Specimens of skin utilized in this study came from various mammals, Bovine (Holstein), canine (standard poodle, rottweiler, German shepherd dog), equine (quarter horse and thoroughbred), feline (domestic short hair), porcine (mixed breed) and ovine (mixed breed) skin were obtained from either recently euthanized animals or from biopsy specimens submitted to the Animal Health Diagnostic Laboratory. Euthanized animals had a normal pelage at the time of death. Specimens of rat (Sprague-Dawley; Cr1:CD(SD)BR) skin came from freshly euthanized animals that had been part of a study that concerned toxicologic effects on the lung. Mice (Swiss) skin samples came from freshly euthanized 21 day old animals that served as untreated controls. Murine tissues were compared with histologic sections taken from normal skin of 8 mice (C57BL/6J) in each of the following categories: 1–7 days old (neonates), 20–24 days old (weanlings), 50–65 days old (young adults) and greater than 90 days old (adults). Because muscles of hair erection in haired mammals are largest over the dorsal lumbar region, our observations in all of the species examined were made on skin taken from that anatomic site.

Tissue freshly obtained from each species was fixed in 10% neutral buffered formalin, except for samples from the eight mice that were fixed in Tellyesniczky-Fekete solution. Tissues were trimmed longitudinally and transversely, and embedded cut side down. For each block, 10 serial sections were made and stained with hematoxylin and eosin. If those sections did not include complete insertion of a muscle of hair erection during the period between early anagen and late telogen, 10 more sections were cut serially. This process was repeated until sections were obtained that, throughout the course of the follicular cycle, showed morphologic aspects of the isthmus and of the arrector pili muscle.