Electron microscopy and immunocytochemistry on endothelial progenitor cells during healing process of burn injury in adult rats

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Vasculogenesis is defined as a neovascularization process by transformation of vasoformative mesenchymal cells to endothelial progenitor cells (EPCs). Although recent researches insisted that bone marrow-derived EPCs via peripheral blood vessels are mainly involved in vasculogenesis during wound healing process of adult tissues, we have reported that multipotent mesenchymal cells derived from tissue fibroblasts located in several adult tissues transform to EPCs and participate in the acceleration of the vascular growth. This study deals with ultrastructural and immunocytochemical analysis on EPCs during healing process of burn injury. Deeply anesthetized adult Wistar rats received burn injury in their back skin by exposure to heated cylinder for 1 min at 100℃, and then injured tissue were prepared for light microscopic immunocytochemistry and electron microscopy, respectively. At 3 days after receiving burn injury, migration of tissue fibroblasts from intact dermis to injured dermis was often seen. These cells were occasionally associated with endothelial lining of growing capillaries, or in contact with the neighboring ones. They often formed solid cell cords that eventually make contact with the growing capillaries. Since these cells show immunoreactivities for von Willebrand factor and possess many Weibel-Palade (WP) bodies, we defined them as phenotypes of EPCs. Immunoreactivities for VEGF are seen along endothelial lining of growing capillaries and in such EPCs and these for VEGF receptors are detected in the EPCs suggesting that VEGF produced by the endothelial cells activates transformation of tissue fibroblasts to EPCs via receptor mediated fashion. In addition, immunoreactivities for endothelia (ET)-1, big ET-1 and ET converting enzyme (ECE)-1 are seen in such EPCs. Since it is widely accepted that ET-1 and ECE-1 are stored in WP bodies, we consider now that EPCs which are characterized by possessing WP bodies have ability to synthesize ET-1.