Hair

Ghassan Shaker and Dominique Van Neste

Skinterface sprl, Tournai, Belgium

INTRODUCTION

Hair is a symbol of good looks and beauty in some areas of the human body. So much time, effort, and money are spent in caring for it, especially in the case of scalp hair. In some other areas, like the beard, daily care by shaving is necessary for the majority of males. In females, abundant scalp hair is very much welcomed, unlike leg hair, facial hair, and armpit (axillary) hair. Hair distribution in certain body regions is a secondary sex characteristic and starts to appear around puberty as the beard, moustache, and body hair in males, and pubic and axillary hair in both sexes.

The social meaning of hair is very important. So many old and present social and/ or religious practices deal with hair. Enforced shaving of scalp hair has long been used as a sign of punishment and in certain religious practices as a sign of obedience. The Romans completely shaved the scalps of prisoners, adulterers, and traitors. Scalping the warring enemies, which was long practiced by some primitive societies was meant to express victory and revenge [1].

Hair styling can serve as a form of expression. Rebellion of youth to the existing social order is often manifested as a change in appearance, and especially change of hair style, e.g., long hair on males, shaved hair (skinheads), and dyed hair (punks) [1].

Hair also plays a role as a distinguishing sign of one's ethnicity, varying from straight to curly in form and from dark to blond in color. There is also a difference in the amount of body hair between races. Hair is generally subject to so much interracial and interindividual variation that it can be said that, apart from the hair follicle, there is no organ in the human body that is morphologically so much variable as hair.

Although hair is not vital to human existence, it is greatly important to one's psychological equilibrium [2-4]. Psychological problems of hair loss occur in both sexes, and more among women because of the relevance of physical attractiveness [5]. Hair is closely related to physical attractiveness and the difference between male and female hair patterning provides a recognition phenomenon. In general, baldness leads to overestimation of age of affected males [1].

In addition to the aesthetic function of hair, it has more natural functions, which are becoming less important because of the anthropological evolution and technical prog

ress of mankind. Scalp hair protects against certain environmental conditions like sun rays and cold. Body hair in man is very much reduced in comparison with other mammals, and many theories have been postulated to explain this fact; most are based on temperature and thermal regulation of the human body all along the course of the evolution of mankind. Nasal hair protects against dust and acts as an air filter. Axillary and perineal hair reduce the friction during body movement and also serve for the wider or prolonged dissemination of apocrine gland odor. Pubic hair is said to have some excitatory functions during sexual intercourse.

Innumerable are the cosmetic products intended for use in hair care to remove sebum and dirt and to improve the look, shininess, uniformity, softness, color, odor, and ease of comb of the hair, as well as deposition of conditioning molecules and reduction of static ''fly-aways'' (e.g., shampoos, conditioners, hair dyes, fixation sprays, gels, creams, etc.) There are also many products that have been marketed and used by people as anti-hair loss preparations and/or hair growth-promoting agents. Many have not stood the test of time. Ancient medical literature is full of pharmaceutical prescriptions and formulas to be used to treat hair loss or to promote hair growth. They are so diverse in source and nature that any attempt to categorize them seems useless.

In addition to scalp hair formulas, many other compounds are intended to remove or to assist the removal of hair from other parts of the body, e.g., preshave and aftershave preparations, depilatories, and so on. Other products aim to decrease the contrast of hair with the skin, making hair less visible, e.g., bleaching agents. Besides the variable efficacy of these products, consumers may develop many nonintended effects on the hair and skin such as hair damage, hair loss, skin irritation, and/or allergy and photoreactions attributable to some active ingredients and/or their additives. In order to understand hair production, it is necessary to revisit the embryogenesis and to have an idea about the structure and functional activity of the hair follicle. These aspects will now be briefly described.

