of the AmeRiCaN A.CaDemy OF JOURNAL

DerMaTOLOGY VOLUME 25

Continuing

NUMBER 1 PART 1 JULY 1991

medical education

Hair anatomy for the clinician Leonard C. Sperling, MAJ, MC, USA Washington, D. C. The rational evaluation of hair disorders requires familiarity with follicular anatomy. Hair structure can be easily examined by studying clipped hair shafts, entire hairs gently pulled or forcibly plucked from the scalp, and scalp biopsies (sectioned vertically or transversely). Anatomic features will be different depending on whether a given hair is in the anagen, catagen, or telogen phase. Follicle size will also vary, from the minute vellus hair to the long, thick terminal hair. Each follicle can be divided into distinct regions-bulb, suprabulbar zone, isthmus, and infundibulum. Activity growing (anagen) hairs are characterized by a hair matrix surrounding a dermal papilla; inner and outer root sheaths are present and well developed. A catagen hair can be identified by its markedly thickened vitreous layer and fibrous root sheath, which surrounds an epithelial column; above this column, the presumptive club forms. A telogen hair is distinguished by its fully keratinized club, which is surrounded by an epithelial sac. Below this lies the secondary hair germ and condensed dermal papilla, waiting for the mysterious signal that initiates a new life cycle. (J AM ACAD DERMATOL 1991;25:1-17.)

Despite the technologic revolution of the past few decades, the human hair follicle remains a mysterious and intriguing structure. Some of the most basic questions about hair growth and hair disease remain unanswered. In what way do the dermal papilla and the hair matrix interact to regulate hair growth? What signals the end of anagen and the follicle's odyssey through catagen into telogen? How do androgenic hormones stimulate follicular hypertrophy in an adolescent boy's beard while causing hair miniaturization on a young man's balding scalp? The CME articles are made possible through an educational grant from the Dermatological Division, Ortho Pharmaceutical Corporation, From the Dermatology Service, Walter Reed Army Medical Center. The opinions or assertions contained herein are the private views of the author and are not to be construed as official or as reflecting the views of the U,S, Army or the Department of Defense. Reprint requests: Leonard C. Sperling, MD, Dermatology Service, Walter Reed Army Medical Center, Washington, DC 20307.

16/2/27652

The answers to such questions seem remote, but the hair follicle itself is remarkably accessible to observation and study. Understanding hair growth and disease requires familiarity with follicular anatomy, just as mastering human physiology and pathology begins with a knowledge of normal gross anatomy. Although some features of hair shaft structure are visible with the naked eye or a hand-held magnifying lens, the study of follicular anatomy requires the use of a light microscope. This article reviews the normal structure of hair, with an emphasis on those anatomic features that can be readily observed by a clinician.

TECHNIQUES FOR STUDYING HAIR MORPHOLOGY Clipping a few hairs near the scalp surface provides samples for microscopic detection of various hair shaft deformities. Several hairs should be sampled because all hairs may not be affected. The choice of proper hairs can often be enhanced with a hand-held magnifying lens. Ifan area of fairly normal hair growth is present, it should also be sampled 1

Journal of the American Academy of Dermatology

2 Sperling

Fig. 1. The forcible hair pluck (trichogram). Use of a rubber-tipped hemostat facilitates plucking and reduces artifactual trauma to the hair. Ifthe hair shafts are taped to a glass slide before the hemostat is released, most telogen hair bulbs will be segregated together at a more superficial level than the anagen roots.

for the purpose of comparison. The locks of hair are placed onto microscope slides that have been coated with a copious amount of immersion oil, and a second slide is placed on top. I This flattens the hairs into one plane while the oil eliminates optical refraction. Both proximal and distal portions of the hair shaft may need to be examined in conditions that affect the hair focally (e.g., trichorrhexis nodosa). Lowering the condenser of the microscope (as is routinely done when searching for dermatophyte infection) increases the contrast between various planes of differing keratinization and is useful in identifying many hair structures and defects. The use of a polarizing lens can be helpful in evaluating disorders such as trichothiodystrophy and pili torti. Hairs can be extracted from the scalp for evaluation of the root structure by one of two methods. The first is the "gentle hair pull," in which locks of hair are held firmly between thrumb and forefinger, and a firm, steady traction is placed on the hairs. This test is done to see which hairs are easily epilated. The method of Caseri02 can be used for precise quantification, but normally only a few (less than four) telogen hairs can be extracted per pull on the

normal scalp. More than six hairs per pull is probably abnormaL Ifthe hair has been recently shampooed, the number will be smaller. A second method of extracting hairs is the "forcible hair pluck," or trichogram, which is used to quantify the ratio of anagen to telogen hairs. To some extent, a hair pluck can also be used to study root morphology. Despite claims that "the hair pluck is a painful technique that is outdated and adds little to the diagnostic evaluation,"3 most authors find the procedure useful. In this test, a given number of neighboring hair shafts (Caseri0 2 recommends 60) are grasped near the scalp by a rubber-tipped hemostat (Fig. 1). The hairs are then forcibly plucked from the scalp in the direction of hair growth. Maguire and Kligman 4 emphasize the importance of proper technique in obtaining consistent results. However, the results seem to depend just as much on the patient-hairs seem to be more firmly embedded in some scalps than in others, resulting in more or less artifactual distortion during extraction. The anatomic features of a "forcible hair pluck" that can be readily identified with light microscopy are presented at the end of this article.

