The calcium-binding protein calretinin is a marker of the companion cell layer of the human hair follicle

Summary Background Ultrastructural studies of the hair follicle show that the outer root sheath (ORS) does not consist of a homogeneous cell population. The innermost cell layer of the ORS, also called the companion layer, is a single cell layer closely associated with the Henle layer of the inner root sheath. Objectives To describe the immunohistochemical expression of calretinin, a calcium‐binding protein, in the human hair follicle. Methods Immunohistochemical studies using two different antisera to calretinin were performed in paraffin‐embedded and in frozen scalp specimens using standard techniques. Results Calretinin immunostaining was consistently and specifically found in the companion cell layer of hair follicles. Conclusions These findings provide further evidence to support the notion that the companion layer is not only morphologically, but also immunohistochemically, different from the other cells of the ORS.