminal differentiation program. In the two-step model, a stem cell divides asymmetrically to preferentially partition proliferation-associated factors into the stem cell daughter while providing differentiation-inducing components to the other daughter, which is fated to become a spinous cell (SP). Depending on the orientation of the spindle, the divisions could either result in detachment of the SP daughter from the basement membrane or, if lateral, would then necessitate subsequent delamination of the committed SP daughter. The spinous cells enter a program of terminal differentiation as they move outward and are eventually sloughed from the skin surface (see Fig. 1). Differentiating cells are continually replaced by a flux of inner cells committing to terminally differentiate and moving outward.

Figure 3.

Predicted roles of Notch signaling in the three different lineages of the epidermis. (A) Model. In the skin, Notch ligands are often basal, whereas Notch receptors are typically suprabasal (Pan et al., 2004; Blanpain et al., 2006; Estrach et al., 2006, 2007; Nguyen et al., 2006a). Notch signaling results in the release of Notch intracellular domain, which acts as a transcription cofactor for the DNA-binding protein RBPj (Pan et al., 2004). Downstream targets of Notch intracellular domain/RBPj include Hes1 and Hey1, which bind DNA and often function as transcriptional repressors (Hurlbut et al., 2007). In the skin lineages, Notch signaling, as indicated by either the expression of Notch intracellular domain (Pan et al., 2004), Notch target genes Hes1 and Hey1 (Blanpain et al., 2006), or loss of function studies (Blanpain et al., 2006; Estrach et al., 2006, 2007), is particularly prominent at the transition of fates from proliferative to differentiating. The arrows denote molecular steps along the pathways for each of the three major epithelial lineages of the skin. Those marked in red indicate the steps at which Notch signaling acts. (B) Immunofluorescence. Immunofluorescence microscopy was used on frozen mouse skin sections of E18.5 (epidermis) and postnatal day 28 (hair follicle bulb) to illustrate the localization of Notch3, Hes1, and Hey1. All samples were counterstained with DAPI (blue) to mark the nuclei. Where indicated, antibodies against β4 integrin (Int) were used to mark the dermo-epidermal boundary, and keratin 14 (Ker14) was used to denote the basal layer of the ORS. The top two panels illustrate the coexpression of Notch3 and Hes1, which is particularly strong at the basal to suprabasal transition of the epidermis. The bottom two panels show that Hes1 is prominent in the differentiating cells of the IRS (left), whereas Hey1 is more broadly expressed in both the precortex and IRS (right). The inset in the image in the bottom right frame shows a close-up view of hair shaft precursor cells that have been labeled with antibodies against hair keratins in green and Hey1 in red to show that nuclear Hey1 colocalizes with cells that generate the hair shaft. Note that the highly proliferative matrix cells at the base of the hair bulb are negative for signs of Notch signaling. For further information and similar images (courtesy of C. Blanpain and W.E. Lowry), see Blanpain et al. (2006). Bar, 45 mm.

Figure 4.

Embryonic stages of hair follicle morphogenesis. The process of follicle morphogenesis occurs relatively late in embryonic development. In the mouse, discrete waves of placodes are seen beginning at approximately E14.5 and ending near birth. Once initiated, newly formed follicles continue to mature postnatally until approximately day 9, at which time matrix (TA) cells at the base of the follicle continue to rapidly proliferate and differentiate, generating hair growth. For a comprehensive analysis of the regional, temporal, and strain-specific variations in the timing of hair follicle morphogenesis, see the reviews Muller-Rover et al., 2001 andSchmidt-Ullrich and Paus, 2005.