Practice question

Intestinal epithelium renews remarkably every three to five days driven by rapidly cycling Lgr5-positive crypt base columnar cells intercalated between Paneth cells at crypt bottom. Paneth cells supply essential Wnt3, epidermal growth factor EGF, and Notch ligands Dll1 and Dll4, sustaining high Wnt beta-catenin activity essential for stem proliferation. Definitive lineage tracing and single-cell organoid formation assays confirm Lgr5 cells are multipotent self-renewing stem cells generating enterocytes, goblet, enteroendocrine, and Paneth progeny. Ablation experiments demonstrate niche dependence: loss of Paneth or Wnt signals triggers rapid stem cell differentiation and crypt collapse.