Integumentary System: Skin, Hair, Nails, and Glands

Overview

The integumentary system is the largest organ system of the body, accounting for 15–20% of total body weight. It is far more than a simple outer wrapping — the skin is a complex, dynamic organ that protects us from the external environment, regulates body temperature, provides sensory information about the world around us, and synthesizes vitamin D. Its surface area averages 1.5–2 square meters in adults.

The system consists of the skin itself (with its three layers: epidermis, dermis, and hypodermis) and its appendages: hair, nails, sebaceous glands, and sweat glands. Each component serves both specialized and overlapping functions. The skin does not merely sit passively on the body’s surface — it actively communicates with the immune system, the nervous system, and the vasculature, and it changes with age, environment, and disease.

Skin

Layers of the Skin

The skin is composed of three distinct layers. From superficial to deep: the epidermis (epithelial layer), the dermis (connective tissue layer), and the hypodermis (subcutaneous fat layer). Although the hypodermis is not technically part of the skin, it is functionally inseparable from it.

Epidermis

The epidermis is the outermost layer of the skin, composed of stratified squamous epithelium. It is avascular — it receives all its nutrients by diffusion from the underlying dermis. The epidermis varies in thickness from 0.05 mm on the eyelids to 1.5 mm on the palms and soles.

Four cell types populate the epidermis:

Keratinocytes are by far the most numerous (approximately 90% of epidermal cells). They produce keratin, a tough, fibrous structural protein that gives the epidermis its protective properties. Keratinocytes originate in the deepest layer of the epidermis and migrate upward over approximately 28 days, undergoing a process of differentiation and eventual death to form the protective outer layer.

Melanocytes are pigment-producing cells derived from the neural crest. They reside in the stratum basale and extend long dendrites to transfer melanin pigment to adjacent keratinocytes. Melanin absorbs and scatters ultraviolet radiation, protecting the DNA of keratinocytes from UV-induced damage. The number of melanocytes is roughly the same across all skin types — what differs is the amount and type of melanin produced.

Langerhans cells are dendritic cells of the immune system that reside primarily in the stratum spinosum. They capture and process antigens that enter through the skin, then migrate to lymph nodes to present them to T-cells, initiating an immune response. They are the skin’s first line of immunological defense.

Merkel cells are mechanoreceptors located in the stratum basale, most abundant in the fingertips and lips. They are associated with sensory nerve endings and detect light touch and texture — they are the reason we can feel fine details like the texture of a fabric.

Layers of the Epidermis

The epidermis is organized into five strata (layers). From deepest to most superficial:

Stratum basale (also called stratum germinativum) is a single layer of columnar or cuboidal keratinocytes attached to the basement membrane by hemidesmosomes. This is where cell division occurs — the stem cells of the epidermis. Melanocytes and Merkel cells are also found here.

Stratum spinosum consists of several layers of polygonal keratinocytes that appear to have “spines” (desmosomes) connecting them — an artifact of histological preparation that gives this layer its name. Langerhans cells are most abundant in this layer.

Stratum granulosum is marked by the presence of dark keratohyalin granules within the keratinocytes. These granules contain proteins that aggregate keratin filaments and begin the process of cell death. This is the layer where keratinocytes undergo the transition from living to dead cells.

Stratum lucidum is a thin, clear layer found only in thick skin (palms and soles). It consists of densely packed, translucent cells filled with eleidin, a precursor of keratin.

Stratum corneum is the outermost layer, composed of 15–30 layers of dead, flattened keratinocytes (squames or corneocytes) that have lost their nuclei and organelles. These cells are filled with keratin and are continuously shed (desquamated). The stratum corneum is the primary barrier of the skin — it prevents water loss and blocks the entry of microorganisms, chemicals, and allergens.

Dermis

The dermis is the connective tissue layer of the skin, located between the epidermis and the hypodermis. It is 0.5–2 mm thick and provides the skin with strength, elasticity, and blood supply. Unlike the epidermis, the dermis is richly vascularized and innervated.

