Embryologic, Histologic, and Anatomic Aspects
Morphologic and qualitative differences in hair, a cornified end-product of matrical cells positioned in the center of a follicular bulb, exist among the races of Man. In general, Caucasians have the most prominent hair, Asians have the least, and Africans in between. Hairs can be divided morphologically into four major categories, namely, straight, spiral, helical, and wavy. Hairs in Asians are straight because follicles are straight, oriented as they are nearly vertical to the skin surface. Hairs in Africans spiral because follicles are curved, their base being aligned nearly horizontal to the skin surface. Different rates of growth of matrical cells along the sides of a bulb may contribute to the spiral. Hairs in Caucasians may be any of the four types, but mostly are wavy or straight.
Hair is different, morphologically and biologically, on different anatomic sites. Moreover, hairs vary in structure, rate of growth, length, and response to various stimuli. As an example of the latter, sex hormones do not affect eyebrows and eyelashes, but at puberty they influence profoundly the characteristics of pubic, axillary, facial, and body hairs.
A fetus is covered by soft, fine, lightly pigmented hairs called “lanugo” (L. lana “wool”). The fine hairs that cover most of the body of children and adults are termed “vellus” (L. vellus “fleece”). Long, coarse, pigmented hairs with a larger diameter are named “terminal hairs” and are situated on the eyebrows, eyelashes, scalp, beard, axillae, and pubes. Terminal hairs are the only type that possess a medulla consistently. Apart from a medulla, and from size overall, both vellus and terminal follicles are made up of the same two parts, i.e., an upper stationary segment, the isthmus, and a lower transient segment, the bulb and the stem. Moreover, the lower segment of both vellus and terminal follicles undergoes repeated and repeatable alterations morphologically during phases known as anagen (growing), catagen (involuting), and telogen (resting), a topic that will be elaborated on later in this chapter.
During its lifetime, a particular follicle may generate all three types of hair. A follicle on the scalp may produce a lanugo hair initially, a terminal hair later, and a vellus hair in baldness. Regardless of size, the structure of these types of follicles, as judged both by inspection grossly and examination by microscopy, is the same. With the onset of puberty and the surge of androgens that accompanies it, vellus follicles in the beard, pubic, and axillary regions become terminal follicles that generate terminal hairs. An analogous situation is seen in hirsute women in whom vellus follicles, especially on an upper lip, come to produce terminal hairs. After birth, no new hair follicles are formed in normal skin.
A hair follicle is continuous with an infundibulum, the funnel-shaped component of epidermis that reaches from the ostium at the surface of the skin above to the uppermost part of the follicle, i.e., the isthmus, below. In a longitudinal section, a mature follicle may be divided histologically (Fig. 1.23) into (1) an upper segment constituted of a single part, i.e., the isthmus that extends from the base of the infundibulum above to the place where corneocytes of the inner sheath desquamate below, and (2) a lower segment that consists of two parts, i.e., the stem, which stretches from the base of the isthmus to the end of the keratogenous zone at Adamson’s fringe, and the bulb, which is the part of a follicle that resides below Adamson’s fringe. Adamson’s fringe is the boundary between nucleated cells of a hair in the bulb of a follicle and anucleate cells of a hair in the stem of a follicle.
A hair follicle can be divided, on the basis of considerations morphologic and biologic, into two segments, namely, the isthmus of the permanent upper segment and the stem and bulb of the transient lower segment. The upper segment does not participate in the follicular cycle, whereas the lower segment does. The infundibulum is not a component of the follicle, but of the epidermis, it being virtually identical histologically to surface epidermis.
