How different is human hair? A critical appraisal of the reported differences in global hair fibre characteristics and properties towards defining a more relevant framework for hair type classification.

This review critically appraises the reported differences in human hair fibre within three related domains of research: hair classification approaches, fibre characteristics and properties. The most common hair classification approach is based on geo-racial origin, defining three main groups: African, Asian and Caucasian hair. This classification does not account sufficiently for the worldwide hair diversity and intergroups variability in curl, shape, size and colour. A global classification into eight curl types has been proposed but may be too complex for reproducibility. Beyond that, hair cross-sectional shape and area have been found to have an inverse relation to curl: straighter fibres are circular with larger cross-sectional area, whilst the curlier fibres are elliptical with smaller cross-sectional area. These geometrical differences have been associated with bilateral vs homogenous distribution of cortical cell in curly vs straight hair respectively. However, there is no sufficient data demonstrating significant differences in hair amino composition, but proteomic studies are reporting associations of some proteins with curly hair. Eumelanin's relative abundance has been reported in all hair colours except for red hair which has a high pheomelanin content. Higher tensile and fatigue strength of straight hair are reported, however, curly hair fragility is attributed to knotting, and crack and flow formations rather than the structural variations. African hair has been found to have the highest level of lipids, whilst the water sorption of Caucasian hair is the highest, and that of Asian hair the lowest. Not all comparative studies clearly report their hair sampling approaches. Therefore, to strengthen the robustness of comparative studies and to facilitate cross-study data comparisons, it is recommended that the following hair defining characteristics are reported in studies: hair cross sectional diameter/area, curl type, hair assembly colour, as well as where possible donor data (age/gender) and sample pooling approach.

Cette revue évalue de façon critique les différences rapportées sur les fibres capillaires humaines dans trois domaines de recherche connexes : les approches de classification des cheveux, les caractéristiques et les propriétés des fibres. L'approche la plus courante de classification des cheveux est basée sur l'origine géo-raciale, définissant trois groupes principaux : les cheveux africains, asiatiques et caucasiens (européen). Cette classification ne prend pas suffisamment en compte la diversité des cheveux dans le monde et la variabilité des boucles, de la forme, de la taille et de la couleur découlant de la mixité de groupes. Une classification globale de huit types de boucles a été proposée, mais elle pourrait être trop complexe pour être reproductible. De plus, il est à souligner que la forme et la surface de la coupe transversale des cheveux ont une relation inverse à la boucle : les fibres plus droites sont circulaires avec une plus grande section transversale, tandis que les fibres plus bouclées sont elliptiques avec une plus petite section transversale. Ces différences géométriques ont été associées à une distribution bilatérale des cellules corticales dans les cheveux bouclés par opposition à une distribution homogène sur les cheveux droits. Cependant, il n'y a pas suffisamment de données démontrant des différences significatives dans la composition aminée des cheveux, mais des études protéomiques relèvent des associations de certaines protéines avec des cheveux bouclés. L'abondance relative de l'eumélanine a été trouvé dans tous les coloris de cheveux, à l'exception des cheveux roux qui ont un contenu élevé en phaeomélanine. Une plus grande résistance des cheveux droits à la traction et à la fatigue est signalée, mais la fragilité des cheveux bouclés est attribuée au nouage, et aux formations de fissures et d'écoulement plutôt qu'aux variations structurelles. Les cheveux africains ont le plus haut niveau de lipides, tandis que l'absorption d'eau des cheveux caucasiens est le plus élevé, et celle des cheveux asiatiques le plus bas. Toutes les études comparatives ne rapportent pas clairement leurs méthodes d'échantillonnage capillaire. Par conséquence, afin de renforcer la solidité des études comparatives et de faciliter les comparaisons de données entre études, il est recommandé que les caractéristiques des cheveux suivantes soient divulguer dans les études : diamètre et surface de la coupe transversale des cheveux, type de boucle, couleur de montage des cheveux, ainsi que, si possible, des données sur les donneurs (âge/sexe) et une approche de regroupement d'échantillons.

Following historical "tracks" of hair follicle miniaturisation in patterned hair loss: Are elastin bodies the forgotten aetiology?

Pattern hair loss (PHL) is a chronic regressive condition of the scalp, where follicular miniaturisation and decreased scalp hair coverage occurs in affected areas. In all PHL cases, there is a measurable progressive shortening of the terminal hair growth duration, along with reduced linear growth rates. In both genders, PHL initially shows an increase in short telogen hairs ≤30 mm in length, reflecting a cycle completion of under 6 months in affected terminal hair follicles. To understand the miniaturisation process, we re-examine the dynamics of miniaturisation and ask the question, "why do miniaturised hair follicles resist treatment?" In the light of recent developments in relation to hair regeneration, we looked back in the older literature for helpful clues "lost to time" and reprise a 1978 Hermann Pinkus observation of an array of elastin deposits beneath the dermal papilla following subsequent anagen/telogen transitions in male balding, originally described by Arao and Perkins who concluded that these changes provide a "morphologic marker of the entire biologic process in the balding scalp." Thus, we have reviewed the role of the elastin-like bodies in hair pathology and we propose that alterations in elastin architecture may contribute to the failure of vellus-like hair reverting back to their terminal status and may indicate a new area for therapeutic intervention.