Volume 25 Number 1, Part 1 July 1991

A biopsy specimen of the scalp allows examination of both follicular and perifollicular detail. The standard method for processing specimens involves vertical sections. These sections are particularly useful in evaluating inflammatory dermatoses because the various levels of the skin (epidermis down to fat) are represented on a single slide. However, hair follicles are not oriented at right angles to the skin surface and most vertical sections will result in tangential cuts of follicles. Although a 4 mm biopsy specimen may contain numerous hairs (20 to 30 on the normal scalp5), only a few of these will lie in the plane ofthe microtome blade. In an effort to sample a larger number of hairs in any given specimen, Headington5 has proposed that transverse (horizontal) sections be studied. Because a horizontal section through the upper third of the dermis theoretically samples all hairs in the specimen, the number and size (diameter) of hairs can be quantified. To interpret such specimens, the observer must be familiar with the transverse anatomy of hairs. Transverse sectioning allows quantification ofinformation, such as the number of anagen (vs telogen and catagen) hairs and the actual hair density.6 However, most dermatologists and dermatopathologists are unfamiliar with transverse sections, and the technique requires considerable time and experience for proper interpretation. Scanning electron microscopy (SEM) is an additional investigative tool for examining hair anatomy, but is not required (or available) in most clinical situations. SEM can prove to be especially useful in studying hair shaft anomalies such as the "uncombable hair syndrome,,7 and the "loose anagen syndrome."s A considerable body of information about the electron microscopic features of hair is available, but will not be discussed in this review. Readers interested in this subject should consult the excellent reviews by Hashimot09 and Montagna and Parakkal. lO EMBRYOLOGY

One of the complexities of hair anatomy (and hair disease) is the changing morphology that occurs as the hair passes through its life cycle (Fig. 2). To a large extent, this life cycle is a recapitulation of a hair's embryonic formation. The hair follicle begins in utero as an epithelial bud projecting downward from the fetal epidermis. This primary epithelial germ, or primary hair germ, is guided in its development by the underlying dermal papilla, an accumu-

Hair anatomy 3 lation of mesenchymal cells (see Fig. 2). The hair germ descends along with the papilla deeper into the fetal skin and differentiates into the various epithelial components of the first fetal anagen hair. The epithelium directly above and surrounding the dermal papilla forms the hair shaft. Lateral bulges of epithelium will develop into the sebaceous gland and duct, the apocrine gland and duct (in those follicles with apocrine gland activity), and the point of attachment of the arrector pili muscle. This latter epithelial protuberance is called "the bulge" (der Wulst).ll The typical scalp anagen follicle continues to produce a hair shaft for approximately 2 to 3 years, and occasionally as long as 7 years. The shorter hairs found on other body parts have correspondingly shorter anagen periods. After several years of active anagen growth, an abrupt and biologically dramatic change occurs: the follicle proceeds from the anagen to the telogen phase. Kligman 12 has eloquently described the changing histologic characteristics of a hair as it moves from anagen to telogen. At the end of anagen (and the beginning of catagen), the now club-shaped, keratinized proximal shaft is "pushed" upwards bya column of epithelial cells. Shortly thereafter, the dermal papilla moves upwards as this epithelial column shortens from below. The catagen stage of the hair cycle, requiring only a few weeks, ends as the catagen hair becomes the "resting" telogen hair. The telogen stage lasts about 3 months. At the beginning of telogen, a remnant of the epithelial column found during catagen remains as a small bud off the base of the hair follicle. This bud (the secondary hair germ), and the underlying dermal papilla, form the telogen germ unit 5 from which the new anagen hair is believed to develop. Catsarelis et aLII have proposed that new anagen growth is initiated by cells that reside in "the bulge," the protuberance of cells that serves as a point of attachment for the arrector pili muscle. During telogen, the "bulge" lies in close proximity to the secondary hair germ. However, in postnatal human terminal hair an actual anatomic bulge is difficult to identify as a distinct structure; the arrector pili attachment serves as the only indication of its location. In any case, further growth mimics the developmental process of the primary epithelial germ. The anagen hairs of postnatal life begin as downgrowths of secondary hair germ cells (or alternatively the metabolically active descendants of

4

Journal of the American Academy of Dermatology

Sperling

The Human Hair Cycle • • ,..w._» • i' #'.'I"I . I. I " , primaryhairgerm

"~ ~

-

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I

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,

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secondary hair germ - dermal paplile

,

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~:

IIbrou8root sheath Malr amer"

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presumptive club hair

ouler root sheath fibrous root sheath - , t

dermal papilla -~---.""

