(A) In the steady state, Ptgs2-expressing epithelial cells are mostly present in the lamina propria of the upper and middle regions of the rectal crypt. (B) Upon DSS-induced injury (i), these Ptgs2-expressing cells migrate to the bottom of the crypt, occupying a position near the stem cell niche (ii). This relocalization is dependent on MyD88 expression by leukocytes, presumably stimulated by TLR recognition of microbial products following barrier disruption. (C) Compensatory proliferation of stem cells and transit-amplifying (TA) epithelial cells after DSS-induced injury is dependent on MyD88 and Ptgs2, whereby a MyD88-dependent signal triggers repositioning of PGE₂-producing cells to the crypt base, adjacent to the stem cell compartment.