The hair follicle is the sac-like housing of the hair shaft. The germinative part of the hair shaft is the hair matrix (see p. 39, Fig. 2.15). Melanocytes migrate into the matrix and the melanins that they produce are responsible for the different colours of hair. Each follicle passes, independently of its neighbours, through regular cycles of growth (anagen) and resting/shedding (lelogen). (he duration of these stages varying in different regions of the body. The anagen phase of scalp hair may last for up to 5 years, accounting for its length. This phase is shorter and the telogen phase longer in eyebrow and sexually determined hair. Moulting does not normally occur in humans unless the hair cycles in neighbouring follicles become synchronised, as sometimes occurs after childbirth and various pyrexic illnesses (telogen effluvium).
There are three different types of hair. Fine long lanugo hairs cover the fetus but are shed about I month before full-term. They are replaced by fine short vellus hairs, which cover much of (he body surface. Terminal hairs then replace vellus hairs on the scalp. At puherty. the vellus hair of the pubic region is replaced by darker, coarser and curlier hair. This begins earlier in girls (average age 11.5 years) than in boys (average age 13.5 years). The development of pubic hair is related to adrenal androgen production (adrenarche). This occurs in the absence of gonadotropin secretion. Thus patients with isolated gonadotrophs deficiency may have pubic hair but no other signs of pubertal development.
Axillary hair appears about 2 years after the start of pubic hair growth and, in boys, coincides with the development of facial hair. Last of all, body hair develops, and its extent increases throughout the years of sexual maturity though there is a wide variation in its pattern.
The amount of hair varies considerably in normal subjects and is influenced by familial and racial factors. Asians tend to have straight hair. Negroids to have curly hair and Europeans to have wavy hair. Mongoloids have sparse facial and body hair. Mediterranean people have more hair than northern Europeans.
□ Inspect the scalp hair for its lustre, calibre, structure, tensile strength and density.
□ If alopecia is present, determine if the abnormality affects the hair shaft or the scalp itself.
□ Inspect for evidence of fungal infection, nits and lice.
□ Examine the body and secondary sexual hair for its nature and distribution.
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