The skin is the body’s largest organ, and is the first line of defence against the external environment. The epidermis – the outermost layer of the skin – is highly specialised and often exhibits unique characteristics depending on its anatomical location and the function it serves. It has long been known that this specialisation is dependent on inductive signals that originate from underlying fibroblasts; however, the exact nature of the signals and their role in maintaining the epidermis is only just starting to emerge. In this issue (p. 1498), John Foley and colleagues identify an oestrogen-regulated TGFβ signalling pathway that is crucial for the maintenance of the highly specialised nipple epidermis. Using a series of grafting experiments, the authors show that fibroblasts taken from the nipple-like skin of mice can induce reprogramming of trunk keratinocytes into nipple-like epidermis. Transcriptional profiling of the nipple-like fibroblasts identifies oestrogen signalling as a strong candidate factor for the maintenance of the nipple epidermis and, indeed, ablation of oestrogen signalling in ovariectomised mice results in an abnormally thin nipple epidermis. The authors further identify Tgfb1 as a direct target of oestrogen signalling and show how ectopic treatment of TGFβ1 protein into the connective tissue of the nipple causes a decrease in epidermal proliferation and a thinning of the nipple epidermis. Taken together, these data represent an important step forward in understanding the signalling network that maintains the specialisation of the nipple epidermis.