Fig. 2. The follicular life cycle. Beginning in utero, the first anagen hairs form from primary hair germs. These eventually enter catagen and are shed as telogen hairs. Thereafter, all new anagen hairs form from secondary hair germs. (From Demis DJ, Thiers BH, Smith BB, et aI, eds. Clinical dermatology. Philadelphia: JB Lippincott, 1991:2.)

"bulge" cells), which envelop and follow the dermal papilla in its descent through the dermis. As the newly forming anagen hair descends, it enlarges and begins to produce a shaft (Fig. 3). Even before the follicle has reached its ultimate destination in the fat, the new, tapered shaft has reached the follicular canal of its predecessor. As the newly forming anagen hair shaft grows up and out of the follicle, the effete telogen hair is shed. Unlike the primary hair germ, the secondary hair germ does not need to develop follicular adnexa (e.g., sebaceous and apocrine glands). After their initial development in utero,

these adnexal glands and the arrector pili "bulge" are permanent fixtures in the follicle. They remain little altered by the dramatic events of the hair cycle. ANAGEN HAIRS

Every day, approximately 50 to 100 hairs from various regions ofthe scalp are shed and replaced by newly growing anagen hairs. Anagen follicles are highly active metabolically (second only to hematopoietic tissue), which explains their sensitivity to nutritional deprivation and chemical insult. The

Volume 25 Number 1, Part 1 July 1991

Hair anatomy 5

Fig. 3. Vertical section of an early anagen hair. By this time the secondary hair germ has differentiated and proliferated to form a new anagen hair bulb, which is already manufacturing a hair shaft (8). The telogen club hair (T) from the previous life cycle is still present within its epithelial coating. As the new shaft reaches the follicular canal, the club hair will be shed. (Hematoxylin-eosin stain.)

vast majority of hairs on the normal scalp represent anagen follicles, but they are so firmly rooted that they can only be examined when subjected to biopsy or when forcibly plucked. Anagen hairs found in the "loose anagen hair syndrome" and in certain highly inflammatory diseases of the scalp are exceptions to this rule. 13 Anagen hairs come in a variety of sizes ranging from the large terminal hairs found in the beard and on the scalp to the fine, minute vellus hairs covering most of the glabrous skin. The size of certain follicles changes dramatically during a person's life span. The downy facial hairs of a boy become the beard

hairs of a man, the vellus hairs of the axillae and pubis lengthen and coarsen at puberty, and many terminal hairs of the scalp miniaturize during the balding process. Both vellus and terminal hairs go through all stages of the follicular life cycle, but the length of anagen is much shorter in vellus hairs. Although vellus hairs cover most of the human body, terminal hairs are of more interest to both clinician and patient. Their presence in hirsutism is the cause of much distress; their absence in androgenetic alopecia is equally disturbing; and their mere existence on the normal scalp accounts for an enormous portion of cosmetic industry. In this discussion of

Journal of the

6

American Academy of Dermatology

Sperling

INFUNDIBULUM

ISTHMUS I'

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arrector pili muscla

medulla or hair shall _-.:m'"r--->ii--'iI

.,

cortex or hair shaft ----"ia_-ir

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Inner root sheath --'tI--'il outer root sheath - - w f t vitreous or "glassy" layer ----."11

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Fig. 4. Schematic diagram of an anagen hair. (From Demis DJ, Thiers BH, Smith EB, et aI, eds. Clinical dermatology. Philadelphia: JB Lippincott, 1991:3.)

follicular anatomy, the focus therefore is on terminal hairs. Anagen hairs continue to produce a hair shaft for years. During this period their structure remains constant. The overall architecture of a fully developed anagen hair is shown schematically in Fig. 4. The dermal papilla is connected to the surrounding fibrous root sheath at its lower pole. Otherwise, the papilla is surrounded on its top and sides by the epidermal cells of the hair matrix. The matrix is the actively growing portion of the hair, whose cells rapidly divide and move upward. Matrix cells are confined to the lowermost portion of the follicle. The expanded, bulbous section of the lower hair follicle, including the hair matrix and the dermal papilla, is known as the hair bulb. The isthmus of the follicle is the short portion lying in between the point of attachment of the arrector pili muscle and the point of entry of the sebaceous gland duct. The infundibulum, derived from the Latin word for funnel, lies above the entry of the sebaceous duct and merges with the surrounding surface epidermis. Anagen follicles are deeply rooted in the fat and reticular dermis and are finnly attached to their surroundings. When such a follicle is forcibly plucked, the anatomic features are always altered

