31: Hair Physiology and Grooming


CHAPTER 31
Hair Physiology and Grooming


Maria Hordinsky1, Sherman Chu2, Ana Paula Avancini Caramori3, and Jeff C. Donovan4


1 University of Minnesota, Minneapolis, MN, USA


2 College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, Northwest, Lebanon


3 Hospital São Lucas, PUCRS, Porto Alegre, Brazil


4 University of Toronto, Toronto, ON, Canada


Definitions


The use of hair cosmetics is ubiquitous among men and women of all ages. Virgin hair is the healthiest and strongest but basic grooming and cosmetic manipulation cause hair to lose its cuticular scale, elasticity, and strength. Brushing, combing, and shampooing can inflict damage to the hair shaft, much of which can be reversed with the use of hair conditioners. In this chapter, the physiology of hair, grooming techniques including the science and use of shampoos, conditioners, and hair camouflage techniques are reviewed.


Physiology


Hair follicle


The hair follicle is a complex structure that demonstrates the ability to completely regenerate its product, the hair fiber – hair grows, falls out, and then regrows. Plucked hairs can regrow. Important cells for the development of hair follicles include stem cells in the bulge region and dermal papilla cells [1]. Hair follicle stem cells are described as being present just below the entrance of the sebaceous duct into the hair follicle. The hair follicle’s complexity is further appreciated when examining the organization of follicles in the scalp and the complexity of its vascular complex and nerve innervation. Scalp hair follicles present in groups of one, two, three, or four follicular units (Figure 31.1). These follicular units typically contain primarily terminal anagen follicles with sebaceous glands and ducts, arrector pili muscles, and sweat ducts.


The hair follicle is defined histologically as consisting of several layers (Figure 31.2). It is the interaction of these layers that produces the hair fiber. The internal root sheath consists of a cuticle which interdigitates with the cuticle of the hair fiber, followed by Huxley’s layer, then Henle’s layer. Henle’s layer is the first to become keratinized, followed by the cuticle of the inner root sheath. The Huxley layer contains trichohyalin granules and serves as a substrate for citrulline‐rich proteins in the hair follicle, that gives strength to the inner root sheath to support and mold the growing hair shaft. The outer root sheath has specific keratin pairs, K5–K14, characteristic of basal keratinocytes, and the K6–K16 pair characteristic of hyperproliferative keratinocytes, similar to what is seen in the epidermis. Keratin K19 has been located in the bulge region [2].


The complexity of the hair follicle is further demonstrated by the fact the follicle cycles from the actively growing phase (anagen), through a transition phase (catagen), and finally a loss phase (telogen). The signals associated with the transition from anagen, catagen to telogen are the subject of current research activities in this field.


Structural differences of hair exist among ethnic groups. African hair follicles are typically asymmetrical with elliptical or oval shapes in cross section. In contrast, Asian hair is round‐shaped and straight, while Caucasian hair is structurally between Asian and African hair [3, 4]. In particular, African hair fibers tend to have flattened or irregular hair shafts, less water content, thinner cuticle layers, and lower follicular density compared to Caucasian hair. These observations have been associated with lower resistance and being more prone to damage [3, 5].

Schematic illustration of (a) Horizontal section of a 4 mm scalp biopsy specimen demonstrating follicular units containing 1, 2, 3, or 5 anagen follicles. (b) Vertical section of a 4-mm scalp punch biopsy specimen from a normal, healthy Caucasian female in her early 1920s demonstrating normal anagen follicles.

Figure 31.1 (a) Horizontal section of a 4 mm scalp biopsy specimen demonstrating follicular units containing 1, 2, 3, or 5 anagen follicles. (b) Vertical section of a 4‐mm scalp punch biopsy specimen from a normal, healthy Caucasian female in her early 1920s demonstrating normal anagen follicles.


Product of the hair follicle: the hair fiber


The hair follicle generates a complex fiber which may be straight, curly, or somewhere in between. The main constituents of hair fibers are sulfur‐rich proteins, lipids, water, melanin, and trace elements. The cross‐section of a hair shaft has three major components, from the outside to the inside: the cuticle, the cortex, and the medulla.


Fibers can be characterized by color, shaft shape – straight, arched, or curly – as well as microscopic features. The cuticle can be defined by its shape – smooth, serrated, or damaged, and whether or not it is pigmented. The cortex can be described by its color and the medulla by its distribution in fibers. It can be absent, uniform, or randomly distributed. Lastly, fibers can be abnormal and present with structural hair abnormalities such as trichoschisis or trichorrhexis nodosa. Both of these structural abnormalities can commonly be seen in patients with hair fiber injury related to routine and daily cosmetic techniques including application of high heat, frequent perming, as well as from weathering, the progressive degeneration from the root to the tip of the hair initially affecting the cuticle, then later the cortex [6].


The cuticle is also composed of keratin and consists of six to eight layers of flattened overlapping cells resembling scales. The cuticle consists of two parts: endocuticle and exocuticle. The innermost endocuticle covers the exocuticle. The exocuticle lies closer to the external surface and comprises three parts: b‐layer, a‐layer, and epicuticle. The epicuticle is the most external layer and is a hydrophobic lipid layer of 18‐methyleicosanoic acid on the surface of the fiber, or the f‐layer. The cuticle protects the underlying cortex and acts as a barrier and is considered to be responsible for the luster and the texture of hair. When damaged by frictional forces or chemicals and subsequent removal of the f‐layer, the first hydrophobic defense, the hair fiber becomes much more fragile.


