A baseline biopsy was performed at the edge of the ulceration in patient 1 just prior to bone marrow aspiration (day 0). Histologic examination of the specimen revealed few dermal cells (mainly fibroblasts), fibrosis, an absence of reticulin fibers, and a decrease in elastic fibers consistent with a dermal scar. On day 11, a second biopsy specimen from the wound edge revealed a dramatic increase in the number and variety of cell types within the dermis (Figure 2), including mature and immature inflammatory cells. Engraftment of bone marrow cells was evidenced by the appearance of immature hematopoietic cells within the dermis. These immature cells are not found in the skin except in the case of an underlying hematopoietic malignancy or a myeloproliferative disorder, and the patient did not have these diseases. The observed immature hematopoietic cells included bandlike and blastlike forms that stained positive for myeloperoxidase, which identifyed them as granulocytic progenitors. Mature inflammatory cells included neutrophils, eosinophils, mast cells, plasma cells, and histiocytes. Many immature, elongated, spindle-shaped cells were noted, particularly in the mid- and deep dermis (Figure 2). These spindle cells had an immature mesenchymal morphology and stained positive for vimentin, which is consistent with a mesenchymal origin. They were not likely to be tissue macrophages, as they did not stain for CD68 or S100 proteins. The immature, mesenchymal-appearing cells may then have been engrafted bone marrow cells, possibly bone marrow stromal cells. The CD34 stain highlighted several blood vessels but did not stain the new immature cells observed in the dermis. Some dendritic cells in the dermis stained with factor XIIIa. However, the immature-appearing, elongated, spindle-shaped cells did not stain for factor XIIIa. The CD31 stain, a marker for endothelial cells, highlighted several cells within the dermis and also revealed many new blood vessels. This finding supported our clinical observation of increased vascularity of the wound bed at that time. In addition, the biopsy specimen was filled with newly laid-down reticulin fibers, and such fibers were absent from the specimen obtained before treatment (Figure 3).