The lowest part of a follicle is named the bulb because it resembles the bulb of a tulip or an onion (Fig. 1.24). The bulk of a follicular bulb consists of matrical cells among which melanocytes are interspersed (Figs. 1.25 and 1.26). The base of a bulb encloses a follicular papilla formed of connective tissue in the shape of an inverted pinecone. When a follicle is in a growing phase (anagen), a single capillary, which is surrounded by abundant mucin, traverses each papilla, the latter structure being thought to organize, direct, and maintain the function of a follicle. The follicular papilla, through a narrow outlet at the distal end of the bulb, is continuous with the connective tissue (perifollicular) sheath that envelops the outer sheath of a follicle; the perifollicular sheath terminates at the level of infundibular epidermis. The arrangement of the connective tissue around the infundibulum is like that of the papillary dermis and is different from that of the perifollicular sheath, which has two components, an outer one with bundles of collagen arranged longitudinally and an inner one with bundles of collagen that encircle the follicle. Collagen in the perifollicular sheath, like that in the papillary dermis, is predominantly Type III. Scattered in the perifollicular sheath are fibrocytes that are responsible for producing it and capillaries that are aligned parallel to bundles of collagen in the outer component of it. A “glassy” basement membrane, periodic acid Schiff positive, lies between the follicular papilla and the perifollicular sheath of fibrous tissue on one hand and the follicle itself on the other. On the inner side of the basement membrane and juxtaposed to it are pale or clear epithelial cells of the outer sheath of the follicular bulb. Those clear cells that abut the basement membrane are columnar and arrayed in a palisade. Nuclei of them are positioned at the distal part of the cell, that is, the part farthest from the basement membrane. During anagen, the outer sheath serves as a sleeve for the inner sheath and hair, which, because of the rapid turnover of matrical cells in the middle of the bulb, move upward with far greater celerity than do cells of the outer sheath.
Three-dimensional view of the bulb and part of the stem of a hair follicle.
A. Bulb and part of the stem of a terminal follicle in anagen. That this follicle was situated on the scalp can be inferred from how deep the base of it is positioned in the subcutaneous fat. Note that the bulb ends at Adamson’s fringe, where cells of the future hair show signs of having cornified completely. B. Boundary of bulb and stem. The boundary is the distal margin of the keratogenous zone, i.e., Adamson’s fringe, where cells of the future hair lose their nuclei and become cornified fully. C. Lower part of bulb. The inferior aspect of a bulb of a follicle consists mainly of matrical cells that mature into cells of the outer sheath, the inner sheath, and the hair. (From AB Ackerman, H Jacubovic. In: Moschella SL, Hurley HJ, eds. Dermatology. 3rd ed. Philadelphia: W. B. Saunders, 1992.)
Strikingly dendritic melanocytes in the upper half of a follicular bulb. (×352)
Matrical cells of a follicular bulb differentiate along seven separate pathways (Figs. 1.27 and 1.28). From outside inward, those lines of differentiation are as follows: (1) outer sheath; (2) Henle’s layer of the inner sheath, one cell thick with striking, brightly eosinophilic trichohyalin granules and the first to cornify; (3) Huxley’s layer of the inner sheath, two cells thick and characterized by numerous trichohyalin granules; (4) cuticle of the inner sheath, one cell thick; (5) cuticle of the hair made up of a single layer of squames that wrap in imbricated fashion around the shaft and interdigitate with cornified cells of the cuticle of the inner sheath; (6) cortex of the hair that comprises the bulk of it; and (7) medulla of the hair, the last to cornify and present only in terminal hairs.
