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The integumentary system

Describe the layers and cells of the skin, and explain how the skin protects, senses and controls temperature.

You can see and touch your own skin. Use that familiarity to look at what it is made of and what it does for the rest of the body.

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Epidermis

The epidermis is the superficial, thinner part of the skin, made of keratinized stratified squamous epithelium. It has no blood vessels, so nutrients reach its cells by diffusion from the dermis. The deepest layer, the stratum basale, holds stem cells that divide and push new keratinocytes towards the surface.

For example

A superficial graze that does not bleed has damaged epidermis only. Once it reaches the dermis, vessels are torn and bleeding starts.

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Epidermis: The epidermis is the superficial, thinner part of the skin, made of keratinized stratified squamous epithelium. It has no blood vessels, so nutrients reach its cells by diffusion from the dermis. The deepest layer, the stratum basale, holds stem cells that divide and push new keratinocytes towards the surface. A superficial graze that does not bleed has damaged epidermis only. Once it reaches the dermis, vessels are torn and bleeding starts.
Keratinocyte: Keratinocytes are about 90% of epidermal cells. They produce keratin, a tough fibrous protein that protects underlying tissue from heat, microbes and chemicals, and lamellar granules that release a water-repellent sealant. The sealant reduces water entry and loss and slows the entry of foreign material. Prolonged friction thickens the epidermis into a callus. More keratin means more protection, but not more sensitivity.
Melanin: Melanocytes produce melanin, a yellow-red or brown-black pigment that contributes to skin colour and absorbs damaging ultraviolet light. Their long projections reach between keratinocytes and transfer melanin granules, which cluster over the nucleus on the side facing the surface. Skin colour differences come mainly from how much melanin is produced and how it is distributed, not from a different kind of cell. After sun exposure, more melanin is produced and the skin darkens. That tan is a response to damage, not proof that the ultraviolet exposure was safe.
Dermis: The dermis is the deeper, thicker part of the skin and is connective tissue. Its thin papillary region lies just under the epidermis, with dermal papillae that interlock with epidermal ridges. Its thicker reticular region holds collagen and elastic fibres, hair follicles, glands, vessels, nerves and most receptors. Fibres from the dermis anchor the skin to the subcutaneous layer. A deep cut through the dermis damages glands and follicles. The epidermis can regrow, but the appendages may not, which is why a scar has no hair or sweat glands.
Appendages: Hair, nails and glands are skin appendages. A hair follicle is a tube of epidermal cells dipping into the dermis, with an arrector pili muscle that raises the hair. Sebaceous glands secrete sebum, which lubricates hair and skin. Sweat glands release sweat through a duct; eccrine glands are widespread, while apocrine glands open into hair follicles. Nails are plates of hard keratinized cells. Goose bumps in the cold are the arrector pili muscles contracting. They raise the hair, but in humans they add little warmth.
Thermoregulation: Skin helps hold body temperature steady. When the body is too warm, blood vessels in the skin widen and sweat glands release sweat, whose evaporation removes heat. When the body is too cold, those vessels narrow and sweat production falls, so less heat is lost. Shivering adds heat by muscle activity. These are negative-feedback responses: the change is reduced, not amplified. During hard exercise the skin flushes and sweating starts. Both changes increase heat loss, so the rise in temperature is limited.

Layers and cells of the skin

The integumentary system is the skin plus its hair, nails, glands and sensory receptors. The skin itself is the largest organ by surface area and weight: about 2 square metres and 4.5–5 kg in an adult, roughly 16% of body weight. It has two parts. The superficial epidermis is epithelium; the deeper, thicker dermis is connective tissue. Deep to the dermis, and not part of the skin, lies the subcutaneous layer, or hypodermis.

From basal layer to surface

The epidermis has no blood vessels of its own. Its deepest layer, the stratum basale, contains stem cells that divide to make new keratinocytes, which are then pushed towards the surface. Keratinocytes are about 90% of epidermal cells; they make keratin, a tough protein, and lamellar granules that release a water-repellent sealant. Thick skin on the palms and soles has five layers; thin skin elsewhere has four. The skin also synthesises vitamin D when it is exposed to ultraviolet light.