THE HAIR FOLLICLE Embryology

In the early stages of hair follicle development in human fetal skin, a simultaneous differentiation of some epidermal and dermal cells takes place between the second and third months of intrauterine life in some areas such as the eyebrows and chin, followed by other body regions in the fourth month. Histologically, it begins as a crowding of cells in the basal layer of the epidermis with a simultaneous aggregation of mesenchymal cells directly beneath the developing epithelial component. Cells in the basal layer elongate to form the hair peg, which grows obliquely downwards in an orientation characteristic for each body region. The broad tip of the hair peg will become slightly concave and carries before it the aggregated mesenchymal cells, which will become the dermal papilla. During the downward course of the hair peg, two swellings appear at the posterior side of the follicle. The upper swelling will form the sebaceous gland, whereas the lower will become the insertion site of the arrector pili muscle. In some body sites, such as the axilla, groin, skin of genitalia, and face, a third swelling is going to develop above the sebaceous gland bud and this will form the apocrine gland [6-8].

Hair follicle development proceeds in a cephalocaudal direction and is completed

by the 22nd week of intrauterine life. These follicles progressively synthesize hair shafts (lanugo hair), which are visible at the cutaneous surface by the 28th week. The first hair coat of fine lanugo hair is shed in utero at about 1 month before birth at full term. The shedding course follows a cephalo caudal direction, which means that frontal hair follicles begin their second hair cycle while occipital hair follicles are still in their first hair cycle. The second coat of lanugo hair is going to shed from all areas during the first 3 to 4 months of life [6-8].

Histology

The hair follicle bulb is composed of a central dermal papilla and a surrounding hair matrix. It undergoes many changes according to the cyclical activity of the hair follicle in health and disease. At the level of attachment of the arrector pili muscle to the follicle is the bulge zone of the root sheaths. This is considered to be the stem cell site from which a new hair cycle is initiated. The hair shaft is enclosed in two sheaths, i.e., the inner root sheath and the outer root sheath. The inner root sheath consists of a cuticle layer on the inside (next to the cuticle layer of the hair cortex), Huxley's layer in the middle, and Henle's layer on the outside. The inner root sheath hardens before the presumptive hair within it, and it is consequently thought to control the definitive shape of the hair shaft [6-8].

The outer root sheath cells have a characteristic vacuolated aspect. This sheath is covered by the vitreous membrane. Next to this layer we can find the connective tissue sheath with its characteristic fibroblasts [6-8].

Cyclical Activity

Production of a hair segment by a hair follicle undergoes a cyclical rhythm. Activity (anagen) is followed by a relatively short transitional phase (catagen) and a resting phase (telogen) (Fig. 1). The duration of activity or anagen varies greatly with species, body region, season, age, and the type of hair (i.e., terminal or vellus).

In adult humans the activity of each follicle is independent of its neighbors (asynchronous). However, during the development of the human embryo as well as the early months of life, there is a more or less synchronous moult of scalp hairs. Each follicle goes through the hair cycle a variable number of times in the course of a lifetime. On average, at any one time about 13% of the scalp hair follicles are in telogen and only 1% or less are in catagen. Telogen ratio may count higher in certain stressful physical and/ or mental conditions such as telogen effluvium and postpartum alopecia [6-8].

HAIR STRUCTURE

Postnatal hair may be divided into two broad categories: vellus hair, which is soft, unme-dullated, occasionally pigmented, and seldom exceeds 2 cm in length; and terminal, which is longer, coarser, and often pigmented and medullated [8]. Before puberty, terminal hair is limited to the scalp, eyebrows, and eyelashes. After puberty, secondary sexual terminal hair is developed from vellus hair in response to androgens. The bulk of any hair segment is formed mainly by the cortex, which is surrounded by a cuticle and may also have a continuous or discontinuous core or medulla [8,9]. The medulla is usually found in thicker

Figure 1 Schematic view of hair cycling of a human hair follicle. The latest steps of the hair-growth phase (anagen 6) during which hair is visible at the skin surface and growing are shown in (A) while the apparent rest phase of the hair cycle (telogen phase) is shown in (B) during which a new hair cycle can be initiated. The legend [between (A) and (B)] helps the reader to orient himself within the various components of the human hair follicle, which are essential to understanding growth and rest.

