The microanatomy of the distal arrector pili: possible role for alpha 1 beta 1 and alpha 5 beta 1 integrins in mediating cell-cell adhesion and anchorage to the extracellular matrix

The arrector pili (AP) muscle is a small band of smooth muscle that attache s proximally to the bulge area of the pilosebaceous apparatus in the reticu lar dermis and extends up toward the epidermis. The distal anatomy of the A P and the anchorage mechanism allowing hair erection have not been previous ly described. Integrins are likely candidates mediating this attachment. Im munohistochemical techniques were used to determine the distribution of the following integrins: alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, alpha 6 and beta 1 as well as fibronectin. Frozen human scalp tissue was sectioned in traditional planes, obliquely and horizontally to visualize microanatomy in three dimensions. Histological examination revealed that the distal por tions of smooth muscle fibers splay extensively between collagen bundles of the upper dermis. Integrin subunits alpha 1, alpha 5 and beta 1 were expre ssed by the AP muscle. Analysis of the relative density of immunoreactivity in digitized sections revealed increased alpha 5 subunit expression at the extracellular matrix (ECM)-muscle interface. These data suggest that ancho rage of the AP muscle to the ECM is via alpha 5 beta 1 integrin and alpha 1 beta 1 integrin functions in muscle cell-cell adhesion. Extensive splaying of smooth muscle fibers may allow increased surface area contact between t he ECM and smooth muscle cells expressing peripherally situated alpha 5 int egrin.