Accessory Structures of the Skin

Core Concepts

Hair: a keratin shaft with a living root

Each hair has a visible shaft above the skin surface and a root buried below it. The root sits inside a hair follicle — an invagination of the epidermis that extends down into the dermis, and in some areas into the hypodermis. At the follicle's base is the hair bulb, which contains the hair matrix, a zone of actively dividing cells that produce the hair. A small bud of connective tissue called the dermal papilla pokes into the bulb and carries the blood vessels that nourish the growing hair; destroy the papilla and that follicle stops producing hair.

The shaft is dead, keratinized cells in three layers: an inner medulla, a cortex holding most of the pigment melanin, and an outer cuticle of overlapping scales. Hair color comes from melanin in the cortex; graying reflects reduced melanin production in aging follicles.

Hair does not grow continuously. Follicles cycle through anagen (active growth), catagen (transition, when growth stops), and telogen (resting, when the old hair is shed). Scalp hairs spend years in anagen, which is why they grow long; body hairs have short anagen phases and stay short.

Two hair types matter for exams: vellus hairs are fine, short, and lightly pigmented across much of the body, while terminal hairs are thick, long, and dark — the scalp, eyebrows, eyelashes, and (after puberty) the axillary and pubic regions.

Nails: keratin plates grown by a matrix

Nails are hard plates of densely packed keratinized cells that protect the fingertips and toes and support fine manipulation. The visible nail plate rests on the nail bed; the growing region, the nail matrix, sits under the skin at the base of the nail. The pale crescent you can often see at the base is the lunula — the visible part of the matrix showing through the plate. The cuticle (eponychium) is the fold of skin that covers the nail root, and the hyponychium is the skin seal under the free edge.

Because the matrix produces the nail, damage there disrupts growth, while damage to the plate alone does not. Fingernails are commonly quoted as growing about 1 mm per week, with toenails slower — treat these as reference approximations, not precise values. Nail color, shape, and surface changes are classic assessment observations; interpreting them is a clinician's job.

Sebaceous glands: the skin's oil producers

Sebaceous glands usually open into hair follicles and secrete sebum, a waxy lipid mixture, through holocrine secretion — cells fill with product, then disintegrate to release it. Sebum coats hair and skin, keeping them lubricated and water-resistant, with mild antimicrobial properties. Sebaceous glands occur almost everywhere except the palms and soles; modified versions appear in the areolae and eyelids (tarsal glands).

When sebum and dead cells plug a follicle, the result is a comedo — the starting point of acne vulgaris, a topic explored in the diseases section of this chapter.

Sweat glands: eccrine and apocrine

Eccrine sweat glands are the most numerous sweat glands, distributed over nearly the whole body with high density on the palms, soles, and forehead. They open directly onto the skin surface and produce a watery secretion (mostly water with dissolved salts and small amounts of metabolic wastes such as urea). Their main job is thermoregulation: when the body overheats, the nervous system triggers increased eccrine output, and the evaporation of water from the skin carries heat away.

Apocrine sweat glands are larger, found mainly in the axillae, pubic region, and around the nipples. They open into hair follicles and release a thicker, milky, protein-rich secretion. Active from puberty onward, they produce little odor themselves: body odor develops when skin bacteria break down the secretion. Some researchers suggest apocrine secretions may carry chemical signals between people, but this remains an area of active study.

Modified sweat glands: ceruminous and mammary

Two familiar structures are modified sweat glands. Ceruminous glands, located in the lining of the external ear canal, produce cerumen (earwax), which traps debris and discourages insects. Mammary glands, which produce milk, are modified apocrine-type sweat glands — an important classification detail that shows up on exams.

How It Works / Step-by-Step Process

Following a hair through one growth cycle:

1. Anagen: Stem cells in the hair matrix divide; new keratinized cells are pushed upward, die, and become part of the shaft. The dermal papilla supplies the glucose and oxygen needed for this rapid cell division.
2. Catagen: Growth signals stop. The follicle shortens, the bulb pulls upward, and the base of the hair becomes a club-shaped structure.
3. Telogen: The follicle rests. The old club hair may be shed (everyone sheds hairs daily — this is normal), and the follicle eventually re-enters anagen, producing a new hair in the same follicle.

Applying the same logic to glands: a gland's secretion route (into a follicle vs. onto the surface), mode of release (holocrine for sebum vs. merocrine for eccrine sweat), and timing of activation (apocrine at puberty) explain most of its function.

Common Confusions