Endothelial DNA synthesis in themicrovasculature of rat skin during the hair growth cycle.

Earlier studies of the vascular patterns around hair follicles revealed that actively growing follicles have an extensive microvasculature relative to that around quiescent follicles. In order to determine whether the establishment of the active perifollicular vascular pattern involves replication of endothelial and perivascular cells, we studied the 3H-thymidine labeling indices of these cells in autoradiographs of rat skin from regions containing either actively growing follicles or recently inactive follicles. In skin containing inactive follicles, the endothelial labeling index was 0.22%, a figure similar to those reported for the endothelium of small blood vessels in several other non-stimulated adult tissues. However, skin containing actively growing follicles displayed an endothelial labeling index of 2.35%. The labeling indices of perivascular cells paralleled those of the endothelial cells. Thus, endothelial and perivascular cells in vessels associated with actively growing follicles undergo significantly more DNA synthesis than those in vessels associated with inactive follicles. Replication of these cells apparently contributes to the extension of the microvascular network around growing follicles.