Cells of the follicular matrix differentiate along seven separate lines. A. Schematic view, from outside inward, shows outer sheath; Henle’s layer, only one cell thick and the first to cornify; Huxley’s layer, two cells thick and characterized by brightly eosinophilic-staining trichohyalin granules; cuticle of inner sheath; cuticle of hair; cortex of hair; and medulla of hair. A follicular papilla enclosed largely by the bulb is continuous, through a narrow outlet, with the connective tissue sheath that envelops the follicle. B. Photomicrograph of follicular bulb and papilla. (×374)
Cross section through different levels of the bulb and stem of terminal follicles in the subcutaneous fat. (×176)
Matrical epithelium of the follicular bulb consists of a pool of undifferentiated cells that have intense metabolic activity. Those cells have crowded, round, pale-staining, finely stippled monomorphous nuclei that display a prominent nucleolus. They are very different from germinative cells whose nuclei are crowded equally but are much smaller and darker, and devoid of a discernible nucleolus. Matrical cells derive ultimately from aboriginal cells of the follicular germ, those germinative cells in an embryo giving rise to an entire follicle (as well as to infundibular epidermis and to apocrine and sebaceous units) and, in postnatal life, to the inferior segment of follicles as those structures develop in anagen anew. Matrical cells of follicles on a scalp turn over in about 39 hours, this rapid rate being evidenced by many mitotic figures in nuclei of them, greater than that of any normal tissue with the possible exception of the bone marrow and of the testes. As matrical cells mature into those of outer sheath, inner sheath, and hair, the latter two of those differentiated cornified cells move upward together, gliding past the relatively passive outer sheath. The flattened cells of the cuticle of the inner sheath and the equally flattened cells of the cuticle of the hair overlap one another like interlocking shingles, thereby ensuring that the inner sheath and the hair ascend together at the same pace.
The boundaries of the follicular bulb, which actually has the shape of a pear, are the base of a follicle and the summit of the high-arched curve of “keratogenous zone” that ends at the discrete border of Adamson’s fringe, the site at which Huxley’s layer loses its trichohyalin granules and begins to cornify in orthokeratotic manner. From outside in, the follicle at the bulb consists of pale and clear cells of the outer sheath, trichohyalin-containing cells of the inner sheath, and nucleated corneocytes of the future hair. It is at “Adamson’s fringe” where cornification of viable keratocytes is recognizable first; there, cells of the inner sheath lose their trichohyalin granules and become bluish gray, compactly arranged corneocytes, and cells of maturing hairs lose their nuclei and become fully cornified hair. Just below Adamson’s fringe often appears a marker helpful morphologically to discerning a site crucial to a follicle biologically, namely, artifactual clefts that form between an inner sheath about to cornify and a hair about to do the same.
Although the process of keratinization proceeds apace in the pyriform bulb of a follicle, complete cornification of inner sheath and hair does not occur there, but at the stem. For this reason, dermatophytes, dependent on cornified cells in order to live, never descend a follicle below Adamson’s fringe.
The stem, the longest section of a terminal follicle in anagen, extends from the summit of the bulb to the base of the isthmus. Throughout most of its course, the stem consists, from outside in, of an outer sheath, an inner sheath, and a hair. When the inner sheath desquamates and is lost completely just above the uppermost part of the stem, the isthmus comes into being, i.e., that portion of the outer sheath able to cornify independently because it no longer is suppressed by the compressing effects of a rigid, cornified inner sheath. The periphery of the outer sheath at the stem is made up of cuboidal keratocytes with abundant pink cytoplasm, not of columnar keratocytes with abundant clear cytoplasm like those of the bulb. The outer sheath, in general, acts as a sleeve for the inner sheath, prior to the disappearance of that cornified casing at the isthmus. Whether the innermost aspect of the outer sheath at the bulb and the stem truly consists of a “companion cell layer,” as some authors have proposed, is moot.
The isthmus is delimited by desquamation of corneocytes of the inner sheath below and entrance of the sebaceous duct at the base of the infundibulum above. “Isthmus” means a narrow strip that connects two larger masses and, in cutaneous histology, the term is applied aptly to that short narrow strip of a follicle that is continuous with the stem of the follicle below and with the infundibulum of the epidermis above. The isthmus is distinctive morphologically, fashioned as it is of epithelial cells arranged in a pattern unique to normal skin, except for that in follicles during the involutional phase of their cycle (catagen) when an appearance indistinguishable from that of isthmic epithelium is assumed by them. That epithelium, both of the isthmus and a follicle well advanced in catagen, is characterized by (1) a basal layer, (2) a spinous zone that is not truly “spinous” because intercellular “spines” are barely detectable between cells replete with pink cytoplasm, (3) absence of a granular zone, and (4) a prominent, brightly eosinophilic cornified layer whose cells are arranged compactly and the surface of which is decorated by corrugations. The isthmus, which is the uppermost part of the follicle, lacks an inner sheath, but is a conduit for a hair en route from its origin as matrical cells in a bulb to the ostium of an infundibulum—and beyond it.
