Practice question

A niche is defined as a local microenvironment that physically anchors, sustains, and instructively controls stem cell behavior through contact, paracrine signals, and extracellular matrix mechanics. Classic examples include basal layer of interfollicular epidermis where hemidesmosomes attach basal keratinocyte stem cells to laminin-rich basement membrane, and adjacent dermal fibroblasts provide Wnt and BMP inhibition to prevent differentiation. Other prototypes are bone marrow endosteal niche, intestinal crypt base, and Drosophila cap cells. Trophoblast, erythrocytes, and mature neurons are differentiated products, not regulatory supportive sites at all.