The dermis is of mesodermal origin and contains blood and lymphatic vessels, nerves, muscle, appendages (e.g. sweat glands, sebaceous glands and hair follicles) and a variety of immune cells such as mast cells and lymphocytes. It is a matrix of collagen and elastin in a ground substance.

The sweat glands

Eccrine sweat glands are found throughout the skin except the mucosal surfaces.
Apocrine sweat glands are found in the axillae, anogenital area and scalp and do not function until puberty.
The sweat glands and vasculature are involved in temperature control.

The sebaceous glands

These are inactive until puberty. They are responsible for secreting sebum or grease onto the skin surface (via the hair follicle) and are found in high number on the face and scalp.

Nerves

The skin is richly innervated. These fibres allow sensation of touch, pain, itch, vibration and change in temperature.

Hair

Hairs arise from a downgrowth of epidermal keratinocytes into the dermis. The hair shaft has an inner and outer root sheath, a cortex and sometimes a medulla. The lower portion of the hair follicle consists of an expanded bulb (which also contains melanocytes) surrounding a richly innervated and vascularized dermal papilla. The hair regrows from the bulb after shedding.

There are three types of hair:

• terminal – medullated coarse hair, e.g. scalp, beard, pubic
• vellus – non-medullated fine downy hairs seen on the face of women and in prepubertal children
• lanugo – non-medullated soft hair on newborns (most marked in premature babies) and occasionally in people with anorexia nervosa.

All hair follicles follow a growth cycle: anagen (growth phase), catagen (involution phase), telogen (shedding phase). At any one time most hairs (> 90%) will be in the anagen phase, which is typically 3-5 years for scalp hair.
Grey hair is due to decreased tyrosinase activity in the hair bulb melanocytes. White hair is due to total loss of these melanocytes.

Nails

Nails are tough plates of hardened keratin which arise from the nail matrix (just visible as the moon-shaped lunula) under the nail fold. It takes 6 months for a finger-nail to grow out fully and 1 year for a toe-nail.

The subcutaneous layer

The subcutaneous layer consists predominantly of adipose tissue as well as blood vessels and nerves. This layer provides insulation and acts as a lipid store.

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