The accessory structures of the integumentary system are hair, nails, glands, and small muscles embedded in the skin. While the skin itself (the epidermis, dermis, and hypodermis) forms the main organ, these accessory structures sit within or extend from it and handle specific jobs: temperature regulation, sensation, protection, and lubrication. Here’s how each one works.
Hair and Hair Follicles
Hair grows from follicles, which are tube-shaped structures that extend down from the skin’s surface into the dermis. Each follicle has three segments. The uppermost portion, called the infundibulum, runs from the skin surface down to where the oil gland opens into the follicle. Below that is the isthmus, which ends at a region called the bulge. The bulge is important because it houses stem cells that help regenerate the follicle. The deepest segment includes the bulb, where a cluster of rapidly dividing cells (the matrix) wraps around a small nub of tissue called the dermal papilla. Blood vessels in the dermal papilla supply nutrients that fuel hair growth.
Hair doesn’t grow continuously. It cycles through three phases. The growth phase (anagen) lasts 2 to 6 years for scalp hair but only a few months for eyebrows and eyelashes, which is why those hairs stay short. Next comes a brief transition phase (catagen) lasting just a few weeks, during which the follicle shrinks. Finally, the resting phase (telogen) lasts about 100 days on the scalp. During this time, the hair is fully formed but no longer growing, and it eventually sheds to make room for a new strand.
Beyond the follicle itself, hair plays a sensory role. Nerve endings wrapped around each follicle can detect even light touch, helping you sense when something brushes against your skin before it reaches the surface.
Nails
Nails are dense plates of hardened protein (keratin) that protect the tips of your fingers and toes and improve your ability to grip and manipulate small objects. The nail unit has four main parts: the proximal nail fold, the nail matrix, the nail bed, and the hyponychium.
The nail matrix, tucked just beneath the proximal nail fold, is where the action happens. It contains rapidly dividing cells that produce the nail plate, pushing it forward as new cells are added. The visible, pale crescent near the base of the nail is the lunula, which is simply the part of the matrix you can see through the nail plate. Overlying the matrix area, the cuticle (a rim of tough skin extending from the nail fold) seals the gap between the fold and the nail plate, blocking water and bacteria from getting underneath. At the other end, the hyponychium is the thickened skin beneath the free edge of the nail, serving a similar protective role.
Fingernails grow at an average rate of about 3.5 mm per month, while toenails grow more slowly at roughly 1.6 mm per month. A completely lost fingernail typically takes around 6 months to regrow, and a toenail can take over a year.
Sebaceous (Oil) Glands
Sebaceous glands are found nearly everywhere on the skin, usually opening into hair follicles. Their job is to produce sebum, an oily mixture that coats the skin and hair to keep them flexible and water-resistant. Sebum is mostly fats: triglycerides and fatty acids make up about 57.5% of it, wax esters account for 26%, and a compound called squalene contributes another 12%. Cholesterol and its related molecules make up the remaining 4.5%.
These glands use a dramatic secretion method called holocrine secretion. Rather than simply releasing their product and surviving, the gland cells spend their entire life cycle filling up with lipid droplets until they literally burst apart, dumping their contents into the follicle. New cells from the gland’s outer edge then replace them and repeat the cycle. When sebaceous glands overproduce or their ducts become blocked, the result is acne. Hormonal changes during puberty, menstrual cycles, or stress can all ramp up sebum production.
Sweat Glands
Humans have two main types of sweat glands, and they differ in location, structure, and what they secrete.
Eccrine glands are by far the most numerous. You have roughly 2 to 4 million of them spread across nearly your entire body, including the palms and soles. They produce a thin, watery sweat composed mostly of water and sodium chloride, along with trace amounts of other chemicals from the surrounding fluid. Their primary purpose is thermoregulation: as sweat evaporates from the skin surface, it carries heat away from the body. Eccrine glands are active from birth.
Apocrine glands are concentrated in specific areas, primarily the armpits, groin, scalp, face, and around the breasts. They produce a thicker, more viscous secretion rich in lipids, proteins, sugars, and ammonia. This secretion is essentially odorless when it first reaches the skin surface, but bacteria on the skin quickly break it down, producing body odor. Apocrine glands become active at puberty, which is why body odor tends to appear during adolescence.
Two specialized variations of sweat glands also count as accessory structures. Ceruminous glands in the ear canal are modified apocrine glands that produce cerumen (earwax), which traps debris and helps protect the eardrum. Mammary glands, responsible for milk production, are also classified as modified sweat glands, though they are far more complex in structure.
When eccrine glands become overactive, the result is hyperhidrosis, a condition marked by excessive sweating that goes well beyond what the body needs for temperature control.
Arrector Pili Muscles
Attached to nearly every hair follicle is a tiny bundle of smooth muscle called the arrector pili muscle. One end connects to the follicle at the bulge region; the other anchors into the upper layer of the dermis. When this muscle contracts, it pulls the hair upright and creates a small dimple on the skin surface, producing what you recognize as goosebumps.
In animals with thick fur, this response traps a layer of insulating air close to the body, providing real warmth. In humans, where body hair is fine and sparse, the thermoregulatory benefit is minimal. Piloerection in humans is triggered mostly by cold temperatures or strong emotions like fear or excitement, and it is largely considered a vestigial response. However, recent research suggests arrector pili muscles may play a role in maintaining the hair follicle’s stem cell environment, giving them relevance beyond simply raising hairs.
Sensory Receptors in the Skin
The skin is packed with specialized sensory receptors, each tuned to a different type of stimulus. These are sometimes grouped with the accessory structures because they are embedded within the integumentary system and are essential to its protective function.
- Merkel cells sit in the basal layer of the epidermis and detect fine details like texture and edges, which is why your fingertips are so good at reading Braille or distinguishing surfaces.
- Meissner corpuscles are found in the upper dermis, especially in the fingertips and lips. They respond to light touch and detect when an object is slipping from your grip.
- Pacinian corpuscles sit deeper in the dermis and respond to vibration and deep pressure.
- Ruffini endings detect sustained pressure and skin stretching, helping you sense joint position and grip force.
Together with the nerve endings wrapped around hair follicles, these receptors give the integumentary system its role as the body’s largest sensory organ. They allow you to distinguish a light breeze from a firm press, detect temperature shifts, and pull your hand away from a sharp edge before you consciously register pain.