The intestinal crypt, a prototype stem cell compartment.

Due to its intense self-renewal kinetics and its simple repetitive architecture, the intestinal epithelium has become a prime model for studying adult stem cells in health and disease. Transgenic mouse models allow in vivo visualization and genetic lineage tracing of individual intestinal stem cells and their offspring. Fluorescently marked stem cells can be isolated for molecular analyses or can be cultured to build ever-expanding "mini-guts" in vitro. These studies are filling in the outlines of a robust homeostatic self-renewal process that defies some of the classical definitions of stem cell behavior, such as asymmetric division, quiescence, and exhaustion.