CD34 immunoreactivity and interstitial cells of Cajal in the human and mouse gastrointestinal tract

Immunoreactivity for the tyrosine kinase receptor Kit (Kit-ir) is an establ ished marker for the interstitial cells of Cajal (ICC) of the gut. Recently , the presence of CD34 immunoreactivity (CD34-ir) has been reported in Kit- ir ICC around the myenteric plexus in human small intestine. Conversely, we observed that CD34-ir labeled Kit-negative fibroblast-like cells, closely adjacent to, but distinct from, the Kit-ir ICC. The existence of cells expr essing both CD34-ir and Kit-ir remains controversial. CD34-ir and Kit-ir we re studied by high-resolution confocal microscopy on cryostat sections of h uman and murine gut as well as murine whole-mounts, using specific antibodi es raised to human and murine CD34, respectively. CD34-ir labeled numerous cells in all parts of the gut, in man and in mouse. CD34-ir was consistentl y observed in Kit-negative cells, distinct from the closely adjacent Kit-ir ICC. Thin processes of both cell types intermingled extensively, often at the limit of resolution for light microscopy. CD34-ir was also observed in Kit-negative mesenchymal cells in the submucosa, in capillaries and in meso thelial cells. CD34-ir is not a marker for Kit-ir ICC in the human and muri ne gut. No CD34-ir, Kit-ir-expressing cells were encountered. Conversely, C D34-ir cells, closely adjacent to, but distinct from, Kit-ir ICC were consi stently identified. The intimate relationship between these cells may offer an alternative explanation for reports of CD34 and Kit co-localization. Th e ontogeny and function of CD34-ir cells in the gut, as well as the origin of gastrointestinal stromal tumors, remain unclear.