The dermis has two layers. The papillary layer is the superficial 20%, composed of loose connective tissue arranged into dermal papillae — finger-like projections that interlock with the epidermis, increasing the surface area for nutrient exchange. These papillae contain capillary loops and Meissner’s corpuscles (tactile corpuscles that detect light touch). The reticular layer forms the deeper 80% of the dermis and is composed of dense irregular connective tissue with thick bundles of collagen and elastic fibers. This layer houses the skin appendages — hair follicles, sweat glands, and sebaceous glands — as well as Pacinian corpuscles (deep pressure and vibration receptors).

The orientation of collagen fibers within the dermis follows predictable patterns known as Langer’s lines (cleavage lines). Surgical incisions made parallel to these lines heal with less scarring than those made across them, because the cutting of fewer collagen bundles results in less wound gape. Surgeons routinely plan incisions along Langer’s lines for cosmetic outcomes.

Hypodermis

The hypodermis (subcutaneous layer) lies deep to the dermis and consists primarily of adipose tissue (fat) with interspersed connective tissue septa. It is not technically part of the skin, but it is functionally inseparable.

The hypodermis serves as energy storage (fat is the body’s most concentrated energy source), thermal insulation (fat conducts heat poorly, helping maintain core body temperature), and shock absorption (the fat pad cushions underlying structures against mechanical trauma). It also anchors the skin to the underlying fascia and muscle, and is the site where most body fat is deposited and mobilized. The thickness of the hypodermis varies dramatically with body region, nutritional status, and sex — women typically have thicker subcutaneous fat than men, especially in the hips, thighs, and breasts.

Skin Appendages

Hair

Hair is found almost everywhere on the body except the palms, soles, lips, and parts of the external genitalia. It is a characteristic feature of mammals and serves multiple functions: protection (the scalp from UV radiation, the eyelashes from debris), sensation (hair follicles are richly innervated and detect light touch), and social communication.

There are two types of hair. Vellus hair is short, fine, and lightly pigmented — it covers most of the body surface. Terminal hair is long, thick, and dark — found on the scalp, eyebrows, eyelashes, and, after puberty, in the axillae, pubic region, and (in men) the face and chest.

Each hair consists of a shaft (the visible part above the skin) and a root (below the skin, enclosed within the hair follicle). The shaft has three layers: the cuticle (outer, overlapping scales), the cortex (middle, provides strength and color), and the medulla (central, variable). At the base of the follicle lies the hair bulb, which contains the dermal papilla — a small invagination of the dermis that supplies blood to the growing hair. The matrix (dividing cells around the dermal papilla) produces the hair.

The arrector pili muscle is a small bundle of smooth muscle fibers attached from the hair follicle to the papillary dermis. When it contracts (triggered by sympathetic nervous system activation — cold or fear), it pulls the hair upright, producing goosebumps (cutis anserina). This reflex is vestigial in humans but useful in other mammals for insulation (trapping air) or intimidation (fluffing the fur).

Hair grows in cycling phases. Anagen is the active growth phase, lasting 2–6 years for scalp hair. Catagen is a brief regression phase lasting 2–3 weeks. Telogen is the resting phase, lasting 3–4 months, after which the hair is shed. At any given time, approximately 85% of scalp hairs are in anagen, 1% in catagen, and 14% in telogen. Normal hair loss is approximately 100 hairs per day.

Nails

Nails are hard, keratinized plates on the dorsal surface of the distal fingers and toes. They protect the fingertips, enhance the sensation of fine touch by providing counter-pressure against the pulp, and serve as tools for scratching and manipulating small objects.

The nail plate is the visible hard structure, composed of densely packed, keratinized cells. It rests on the nail bed, which is a modified region of epidermis. The nail matrix is the germinal portion located under the proximal nail fold; its cells divide and keratinize to produce the nail plate. The lunula is the pale, crescent-shaped region visible at the proximal nail — it is the distal portion of the nail matrix where the cells are still incompletely keratinized.