and sometimes greatly distorted. Therefore plucked anagen hairs can look different from each other, depending on their state of "intactness." 13 An intact, plucked terminal anagen hair reveals the outer root sheath (DRS) as the clear sheath coating the pigmented shaft (Fig. 5). The inner root sheath (IRS) is closely apposed to the shaft, lying just within the DRS. The IRS contains no pigment and, because of a difference in refractile properties, can be easily distinguished from the shaft. The hair in Fig. 5 is exceptional in that the dermal papilla and fibrous root sheath have also been extracted during the plucking process. Usually the dermal papilla and fibrous sheath, both derived from the dermis, remain behind during the plucking process. As the anagen hair is forcibly extracted, the DRS, and often the IRS as well, may be left behind in the dermis. This results in an artifactually "naked" hair shaft and root. Dften those hairs that are extracted without the IRS and DRS show a ruffling of the hair shaft cuticle (Fig. 6). This ruffling occurs because the IRS cuticle cells are ripped away from the interlocking shaft cuticle cells. The interlocking of these two cuticles is described later. Usually the hair matrix is torn away from the underlying dermal papilla, which often results in a

Volume 25 Number I, Part I July 1991

Hair anatomy 7

Fig. 6. Normal anagen hair has lost both inner and outer root sheaths during plucking process, thereby resulting in a "ruflling" (arrows) of the hair shaft cuticle. This occurs when interlocking cells of inner root sheath cuticle are forcibly torn away from the hair shaft cuticle. Hairs such as these have been described as "dystrophic," but "traumatized" would be more accurate.

fig. 5. Plucked anagen hair. This specimen is remarkably intact; normally, the dermal papilla and some of the follicular epithelium are left behind in the skin. FRS, Fibrous root sheath; IRS, inner root sheath; ORS, outer root sheath; SDP, stalk of dermal papilla.

broom-shaped matrix. Rarely, the dermal papilla is extracted along with the hair follicle, as seen in Fig. 5. This is fortunately infrequent, or plucked hairs would lose their ability to regenerate. Itshould be remembered that a plucked hair usually represents only a portion of the normal follicle because some epithelial and all dermal components are normally left behind.} 3 The act of plucking obviously creates

considerable anatomic artifact. This is important to remember because considerable confusion in the literature has resulted from an inability to distinguish artifact from abnormality. Finer architectural detail can be seen byexamining stained vertical and horizontal sections of follicles. A full-thickness scalp biopsy specimen is required. As the follicle is followed upward from the dermal papilla to the epidermal surface, a constantly changing picture is seen (Fig. 7). The dermal papilla is connected to the surrounding dermis at its lower pole and is surrounded on its top and sides by the epidermal cells of the hair matrix. With hematoxylin-and-eosin staining, the papilla is seen as a roughly flame-shaped condensation of spindle-shaped nuclei in a fibrous stroma. The papilla invaginates the

8

Fig. 7. Vertical section of the lower half of an anagen follicle. This portion of the follicle can be divided into bulbar (B) and suprabulbar (8) zones. Readily identified at this power are the dermal papilla (p), hair matrix (m), inner root sheath (i), outer root sheath (0), and fibrous root sheath (j).

overlying bulb matrix like the ball in a ball-andsocket joint. The overlying (and surrounding) hair matrix is composed of roughly cubical, undifferentiated epidermal cells. These cells are usually pigmented, even at the basal cell level, in the region destined to form the hair shaft. The matrix cells rapidly differentiate as they move upward. The cells overlying the top and sides of the papilla keratinize to form the centrally located hair shaft. As they move upwards, these cells elongate with a vertical orientation. Just above the hair bulb, the cells begin to keratinize without the formation of a granular cell layer, and the nuclei gradually fade and then disappear. This pilar type of keratinization is seen in some adnexal tumors such as pilomatrixoma and pilar cysts. The cells of the IRS have already keratinized by this point; because of this, it is possible that the hair shaft is "shaped" as well as guided by the IRS. The hair shaft is composed of three layers. In the very center of some, but not all, hairs lies the hair medulla (Fig. 8). In the lower portion of the follicle,

Journal of the American Academy of Dermatology

Fig. 8. High-power view of the suprabulbar zone at approximately the level labeled "S"in Fig. 7. Starting from

the center of the follicle and moving toward the periphery, the following structures can be identified: hair medulla (M) with medullary granules, hair cortex (c), interlocking cuticles (x) ofthe hair shaft and the inner root sheath, Huxley's layer (Hu) of the inner root sheath, Henle's layer (He) of the inner root sheath, the outer root sheath (0), vitreous or "glassy" layer (v), and the fibrous root sheath (j). (Hematoxylin-eosin stain.)

medulla cells are filled with glycogen-containing vacuoles, as well as medullary granules, whose staining properties are similar to trichohyaline granules (see later ).14 The hair medulla contains structural proteins that are markedly different from other hair keratins. 10 These proteins are resistant to chemical treatment and are not well characterized. Medullary granules contain citrulline, an amino acid that is unique to the medulla and the IRS. Gradually the cells of the medulla become dehydrated, and the vacuoles are replaced with air-filled spaces. On light microscopy, these fragmented columns of air appear darker than the surrounding cortex, because of the refraction of light. In humans, the medulla is a vestigial structure, and many hairs do not possess a medulla. 9 However, the central air canal of the medulla provides an insulating effect to wool and other animal furs.

