Immunohistochemical localization of thrombomodulin in the stratified epithelium of the rat is restricted to the keratinizing epidermis

The expression and function of thrombomodulin (TM), an endothelial cofactor protein for thrombin-mediated protein C activation, in the epithelium are not fully characterized. This report describes the distribution and localiz ation of TM in the various types of epithelia in the rat by light and elect ron microscopic immunocytochemistry. TM showed a limited distribution and w as expressed by the keratinizing stratified epithelia of the skin, tongue, and esophagus, but was not present on the non-keratinizing epithelia of the vagina, ureter, trachea, stomach, or gut. An identical pattern of TM expre ssion was seen in mucocutaneous junctions, transitional zones from a non-ke ratinizing stratified epithelium to a keratinizing epithelium at the edge o f the eyelid and in the anal canal. As the keratinization of the stratified epithelia proceeded, the staining intensity increased in the transitional zones. Within the keratinizing stratified epithelia, TM staining was limite d to the keratinocytes of the spinous layer, the spinous cells. The subcell ular localization of TM on the spinous cells was restricted to the plasma m embrane facing the intercellular spaces. TM was not detectable on the desmo somes or the two membranes making up the junction, presumably the nexus. Th e functional significance of TM in keratinizing epithelia is discussed.