Skin biology and structure

An Overview of Your Skin

Skin biology and structure

Skin is the largest organ on our body, made up of several different parts, including water, protein, lipids (fats), and different minerals and chemicals.

Throughout your life, your skin will change, for better or worse. In fact, your skin regenerates itself approximately every 27 days.

It’s very important to treat your skin properly to maintain the health and vitality of this crucial organ.

This information serves as an overview only, and should not replace a professional’s advice.

What does your skin demand every day?

During the hustle and bustle of your daily routines, it’s easy to skip that glass of water or to skip cleaning. Over time, though, those bad habits can take a toll on your skin. Each day, you should be sure to provide your skin with:

  • Plenty of water.
  • Thorough cleansing, twice a day. At night, make sure you remove all your make-up and cleanse properly before going to bed.
  • Balanced nutrition.
  • Moisturizing. This is a necessary step even for those who have oily skin. There are plenty of moisturizers on the market that are oil-free.
  • Sun protection, sun protection, sun protection. It is absolutely crucial to protect your skin from harmful UVA and UVB light rays. Sunscreen (SPF 30 or higher) is recommended every day. This will help prevent sun damage, skin cancers, and wrinkles.

Over the course of your life, you should pay attention to all parts of your skin. Familiarize yourself with it so that you’ll notice any changes that might occur, such as different moles or patches that might need further attention.

What is the skin’s structure?

Epidermis: The outer layer

The epidermis is the thinnest layer, but it’s responsible for protecting you from the harsh environment, with 5 layers of its own. The epidermis also hosts different types of cells:

  • Keratinocytes, which produce the protein known as keratin, the main component of the epidermis.
  • Melanocytes, which produce your skin pigment, known as melanin.
  • Langerhans cells, which prevent things from getting into your skin.

Dermis: The middle layer

The dermis is the layer that gives your skin its fullness and plumpness. Age and the sun can damage the dermis and lead to wrinkles.

The dermis is a complex combination of blood vessels, hair follicles, and sebaceous (oil) glands.

Here, you’ll find collagen and elastin, 2 proteins necessary for skin health because they offer support and elasticity (your skin’s ability to go back to its original state after being stretched).

Fibroblasts are the cells you’ll find in this layer, because they synthesize collagen and elastin. This layer also contains pain and touch receptors.

Hypodermis: The fatty layer

This layer is also known as the subcutis. It hosts sweat glands and fat and collagen cells, and is responsible for conserving your body’s heat and protecting your vital inner organs. Reduction of tissue in this layer contributes to sagging skin.

What are the skin’s proteins?

  • Collagen: Collagen is the most plentiful protein in the skin, making up 75-80% of your skin. Collagen and elastin are responsible for warding off wrinkles and fine lines. Over time, the environment and aging reduce your body’s ability to produce collagen.
  • Elastin: Think elastic. Elastin is found with collagen in the dermis. It’s another protein, responsible for giving structure to your skin and organs. As with collagen, elastin is affected by time and the elements. Lower levels of this protein cause your skin to wrinkle and sag.
  • Keratin: Keratin is the main protein in your skin, and makes up hair, nails, and the surface layer of the skin. Keratin is what forms the rigidity of your skin and helps with the barrier protection that your skin offers.

Last reviewed by a Cleveland Clinic medical professional on 03/17/2016.


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The Importance of Lipids in Cutaneous Function

Skin biology and structure

  1. Kennenth R. Feingold, Associate Editor

The skin is the largest organ of the body and has a very complex and heterogeneous structure. The innermost layer is the subcutaneous fat layer. Next is the dermis, which contains fibroblasts that produce collagen and elastic fibers.

Localized within the dermis are specialized organelles including sebaceous glands, eccrine glands (sweat glands), and hair follicles. The nerves and blood vessels are also localized to the dermis. The outermost layer of the skin is the epidermis, a highly active lipid synthetic tissue, consisting of several distinct layers.

The innermost layer of the epidermis is the basal layer, where cells are rapidly dividing. The spinous layer is in the mid-epidermis and represents cells that have undergone the initial steps of differentiation, while the stratum granulosum layer consists of cells that are further differentiated and have numerous lamellar bodies.

The outer layer of the epidermis, the stratum corneum, consists of corneocytes (cells that have undergone terminal differentiation) and extracellular lipid membranes.

The skin has a large number of important functions. It provides an interface between a hostile external environment and the host. Thus the major function of the skin is to provide a barrier between the body and the outside environment.

There are numerous barrier functions, and the skin has unique structures that provide for these various barriers. For example, the permeability barrier, which resides in the extracellular lipid membranes in the stratum corneum, prevents the loss of water and electrolytes. Additionally, it prevents the entry of toxic compounds.

The skin surface is also, to a large extent, how we visualize other individuals, and thus the skin has not only biological functions but social functions. The cosmetic industry is built upon the aim of making the skin more attractive. Furthermore, skin diseases are very common.

For example, atopic dermatitis affects 10-20% of children, acne affects almost all adolescents, and psoriasis affects 2% of the adult population.

While the skin is a vital organ, studies of lipid metabolism in the skin have not been in the mainstream of lipid research.

However, now that molecular geneticists have shown a primary role for permeability barrier abnormalities as the cause of atopic dermatitis and perhaps other common cutaneous diseases, increasing attention is focusing on the factors that regulate and maintain permeability barrier function.

Additionally, the key role of sebaceous gland lipids in acne, the marked dysfunction of sebaceous glands in genetically engineered mice with a deficiency of enzymes required for fatty acid and triglyceride metabolism, and mutations in lipid enzymes as the etiology of many of the types of ichthyosis (skin changes resembling fish-scales) have also heightened interest in cutaneous lipid metabolism. Therefore, I think it is timely that a thematic review series provide an up-to-date discussion on lipid metabolism in the skin and hopefully interest other investigators to study this complex but essential organ.

