Hair Morphology 2

Clinical Features

Differences in head hair morphology among different human populations have long been recognized (Trotter, 1938). Hrdy (1973) reported quantitative variation in hair from among 7 populations worldwide, including individuals from northwest Europe, Africa, the Solomon Islands, and Japan. Assessments included differences in diameter, medullation, cuticular scales, kinking, and curvature. The greatest hair curvature and kinking was observed among Africans, and the greatest diameter (see 612630) was observed among Japanese. Hrdy (1973) concluded that a small number of genes with varying expression resulting from environmental forces determine differences in hair morphology among different populations.

Franbourg et al. (2003) used several methods to examine the differences between head hair from persons of Caucasian, Asian, and African descent. X-ray analysis showed that all 3 hair types were similar in keratin protein structure down to coiled-coil molecules. Radial water swelling studies showed that African hair had the lowest increase in diameter when water was applied. Caucasian and Asian hair showed similar radial swelling changes to one other. Three-dimensional structure analysis showed that Asian hair had the greatest diameter and that African hair had the greatest sectional variability and variability of ellipticity along the fiber. A tensile study showed that African hair had the earliest breaking time and lowest stress requirement at breaking. The findings demonstrated that there are measurable differences in hair morphology among these 3 ethnic groups.

In a study of scalp biopsies from African and Caucasian volunteers, Thibaut et al. (2005) observed that dermal implantation of hair follicles obtained from scalp biopsies of African individuals was curved with a retrocurvature at the level of the bulb, as opposed to a straight shape observed in follicles derived from Caucasian scalp biopsies. In the African samples, the bulb itself was bent, and both the outer root sheath and the connective tissue sheath were thicker on the concave side of the follicle. In vitro growth of African curly hair follicles was slightly slower than that of Caucasian follicles, but the curvature was maintained in the hair shaft produced in vitro. Immunohistochemical studies showed that the proliferative matrix compartment of curly hair follicles was asymmetrical. The authors concluded that hair curliness is programmed from the bulb and is linked to asymmetry during differentiation.

De la Mettrie et al. (2007) proposed a classification of 3-dimensional human hair curve shape by measurement of 3 accessible descriptors: curve diameter, curl index, and number of waves per hair sample, instead of basing certain hair types on ethnicity. The authors also stated that descriptions using terms such as straight, wavy, curly, frizzy, kinky, woolly, or helical are imprecise and often overlapping. Using the 3 descriptor variables to assess hair samples from 1,442 volunteers from 18 countries, the authors identified 8 different categories of hair shape, ranging from straight (I) to very curly (VIII). Four groups (I-IV) could be distinguished by curve diameter, whereas the other 4 groups (V-VIII) could be differentiated by curl index/number of waves. At the extremes, 'African' hairs were mainly classified as types V to VIII; hairs collected from China were mainly types I and II.

Inheritance

Davenport and Davenport (1908) studied the mendelian inheritance of hair morphology, including straight, curly, and wavy hair, in 230 Caucasian families. The results suggested that straight hair was recessive to curly hair, and that wavy hair may represent a heterozygous condition.

Bean (1911) reported a study of hair morphology among Filipinos and reached conclusions different from that of Davenport and Davenport (1908), i.e., that straight hair may sometimes act as a dominant trait. However, the inheritance of hair morphology did not follow simple mendelian inheritance.

Rostand and Tetry (1964) claimed that curly hair was dominant to straight hair, or semidominant in cases where heterozygotes show intermediate wavy hair. A large number of other so-called normal morphologic traits were tabulated by these authors, including hooked versus straight nose, brown versus blue eyes, long versus short eyelashes, cleft chin (119000), dental diastema (125900), and others that many consider too complex for mendelian interpretation.

Mapping

Medland et al. (2009) performed a genomewide association scan for hair morphology (straight, wavy, curly) in 3 Australian samples of European descent totaling 4,845 individuals and found the most significant association with 4 SNPs on chromosome 1q21.3 (combined p values from 3.12 x 10(-28) to 1.50 x 10(-31)), accounting for approximately 6% of the variance. One of the SNPs, rs11803731, represents an A-T transversion in exon 3 of the trichohyalin gene (TCHH; 190370), resulting in a leu790-to-met (L790M) substitution in the protein. In a sample of 1,668 unrelated individuals, the authors found that with more T alleles, the proportion of straight hair increased. Noting that in silico analyses predicted this change to be 'benign' or 'neutral,' Medland et al. (2009) stated that more work was required to confirm that this was the causal variant.