(A) From growth to rest: The same hair follicle is represented at various times (days) at the very end of the growth phase. At the skin surface, there is normal pigmented hair production (days a-b and b-c) representing the constant daily hair production (L1 and L2). Then, the pigmentation of the newly synthesized hair shaft (appearing at the bottom of the hair follicle) is decreased (c). This early event announces the regression of the impermanent portion of the hair follicle and is followed by terminal differentiation of cells in the proliferation compartment (d) and shrinkage of the dermal papilla (e). The latter starts an ascending movement together with the hair shaft (f-h; 21 days). This characterizes the catagen phase (d-h). The apparent elongation of the hair fiber (L3) reflects the outward migration of the hair shaft. What is left after disappearance of the epithelial cells from the impermanent portion of the hair follicle is, first, basement membranes, followed by dermal connective tissue usually referred to as streamers or stelae (***). The true resting stage begins when catagen is completed, i.e., when the dermal papilla abuts to the bottom of the permanent portion of the hair follicle. In the absence of physical interaction between dermal papilla and bulge the next cycle (see B) is definitely compromised. As from now no hair growth is observed at the surface (h-i).

90 days 0 to SO days 3 days 1 day a b c d e f g h i

Figure 1 Continued (B) From rest to growth: During this stage, one notices absence of hair growth at the skin surface (a-g) but significant changes occur in the deeper parts of the hair follicle. The dermal papilla expands and attracts epithelial cells from the bulge (stem cell zone) in a downward movement (a-b). To create space, previously deposited materials have to be digested (a-b, ***). The epithelial cells then start differentiation in an orderly fashion starting with the inner root sheath (c) and the tip of the cuticle and hair cortex of the newly formed unpigmented hair fiber (d). The resting hair remains in the hair follicle for approximately 1 to 3 months (a-e), then the detached hair is shed (f). The shiny root end of the shed hair is the club. Before, during, or after hair shedding there may be replacement by a new hair shaft (e-f-g). Indeed, under physiological conditions, the follicle proceeds immediately or only slowly with new hair production (from f to g;maximum 90 days). Certain conditions are characterized by a much longer interval before regrowth is visible. Usually, a nonpigmented hair tip is seen first (h), followed by a thicker, more pigmented, and faster-growing hair fiber (i) depending on the many regulatory factors controlling the hair follicle. (Reproduced with permission from H.A.I.R. Technology [Skinterface sprl, Tournai, Belgium].)

90 days 0 to SO days 3 days 1 day a b c d e f g h i

Figure 1 Continued (B) From rest to growth: During this stage, one notices absence of hair growth at the skin surface (a-g) but significant changes occur in the deeper parts of the hair follicle. The dermal papilla expands and attracts epithelial cells from the bulge (stem cell zone) in a downward movement (a-b). To create space, previously deposited materials have to be digested (a-b, ***). The epithelial cells then start differentiation in an orderly fashion starting with the inner root sheath (c) and the tip of the cuticle and hair cortex of the newly formed unpigmented hair fiber (d). The resting hair remains in the hair follicle for approximately 1 to 3 months (a-e), then the detached hair is shed (f). The shiny root end of the shed hair is the club. Before, during, or after hair shedding there may be replacement by a new hair shaft (e-f-g). Indeed, under physiological conditions, the follicle proceeds immediately or only slowly with new hair production (from f to g;maximum 90 days). Certain conditions are characterized by a much longer interval before regrowth is visible. Usually, a nonpigmented hair tip is seen first (h), followed by a thicker, more pigmented, and faster-growing hair fiber (i) depending on the many regulatory factors controlling the hair follicle. (Reproduced with permission from H.A.I.R. Technology [Skinterface sprl, Tournai, Belgium].)

hair, and its protein composition contains trichohyaline. Above the level of the epidermis some medullar cells dehydrate, forming air-filled vacuoles, which are responsible for the interrupted appearance of the medulla because of the reflection of light on these air-filled spaces. The mature cortex consists of closely packed spindle-shaped cells separated by intercellular lamella cementing the cells together. Within the cells most of the microfibrils are closely packed and oriented longitudinally [8,9].