Bulges from the lower half of the isthmus and the uppermost part of the stem are protrusions of the follicle to which muscles of hair erection are tethered by a tendon of fibrous tissue. Another important anatomic boundary alluded to previously is that marked by the entry of a sebaceous duct; that site demarcates the isthmus from the infundibulum, that is, follicular from epidermal epithelium.
An infundibulum, as its name denotes, has the shape of a funnel. The upper two thirds of it consist of the cone of the funnel, whereas the lower one third, sometimes referred to inaccurately as the infrainfundibulum, is formed by the narrow tube of the funnel. The infundibulum is not part of the outer sheath, but is integral to epidermis; histologically, it is not epidermoid, as often is said, but epidermal. Although the epithelium of the lower tubular part of an infundibulum differs from that of the upper cone-like portion by having walls parallel to one another, a slightly thinner granular zone, and fewer corneocytes, for practical purposes, infundibular epidermis is identical morphologically to surface epidermis, with which it is continuous.
The length of infundibula varies greatly on different anatomic sites, being particularly long and dwarfing vellus follicles on a face, and being much shorter on a leg, for example. On one anatomic site, infundibula are unaffiliated with a follicle, namely, the nipple where a lactiferous (apocrine) duct enters directly into infundibula.
Cornified cells of infundibula, especially those on the scalp, face, and upper part of the trunk, are host normally to a variety of microorganisms, among them being bacteria, i.e., Staphylococcus epidermidis and Propionibacterium acnes; yeasts, i.e., Pityrosporum; and a mite, Demodex folliculorum (Fig. 1.29). That potpourri of microorganisms, not present in the cornified layer of normal surface epidermis, in conjunction with sebum, may produce chemicals that insects find attractive; not uncommonly, an infundibulum is the actual site of an arthropod “bite.”
Normal inhabitants of an infundibulum on a face. A. Spores of Pityrosporum. (×623) B. Mite of Demodex folliculorum. (×670)
What generally is designated “folliculitis” really is infundibulitis, the infundibulum rather than the follicle being the epithelium involved. Infundibulitides usually are suppurative, e.g., noninfectious, as in acne vulgaris, and infectious, as in staphylococcal pustulosis. In those conditions, infundibula are filled with neutrophils. Much less common is spongiotic infundibulitis, e.g., so-called infundibulofolliculitis, in which spongiosis mediated by lymphocytes is present in the wall of infundibula. Different from spongiotic infundibulitis, but associated nonetheless with lymphocytes in an infundibulum, is so-called follicular mucinosis, a distinctive pattern of cutaneous epithelium in which copious quantities of mucin are deposited in infundibula especially, but also at times in sebaceous lobules and in follicular epithelium. “Follicular” mucinosis is more accurately termed “epithelial mucinosis”; the epithelium most often affected is epidermal (infundibular), not follicular. Epithelial mucinosis is the sine qua non for diagnosis of alopecia mucinosa, which is but one of many variants morphologic of mycosis fungoides. Epithelial mucinosis also is encountered often in “folliculitis” with eosinophils (badly termed “eosinophilic folliculitis,” which, in actuality, is infundibulitis with eosinophils) and especially in that condition designated Ofuji’s disease.
