Hair (overview)

Thread-like skin appendage consisting of keratin. Humans have about 5.0 million hairs or hair follicles, about 100,000 to 150,000 on the head (hairy head = capillitium). The daily loss rate is 30 to a maximum of 100 hairs. They grow back after a resting period. The amount of hair varies with hair colour: blondes have an average of 140,000 hairs, which is more than the dark-haired (about 100,000 hairs). Blonde hair is often slightly finer, red and black hair is often stronger.

Hair can cover the entire integument except for the palms of the hands and soles of the feet, lip red and eyelids. They served primarily as protection against external influences such as sun, heat and cold. These functions have now been lost by the hair coat. Nowadays, terminal hair on the extremities or on various parts of the face is rather seen as a cosmetic problem.

The hair is composed of the hair shaft and the hair root.

The hair colour (see below hair, colour changes) is determined by melanin and physical factors. There are 3 known melanin classes:

• Eumelanin
• Pheomelanin
• Neuromelanin.

Decisive for the hair colour are the black-brown eumelanins, which are formed by enzymatic oxidation of tyrosine, and the red-brown phaeomelanins, which are formed by reaction with cysteine. Hair of red-haired people contains trichromes, which can be assigned to the phaeomelanins. In a large European genome-wide association study (Hysi et al. 2018) 123 autosomal loci and a single X-linked locus were found in 300,000 participants, which are significantly related to hair color. The single nucleotide polymorphisms associated with hair colour explain the heritability of hair colour:

• 34.6% red hair
• 24,8% blond hair
• 26.1% black hair

There are three types of human hair known:

• Lanugo Hair
• Vellus hair
• Terminal hair.

1. Hysi PG et al (2018) Genome-wide association meta-analysis of individuals of European ancestry identifies new loci explaining a substantial fraction of hair color variation and heritability. Nat Genet 50:652-656.