The hair cuticle consists of five to 10 overlapping cell layers imbricated like roof tiles and aimed outwards (towards the distal end of the hair). The mature cells are thin scales consisting of dense keratin. Over the newly formed part of the hair the scale margins are intact, but as the hair emerges from the skin they break off progressively. The outer surface of each cuticular cell has a very clear A-layer, which is rich in high-sulfur protein; this layer protects the cuticular cells from premature breakdown caused by chemical and physical insults [8,9].

Keratins are a group of insoluble cystine-containing helicoidal protein complexes produced in the epithelial tissues of vertebrates. Because of the resistance of these protein complexes, hairs have been said to contain hard keratins as opposed to the soft keratins of desquamating tissues [9].

CLINICAL HAIR-GROWTH-EVALUATION METHODS

Subjective evaluation and personal satisfaction of people using hair-growth modulators and/or cosmetics on a wide scale are the most important factors for the survival of these products in the market. This evaluation will be based on whether they are perceived as efficacious, especially when the benefit is cosmetic in nature (acknowledging the massive placebo effect and the possible bias). Hence, before they reach the hands of consumers, safety and efficacy testing have to be performed according to the science, ethics, and rules of good clinical practice and medical research in order to adequately support the claims made to the patient and the consumer.

For an evaluation method to be considered valuable, it should provide information about the following variables: hair density, which is the number of hairs per unit area (usually number/cm2); linear hair growth rate (LHGR) as millimeters per day; percentage of anagen growth phase (%A); hair diameter in micrometers; and time to hair regrowth after completion of telogen phase [10]. For many evaluation techniques, the methodology details are lacking as well as information about sensitivity and reproducibility usually required for clinical investigative techniques [11]. Much effort is needed for the standardization of evaluation methods in order to make it possible to compare different methods, or different results from different centers using the same method. For classification purposes these methods can be categorized as invasive, semi-invasive, and noninvasive.

Invasive methods

Biopsy

In addition to the ordinary vertical sectioning of skin biopsies which permits the study of longitudinal follicular sections, horizontal sectioning (parallel to the skin surface) of scalp biopsies offers further diagnostic opportunities. First described by Headington [12], it has been demonstrated that horizontal sectioning may provide a better diagnostic yield than vertical sectioning [13,14]. Horizontal sectioning allows the study of larger number of follicular structures. Inflammatory infiltrates are more easily seen and their relationship to the follicular structures is more obvious than in vertical sectioning. Fibrous tracts, which are often difficult to visualise on vertical sectioning, become much more apparent on horizontal sectioning. It is possible as well to distinguish vellus from terminal hairs, to identify the stages of all hairs in one section and to classify them into anagen, telogen or catagen follicles.

Semi-invasive Methods

Trichogram

The idea of estimating changes affecting hair growth by examining hair roots was first suggested by Van Scott et al. [15]. In order to examine hair root status necessary to diagnose hair disorders, at least 50 hairs should be plucked in order to reduce sampling errors. The roots are examined under a low-power microscope. The root morphology is stable and hairs can be kept for many weeks in dry packaging before analysis. Due to the relative values generated telogen/anagen (T/A) ratio, this technique is a relatively poor indicator of disease activity and/or disease severity in androgen-dependent alopecia in women [16]. In our center this method has been abandoned because it generates only relative values as compared with the method described in the following section.

Unit Area Trichogram

The unit area trichogram (UAT) is a technique in which all the hairs within a defined area (usually 60 mm2) are plucked and mounted onto double-sided tape attached to a glass slide. Optical microscopical examination of these slides estimate various hair variables as hair density, anagen%, hair length and hair diameter. The scalp area to be sampled should first be degreased (with an acetone/isopropanol mixture) and then delineated with a roller pen. All hairs contained in the area are epilated individually (one by one). Each hair is grasped at a uniform point above the scalp and the forceps are rotated to ensure firm grasp. Epilation should be performed rapidly in a single action in the direction of hair growth orientation, in order to minimize trauma to the roots [17].

