Hair Follicle                                      Ch13P5-L3
(constant & inconstant portions)

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F1Ch13: The superficial portion of a follicle is represented. Above the blue arrows, the cells outlining the lumen (beneath a thin keratin layer) contain keratohyaline granules in their cytoplasm (keratiniization of a type similar to that of the surface epidermis). This portion of the channel is spoken of as the infundibulum. The canal below the blue arrows is outlined by cells that contain scant keratohyaline granules; the lumen is outlined by a thin layer of compacted, acidophilic, keratinized debris. This portion is the common canal for the hair shaft and sebum. The opening of a sebaceous duct is represented on the right side of this canal. Focally, the keratin layer along the lumen of the common canal has a corrugated quality.

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F2Ch13: This is a weakly stimulated follicle. The basic features of a mature anagen follicle are represented but in miniature; the follicle is miniaturized. Two epithelial “wings” hang down, one on each side of the follicle. They are club-shaped at their extremities as if in anticipation of sebaceous differentiation. Assuming that the wings are primitive sebaceous glands, then the green line might be accepted as a marker for the upper boundary of the isthmus. At the limits of the outer root sheath there is a yellow line; this line might be accepted as the lower limits of the isthmus. Note that the epithelium of the isthmus, having been penetrated by the epithelial wedge of the inconstant portion of the follicle, extends a short distance along the surface of the portion of the wedge formed by the inner root sheath, and the developing hair shaft. Henle’s layer; the epithelial precursors of the cuticles of the inner root sheath, and the hair shaft are not clearly defined in this stage of the developing follicle. A poorly developed outer root sheath envelopes the wedge. Yellow arrows point to Huxley’s layer. Red arrows identify dark germinative cells of the bulb. Blue arrows identify pale germinative cells of the bulb. In better developed follicles, the dark cells appear to be the precursors of cells that, by keratinizing, become the cuticle of the hair shaft. The pale cells could be interpreted as the precusors of the file of cells that will become the cuticle of the inner root sheath. From my perspective, Huxley’s layer is the precursor of keratinizing cells of both Henle’s layer and the cuticle of the inner root sheath; it is a tube that keratinizes along it inner and outer surfaces by forming, as intermediaries, two files of keratinizing cells; the inner file give rise to the cuticle of the inner root sheath, and the outer file gives rise to Henle’s layer.

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F3Ch13: In the early anagen phase, a solid epithelium column descends into the dermis; its course is marked by the fibrous sheath, usually a remnant of the travels of the inconstant portion of a prior anagen follicle. In this photo, representing a eccentric section, the hair bulb and the inner root sheath have an “arrow head” configuration. It is tempting to propose that this configuration is ideally suited, as the inner root sheath and the hair shaft lengthen, to function as a wedge. In the act, the epithelial wedge would penetrate the column of cells forming the outer root sheath (in the microcosm of follicular kinetics, this proposition embodies the principles of the Ying and the Yang). Furthermore, in its longitudinal growth, it would eventually establish continuity with the lumen of the non-cycling portion of the follicle; arriving at such a location, the travel of the non-trichogenic, inconstant portion of an anagen follicle (the inner root sheath) would have been completed.

In the plane of this section, Henle’s layer (as a layer of viable cells) appears to terminate at a level that is deeper than the termination of the cuticle of the inner root sheath (again as a layer of viable cells). In fact, Henle’s layer, Huxley’s layer, and the inner root cuticle form the wall of a tube that embraces the developing hair shaft (i.e., the root). At the superficial extremity of this tube, the trichohyalin-laden cells of Huxley’s layer terminally keratinize; at this extremity, the keratinized product of Henle’s layer, Huxley’s layer, and the cuticle of the inner root sheath all come together to form the hard leading layer at the apex of the epithelial derivatives of the    non-trichogenic components of the hair bulb. The hair shaft separates from the inner root sheath at about the same level that the trichohyalin-laden cells convert to keratinized debris. On the other hand, the hard keratin, as a product of terminal differentiation in the inner root sheath, remains attached to the adjacent epithelium of the outer root sheath up to the level of the isthmus. The level, at which this layer of hard keratin (closely attached to the outer root sheath) terminates, tends to mark the arbitrary boundary between the inner root sheath and the isthmus. From the non-trichogenic portion of the bulb, the division of pale germinative cells appears to be in continuity with “Huxley’s” layer. The division of dark cells of the non-trichogenic portion of the bulb appears to contribute the “cuticle” of the hair shaft (In this usage of “cuticle,” anatomic level along a follicle is pertinent; the “cuticle is a file of viable cells early on; these dark cells, at higher levels, keratinize in an irregular fashion but eventually form a row of non-viable cells. The non-viable cells retain the general outlines of their viable precursors (see F5Ch13).

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F4Ch13: The outline of the luminal surfaces of the sebaceous gland ducts is irregular (corrugated). Blue arrows point to an area in which the ridges are prominent. In part, this configuration allows sebum that is released by the disintegrating sebaceous cells to collect among the ridges. As the keratinized product is released into the central lumen of the common duct, sebum is carried with it.

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F5Ch13: This is a bifid follicle with the two lumens of the inconstant portion of the follicle extending to the margin of the field on the left. Actually, in this plane, the bottom lumen is clearly represented. Above the clearly defined lumen, there is, on the left, a cuticle of hard keratin (cuticle of the inner root sheath - at this level, representing the combined product of Henle’s layer, Huxley’s layer, and the cuticle of the inner root sheath). The portion of epithelium attached to this layer of keratinized debris is a layer of the inner root sheath; at this level, the epithelium is common to the two components of the bifid, inconstant portion of this follicle. At this level, nearer the top of the field, another layer of hard keratinized debris is represented. The plane of the section is eccentric for this second component; the lumen is not represented. The site where the intensely acidophilic keratin begins, also marks the beginning of the isthmus for this follicle. This is a diseased follicle with perifollicular sclerosis in the vicinity of the isthmus. Perifollicular fibrosis of this type, involving the isthmus of a follicle, is a harbinger of a form of scarring alopecia. Green arrows identify altered keratin (for this follicle in this portion, the product is parakeratotic) that identifies the isthmus. The glassy membrane is not represented at this level. At the interface between the isthmus and the outer root sheath, the alteration in the character of the keratinized product results in the detachment and desquamation of the layer of hard keratin (the pale, angulated, “plate-like,” keratinized [hard keratin] cells along the lumen to the left). 

The patterns in the so-called pilar (tricholemmal cyst) tend to recapituate those seen in the inconstant portions of the catagen hair bulb. This observation does not then imply that pilar cyst is a derivative of germinative epithelium; its patterns recapitulate those of the squamous epithelium of the regressing portion of catagen follicle below the level of the clubbed hair shaft.

In chapter 34, the patterns of a vellus hair cyst are presented.

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