The localization and distribution of cells labeled by a somatic stem cell-recognizing antibody (A3) in rat colon development; possible presence of a new cell type forming the intestinal stem cell niche.

A3, generated as a monoclonal antibody against rat malignant fibrous histiocytoma (MFH)-derived cloned cells, recognizes somatic stem cells (bone-marrow/hair follicle stem cells). We investigated the distribution of cells immunoreactive to A3 in the developing rat intestine (particularly, the colon), focusing on the ontogenic kinetics of A3-positive cells. In the rat intestine, A3 labeled spindle-shaped stromal cells localized in the submucosa and labeled endothelial cells of capillaries in the lamina propria forming villi in the early development stage. With development progression, A3-positive cells were exclusively localized around the crypts of the colon. Double immunofluorescence revealed that A3-positive cells around the crypts reacted to vimentin (for mesenchymal cells) and Thy-1 (for mesenchymal stromal cells) but not to α-SMA (for mesenchymal myofibroblastic cells) or CD34 (for hematopoietic stem cells), indicating that A3-positive cells around the crypts may have characteristics of immature mesenchymal cells. In addition, A3 labeled a few epithelial cells at the base of colon crypts. Furthermore, immunoelectron microscopy revealed that A3-positive cells lay inside myofibroblasts adjacent to the epithelium of the crypts. A3-positive cells were regarded as a new type of immature mesenchymal cells around the crypts. Collectively, A3-positive cells might take part in the stem cell niche in the colon, which is formed through epithelial-mesenchymal interaction.