The unit area trichogram is one of the rare exceptions to a strange general rule or law in trichology; indeed, most methods are promoted along with a new drug or a new cosmetic efficacy evaluation program. The exception in the unit area trichogram is that the method has been evaluated independently in terms of reproducibility and clinical relevance. Therefore, it could serve for comparative purposes. Most hair-growth variables estimated through unit area trichogram and the phototrichogram are comparable. However, the unit area trichogram has the advantage in that it can be used reliably in subjects in whom there is no contrast between hair and skin color [18].

Noninvasive Methods

Global Methods

Scoring Classification Systems The patterns produced by the gradual process of scalp hair loss in male pattern baldness were first described by Hamilton in 1951. In 1975, Norwood proposed a modification of Hamilton's classification. In this modification he mentioned three patterns that referred to women. Finally, in 1977 Ludwig published the stages of female androgenetic alopecia in three patterns. For more details we refer the interested reader to the following references: Camacho F, Montagna W [19] and Ludwig E, Montagna W, Camacho F [20]. Although static by definition, such diagrams can be enriched by more gradual variations [8], an updated version of which appears in Figure 2, but these will only rarely match the continuum that one observes in the hair clinic.

Global Photography Global photography apprehends all factors involved in hairiness at once and can be used for drug efficacy evaluation provided that adequate scalp preparation and hair style are maintained throughout the study. This is the most patient-friendly photographic method. This method is used in the clinic under standardized conditions of exposure [21]. Processing and rating have to be performed under controlled (i.e., blinded as to treatment and/or time) conditions. Trained raters could generate reproducible data.

Daily Collection of Shed Hair The cyclic hair growth activity results in a daily shedding process in which telogen hairs are shed to be replaced by anagen hairs. The reported normal average daily loss of hair ranges somewhere between 40 to 180 hairs per day. In a study of 404 females without hair or scalp disease, lost hair was collected daily over 6 weeks in the aim of comparing two shampoos. Results showed mean hair loss rates ranging from 28 to 35 per day. No significant differences were noted in the mean daily hair loss rates during the 2-week baseline and the 4-week treatment period [22].

Figure 2 Scoring of androgen-dependent alopecia (ADA) in men. The present classification shows ADA patterns that affect the scalp of genetically susceptible male subjects after puberty. They are subdivided in six stages from mild to severe balding (1-6). The anterior pattern (A) indicates a backward progression of hair follicle miniaturization and deficient hair production with the ensuing bald appearance. The vertex type (V) indicates isolated regression occurring on the vertex but this is usually combined with the involvement of the frontal temporal areas. (Reproduced with permission from H.A.I.R. Technology [Skinterface sprl, Tournai, Belgium].)

Quantitating daily hair loss in women was assessed in another study of 234 women complaining of hair loss among which 89 had apparently normal hair density. They have found that subjects with normally dense hair (although complaining about hair loss) shed less than 50 hairs a day [16]. So the magic number of 100 so often referred to in textbooks and found in the lay press should be seriously revisited. Less than 50 hairs can be significantly abnormal in a patient having lost 50% of his hair. Further standardization studies are currently being run in our laboratory.

Hair Weight and Hair Count The efficacy of hair-growth-promoting agents can be established by comparing the total hair mass (weight) and counts of grown hair in a small, carefully maintained area of the scalp [23,24]. A plastic sheet with a 1.2 cm2 hole was placed over the selected site. All hairs within the square hole were pulled through it and hand clipped to 1 mm in length. The apparent advantage of this method is that it provides a global measurement of growth on a small sample size for the detection of drug effects and between treatment regimens (e.g., 2 vs. 5% minoxidil) [24]. One must be aware of the technical skills necessary to handle the samples in the proper way to avoid the loss of some hairs between the clinic and the laboratory. Again, as for many of these

techniques, the methodological comparisons are lacking and there are no evaluations of the reproducibility and sensitivity usually required for laboratory evaluation methods because they were introduced on the occasion of drug evaluation protocols. The major limitation of this method is that it generates a global index of growth, the individual components of which cannot be analyzed separately.

Hair-Pull Test The hair-pull test is based on the idea that ''gentle'' pulling of the hair brings about the shedding of telogen hairs [16]. It is a very rough method and difficult to standardize because it is subject to so much interindividual variation among the investigators. Physically speaking, the pulling force is not uniformly distributed over the whole hair bundle, thereby creating variation in the pulling force from one hair to another. It seems to be useful only in acute and severe conditions, not in chronically evolving conditions like androgen-dependent alopecia.

