The basement membrane of hair follicle stem cells is a muscle cell niche

The hair follicle bulge in the epidermis associates with the arrector pili muscle (APM) that is responsible for piloerection ("goosebumps''). We show that stem cells in the bulge deposit nephronectin into the underlying basement membrane, thus regulating the adhesion of mesenchymal cells expressing the nephronectin receptor, alpha 8 beta 1 integrin, to the bulge. Nephronectin induces alpha 8 integrin-positive mesenchymal cells to upregulate smooth muscle markers. In nephronectin knockout mice, fewer arrector pili muscles form in the skin, and they attach to the follicle above the bulge, where there is compensatory upregulation of the nephronectin family member EGFL6. Deletion of alpha 8 integrin also abolishes selective APM anchorage to the bulge. Nephronectin is a Wnt target; epidermal beta-catenin activation upregulates epidermal nephronectin and dermal alpha 8 integrin expression. Thus, bulge stem cells, via nephronectin expression, create a smooth muscle cell niche and act as tendon cells for the APM. Our results reveal a functional role for basement membrane heterogeneity in tissue patterning.