The human progenitor cell antigen (CD34) is localized on endothelial cells, dermal dendritic cells, and perifollicular cells in formalin-fixed normal skin, and on proliferating endothelial cells and stromal spindle-shaped cells in Kaposi's sarcoma.

The human progenitor cell antigen (CD34) is selectively expressed on hematopoietic progenitor cells in the bone marrow. In either cryostat sections of snap-frozen skin, or formalin-fixed paraffin-embedded sections of normal skin, anti-CD34 monoclonal antibody immunostained vascular endothelial cells and perivascular/interstitial dendritic cells, particularly in the reticular dermis. A distinctive population of perifollicular spindle-shaped cells in the midportion of the follicle (ie, bulge area), which is the site of the putative hair follicle stem cells, were CD34 positive, as were spindle-shaped cells in and around the eccrine glands accentuating their basement membrane zone. In patch/plaque--and tumor-stage acquired immunodeficiency syndrome-associated Kaposi's sarcoma lesions, CD34 expression was present on both the proliferating endothelial cells as well as the spindle-shaped stromal cells. CD34 positive endothelial cells and spindle-shaped stromal cells may play important participatory and supportive functions in both normal and diseased skin.