Pigment, immunity and touch in the epidermis

Melanocytes are about 8% of epidermal cells and produce melanin, the pigment that gives skin its colour and absorbs damaging ultraviolet light. Two other cell types matter. Langerhans cells come from red bone marrow and take part in immune responses against microbes that enter the skin. Merkel cells sit in the deepest epidermal layer and, with the ending of a sensory neuron, form a disc that detects touch.

Dermis, appendages and temperature control

The dermis contains vessels, nerves, glands, hair follicles and receptors, and it has a thin papillary region under the epidermis and a thicker reticular region below. Hair follicles dip into the dermis; sebaceous glands lubricate hair and skin with sebum; sweat glands release sweat that cools the body as it evaporates. Nails are hardened keratin plates. Receptors for light touch, pressure, pain and temperature report the outside world, and skin blood flow and sweating adjust heat loss.

Explore the illustration

Look at the whole view first. Then select one label and connect it to the idea you just read.

Skin layers, cells and appendages

Schematic skin section, not to scale: the surface is at the top. The epidermis sits over the dermis, which sits over the subcutaneous layer. Hair, glands, a nail unit and several receptor types are shown in place.

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01 / 16 Epidermis

The epidermis renews itself from its base

Guided anatomy tour: each step identifies one structure and explains its relationship to the next. The highlight moves; the anatomy stays still. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.

Epidermis

The epidermis is the superficial, thinner part of the skin, built of keratinized stratified squamous epithelium. It has no blood vessels of its own, so its cells receive nutrients by diffusion from the dermis. Its thickness varies: thin skin has four strata and thick skin, on the palms and soles, has five.

Go deeper

In-depth reading

Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.

Worked example: what happens to skin on a hot day

You work outside on a hot day. Receptors in the skin and the body’s core temperature rise above the usual set point, and the hypothalamus coordinates the response.
Blood vessels in the dermis widen. More warm blood reaches the surface, where heat can leave the body by radiation and convection. This is why the skin looks flushed.
Sweat glands in the dermis release sweat through ducts to the skin surface. As the water in sweat evaporates it takes heat from the skin, so cooling continues while sweating continues.
Both changes lower body temperature back towards the set point, so the signal for sweating and vasodilation is reduced. This negative feedback is the reason the rise in temperature is limited rather than endless.

Explain it yourself

You move to a cold room. What happens to the blood vessels in the dermis and to sweat production, and why does that help?

Show the explanation

The vessels in the dermis narrow and sweat production falls. Less warm blood reaches the surface and less water evaporates, so less heat leaves the body and the temperature is conserved. If the fall continues, skeletal muscles shiver to generate extra heat. This is still negative feedback: the body responds to a change by reducing it.

More detail — when you are ready

These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.

Two layers and a layer beneath them

The skin, also called the cutaneous membrane or integument, is the largest organ of the body by surface area and weight. In adults it covers about 2 square metres and weighs 4.5–5 kg, roughly 16% of total body weight. Thickness varies widely: about 0.5 mm on the eyelids and 4.0 mm on the heels, but 1–2 mm over most of the body.

It has two parts. The epidermis is the superficial, thinner portion and is epithelial tissue. The dermis is the deeper, thicker portion and is connective tissue. Deep to the dermis, and not part of the skin, lies the subcutaneous layer or hypodermis, made of areolar and adipose tissue. Fibres from the dermis anchor the skin to this layer, which in turn attaches to the fascia around muscles and bones. The subcutaneous layer stores fat and carries large blood vessels.

The integumentary system is more than the skin. It also includes hair, oil and sweat glands, nails and sensory receptors. Its main contributions to homeostasis are protection, temperature regulation and sensory information about the environment. Because skin is so visible, changes in its colour can also signal problems elsewhere: a bluish colour can accompany oxygen deficiency at the tissue level.

The epidermis: four cells, four or five layers

The epidermis is keratinized stratified squamous epithelium and contains four principal types of cell. About 90% are keratinocytes, arranged in four or five layers. They produce keratin, a tough fibrous protein that protects underlying tissue from heat, microbes and chemicals, and lamellar granules that release a water-repellent sealant. The sealant decreases water entry and loss and inhibits the entry of foreign materials.