Volume 25 Number 1, Part 1 July 1991

Hair anatomy 9

Fig. 9. Oblique section through the isthmus. As the inner root sheath (i) desquamates and disappears, outer root sheath (0) begins to keratinize without the formation of a granular cell layer, that is, trichilemmal keratinization (tk). The isthmus ends superiorly as the sebaceous duct (sd) enters the central follicular chamber. (Hematoxylin-eosin stain.) Fig. 10. Transverse section through the bulb of an anagen follicle. (Hematoxylin-eosin stain.) Dermal papilla (p) is surrounded by the hair matrix (m). Outer root sheath (0) is one cell thick at this level; and artifactual cleft has disrupted the outer root sheath in this figure. The vitreous layer (v) and fibrous root sheath (f) surround the follicular epithelium.

External to the medulla lies the hair cortex, which constitutes the bulk ofthe hair. The outermost layer, whose flattened cells interlock with those of the IRS cuticle, is called the hair cuticle (see Fig. 8). Electron microscopy has shown that the hair cuticle is composed of a single layer of cells. However, these cells are flattened and overlap, like shingles on a roof, so that in cross section there appear to be 5 to 10 layers of compacted cells. 9 The free edges of cuticular cells point upward and outward. Just external to the shaft are the three layers forming the IRS (see Fig. 8). The innermost layer, lying against the cuticle of the hair, is the IRS cuticle. Its cells, the free edges of which point downward and inward, rapidly flatten and interlock with those of the hair cuticle. The two cuticles are so completely integrated that the interlocking cells appear to be a single layer when viewed with a light microscope. External to the IRS cuticle is the Huxley layer of the IRS. The outermost layer, keratiniz-

ing first (i.e., lower in the follicle) is the Henle layer. Keratinization is achieved through the formation of trichohyaline granules, which are more eosinophilic than the granules of the epidermal granular cell layer. Although the three layers of the IRS look distinct just above the dermal papilla, they all keratinize low in the follicle, and blend together as a single functional and histologic unit at higher levels. The IRS stains deeply with a variety of histologic stains, such as toluidine blue6 and 4-dimethylaminocinnamaldehyde. 1S This makes it easy to identify in both vertical and horizontal sections. The special staining qualities ofthe IRS are probably due to the presence of citrulline, which is present in high concentration in the IRS. IS The IRS coats and sup: ports the hair shaft up to the level of the isthmus (which begins at the attachment of the arrector pili muscle). In the isthmus, the IRS disintegrates and its keratinized cells desquamate into the infundibular space (Fig. 9).

Journal of the

American Academy of Dermatology

10 Sperling

Fig. 11. Transverse section through the suprabulbar zone of an anagen follicle. (Hematoxylin-eosin stain.) All layers of the inner root sheath (i) have keratinized at this level, but the outer root sheath (0) shows no evidence of keratinization. This particular hair shaft has a medulla (m) as well as a cortex (c). Surrounding the fibrous root sheath (f) is subcutaneous fat. Cleft between the hair shaft and the inner root sheath (also seen in Fig. 12) is an artifact of processing; it does not occur in vivo. Fig. 12. Transverse section through the suprabulbar zone of an anagen hair. Inner root sheath stains an intense blue, which is easily identified even at low power. Even the nuclear material of the outer root sheath (0) does not possess as great an affinity for toluidine blue. Hair shaft within this follicle has no medulla. (Toluidine blue stain.) Fig. 13. Transverse section through the infundibulum. Desquamated cells, sebum, and microorganisms fill the space (8) between the outer root sheath and the hair shaft. Keratinization occurs with the formation ofkeratohyaline granules (arrows), resulting in a stratum corneum (sc). (Toluidine blue stain.)

External to the IRS is the clear-celled ORS. It is also known as the trichilemma, a word derived from the Greek for "coating or sac around the hair." The ORS is thickest at the isthmus, and narrowest in the lower portion of the bulb, where it is only one cell

layer thick. Unlike the cells of the hair shaft and IRS, which are known to move upward and out of the follicle, the cells of the ORS may remain relatively stationary. Some authors state that the growing IRS "glides" past the opposing ORS.16

Volume 25 Number 1, Part 1 July 1991

Hair anatomy 11

Fig. 14. Vertical section of a mid-catagen hair. Markedly thickened vitreous layer (V) and the undifferentiated epithelial column (EC) below presumptive club (P) are markers for this phase of hair growth. (Hematoxylin-eosin stain.) Fig. 15. Transverse section ofa catagen follicle below the level of the presumptive club. The dramatic thickening ofthe vitreous layer (V) is especially striking when the follicle is sectioned in this fashion. The fibrous root sheath (j) is also thickened. Degenerating cells with pyknotic nuclei (arrows) are present within the epithelial column. (Hematoxylin-eosin stain.)