Age-Related Changes in Female Scalp Dermal Sheath and Dermal Fibroblasts: How the Hair Follicle Environment Impacts Hair Aging.

In women, aging leads to reduced hair density and thinner fibers and can result in female-pattern hair loss. However, the impact of the aging dermal environment on female scalp hair follicles remains unclear. In this study, we document in situ changes in 22 women (aged 19-81 years) and primary cultures of dermal fibroblast and dermal sheath cells. In situ, the papillary reticular boundary was indistinguishable in the young scalp but prominent in the scalp of those aged >40 years, accompanied by reduced podoplanin (PDPN) expression, increased versican expression, and changes in collagen organization. Hair follicles were shorter, not reaching the adipose layer. Hyaluronic acid synthase 2 was highly expressed, whereas matrix metalloproteinase 1 was elevated in the dermal papilla and dermal sheath in situ. Primary dermal fibroblast cultures confirmed that matrix metalloproteinase 1 mRNA, MMP1, increased with aging, whereas in dermal sheath cells, hyaluronic acid synthase 2, HAS2, and PDPN increased and α-smooth muscle actin αSMA mRNA decreased. Both exhibited increased cartilage oligomeric protein, COMP mRNA expression. Proteomics revealed an increase in dermal sheath proteins in the dermal fibroblast secretome with aging. In summary, aging female scalp shows striking structural and biological changes in the hair follicle environment that may impact hair growth.

Cryptochrome 1 is modulated by blue light in human keratinocytes and exerts positive impact on human hair growth.

Photoactivation of cryptochrome-family proteins by blue light is a well-established reaction regulating physiology of plants, fungi, bacteria, insects and birds, while impact of blue light on cryptochrome synthesis and/or activity in human non-visual cells remains unknown. Here, we show that 453 nm blue light induces cryptochrome 1 (CRY1) accumulation in human keratinocytes and the hair follicle. CRY1 is prominently expressed in the human anagen hair follicle, including epithelial stem cells. Specific silencing of CRY1 promotes catagen, while stimulation of CRY1 by KL001 prolongs anagen ex vivo by altering the expression of genes involved in apoptosis and proliferation. Together, our study identifies a role for CRY1 in sustaining human hair growth. Previously, we demonstrated positive effects of 453 nm blue light on hair growth ex vivo. Taken all together, our study suggests that CRY1 might mediate blue light-dependent positive effects on hair growth.

The biology and genetics of curly hair.

Hair fibres show wide diversity across and within all human populations, suggesting that hair fibre form and colour have been subject to much adaptive pressure over thousands of years. All human hair fibres typically have the same basic structure. However, the three-dimensional shape of the entire fibre varies considerably depending on ethnicity and geography, with examples from very straight hair with no rotational turn about the long axis, to the tightly sprung coils of African races. The creation of the highly complex biomaterials in hair follicle and how these confer mechanical functions on the fibre so formed is a topic that remains relatively unexplained thus far. We review the current understanding on how hair fibres are formed into a nonlinear coiled form and which genetic and biological factors are thought to be responsible for hair shape. We report on a new GWAS comparing low and high curl individuals in South Africa, revealing strong links to polymorphic variation in trichohyalin, a copper transporter protein CUTC and the inner root sheath component keratin 74. This builds onto the growing knowledge base describing the control of curly hair formation.

The biology of hair diversity.

Hair diversity, its style, colour, shape and growth pattern is one of our most defining characteristics. The natural versus temporary style is influenced by what happens to our hair during our lifetime, such as genetic hair loss, sudden hair shedding, greying and pathological hair loss in the various forms of alopecia because of genetics, illness or medication. Despite the size and global value of the hair care market, our knowledge of what controls the innate and within-lifetime characteristics of hair diversity remains poorly understood. In the last decade, drivers of knowledge have moved into the arena of genetics where hair traits are obvious and measurable and genetic polymorphisms are being found that raise valuable questions about the biology of hair growth. The recent discovery that the gene for trichohyalin contributes to hair shape comes as no surprise to the hair biologists who have believed for 100 years that hair shape is linked to the structure and function of the inner root sheath. Further conundrums awaiting elucidation include the polymorphisms in the androgen receptor (AR) described in male pattern alopecia whose location on the X chromosome places this genetic contributor into the female line. The genetics of female hair loss is less clear with polymorphisms in the AR not associated with female pattern hair loss. Lifestyle choices are also implicated in hair diversity. Greying, which also has a strong genetic component, is often suggested to have a lifestyle (stress) influence and hair follicle melanocytes show declining antioxidant protection with age and lowered resistance to stress. It is likely that hair research will undergo a renaissance on the back of the rising information from genetic studies as well as the latest contributions from the field of epigenetics.