The following summarizes matters morphologic that are a manifestation of considerations biologic: In sections oriented vertically, the walls of the stem appear to be parallel to one another until they near the isthmus, where they narrow slightly. The walls of the isthmus, also parallel to one another, become continuous with the walls of the tubular segment of infundibular epidermis that then flare to become the lateral margins of the upper two thirds of that epithelium. When the outer sheath is released from the compressing effects of a rigid cornified inner sheath at the advent of the isthmus, the pink cells of the outer sheath, now freed, are able to cornify independently—and they do. The newly emancipated isthmic epithelium produces a layer of compactly arranged corneocytes whose luminal border is distinctly notched, as is the surface of the viable epithelium beneath it. Therefore, the outer sheath extends all the way from the bottom of the bulb to the base of the infundibulum. The infundibulum, being epidermis, is continuous with surface epidermis. Soon after a follicle enters the involutional stage of a cycle, i.e., catagen, changes occur in the lowest part of the outer sheath that cause that epithelium to resemble the outer sheath at the isthmus. The inner surface of the outer sheath of a follicle in catagen has a notched appearance, just like that of the outer sheath at the isthmus. In brief, findings histologic of outer sheath epithelium at the isthmus and in follicles in the midst of catagen are just like one another, probably because in both situations an outer sheath has retracted from an inner sheath.
In contrast to the outer sheath of a growing follicle that traverses the distance from the base of a bulb to the base of an infundibulum, the inner sheath extends for a shorter distance, i.e., from the apex of a bulb to the isthmus, i.e., the length of the stem. In the act of desquamation, the inner sheath not only announces its own demise but also creates the isthmus, which is the uppermost portion of the outer sheath devoid of a contiguous inner sheath. In some follicles, paradoxically, an inner sheath does not desquamate completely at the isthmus but continues upward for a very short distance into the isthmus.
What usually is referred to as perifolliculitis really is peri-infundibulitis, affecting as it does the infundibulum mostly. At times, however, infiltrates of inflammatory cells not only surround the infundibulum, but extend along the entire course of a follicle. When a peri-infundibulitis/perifolliculitis is long-lasting, as is the case for some examples of lichen planus (lichen planopilaris) and discoid lupus erythematosus, follicles may be destroyed by the effects of products of lymphocytes, the result being permanent alopecia. One inflammatory type of alopecia is characterized by infiltrates of lymphocytes solely around the follicular bulb, i.e., alopecia areata and its variants, to wit, alopecia totalis and alopecia universalis.
The flattened cells of the cuticle of a hair overlap one another in imbricated fashion around the cylindrical cortex. The free edge of those cuticular cells point upward, thereby enabling them to interlock with cells of the cuticle of the inner sheath, which point downward. The cornified cuticles of the two adjacent structures remain locked, an arrangement that enables the hair to be held tightly in a follicle. The cells of the medulla, cortex, and cuticle cornify in the absence of keratohyaline granules (unlike the situation in infundibular and surface epidermis) and of trichohyalin granules (unlike the circumstance in the inner sheath).
Just as epidermal keratocytes incorporate melanosomes by clipping off the ends of dendrites of melanocytes by a mechanism termed apocopation, matrical cells of a follicle do the same to dendrites of melanocytes positioned in the bulb. Color of hair depends primarily on the amount and distribution of melanosomes in those filaments of corneocytes. Dark brown or black hair contains large, ellipsoidal, markedly melanized eumelanosomes, whereas red hair houses spherical pheomelanosomes. In blond hair, melanocytes in bulbs produce relatively few or incompletely melanized melanosomes. In gray and white hair, melanocytes in bulbs are reduced in number and melanosomes are poorly melanized. Cells of the cuticle of a hair do not contain melanosomes. Although melanocytes in considerable numbers are interspersed among matrical cells in the bulb, few melanocytes are found along the course of the stem and the isthmus; in the infundibulum, melanocytes are disposed in a manner like that of surface epidermis.
