FIGURE 1.

Tissue architecture of three stem cell niches. (A) In the Drosophila testis niche, two stem cell populations, the GSCs and CySCs, intermingle around a cluster of cells called the hub. When the stem cell populations divide, daughters that move away from the hub differentiate, and the differentiating germ cells, which begin to undergo TA, become encysted by the differentiating cyst cells. In this niche, the CySCs produce signals promoting the self-renewal of neighboring GSCs. (B) In the mammalian hair follicle, the bulge and sHG HFSCs reside next to the dermal papilla during telogen, and the MlSC intermingle with the HFSCs. During anagen, the HFSCs and MlSCs divide to produce matrix cells and melanocytes, respectively, which cluster around the dermal papilla and contribute to growth of the new hair. The HFSCs provide molecular signals a different stages of the hair follicle cycle which regulate the behavior of the MlSCs. (C) In the mammalian bone marrow, HSCs have been identified next to sinusoids (blood vessels) and next to the endosteum (osteoblasts). MsSCs, which are innervated by the sympathetic nervous system, cluster around the sinusoids, and are required for HSC maintenance. Other cell populations with reported contributions in this niche are the endothelial cells, macrophages, osteoclasts, and other perivascular stromal cells.