Modulation in proteolytic activity is identified as a hallmark of exogen by transcriptional profiling of hair follicles.

Exogen is the process by which the hair follicle actively sheds its club fiber from the follicle. However, little is known about signals that govern the cellular mechanisms of shedding. Here, we have identified factors that are important in regulating either the retention or release of the hair club fiber from its epithelial silo within the follicle. Using the vibrissa follicle as our model, we isolated follicle segments containing club fibers and surrounding follicle tissue at different time points before their natural release from the hair follicle. We then performed microarray analysis to identify key molecular changes as the club fiber approached final release. Among the different classes of genes that were identified, we found changes in the expression pattern of protease inhibitors and proteases, suggesting that proteolysis may mediate fiber release, either through terminal differentiation or proteolytic cleavage. We validated transcriptional changes using reverse transcription-PCR, and further immunofluorescence analysis indicated that protease inhibitors surrounding the club fiber may have an important role in regulating the process of club fiber shedding. Our findings also highlighted that molecular differentiation of the innermost layer of cells immediately surrounding the club fiber, the companion(CL), is likely to be important in hair shedding.

Modelling the hair follicle dermal papilla using spheroid cell cultures.

Human dermal papilla (DP) cells grown in two-dimensional (2D) culture have been studied extensively. However, key differences exist between DP cell activities in vivo and in vitro. Using a suspension method of cell culture to maintain DP cells, we created three-dimensional (3D) dermal spheres morphologically akin to intact (anagen) DPs. Analysis of these spheres using immunocytochemistry demonstrates that they have expression profiles different from papilla cells cultured in 2D but with many similarities to intact DPs. This method of DP cell culture may provide us with a tool to elucidate our understanding of signalling within the DP as it relates to induction, maintenance or even inhibition of hair growth.

Exogen involves gradual release of the hair club fibre in the vibrissa follicle model.

Exogen is a distinct phase of the hair cycle describing the process by which the hair club fibre is shed from the follicle. This process is difficult to study in human skin and little is known about the mechanisms involved in the release of club fibres. We sought an alternative model system to study exogen in more detail, and therefore utilised the vibrissa system on the rodent mystacial pad. The time at which a vibrissa club hair will be lost can be predicted, based on the relative lengths of the new growing fibre and old club fibre. This timing phenomenon was exploited to investigate the club fibre within the follicle as it approaches final release, revealing key changes in the adhesive state of the club fibre within the epithelial sac as it approached release. We propose that exogen should be subdivided to represent variations in the club fibre status.

From telogen to exogen: mechanisms underlying formation and subsequent loss of the hair club fiber.

The hair follicle has the unique capacity to undergo periods of growth, regression, and rest before regenerating itself to restart the cycle. This dynamic cycling capacity enables mammals to change their coats, and for hair length to be controlled on different body sites. More recently, the process of club fiber shedding has been described as a distinct cycle phase known as exogen, and proposed to be an active phase of the hair cycle. This review focuses on the importance of the shedding phase of the hair cycle and, in the context of current literature, analyzes the processes of club fiber formation, retention, and release, which may influence progression through exogen, particularly in relation to human hair.

Temporary hair removal by low fluence photoepilation: histological study on biopsies and cultured human hair follicles.

BACKGROUND AND OBJECTIVES: We have recently shown that repeated low fluence photoepilation (LFP) with intense pulsed light (IPL) leads to effective hair removal, which is fully reversible. Contrary to permanent hair removal treatments, LFP does not induce severe damage to the hair follicle. The purpose of the current study is to investigate the impact of LFP on the structure and the physiology of the hair follicle. STUDY DESIGN/MATERIALS AND METHODS: Single pulses of IPL with a fluence of 9 J/cm(2) and duration of 15 milliseconds were applied to one lower leg of 12 female subjects, followed by taking a single biopsy per person, either immediately, or after 3 or 7 days. Additionally, we present a novel approach to examine the effects of LFP, in which ex vivo hairy human scalp skin was exposed to IPL pulses with the same parameters as above, followed by isolation and culturing of the hair follicles over several days. Samples were examined histologically and morphologically. RESULTS: The majority of the cultured follicles that had been exposed to LFP treatment showed a marked treatment effect. The melanin containing part of the hair follicle bulb was the target and a catagen-like transformation was observed demonstrating that hair formation had ceased. The other follicles that had been exposed to LFP showed a less strong or no response. The skin biopsies also revealed that the melanin-rich region of the hair follicle bulb matrix was targeted; other parts of the follicle and the skin remained unaffected. Catagen/telogen hair follicles were visible with unusual melanin clumping, indicating this cycle phase was induced by the IPL treatment. CONCLUSIONS: Low fluence photoepilation targets the pigmented matrix area of the anagen hair follicle bulb, causing a highly localized but mild trauma that interrupts the hair cycle, induces a catagen-like state and eventually leads to temporary loss of the hair.