The growth of hair is cyclical (Figs. 1.30 –1.38). The three phases in that cycle are (1) growing (anagen), (2) involuting (catagen), and (3) resting (telogen). The follicle beneath the isthmus, i.e., bulb and stem, is transitory in the sense that it disappears during the involutional stage (catagen) of the follicular cycle and re-forms during the growth phase (anagen) of it. In contrast, the part of the follicle that is stationary, i.e., the isthmus, is not involved at all in the follicular cycle. If a follicle can be divided histologically into an isthmus (upper segment) and a stem and bulb (lower segment), then it also can be conceived of biologically, i.e., functionally, as consisting of an upper fixed isthmus and a lower mobile stem and bulb. The isthmus, which histologically is an integral component of the outer sheath, does not join the rest of the outer sheath during the follicular cycle in a to-and-fro migration along a fibrous track, but is a mere bystander of movements that take place below it. The fibrous track consists of converged perifollicular fibrous sheath, that meeting being a consequence of retreat upward of the lower segment of a follicle during catagen.
Phases in the cycle of a follicle recapitulate events during formation of a follicle in an embryo. A. Anagen begins with renewal of the intimate relationship between a follicular papilla and germinative cells situated at the base of an isthmus. B. As anagen proceeds, matrical cells generate an outer sheath, inner sheath, and hair. C. Mature anagen follicle consists of upper and lower segments. D. During catagen, the entire lower segment of a follicle shrivels into a thin cord of epithelial cells that is followed upward along a fibrous track by a follicular papilla. E. During telogen, an ill-defined follicular papilla reposes immediately beneath the isthmus in readiness for initiating the follicular cycle anew.
Fully developed anagen. The bulb of a follicle at the prime of anagen envelops largely a follicular papilla and consists mostly of matrical cells, which mature into hair, inner sheath, and outer sheath.
Early catagen. The follicular bulb no longer is extant, and the follicular papilla has become contiguous with the flattened base of the inferior segment of the involuting follicle. A markedly thickened basement membrane surrounds the atrophic lower segment of the follicle and separates it from the perifollicular sheath.
Well-advanced catagen. The lower segment of the involuting follicle now consists of an effete column of epithelial cells surrounded by a strikingly thickened, corrugated basement membrane. At the base of the column resides an ill-formed follicular papilla.
Far-advanced catagen. Epithelial cells at the base of a shrunken column of epithelial cells, that contracted column representing the residuum of the lower segment of a follicle, form a pincer around a well-defined follicular papilla. The lower segment is surrounded by a markedly thickened, corrugated basement membrane. Below the involuting follicle is a fibrous track that serves as a kind of railroad track along which the events of the follicular cycle proceed.
Late catagen. The remnant of follicular epithelium at the end of catagen resembles that of normal isthmus. The follicular papilla is not recognizable as a discrete structure, but only as scattered fibrocytes at the base of a follicle now nearing its resting stage.
Telogen. A follicle at rest consists only of an upper segment, namely, an isthmus. Columnar cells situated at the periphery of the isthmus are aligned in a palisade. At the base of the isthmus is what remains of a follicular papilla, to wit, a few scattered fibrocytes. Anagen will commence when a revivified papilla induces germinative cells at the base of the isthmus to form a discrete germ. The bowed cords of undifferentiated epithelial cells that emanate from the junction of infundibulum and isthmus are mantles, i.e., anlagen of sebaceous glands and ducts in prepubescents and residua of them in senescents.
Very early anagen. At the base of the isthmus is a new follicular germ and beneath it sits an incipient follicular papilla. The germ-like structure, like the germ of the infundibuloapocrine-sebaceous- follicular unit in an embryo, is characterized at its periphery by columnar cells aligned in a palisade and, in its center, by germinative cells whose nuclei are crowded and monomorphous. A mitotic figure is present above the basal layer. Note that melanocytes, cells with small dark nuclei surrounded by a cleft, are relatively equidistant from one another in the basal layer of the newly formed germ. This germ, however, unlike the one in the embryo, gives rise only to the lower segment of a follicle.
Early anagen. Continuous with the base of the isthmus is a new elongated germ whose arc-like base is contiguous with a discrete follicular papilla. Soon, the base of the evolving follicle will envelop partially the follicular papilla, an arrangement that is seen most dramatically in a fully formed anagen follicle in which the papilla is enclosed nearly entirely by the bulb.