The cortex is the major component of the hair shaft and makes up the majority of hair fiber composition. It lies below the cuticle and contributes to the mechanical properties of the hair fiber, including strength and elasticity. The cortex consists of elongated shaped cortical cells rich in keratin filaments as well as an amorphous matrix of sulfur proteins. Cysteine residues in adjacent keratin filaments form covalent disulfide bonds, which confer shape, stability, and resilience to the hair shaft. Other weaker bonds such as the Van der Waals interactions, hydrogen bonds, and coulombic interactions, known as salt links, have a minor role. These bonds can be easily broken just by wetting the hair. Melanin is also present in the cortex and it is the presence of this protein that gives hair color; otherwise, the fiber would not be pigmented.


The medulla appears as continuous, discontinuous, or absent under microscopic examination of human hair fibers. It is viewed as a framework of keratin supporting thin shells of amorphous material bonding air spaces of variable size. Fibers with large medullas can be seen in samples obtained from porcupines or other animal species. Other than in gray hairs, human hairs show great variation in their medullas.


Human hair keratins


Human hair keratins are complex [7]. The hair keratin family has been described as consisting of 17 members, eleven type I acidic and six type II basic keratins. These keratins exhibit a particularly complex expression pattern in the hair‐forming compartment of the follicle [7] (Figure 31.2). In addition, 26 human keratin genes have been described to be largely restricted to the hair follicle [8].

Schematic illustration of not quite sure how this is handled. The figure below from this journal is the one we would like to use. Do I need to come up with a new legend or can we use what is available and just acknowledge? The article and pictures are all readily available.

Figure 31.2 Not quite sure how this is handled. The figure below from this journal is the one we would like to use. Do I need to come up with a new legend or can we use what is available and just acknowledge? The article and pictures are all readily available.


The human type I hair keratin subfamily is comprised of 11 members that are subdivided into several groups. Both group A (hHa1, hHa3‐I, hHa3‐II, hHa4) and group B (hHa7, hHa8) contain highly related hair keratins, whereas group C (hHa2, hHa5, hHa6) is comprised of structurally unrelated hair keratins [9]. The newer type I members, Ka35 and Ka36, are grouped independently of the other type I hair keratins but have been associated with group C and group B, respectively [10, 11].


The human type II hair keratin subfamily consists of six individual members which are divided into two groups. Group A members hHb1, hHb3, and hHb6 are structurally related, while group C members hHb2, hHb4, and hHb5 are considered to be rather distinct. Both in situ hybridization and immunohistochemistry on anagen hair follicles have demonstrated that hHb5 and hHb2 are present in the early stages of hair differentiation in the matrix (hHb5) and cuticle (hHb5, hHb2), respectively. Cortical cells simultaneously express hHb1, hHb3, and hHb6 at an advanced stage of differentiation. In contrast, hHb4 has been undetectable in hair follicle extracts and sections but has been identified as the most significant member of this subfamily in cytoskeletal extracts of dorsal tongue [6, 8].


Grooming


Shampoos: formulations and diversity


Cleaning hair is viewed as a complex task because of the area that needs to be treated. The shampoo product has to also do two things – maintain scalp hygiene and beautify hair. A well‐designed conditioning shampoo can provide shine to fibers and improve manageability, whereas a shampoo with high detergent properties can remove the outer cuticle and leave hair frizzy and dull [12]. This balancing act between good cleaning and beautifying the hair is an art achieved by mixing various ingredients in the correct proportion in the shampoo preparation.


Formulations


Shampoos contain molecules with both lipophilic and hydrophilic sites. The lipophilic sites bind to sebum and oil‐soluble dirt and the hydrophilic sites bind to water, permitting removal of the sebum with water rinses. There are five basic categories of shampoo detergents: anionics, cationics, amphoterics, nonionics, and natural (Table 31.1). Each of these groups possesses different hair cleansing and conditioning qualities. Modern shampoos contain a mixture of surfactants (usually between two and four) for providing optimum cleaning levels according to hair type and requirement – normal, oily, dyed, permed, colored, or damaged hair. The detergent listed first denotes the primary cleanser which is in highest concentration and the detergent listed second is usually the secondary cleanser designed to offset the shortcomings of the primary detergent.


Anionic detergents


Anionic detergents have a negatively charged hydrophilic polar group and are quite good at removing sebum; however, they tend to leave hair rough, dull with frizz, and subject to static electricity. Anionic detergents can be grouped into different classes that have different selective properties and are typically indicated for patients with oily scalp skin and are not recommended for those with bleached, damaged, and frizzy hair. The classes of anionic detergents include lauryl sulfates, laureth sulfates, sarcosines, and sulfosuccinates and are described in more detail below [12].

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Nov 13, 2022 | Posted by in Dermatology | Comments Off on 31: Hair Physiology and Grooming
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