F1Ch12 (R4564): This section includes only a small portion of the outer surface of the hair bulb. There is a kink in the follicle and the bulb is directed out of the plane of the section (kinks of this type correlate with kinky hair, and are a feature of the hair of Blacks). The outer root sheath is composed of uniform cells that have clear cytoplasm; they are rich in glycogen. The termination of the outer root sheath is represented. The inner root sheath is composed of what is generally characterized as three sheaths - each of the three sheaths (“layers”) has been characterized as representing a distinctive epithelium (but the keratinized product is basically identical for all three). The sheath of the hair shaft, as represented near the center of the field (blue arrows), is partially within the plane of the section. Green arrows identify Henle’s layer; bright red trichohyaline granules identify Huxley’s layer; and yellow arrows identify the “cuticle” of the inner root sheath. Red arrows identify the dark cells of the “cuticle” of the hair shaft - appearing as a file of cells at the level of the section . This terminology is open to criticism, but is accepted without question. The term, cuticle, is most appropriate when the product is hard keratin; a structural cuticle about the cortex of the hair is seen along the hair shaft only after the shaft has separated of the inner root sheath (i.e., at the terminus of the latter).
At the distal extremity of the inner root sheath, as represented in this plane, a cellular sheath (consisting of cells of the “cuticle of the inner root sheath) appears as a solid layer (area defined by 5 blue arrows) in which the nuclei are elongated and thin; in the area defined by the blue arrows, the orientation of the long axis of each of these nuclei appears to be perpendicular to the longitudinal axis of the hair shaft. The bright red cytoplasmic granules of the cells of the multi-layered zone of the inner root sheath (Huxley’s layer) are trichohyalin granules. The innermost sheath (inner root “cuticle”) is not well defined at this level. The outermost sheath (Henle’s layer)of this layer, being a single cell layer in thickness, appears as a file of small cells with pyknotic nuclei and pale acidophilic cytoplasm; it defines the boundary between the inner and the outer root sheaths. At a higher level, a single row of similar flattened cells (“cuticle” of the inner root sheath) (yellow arrows) separates the sheath of granular cells from a sheath of darker acidophilic cells (“cuticle” of the shaft) that is closely applied to the cortex of the hair shaft. The inner most structure in the above figure is the proximal end of the growing hair shaft (the root). In this area, for this example, the keratinizing cells of the hair root are still nucleated; the long axis of each thin, elongated nucleus is parallel to the long axis of the follicle. A thin layer of loosely cellular, loose fibrous tissue outlines this portion of the follicle. The external hyaline membrane that abuts upon the fibrous sheath is characterized as the vitreous, or glassy membrane.
The above is description differs somewhat from classic characterizations. The inner root sheath should be accepted as a single population of keratinizing cells with keratinization being associated with the formation of trichohyaline granules. The inner and outer layers may not be separate and distinct populations of cells; they may be the expression of terminal differentiation, with the cells of “Huxley’s” layer being the progenitors. The inner root sheath is a population of cells, derived from the population of pale cells of the hair bulb. It grows with the hair shaft as a sleeve but appears on a histologic section as a column; it has a multi-layer core of cells rich in trichohyaline granules; keratinization finds expression in the formation of a single layer of keratinizing cells on the inner and outer surface of the trichohyalin-rich column.
Guide (arrows): red - cuticle of hair shaft (dark cells); yellow - “cuticle” of inner root sheath; green - Henle’s layer; blue - en face representation of dark cells of cuticle of hair shaft.