093 Mesenchymal cell dynamics during the murine hair growth cycle

D.J. Tobin1, M. Magerl1, A Gunin2, R. Paus2. Dept. of Biomedical Sciences, University of Bradford, Bradford, England1 and Dept. of Dermatology, University Hospital Eppendorf, Hamburg, Germany2

Dramatic changes are seen in both the HF epithelium and mesenchyme during the hair growth cycle. However, the relative contributions of the HF fibroblast sub-populations, i.e. the dermal papilla (DP) and the connective tissue sheath (CTS), to this process remain unclear. Here, we studied changes in total HF length, total DP size, and DP and CTS mitotic indices during the C57BL/6 hair growth cycle. Proliferation and apoptosis were also examined by immunohistochemistry (Ki67, PCNA, BrdU), mitotic index after cholcichine injection, and by light and electron microscopic analysis. Early induced anagen was associated with a very rapid increase of total DP size beginning at day 3 (anagen II;500µm2) and reaching a maximum at day 12 (anagen VI;7200µm2). This 14-fold increase occurred with only a 2-fold increase in DP cell number; indicating the major contribution of extracellular matrix production to DP size. DP enlargement occurred significantly earlier than a corresponding increase in total HF length. This increase occurred initially via greater numbers of DP cells and before evidence of intra-DP cell proliferation, yet coincided with mitosis in the CTS. This suggests that CTS fibroblasts proliferate and thereafter migrate into the DP. DP and CTS organization and cellular activity was also modulated during the hair cycle. Telogen DP, consisting of tightly-packed inactive cells, displayed heterochromatic nuclei and limited cytoplasm. Anagen onset correlated with increasing numbers of randomly distributed cells of variable activity, but with increasing nuclear euchromatism. While DP mitosis was rare (anagen I-IV), most cells were highly active by anagen IV, aligning perpendicular to the hair growth direction. The DP was fully extended, with fibroblasts oriented parallel to the growth axis, in anagen VI. DP extracellular matrix and cytoplasmic volume were reduced in early catagen, with a corresponding increase in nuclear heterochromatism. Late catagen/telogen DP contained fewer cells with increasingly pleomorphic nuclei. Since the reduction in DP volume and cell number in catagen/telogen occurred without apoptosis, fibroblasts likely migrate out of the late anagen DP back into the proximal CTS. This study suggests that HF mesenchymal cells exhibit a greater proliferative and migratory capacity that previously thought and suggests dynamic exchange of HF fibroblast sup-populations during the hair growth cycle.