Others believe that it is not known whether the ORS cells move upward with the IRS cells, as they do in mice and sheep.l0 In the lower portion of the follicle (below the isthmus), the ORS does not keratinize. The nonkeratinized inner layer of cells lie against the fully keratinized IRS. At the level of the isthmus, when the IRS disintegrates, the ORS keratinizes, but without the formation of a granular cell layer (see Fig. 9). This is known as "trichilemmal keratinization." Abov~ the level of the sebaceous duct entry (i.e., the infundibulum), keratinization of the ORS switches to the "epidermal" mode, with formation of a stratum granulosum and stratum corneum. At this point, the ORS becomes indistinguishable from the surrounding epidermis. The glassy or vitreous layer is the acellular, eosinophilic zone surrounding the follicle, just external to the ORS (see Fig. 8). Like the epidermal basement membrane zone, with which it is continuous, the vitreous layer is periodic acid-Schiff (PAS)

thicker and visible with routine stains. During catagen, the vitreous layer becomes markedly thicker and corrugated in appearance. This thickened layer then serves as a marker of the catagen phase. Surrounding the vitreous layer is the fibrous root sheath, the outermost layer of the hair follicle. Itis composed of thick collagen bundles and represents a specialized dermal coating for the epithelium-derived hair follicle. The fibrous root sheath extends down into the fat to coat deeply placed terminal hairs, such as scalp hairs. This collagenous sheath is continuous with the dermal papilla below and blends imperceptibly with the papillary dermis above. As discussed earlier, transverse sections of scalp biopsy specimens offer several advantages over routine, vertical sections. Step sections of a single specimen, from papillarY dermis to fat, provide ample materialfor study. For the purpose ofthis discussion, only a few levels will be presented. The most important "landmark" levels are cross sections through the

12 Sperling

Journal of the American Academy of Dermatology

Fig. 16. Two catagen follicles, sectioned transversely. Follicle at left has been sectioned through the presumptive club (PC), which has started to keratinize but still retains nuclear remnants. The presumptive club is surrounded by outer root sheath-type epithelial cells (0). Follicle at right was sectioned just below its presumptive club, where the epithelial column (Ee) is thickest. Fig. 17. Plucked catagen hair. Although a keratinized presumptive club (P) has formed, portions of both inner and outer (0) root sheaths still coat the proximal hair shaft. Inner root sheath in this section (i) appears dark and folded, having been compressed downward like the paper wrapper of a soda straw; this is an artifact of plucking. The epithelial column of this catagen hair was lost during the plucking process. Figs. 9 through 13 show the typical features of hairs transected at these levels. At the level of the dermal papilla (Fig. 10), the Henle layer has already begun to keratinize, as demonstrated by the presence of trichohyaline granules. The ORS is only one cell layer thick. The matrix cells are undifferentiated and have not keratinized. Fig. 11 is a cross section through the suprabulbar zone. At this level, the follicle is still surrounded by fat. All three layers of the IRS have keratinized and are seen as one homogeneous unit, although some nuclear material still persists. The ORS is much thicker at this point, but even the innermost cells lying against the IRS do not keratinize. The hair shaft is keratinized, but retention of residual nuclear material is evident. The medulla is seen as a central oval differing in tinctorial quality. Fig. 12 is also a cross section through the suprabulbar zone, but at a slightly higher level where shaft keratinization is complete.

Because the isthmus is a region of marked transition, cross sections through different portions of this short segment will show different types of keratinization. Fig. 9 is an oblique section from the bottom to the top of the isthmus that reveals all the histologic features of this segment. The deeper portion of this section shows the uppermost portion of the IRS. As the section becomes more superficial (approaches the epidermis), the IRS disappears, and the ORS begins to keratinize in the "trichilemmal" fashion (without a granular cell layer). At the uppermost portion of the isthmus, the sebaceous duct can be seen entering the follicle. At this point, the ORS begins to develop a granular layer as it switches to "epidermal" keratinization. Fig. 13 shows a cross section through the infundibulum. Keratinization is of the epidermal type, with formation of a granular cell layer. The infundibulum contains not only the hair shaft, but also sebum, desquamated epithelial cells, and a variety of

Volume 25 Number I, Part 1 July 1991

Hair anatomy 13

Fig. 18. Vertical section oftelogen hair. Secondary hair germ (G) and dermal papilla (P) are evident at the base of the follicle, below the keratinized club (C). (Toluidine blue stain.)

