Basement membrane and niches. Specialized and nonspecialized niches are anatomically distinct in relation to the basement membrane (bm). The specialized niche in close proximity to stem cells arrays on the basement membrane in the epithelium. In concept, the specialized niche is composed of particular cell types directly regulating survival and proliferation of stem cells. The nonspecialized niche, composed of numerous cell types widely distributed among connective tissues, directly or indirectly regulates stem cells in a region. (A) ISCs and specialized niche cells, Paneth cells, form a monolayer of the epithelium on the basement membrane. The lamina propria under the basement membrane may function as the nonspecialized niche. (B) IFESCs and specialized niche cells in the basal layer lie on the basement membrane of the epidermis. The dermis may function as a nonspecialized niche. (C) Spermatogonia, lying on the basement membrane, give rise to spermatocytes and spermatids in the seminiferous epithelium of the testis. Sertoli cells most likely serve as specialized niche cells. Leydig cells, myoid cells, and vascular endothelial cells under the basement membrane may serve as a nonspecialized niche. (D) In the bone marrow, the basement membrane is found underneath the vascular endothelium. HSCs do not lie on the basement membrane with endothelial cells. HSCs lie among the nonspecialized niche.