Crystal Aguh - Fundamentals of Ethnic Hair: The Dermatologists Perspective

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Part I
Structure and Function of Hair

Springer International Publishing Switzerland 2017

Crystal Aguh and Ginette A. Okoye (eds.) Fundamentals of Ethnic Hair 10.1007/978-3-319-45695-9_1

1. Chemical and Physical Properties of Hair: Comparisons Between Asian, Black, and Caucasian Hair

Alice He 1 and Ginette A. Okoye 2

(1)

Johns Hopkins University School of Medicine, 733 N. Broadway, Baltimore, MD 21205, USA

(2)

Department of Dermatology, Johns Hopkins University School of Medicine, 5200 Eastern Avenue, Suite 2500, Baltimore, MD 21224, USA

Keywords

Hair structure Disulfide bonds Cuticle Cortex Cystine Curl type Curl pattern

Introduction

Although the fundamental structure and function of the hair are similar among all races, there are important anatomic and molecular differences that contribute to the unique characteristics of ethnic hair and impact its health and management. Hair researchers have generally classified hair into African, Asian, and Caucasian subgroups []. Although this may be an oversimplification, this classification scheme is used in this chapter for the sake of uniformity. These racial subgroups may hold true when discussing the structure of the hair, but when discussing hair management and cosmetic product selection, the relative curliness of the hair may be more important than race.

Hair Structure

The epidermal component of the hair, called the hair shaft, is the portion of the hair that exits the scalp. The dermal components of the hair include the hair follicle (also called hair bulb or hair root) with its stem cells, blood supply, sebaceous (oil) glands, and inner and outer root sheaths (Fig. ).

Fig. 1.1

( a ) Longitudinal section of the hair depicting the epidermal and dermal components. ( b ) Cross-section of a hair shaft depicting the relationship between the three layers of the hair shaft (epidermal component)

Anatomy of the Hair: Cuticle, Cortex, and Medulla

The hair shaft is the part of the hair that is most susceptible to the effects of environmental conditions and cosmetic preparations and procedures. From the external surface inwards, the hair shaft comprises the cuticle, cortex, and medulla (Fig. ).

The Cuticle

The cuticle is the outermost layer of the hair shaft and is composed of the protein keratin. It protects the underlying cortex by providing a barrier to chemicals and water []. A healthy, intact cuticle has a smooth surface and low friction in the root to tip direction and contributes to the sheen associated with healthy hair. A damaged cuticle results in hair that is frizzy, dull, and prone to breakage.

Fig. 1.2

Electron micrograph showing the overlapping, scale-like cells of the cuticle layer. (Reprinted from: Wolfram LJ. Human hair: a unique physicochemical composite. J Am Acad Dermatol. 2003;48(6 Suppl):S106-14, with permission from Elsevier)

Although the chemical composition of the cuticle is similar in all hair types, there are decreasing numbers of cuticular cell layers in Asian, Caucasian, and African hair [].

The outer aspect of the cuticle contains lipids (fatty acids, ceramides, and cholesterol) that contribute to the barrier function of the cuticle and promote the hydrophobicity and low friction of healthy hair [).

The Cortex

The majority of the mass and the tensile strength of the hair shaft can be attributed to the cortex [].

There is a strong adhesive layer between the cells of the cortex, known as the cell membrane complex (CMC ). The CMC is vulnerable to cosmetic chemical hair treatments, such as bleaching, dyeing, straightening, and perming [].

Cortical cells in human hair are divided into different regions termed orthocortex, paracortex, and mesocortex [].

Cystine and Chemical Bonds in the Cortex

There is no difference in the cystine content of keratin proteins between African hair and that of other racial groups [].

In addition to disulfide bonds, keratin proteins are also linked by weaker bonds, such as hydrogen bonds, which can be easily disrupted by water to create temporary hair styles, e.g., using rollers on wet hair to create curls (wet-setting) [].

The Medulla

The medulla forms the porous, empty center of the hair fiber (Fig. ].

In summary, a healthy hair shaft has an intact, smooth cuticle with high lipid content from root to tip and a strong cortex with intact CMCs and many disulfide bonds. These basic building blocks of healthy hair are similar in all races/ethnicities, but are vulnerable to disruption by cosmetic products and styling practices. Significant changes in the disulfide and hydrogen bonds in keratin is crucial to nearly any modification to hair, including permanent curling/straightening procedures, bleaching, wet-setting, and even daily grooming procedures such as shampooing.

Anatomy of the Hair: Dermal Structures

The Inner and Outer Root Sheaths

In the dermis, the inner root sheath (IRS) surrounds the hair shaft cuticle layer (Fig. ). External to the IRS is the outer root sheath (ORS) .

Sebaceous Glands

Conclusions from studies on racial differences in sebaceous gland size and activity are conflicting. Current opinion suggests that there are very few differences among different racial groups in this regard. However, in African hair and other curly hair types it is more difficult for sebum to make its way from the scalp down the hair shaft. Thus, curly hair types tend to be relatively dry and require regular application of cosmetic products to promote moisture retention.

Blood Supply

Studies on the racial differences of cutaneous blood supply have also shown conflicting results. However, it has been suggested that blood flow to the hair follicle in blacks is lower compared to whites, and this may contribute to the increased prevalence of scarring alopecia in black women [].

Elastic Fibers

Differences in elastic fibers among racial groups have been reported. Black patients had fewer elastic fibers anchoring their hair follicles as compared to whites []. This observation may explain black patients susceptibility to traction alopecia.

Physical Characteristics of the Hair

Growth Properties

The human hair follicle grows in a continuous cyclical pattern characterized by a period of growth (anagen) followed by involution (catagen) and resting (telogen) periods [].

Fig. 1.3

The hair growth cycle

At any given time, approximately 90 % of the hairs on the scalp are in anagen phase, and approximately 1015 % of hairs are in telogen phase []. However in telogen effluvium, a common cause of non-scarring alopecia, there is an increase in the number of hairs in telogen phase which results in shedding of a larger than normal number of hairs. This is often a temporary condition that resolves over time without treatment.

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