For many years, speculation has abounded concerning the origin of cells responsible for formation of a new follicle at the end of telogen. It long was an article of faith that matrical cells were primordial in that regard. During the last decade of the 20th century, the “bulge-activation hypothesis” gained acceptance, the supposition being that the bulge of a follicle, which serves as a site for attachment of a muscle of hair erection, serves also as a reservoir for stem cells that, at the outset of anagen, give rise to a new inferior segment of a follicle. Our studies of sections of tissue of normal follicles, cut in both vertical and horizontal directions, have led us to a different conclusion. For one, the bulge is very different structurally from that pictured and described by proponents of the “bulge-activation hypothesis,” i.e., it is not a single knobby protuberance that emanates from a discrete locus on one side of a follicle, but rather numerous finger-like projections that emerge along more than half the circumference of it (Fig. 1.39). For another, bulges are irrelevant to the follicular cycle; the cells that become germinative, form a follicular germ, and soon transform into matrical cells en route to producing a new lower segment in anagen, derive from cells left behind at the base of the isthmus at the end of catagen, those cells lying dormant throughout telogen, only to be reawakened by a call from mesenchymal cells that reside immediately below and that come together to form a new follicular papilla. Each of the bulges is attached to a fascicle of smooth muscle whose sole purpose is to enable hairs to become erect (Figs. 1.40 and 1.41).
Bulges of a follicle. Protrusions of isthmic and stem epithelium serve as sites of attachment for fascicles of muscles of hair erection. These epithelial protuberances encircle much of the follicle. Bulges are irrelevant to the follicular cycle.
Bulges of a follicle in vertical section. Bulges serve only as sites of attachment for fascicles of smooth muscle, i.e., those of hair erection; they are not reservoirs for cells that eventuate in the lower segment of a follicle in anagen.
Bulges of a follicle in vertical section. Bulges, which are protrusions of isthmic and stem epithelium, function as sites of attachment for muscles of hair erection, but in a random section, such as this one, muscles of hair erection may not be seen to connect with them.
Unlike certain animals whose coat of hair is shed in synchronous waves, hairs in humans normally are lost randomly and inconspicuously because adjacent follicles are in different phases of the follicular cycle, most of them being in anagen. An intriguing feature of the follicular cycle is differences in interval of time among the growing, involuting, and resting phases. Hairs in different regions of the body spend different amounts of time in anagen, and the result of those differences is variations in length of hairs. Scalp hairs, for example, grow for about 3 to 10 years, involute over a period of approximately 3 to 4 weeks, and rest for nearly 3 to 4 months. In healthy young adults, at least 85% of all follicles on a scalp, at any moment, are in anagen. Of the approximately 100,000 follicles present on the average scalp, at least 70 to 100 telogen hairs are shed normally each day. As a rule, the growing phase of follicles (and, therefore, of hairs) on the eyebrows, trunk, and extremities does not exceed 6 months, and the duration of the resting phase is roughly half that length of time. When hairs are extracted with force manually, the root of an anagen hair is noted to be deeply pigmented, whereas the bulbous tip of a telogen hair is unpigmented (Fig. 1.42). The clubbed appearance of such a hair is the result of thickening at the base consequent to adherence of the inner sheath to the hair itself. Whenever a hair is yanked from a follicle in anagen, that follicle goes into catagen immediately.
The root of an anagen hair is pigmented and surrounded by a translucent inner sheath, in contrast to the base of a telogen hair, which is unpigmented and is enveloped by an inner sheath.
Although scalp hair does not perform a “vital” function in humans, it does serve as an ornament for sexual attraction. Too much or too little hair and abnormal types of hair can be sources of concern, discomfort, and anxiety for both men and women. In addition to its decorative value, however, hair screens the nasal passages from irritants, protects the scalp from the sun’s rays, shields, as brows, the eyes from sunlight and drops of sweat, and reduces loss of heat in cold weather. It may help, too, to reduce friction in intertriginous areas and contribute to perception of tactile stimuli.