Purinergic Signalling (2008) 4:331–338 335 Fig. 2 Expression of P2Y1 and P2Y2 receptors in human anagen hair follicles. a P2Y1 receptors were found in the outer root sheath (ORS) and bulb of ana- gen hair follicles in longitudinal section but not in the inner root sheath (IRS). Scale bar=40 μm. b In transverse section, P2Y1 receptors were only seen in the outer root sheath (ORS). Scale bar=40 μm. c P2Y2 receptors were found in the cortex (CTX). Scale bar=40 μm. d Transverse section of anagen hair follicle: P2Y2 receptors were seen in the cortex (CTX) but not in the central medulla (M), inner (IRS) or outer root sheaths (ORS), or in the surrounding adventitial layer (A). Scale bar=40 μm. e, f Controls: the immunoreaction was abolished after preabsorp- tion of the e P2Y1 and f P2Y2 receptor antibodies with the corresponding peptides, con- firming the specificity of the immunoreaction. Scale bars=100 μm interfollicular epidermis confirmed the presence of P2Y1 receptors in proliferating cells [12]. The P2Y1 receptor- selective agonist 2-methylthio ADP caused an increase in the number of human keratinocytes in vitro. It is therefore consistent that P2Y1 receptors would have a proliferative role in the anagen hair follicle ORS. In anagen hair follicles, P2Y2 receptors were confined to a spatially distinct group of cells in the cortex and at the edge of the medulla, where there was no double labelling with proliferation markers. Commitment to the formation of the three hair shaft cell lineages (medulla, cortex, cuticle) takes place in a subcompartment of the hair matrix known as the precortex [1]. P2Y2 receptors seemed to be expressed on early differentiated cells in the hair shaft, because they were found on living cells of the cortex but not in the differentiated hair cuticle. Previous work on normal inter- follicular epidermis has shown that P2Y2 receptors are expressed in all cells of the basal layer and in a few cells within the stratum spinosum [12]. Stem cells in undamaged epidermis divide infrequently, and it is the nonstem daughters of stem cells, i.e. transit amplifying cells, which are actively dividing [1]. Double labelling of P2Y2 receptors with keratinocyte proliferation markers Ki-67 and PCNA in the interfollicular epidermis confirmed the presence of P2Y2 receptors in proliferating cells [12]. The P2Y2 receptor agonist uridine 5’-triphosphate also caused significant proliferation in keratinocytes [11, 12] and HaCaT keratinocytes [23]. P2Y2 receptors stained all the different cell populations within the basal layer of the epidermis (i.e. stem cells, transit amplifying cells and postmitotic cells). P2Y2 receptors may then cause prolifer- ation in transit-amplifying cells in the epidermis. Although