35) How is the stem cell niche created and maintained?
a. The mechanism depends on a complex interplay of signals between the epithelium and the underlying connective tissue. Exchange of Wnt, Hedgehog, and PDGF signals between the two tissues, and between different regions of the crypt-villus axis, leads to a restriction of Wnt signaling to the neighborhood of the crypts. The epithelial cells in the crypts produce both Wnt proteins and the receptors that respond to them, creating a positive feedback loop that presumably helps to make Wnt pathway activation in this region self-sustaining. At the same time, signals exchanged with the connective tissue lead to expression of BMP proteins in the connective-tissue cells forming the core of the villi. These cells signal to the adjacent villus epithelium to inhibit the development of misplaced crypts: blocking BMP signaling disrupts the whole organization and causes misplaced crypts to form as invaginations of proliferating epithelium along the sides of the villi