SKIN STEM CELLS

• The epidermis maintains a single inner (basal) layer of proliferative cells that adhere to an underlying basement membrane rich in ECM and growth factors. Basal cells express several characteristic markers, including keratins and transcription factors. Periodically, these cells withdraw from the cell cycle, commit to differentiate terminally, move outward, and are eventually shed from the skin surface.
• Upon commitment to terminally differentiate, an epidermal keratinocyte progresses through three distinct differentiation stages:

  • Spinous
  • Granular
  • Stratum corneum
• Major changes in transcription, morphology, and function occur at the basal/spinous layer transition and again at the granular/stratum corneum transition such that differentiated cells reaching the surface are enucleated cellular skeletons that are packed with cables of keratin filaments encased by a γ-glutamyl-ε-lysine cross-linked cornfield envelope of proteins
• Self renewing capacity of epidermal stem cells is enormous and within 4 weeks, a basal cell has rerminally differentiated and exited at the skin surface
• The epidermal proliferative unit (EPU) has been architecturally defined as a bed of 10 tightly packed basal cells yielding a stack of increasingly larger and flatter fells that culminate with a single hexagonal surface cell. This has led to the hypothesis that there is one self renewing stem cell per EPU and the other basal cells are so-called transit-amplifying (TA) cells
• TA cells or transit-amplifying cells are committed cells that divide several times and then exit the basal layer and terminally differentiate
• Where do epidermal stem cells reside?

  Adult hair follicle, sebaceous gland, and epidermia