microorganisms such as bacteria, Demodex, and fungal spores. CATAGEN HAIRS

Eventually the anagen hair receives a signal to cease growth. The source and nature of this message are unknown. The follicle then proceeds to undergo the remarkable metamorphosis seen in the catagen phase. The dramatic changes seen during catagen are condensed into a brief period of 2 or 3 weeks. Kligman 12 beautifully describes the morphologic progression of the catagen hair. For purposes of this discussion, only the anatomic features of the well-developed, mid-catagen hair will be described. The formation and development of the catagen hair have been described in the "Embryology" section. Catagen follicles have several anatomic features that are peculiar to this phase of the life cycle (Figs. 14 and 15). The matrix cells retract from the

dermal papilla and leave the papilla "naked" except for a thin cap of epithelial cells. The papilla itself appears to be stalked at its base. Above the papilla, the follicular epithelium remains as a thin column of pale-staining, nonpigmented, undifferentiated cells. Degenerative changes as evidenced by pyknotic nuclei affect many cells within the epithelial column. The sides of this column may be wavy and irregular. Surrounding the upper portion of the papilla and the entire epithelial column is the vitreous membrane, which now has become massively thickened and corrugated. The fibrous root sheath is also somewhat thickened, but not nearly to the extent of the vitreous membrane. Just above the epithelial column and its corrugated casing of vitreous membrane, a bulbous mass of former matrix cells forms the presumptive club hair. Early in catagen the cells of the presumptive club are still nucleated (Fig. 16), but the nuclei rapidly disappear as the club fully keratinizes. A thick mantle of cells derived from the ORS

Journal of the American Academy of Dermatology

14 Sperling

Fig. 19. Transverse sections through a telogen hair. (Hematoxylin-eosin stain.) A, Section through the keratinized club (kc) surrounded by its epithelial sac or coating (es). Depth of this section corresponds to level "e"in Fig. 18. B, Section through the secondary hair germ at the base of the telogen hair, corresponding to level "0" in Fig. 18. C, Section through the dermal papilla, corresponding to level "P" in Fig. 18. Some melanin pigment is also found in this papilla; such incontinence of pigment is fairly common among black patients and in various inflammatory diseases of the scalp.

encases the club, which remains relatively unchanged until the end of telogen. As catagen progresses, the undifferentiated epithelial column and the surrounding vitreous membrane shorten and disintegrate from below. The dermal papilla follows the remainder of the receding epithelial column and so moves higher and higher into the dermis. Finally the papilla reaches a position just below the presumptive club hair, where the remainder of the epithelial column forms a bud of cells that will become the secondary hair germ. Catagen hairs can also be identified among plucked hairs (Fig. 17). The presumptive club begins to keratinize while the follicle still possesses a significant portion of the IRS and ORS. The epithelial column below the club, although fragile, is some-

times extracted as a clear "tail" of tissue clinging to the keratinized club. These features distinguish plucked catagen hairs from telogen hairs. Because catagen hairs normally comprise less than 1% of scalp follicles, it is unusual to extract a catagen hair. However, in conditions where increased numbers of hairs are entering the catagen phase (such as trichotillomania and the "preshedding" phase of telogen effluvium), catagen hairs may be represented in a plucked sample. TELOGEN HAIRS

By the end of catagen, the epithelial column and thickened vitreous membrane have disappeared. The keratinized club of the new telogen hair (Figs. 18 and 19) has an epithelial coating or sac derived

Volume 25 Number I, Part 1 July 1991

Hair anatomy 15

Fig. 20. Plucked (early) telogen hair. Epithelial sac (ES) still coats the keratinized club. Fig. 21. Spontaneously shed telogen hair. Epithelial coating around the keratinized club has disintegrated.

from the ORS. At the base ofthis epithelial coat, an irregularly shaped protuberance of cells identifies the secondary hair germ. Below this lies the "naked" dermal papilla whose nuclei are arranged in a compact ball. Early in telogen, the keratinized club and its epithelial coating are firmly attached to each other. This anchors the hair in the scalp, although not nearly as firmly as an anagen hair. As telogen progresses over a period of about 3 months, the epithelial coating thins and its hold on the keratinized club weakens. The follicle's grip on the telogen club is further weakened by the newly forming hair shaft moving up into the follicular canal (see Fig. 3). A telogen hair plucked (or vigorously brushed or pulled) during the early part ofthe telogen phase will be extracted with a portion of its epithelial sac intact (Fig. 20). Near the end of telogen, gentle pulling or spontaneous shedding produces the classic "club hair" with a rough, bulbous, keratinized root (Fig. 21). THE FORCIBLE HAIR PLUCK

