Multiunit arrector pili muscular structure as a variation observed by using computer-based three-dimensional reconstruction.

The arrector pili (AP) muscle was recently suggested to comprise one AP muscular unit inserted into all the hair follicles contained within the follicular unit (FU). This study investigated the presence of variations in AP muscular units associated with the FUs. Human cadaver scalp skin specimens were serially sectioned and stained with Masson's trichrome. Three-dimensional reconstructions of the serially sectioned images were analyzed on a computer, which revealed two types of variation in 20 FUs, one of which was the multiunit AP muscular structure, in which the AP muscular unit was inserted into two or more FUs.