Analytical Methods

Phototrichogram The basic principle of the phototrichogram (PTG) consists of taking a photograph of a certain area of the scalp in which the hair is cut in preparation for the photograph, and to repeat this photographic documentation after a certain time period. This period of time should be long enough to permit the evaluation of the growth of a hair segment (which is usually between 24-72 h). The growth is then evaluated by comparing the two pictures. Hairs that have grown are in anagen phase and those that have not are in telogen phase (Fig. 3).

The assessment is made on defined scalp sites considered representative of the condition. The data that can be generated from a PTG include the total number of hairs present in a certain surface area, which allows us to calculate hair density (N/cm2). Hair density is a quantitative element through which we can estimate the degree of hair loss.

Also from a PTG, we can determine the percentage of hairs in the growth phase

Figure 3 Day 2 picture of scalp hair (48 hrs after clipping short all scalp hairs from the photographed scalp site): long growing hairs represent follicles in anagen phase;shorter nongrow-ing hairs represent follicles in telogen.

(anagen%) and can calculate the LHGR. Meanwhile, the reliability of the evaluation of hair thickness has been the subject of detailed analysis. The most precise instrument used for hair-diameter evaluation remains the microscope.

One of the main advantages of the PTG is that first of all it is a patient-friendly method. Secondly it is a totally noninvasive method so it does not affect the natural process of hair growth/loss by itself. However, many patients are afraid of the idea of having their hair cut at one or more given scalp surface sites (area ± 1 cm2 in our protocol). Most are reassured by the fact that this process cannot prevent them from enjoying a normal private and social life. Finally, PTG also permits the chronological follow-up of exactly the same area under the study, and this has been shown to bring about a lot of valuable information [25]. Some technical improvements have been introduced during the course of evolution of the PTG technique. For example, the application of a frontal window with a glass slide mounted on it has been considered a major improvement [26,27]. It reduces the curvature of the scalp and permits a better image clarity.

Some technical photography problems have been identified during the course of the evolution of the PTG, and a series of detailed analysis performed at our laboratory and clinic have pinpointed a number of them, including the primary enlargement factor (PEF), which is one of the factors responsible for the ''visibility'' of hair on a photograph [28]; the secondary enlargement factor (size of printouts); and the experience of technicians. A further improvement was the development of scalp immersion proxigraphy (SIP), which is routinely used at our hair clinic and permits a better diffusion of light through a medium of lower optic heterogeneity [29].

After comparison with UAT [18], weak points of the method have been considered with great care, and using photography in combination with hair-micrometry results in a valid method for global hair perception while allowing an analytical description of all variables intervening in hair-quality evaluation.

Variants of Phototrichogram

Video PTG In this method, the photographic camera is replaced by a video camera equipped with specific lenses. In fact, recent reports in which this method has been used have been on Asians. In these subjects the contrast between hair and scalp seems favorable for the application of this method. Moreover, the reported low figures of hair density could possibly be racial in origin. However, we advise taking these factors into account in order to keep the biological variation as low as possible [30]. The recent introduction of cheap CCD cameras will certainly contribute to further developments in this field.

Traction PTG This test is based on the fact that hairs that can be easily pulled from the scalp are in telogen and those resisting pull are in anagen [31]. This test has been performed on a surface area of 0.25 cm2. Hairs present at this surface area are held gently between the thumb and index fingers and pulled repeatedly. Hairs that can be easily pulled are counted and their number is considered the number of telogen hairs. Those resisting pulling are clipped and counted, and their number represents the anagen hairs. Through this method, we can calculate the hair density per unit area as well as the anagen%.

It is necessary to evaluate this semi-invasive method more critically to define its reproducibility through the standardization of the pulling technique. Other comparative studies may be essential as well to estimate the sensitivity and specificity of this method,

which as it stands today would be rated as flawed with many weak points (e.g., small surface area, lack of control on traction forces etc.).

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