Homeostasis and repair of the adult tissues depends on tissue-specific stem cells. (a) The architecture of the hair follicle stem cell niche. The hair follicle stem cells are marked by CD34 staining (in green). One of their important niche components is the inner layer of the bulge, marked by K6 staining (in red) and composed of differentiated hair follicle stem cell progeny that underwent the transition from slow-cycling to faster-cycling. This feature was exploited by bromodeoxyuridine (BrdU) nucleotide pulse-chase to mark the inner layer cells with blue BrdU staining in the figure. This inner layer of bulge cells plays a key role in maintaining the quiescence of the outer layer of hair follicle stem cells. This image is courtesy of Y-C Hsu and E Fuchs. (b) The hair follicle stem cells are marked by CD34 staining (in green) and are quiescent, due to the high level of bone morphogenetic protein (BMP) signaling within the niche, as shown here by the nuclear staining for phosphorylated Smad1 (in red), the transcriptional effector of the BMP pathway. The nuclei of the skin cells are marked here in Keratin-5 (blue), which reveals the presence of the emerging hair follicle below the activated stem cell niche. This is a classical sign of entry into the growth phase of the new hair cycle. This image is courtesy of N Oshimori and E Fuchs.