Cortical Cells

Cortex cells come from the concentric ring of germ cells localized in the bulb immediately above the apex of the papilla (Figure 3). Their successive keratinization occurs in the keratinization zone (Figure 4). In the lower segment of the keratinization zone, these spindle- shaped cells produce cytoplasmic filaments that are parallel with both the long axis of the cell and the hair follicle. Intense protein synthesis is evidenced by the occurrence of large numbers of polysomes and by strong nucleolar and cytoplasmic staining of RNA.19 The filamentous material is generally high in molecular weight (45,000 to 60,000 daltons) and low in sulphur content; it also has between 30% and 60% helical content (as measured by optical rotary dispersion or circular dichroism).These materials aggregate into dense alpha-keratin fibrils that have no obvious connection with the tonofibrils.

Matrix material forms a bed in which the filaments are arranged in an organized fashion. It is thought that these two types of materials are connected by disulfide bonds.

The matrix material appears to be (1) much more heterogeneous, (2) lower in molecular weight, and (3) of consistently higher sulphur content than that found in the filaments. Halfway up the keratinization zone, tonofibrils begin to increase in number, and the rate of protein synthesis decreases. The amount of RNA diminishes and finally disappears at the distal end of the keratinization zone. The quantities of cysteine and phospholipids increase as an apparent consequence of cell membrane degradation. During cytolysis, nuclei lose their DNA, mitochondria and ribosomes degenerate, and incomplete nuclear-membrane structures are left behind in the cytoplasm. Above the keratinization zone, cysteine is converted to cystine, and the cell membrane becomes thicken The diameter of the fully keratinized hair decreases by 25% because of (I) the loss of water that results from the permeability of the plasma membrane and (2) the contraction of the keratin complex. The fully keratinized, dead cortical cells retain a membranous nuclear outline (nuclear ghost) that persists into the hair shaft.  At this level, the prevalent -SH groups in the pre-keratin are replaced by S-S bonds.


Hair Cuticle

Hair cuticles originate from primordial bulb cells that contain amorphous cytoplasm granules. The cuticular cells elongate in the suprabulbar region and become flattened (Figure 4). During differentiation, the cells increasingly overlap. Tonofibrils and desmosomes are present, but no alpha-keratin fibrils are observed. During hardening and keratinization, dense cytoplasmic granules are visible, and cystine disulfide groups are detectable. These groups form a matrix rather than fibrillar protein structures. The cuticular cells contain no fibrils, and the cystine matter in their cytoplasm is amorphous. The overlapping cells of the cortex’s cuticle are directed outward, and they interdigitate with the cuticular cells of the inner root sheath.

The cells of the hair medulla, cortex, and cuticle all cornify without prior formation of keratohyalin granules or trichohyalin granules.


Inner Root Sheath

The inner root sheath consists of three layers: the cuticle, Huxley’s layer, and Henle’s layer The cuticle is one cell-layer thick, the thickness of Henle’s layer is one to two cells, whereas Huxley’s layer is several cells deep (Figure 4). All three layers are formed from the peripheral mass of matrix cells in the hair bulb (Figure 3), and they undergo differentiation and hardening at different rates. These changes occur first within Henle’s layer, then within the cuticle, and finally within Huxley’s layer. The final stage of differentiation involves the disintegration of the nucleus while other organelles and the trichohyalin become diffusely distributed as dense materials between keratin filaments. Complete hardening and differentiation occur in the inner root sheath before they occur in the layers of the developing hair. The hardened regions of the medulla and inner root sheath strongly indicate the presence of citrulline whereas trichohyalin is no longer demonstrable.

When stain tests are used to detect the presence of arginine, the trichohyalin stains intensely. During the final stages of differentiation, some of the protein-bound arginine residues of trichohyalin are converted into protein-bound citrulline of the hardened proteins. This is particularly evident in the cuticle of the inner root sheath.

The junction between the outer root sheath and the Henle layer is maintained by desmosomes and gap-junctions; and at the end of differentiation, this junction is maintained by intercellular cement and interdigitation between cells. Upon maturation, the inner-root-sheath cells deposit amorphous intercellular material and cause thickening of the plasma membranes. Cells shrink during keratinization, and the mature inner root sheath becomes a rigid cylindrical tube that surrounds the soft, ascending hair structure. The primary function of the inner root sheath is to shape the hair contained within it. Because the cuticles of the hair and the inner root sheath are closely apposed, the fully keratinized hair assumes the shape of the inner root sheath. At the level of the follicular canal, desmosomal contacts between adjacent cells begin to break; and the cells, either singly or in groups, are shed into the follicular canal (Figure 5).


Outer Root Sheath

The outer root sheath surrounds the hair follicle (much like a sleeve), is several layers thick, and is continuous with the epidermis (Figure 4). It has two characteristic proliferation zones: (1) in the bulb and (2) in the basal layer of the epidermis. Two layers of cells surround the bulb; and during formation of the anagen follicle, vertical upward growth predominates. The outer layer of the cell is germinative and continuous with the epidermal basal cells, and differentiation occurs by the horizontal movements of cells from the basal layer of the outer root sheath to the center of the follicle.

Subdivision of the two proliferative zones reveals that the cells are significantly different: (1) those cells derived from the bulb are cylindrical and their long axis parallels the direction of hair growth and (2) those cells derived from the basal layer are irregularly shaped, and their cytoplasm contains many vacuoles. The outer-root-sheath cells nearest Henle’s layer flatten and undergo autolysis. The exact fate of these cells is not known; however, movement toward the surface probably occurs, and they are probably shed into the follicular canal along with the inner root sheath (Figure 5).

Keratinization of the outer root sheath occurs in those areas of the hair follicle where it is not apposed to the inner root sheath.These areas are (1) in anagen hair, located between the insertion of the hair erector pili muscle and the opening of the sebaceous duct (Straile’s zone of sloughing) and (2) in catagen hair the sac of epithelium that surrounds its lower end after the inner root sheath has disappeared. This process is called trichilemmal keratinization, and it is the end product of the outer sheath; therefore, it is not derived from the hair matrix but from stratified squamous epithelium, which is transformed into nonnucleated keratinized cells without forming a keratohyalin layer. In catagen hair a trichilemmal sac surrounds the lower end of the dying hair shaft, and there it forms the club of telogen hair.  Also in catagen hair as the outer root sheath undergoes trichilemmal keratinization, it converges on and occasionally fuses to the cortex of the remaining hair. This “brush” consists of keratinizing cells of the outer root sheath, which becomes elongated rather than flattened similar to the zone of sloughing of the anagen follicle. The function of the outer root sheath is not known.