The eponychium (cuticle) is a fold of stratum corneum that extends from the proximal nail fold onto the nail plate, sealing the gap between the nail and the skin. The hyponychium is the thickened epidermis beneath the free distal edge of the nail plate.

Fingernails grow at approximately 3 mm per month (about 0.1 mm per day); toenails grow at about 1 mm per month. Complete replacement of a fingernail takes 4–6 months; a toenail takes 12–18 months. Nail growth slows with age and is affected by nutrition, illness, and circulation.

Sebaceous Glands

Sebaceous glands are simple, branched alveolar glands that secrete sebum — a mixture of triglycerides, wax esters, squalene, and cholesterol. They are found everywhere on the body except the palms and soles. Most open into hair follicles (the pilosebaceous unit), but a few (such as those on the lips, nipples, and external genitalia) open directly onto the skin surface.

Sebum lubricates the skin and hair, prevents water loss, and has antimicrobial properties. Sebaceous gland activity is stimulated by androgens (particularly testosterone and DHT), which is why they are most active after puberty and why they are concentrated in areas with the highest sebum production: the face, scalp, and chest.

Sweat Glands

The human body has 2–4 million sweat glands, classified into two types that differ in distribution, secretion, and function.

Eccrine sweat glands are the most numerous (2–4 million) and are distributed across nearly the entire body surface. They are simple, coiled tubular glands that secrete a watery, hypotonic solution (99% water, with trace amounts of sodium chloride, urea, and lactate). The secretory coil lies deep in the dermis or hypodermis, and a straight duct ascends through the dermis and epidermis to open at the skin surface as a pore. Eccrine glands function primarily in thermoregulation — sweat evaporation cools the body. They are activated by cholinergic sympathetic nerve fibers.

Apocrine sweat glands are larger than eccrine glands and are found only in specific regions: the axillae, areolae, perineum, and external ear (where they produce cerumen — earwax). They develop at puberty and secrete a thicker, organic-rich fluid (proteins, lipids, steroids) into the hair follicle, not directly onto the skin. The secretions are odorless when produced; the characteristic body odor results from bacterial breakdown of the organic components on the skin surface. The function of apocrine glands in humans is unclear — they are widely considered vestigial scent glands, analogous to the scent-marking glands of other mammals.

Functions of the Skin

The skin performs a remarkable range of functions that are essential for health and survival:

Barrier function — The skin is the body’s primary physical barrier. The stratum corneum resists mechanical trauma, the acidic pH (approximately 4.5–6.0) inhibits microbial growth, antimicrobial peptides (defensins, cathelicidins) kill pathogens, and Langerhans cells provide immunological surveillance. The skin also prevents excessive water loss — without an intact barrier, the body would lose approximately 20 liters of water per day through evaporation.

Thermoregulation — The skin actively regulates body temperature through three mechanisms. Sweat evaporation dissipates heat (each liter of evaporated sweat removes approximately 580 kcal of heat). Dermal blood flow adjusts — vasodilation increases heat loss, vasoconstriction conserves heat. Subcutaneous fat provides insulation, particularly important in cold environments.

Sensation — The skin contains a dense array of sensory receptors that detect touch (Meissner’s corpuscles), pressure (Pacinian corpuscles), pain (free nerve endings), cold (Krause bulbs), and heat (Ruffini endings). This sensory information is essential for interacting with the environment and avoiding injury.

Vitamin D synthesis — When exposed to UVB radiation (wavelength 290–315 nm), 7-dehydrocholesterol in the epidermis is converted to previtamin D₃, which is then converted to vitamin D₃ (cholecalciferol). This process is essential for calcium homeostasis and bone health.

Pigmentation — Melanin produced by melanocytes protects the DNA of keratinocytes from UV damage. Darker skin has more eumelanin (brown-black); lighter skin has more pheomelanin (red-yellow) or less total melanin.