Hair are keratinous filaments derived from invaginations of the germinative layer of epidermis into the dermis. These are peculiar to mammals (like feathers to the birds), and help in conservation of their body heat. However, in man the heat loss is prevented b the cutaneous sensation of touch
Distribution of hair:
Hair are distributed all over the body, except on the palms, soles, dorsal surface of distal phalanges, umbilicus, glans penis, inner surface of prepuce, the labia minora, and inner surface of labia majora. The length, thickness and color of the hair vary in different part of the body and in different individuals.
Structure of hair:
- Each hair has an implanted part called the root, and a projecting part called the shaft.
- The root is surrounded by a hair follicle (a sheath of epidermis and dermis), and is expanded at its proximal end to form the hair bulb.
- Each hair bulb is invaginated at its end by the hair papilla (vascular connective tissue) which forms the neurovascular haunt of the hair and its sheath.
- Hair grows at the hair bulb, by proliferation of its cells capping the papilla.
- The hair follicles, enclosing hair roots lie obliquely to the surface of the skin, which is responsible for characteristic hair strearns in different parts of the body.
Arrectores Pilorum Muscles:
The arrectores pilorum muscles (smooth muscles supplied by sympathetic nerves) connect the undersurface of the follicles to the superficial part of the dermis. Contraction of these muscles leads to erection of hair, squeezes out the sebum and produces “goose skin”.
The fetal skin is covered by fine hair called lanugo (primary hair). These are mostly sited by birth and are replaces during infancy by another set of fine hair called his secondary hair). These are retained in most parts of the body, but are replaced by the thick and dark terminal hair of the scalp and eyebrows, and other hairy areas of the adult skin.
Growth rate and life span:
The hair grow at the rate of about 1.5-2mm per week. Their growth is controlled by hormones. The life span of the hair varies from 4 months (eyelashes, axillary hair) to 4 years (scalp hair)