About 8% are melanocytes, which produce melanin, a yellow-red or brown-black pigment. Their long slender projections reach between keratinocytes and transfer melanin granules. Inside a keratinocyte the granules cluster over the nucleus on the side facing the surface, shielding nuclear DNA from ultraviolet light. Melanocytes themselves are particularly susceptible to ultraviolet damage.

Langerhans cells arise from red bone marrow and migrate into the epidermis. They are a small fraction of the cells and take part in immune responses against microbes that invade the skin, helping other immune cells recognise and destroy the invader. They too are easily damaged by ultraviolet light.

Merkel cells are the least numerous. They lie in the deepest epidermal layer and contact the flattened process of a sensory neuron, forming a Merkel (tactile) disc that detects touch sensations.

In most regions the epidermis has four strata: stratum basale, stratum spinosum, stratum granulosum and a thin stratum corneum. Where friction is greatest, as on fingertips, palms and soles, there is a fifth layer, the stratum lucidum, and a thick stratum corneum. That is the difference between thin and thick skin. The deepest layer, the stratum basale, is a single row of cuboidal or columnar keratinocytes; some are stem cells that divide to produce new keratinocytes continually. It is also called the stratum germinativum because of that role. New cells are pushed towards the surface, so the outermost layers are the oldest.

The dermis and its appendages

The dermis is connective tissue and is much thicker than the epidermis. It has two regions. The papillary region is the thin superficial part, with an uneven surface that forms dermal papillae. These interlock with epidermal ridges, increasing the surface area for exchange and giving fingerprints their pattern. Capillary loops here bring blood close to the epidermis, which has no vessels of its own. Some papillae also contain tactile corpuscles, which are touch receptors close to the surface.

The reticular region is the deeper, thicker part. It contains dense connective tissue with collagen and elastic fibres, and it holds hair follicles, sebaceous and sweat glands, nerves, most sensory receptors and the larger blood vessels. Deep in this region, and sometimes in the papillary region, lie lamellated (pacinian) corpuscles, which are sensitive to pressure.

Hair is a keratinized structure that grows from a follicle, a tube of epidermal cells dipping into the dermis. The root sits inside the follicle; the shaft projects beyond the surface. An arrector pili muscle attaches to the follicle, and its contraction raises the hair. Sebaceous glands are usually connected to follicles and secrete sebum, an oily mixture that lubricates hair and skin. Sweat glands release sweat through a duct; eccrine glands are widespread and open onto the skin surface, while apocrine glands open into hair follicles. Nails are plates of tightly packed, hard keratinized epidermal cells that protect the ends of fingers and toes.

Receptors in the skin divide the work. Tactile corpuscles report light touch, lamellated corpuscles report pressure and vibration, and free nerve endings detect pain and temperature.

Functions, and the difference between skin colour and skin damage

The integumentary system performs several jobs at once. It regulates body temperature, stores blood, protects the body from the external environment, detects cutaneous sensations, excretes and absorbs small amounts of substances, and synthesises vitamin D when skin is exposed to ultraviolet light.

Temperature control works by negative feedback. When the body becomes too warm, vessels in the dermis widen and sweat glands release sweat; evaporation of that sweat removes heat. When the body becomes too cold, those vessels narrow and sweat production falls, so less heat is lost, and shivering adds heat through muscle activity. In both directions the response reduces the original change.

Skin colour is often misunderstood. Melanin production and distribution explain much of the normal variation between people, and the amount present changes with sun exposure. A tan is a response to ultraviolet damage rather than evidence that the exposure was safe, and melanocytes themselves are among the cells most easily damaged by ultraviolet light. Changes in colour can also indicate problems elsewhere in the body, and any persistent or unexplained skin change is a clinical finding that needs proper assessment rather than self-diagnosis.

One practical consequence of the skin’s structure concerns healing. New skin cannot regenerate if an injury destroys a large area of the stratum basale and its stem cells, so such wounds need a skin graft. A deep wound that destroys follicles and glands heals with scar tissue that has neither hair nor sweat glands, because the epidermis can regrow but the appendages may not.