Acta Histochem. Cytochem. 39 (4): 113–123, 2006 doi:10.1267/ahc.06004 Localization of Myosin and Actin in the Pelage and Whisker Hair Follicles of Rat Kiyokazu Morioka1, Toshiyuki Matsuzaki2 and Kuniaki Takata2 1EM Laboratory, The Tokyo Metropolitan Institute of Medical Science, Bunkyo-ku, Tokyo 113–8613, Japan and 2Department of Anatomy and Cell Biology, Gunma University Graduate School of Medicine, Maebashi, Gunma 371–8511, Japan Received February 2, 2006; accepted June 26, 2006; published online July 27, 2006 The combined effects of myosin II and actin enable muscle and nonmuscle cells to generate forces required for muscle contraction, cell division, cell migration, cellular morphological changes, the maintenance of cellular tension and polarity, and so on. However, except for the case of muscle contraction, the details are poorly understood. We focus on nonmuscle myosin and actin in the formation and maintenance of hair and skin, which include highly active processes in mammalian life with respect to the cellular proliferation, differentiation, and movement. The localization of nonmuscle myosin II and actin in neonatal rat dorsal skin, mystacial pad, hair follicles, and vibrissal follicles was studied by immunohistochemical tech- nique to provide the basis for the elucidation of the roles of these proteins. Specificities of the antibodies were verified by using samples from the relevant tissues and subjecting them to immunoblotting test prior to morphological analyses. The myosin and actin were abundant and colocalized in the spinous and granular layers but scarce in the basal layer of the dorsal and mystacial epidermis. In hair and vibrissal follicles, nonmuscle myosin and actin were colocalized in the outer root sheath and some hair matrix cells adjoining dermal papillae. In contrast, most areas of the inner root sheath and hair matrix appeared to comprise very small amounts of myosin and actin. Hair shaft may comprise significant myosin during the course of its keratinization. These results suggest that the actin-myosin system plays a part in cell movement, differentiation, protection and other key functions of skin and hair cells. Key words: myosin, actin, hair, skin, rat I. Introduction The skin of most mammals has hair that grows to a length of several millimeters and sometimes to more than 1 cm in the course of a month. This means that there are thousands of hair cells, whose diameter and height may be 10 µm, that must be generated in a hair bulb each day. The newly produced hair epidermal cells differentiate and form at least three inner root sheath layers, as well as the hair cuti- cle, hair cortex, and medulla in the follicle [18, 20]. This busy, active process requires the rapid replacement of basal cells by the next group of dividing cells so as to maintain a constant rate of hair growth as well as the voracious uptake Correspondence to: Dr. Kiyokazu Morioka, EM Laboratory, The Tokyo Metropolitan Institute of Medical Science, 3–18–22 Honkoma- gome, Bunkyo-ku, Tokyo 113–8613, Japan. E-mail: morioka@rinshoken.or.jp not only of the necessary nutrients but also of hormone-like regulatory substances. In the skin epidermis, the spinous layer consists of amorphous cells with protrusive membrane structures [7, 16]. As the skin and hair cells that are under- going differentiation are sandwiched between a highly proliferative cell mass and a keratinized hard wall, they require a means of resisting the resultant compression. This situation suggests that a force-generating mechanism, such as that provided by the combination of nonmuscle myosin and actin [15] to achieve cell movement, facilitate changes in cell shape, and reinforce the cytoskeleton, may be at work in the skin and hair follicles. Recent advances in the study of the myosin superfamily have shown that it consists of at least 18 classes; one of these, the class II conventional myosin group in humans, contains the products of 16 genes, including 6 skeletal mus- cle, 2 cardiac muscle, 1 smooth muscle, and 3 nonmuscle myosin heavy chains [3, 9]. Herein, we focused on the non- © 2006 The Japan Society of Histochemistry and Cytochemistry