A detailed description of the hair pluck has been postponed until this point because an acquaintance

with anagen, catagen, and telogen anatomy is required for the proper analysis of plucked hairs. Fig. 22 shows a forcible hair pluck in which there is relatively little distortion of follicular anatomy. Most anagen hairs possess an IRS and an ORS, and the telogen bulbs and anagen bulbs are segregated at different levels (as expected, the telogen roots lie at a more superficial level than the anagen roots). In most specimens of plucked hairs, at least some anagen hairs lose their root sheaths as an artifact of plucking. Such hairs have been referred to as "dystrophic,"4 but they should not be confused with truly dystrophic hairs-the malformations associated with true disease states such as alopecia areata. Fig. 23 is perhaps more representative of a typical hair pluck. Some hairs have lost some or both oftheir root sheaths. Nevertheless, this specimen was obtained from a perfectly normal scalp with the identical technique employed for obtaining the specimen as shown in Fig. 22. In almost all cases it is possible to determine the telogen count (the ratio of telogen hairs to total hairs) from a plucked specimen, assuming a sufficient number of hairs have been extracted. The sim-

Journal of the American Academy of Dermatology

16 Sperling

Fig. 22. Hair pluck from a normal scalp. Inner and outer (0) root sheaths of the anagen hairs are fairly intact. Hair matrices (M) ofseveral anagen hairs appear to have a broomlike shape, which is an artifact of plucking. Inner root sheaths (i) of several follicles show an accordionlike artifactual folding (also seen in Fig. 17). Telogen roots (t) are segregated at a more superficiallevel than the anagen roots. Fig. 23. Typical hair pluck from a normal scalp. Although most plucked hairs usually retain a portion of both root sheaths (B), often hairs are extracted without either root sheath (N), or with just the inner root sheath (I). Occasionally the shaft of an anagen hair will fracture cleanly, leaving its entire root behind. These changes are artifacts of plucking and should not be interpreted as abnormalities. Telogen root (T) can be seen at top.

plest method is to first count the number of telogen hairs, which are extracted with little distortion. The total number of hairs is determined by counting the total number of hair shafts. Normal-appearing shafts that have cleanly snapped off above the roots, leaving the bulbs and root sheaths behind, are assumed to be anagen hairs. Hair shafts that taper to a point before breaking are always abnormal and indicative of hair disease. At any given time, an average of 10% to 15% of hairs are in the telogen phase. However, this is the average telogen count for humans as a group. A normal person may have anywhere from 4% to 25% of hairs in telogen; greater than 25% of hairs in telogen is probably abnormal. 8 Different regions of the scalp also vary in the percentage of telogen hairs

present, with the frontal and vertex regions showing the highest counts. REFERENCES

1. Sperling LC. Introduction to diseases ofthe hair; vall, unit 2-0. In: Dcmis DJ, Thiers BH, Smith EB, et aI, eds. Clinical dermatology. Philadelphia: JB Lippincott 1991:1-17. 2. Caserio RJ. Diagnostic techniques for hair disorders. Part III: clinical hair manipulations and clinical findings. Cutis 1987;40:442-8. 3. Pariser DM, Caserio RJ, Eaglstein WH. Techniques for diagnosing skin and hair disease. New York: Thieme, 1986:143-79. 4. Maguire HC, Kligman AM. Hair plucking as a diagnostic tool. J Invest Dermatol 1964;43:77-9. 5. Headington JT. Transverse microscopic anatomy of the human scalp. Arch Dermatol 1984;120:449-56. 6. Sperling LC, Winton G. The transverse anatomy of androgenetic alopecia. J Dermatol Surg Oncol 1990; 16: 1127-33.

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7. McCullum N, Sperling LC, Vidmar D. The uncombable hair syndrome. Cutis 1990;46:479-83. 8. Price V, Gummer CL. The loose anagen hair syndrome. J AM ACAD DERMATOL 1989;20:249-56. 9. Hashimoto K. The structure of human hair. Clin Dermatol 1988;6:7-21. 10. Montagna W, Parakkal PF. The structure and function of skin. 3rd ed. New York: Academic Press, 1974:232. 11. Cotsarelis G, Sun T-T, Lavker RM. Label-retaining cells reside in the bulge area of pilosebaceous unit: implication for follicular stem cells, hair cycle, and skin carcinogenesis. Cell 1990;61:1329-37. 12. Kligman AM. The human hair cycle. J Invest Dermatol 1959;33:307-16.

Hair anatomy 17 13. Sperling Le, Samlaska CPo Hair anatomy as seen by light microscopy. J Assoc Mil DermatoI1989;15:16-28. 14. Ebling FJG, Dawber R, Rook A. The hair. In: Rook A, Wilkinson DS, Ebling FlO, et ai, eds. Textbook of dermatology. Oxford: Blackwell Scientific, 1986. 15. Baden HP, Kubilus J, Baden L. A stain for pluckedanagen hairs. JAM ACAD DERMATOL 1979;1:121-2. 16. Jakubovic HR, Ackerman AB. Structure and function of skin: development, morphology and physiology, In: Moschella SL, Hurley Hl, eds. Dermatology. 2nd ed. Philadelphia: WB Saunders, 1985:1-75.

Hair anatomy for the clinician.

The rational evaluation of hair disorders requires familiarity with follicular anatomy. Hair structure can be easily examined by studying clipped hair...
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