Microtubule associated protein (MAP-2) expression defines the companion layer of the anagen hair follicle and an analogous zone in the nail unit.

Abstract

Microtubule associated protein 2 (MAP-2) is one of a group of polypeptides that are an integral component of the microtubular cytoskeletal structure of the central and peripheral nervous system. During the course of another investigation that utilized immunohistochemistry, MAP-2 expression was observed in the hair follicle, almost exclusively in the innermost layer of the outer root sheath of the anagen follicle. This innermost layer, the so-called companion layer, has unique properties that clearly distinguish it from the rest of the outer root sheath. Among these are diminished glycogenation and an intimate association with the Henle's layer, which it directly apposes. Numerous intercellular connections exist between the companion layer and Henle's layer and, in fact, the companion layer accompanies Henle's layer during its vertical ascent in the follicle. Circumferentially oriented keratin filaments have also been demonstrated between the companion layer and Henle's layer, apparently providing structural support to the inner root sheath. Experimentally, disturbances in the keratin filaments of the companion layer in animals ultimately results in destruction of the hair follicle and an alopecia. Immunohistochemical studies for MAP-2 were performed on 25 additional paraffin-embedded scalp specimens using standard techniques. Because of the parallels between the follicle and nail, MAP-2 expression in the nail unit was also investigated in three specimens. The presence of MAP-2 in the companion layer was confirmed in all cases. Intense MAP-2 expression in the companion layer begins at the B-fringe (the start of the keratogenous zone with cornification of the Henle's layer of the inner sheath) and extends to the level of the isthmus where the inner root sheath exfoliates. MAP-2 is also expressed in the upper layers of the nail matrix. The expression of MAP-2 almost exclusively in the companion layer is probably related to the unique cytoskeletal structure of this microanatomic layer. Since experimental evidence has shown that the cell cytoskeleton is important to the integrity of the hair follicle, it is probable that MAP-2 expression is also important and disturbances in its expression could play a role in the pathogenesis of some alopecias. MAP-2 may play a similar role in the nail matrix.