Figure 1.

Stem cells niches. (a) Stromal type. HSCs are found adjacent to sinusoidal blood vessels as well as at or near the endosteum. Ostoblasts and mesenchymal progenitors have been proposed to produce factors that regulate HSC maintenance. (b) Epithelial type. Stem cells located within the bulge region of the hair follicle give rise to new matrix cells located at the base of the hair and adjacent to the dermal papilla that supports hair growth. Bulge cells can also help maintain the sebaceous gland and the epidermal stem cells. (c) The vascular wall as a reservoir for stem and progenitor cells. Capillaries are constructed by endothelial cells lining the lumen and pericytes, which cover the endothelial tube. Vascular wall-resident endothelial progenitor cells, HSCs and MSCs, are localized in the subendothelial space and are probably located between or around pericytes. HSCs, haematopoietic stem cells; MSCs, mesenchymal stem cells.