I will lead off this series with a review of the role of epidermal lipids in permeability barrier function. For terrestrial life it is essential to have a functional permeability barrier that prevents the loss of water and electrolytes.

This permeability barrier is comprised of extracellular lipid-enriched membranes in the stratum corneum that contain ceramides, cholesterol, and free fatty acids.

I will review how these lipids are synthesized and the regulation of their synthesis in response to barrier perturbations.

Dr. Diane Thiboutot and colleagues will review lipid metabolism in sebaceous glands, which secrete a variety of different lipids including squalene, wax esters, and triglycerides onto the surface of the skin.

Acne is dependent on lipid production by sebaceous glands, and strategies that reduce sebaceous gland lipid secretion could be useful in the treatment of acne.

Additionally, the role of sebaceous gland lipids in skin hydration will be discussed.

Dr. Phillip Wertz and colleagues will review the role of skin lipids in preventing infections, one of the key barrier properties of skin.

Although largely attributed to antimicrobial peptides, such as the β-defensins and cathelicidins, skin lipids, produced by both the epidermis and sebaceous glands, also play a key role in preventing infection, i.e., they are active participants in the innate immune system.

Dr. Peter Elias and colleagues will review the role of lipids in regulating desquamation. The epidermis is a rapidly dividing tissue that is constantly renewing itself. It is essential that the production of new keratinocytes is balanced by the shedding of corneocytes from the skin surface.

Either failure to desquamate and/or overproduction of corneocytes results in thickening of the stratum corneum and thus ichthyosis. The appearance of the skin resembles fish scales, and the term ichthyosis is derived from the Greek word ichthys which means fish.

A number of genetic abnormalities in lipid metabolism have been shown to cause ichthyosis, and these disorders have provided insights into the role of lipids in regulating cohesion and desquamation in normal skin.

Dr. Walter Holleran and colleagues will review sphingolipid metabolism in the epidermis.

It is now recognized that the epidermis produces both large quantities and a great diversity of sphingolipids and that these sphingolipids play key roles in the formation of the extracellular lamellar membranes in the stratum corneum that account for the permeability barrier.

Additionally, unique sphingolipids also play an essential role in the formation of the cornified lipid envelope that links the corneocyte with the extracellular lamellar membranes.

Finally, Dr Matthias Schmuth and colleagues will review the role of PPARs and LXRs in skin biology.

The skin in general, and the epidermis and sebaceous glands in particular, are important sites of lipid synthesis, and therefore it is not surprising that the nuclear hormone receptors that are activated by lipids (liposensors) are present in the skin and regulate a wide variety of skin functions. The role of PPARs and LXR in regulating sebaceous gland and epidermal function will be discussed in that review.

Together, this series of articles should provide an up-to-date review of cutaneous lipid metabolism. Hopefully, the reader will develop a greater appreciation for the key role of lipids in skin biology, and perhaps he/she will be attracted to applying his/her expertise to further elucidate the key roles of lipid metabolism in skin biology and disease.


The Skin (Human Anatomy): Picture, Definition, Function, and Skin Conditions

Skin biology and structure

© 2014 WebMD, LLC. All rights reserved.

The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.

Skin has three layers:

  • The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
  • The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
  • The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.

The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis.

  • Rash: Nearly any change in the skin’s appearance can be called a rash. Most rashes are from simple skin irritation; others result from medical conditions.
  • Dermatitis: A general term for inflammation of the skin. Atopic dermatitis (a type of eczema) is the most common form.
  • Eczema: Skin inflammation (dermatitis) causing an itchy rash. Most often, it’s due to an overactive immune system.
  • Psoriasis: An autoimmune condition that can cause a variety of skin rashes. Silver, scaly plaques on the skin are the most common form.
  • Dandruff: A scaly condition of the scalp may be caused by seborrheic dermatitis, psoriasis, or eczema.
  • Acne: The most common skin condition, acne affects over 85% of people at some time in life.
  • Cellulitis: Inflammation of the dermis and subcutaneous tissues, usually due to an infection. A red, warm, often painful skin rash generally results.
  • Skin abscess (boil or furuncle): A localized skin infection creates a collection of pus under the skin. Some abscesses must be opened and drained by a doctor in order to be cured.
  • Rosacea: A chronic skin condition causing a red rash on the face. Rosacea may look acne, and is poorly understood.
  • Warts: A virus infects the skin and causes the skin to grow excessively, creating a wart. Warts may be treated at home with chemicals, duct tape, or freezing, or removed by a physician.
  • Melanoma: The most dangerous type of skin cancer, melanoma results from sun damage and other causes. A skin biopsy can identify melanoma.
  • Basal cell carcinoma: The most common type of skin cancer. Basal cell carcinoma is less dangerous than melanoma because it grows and spreads more slowly.
  • Seborrheic keratosis: A benign, often itchy growth that appears a “stuck-on” wart. Seborrheic keratoses may be removed by a physician, if bothersome.
  • Actinic keratosis: A crusty or scaly bump that forms on sun-exposed skin. Actinic keratoses can sometimes progress to cancer.
  • Squamous cell carcinoma: A common form of skin cancer, squamous cell carcinoma may begin as an ulcer that won’t heal, or an abnormal growth. It usually develops in sun-exposed areas.
  • Herpes: The herpes viruses HSV-1 and HSV-2 can cause periodic blisters or skin irritation around the lips or the genitals.
  • Hives: Raised, red, itchy patches on the skin that arise suddenly. Hives usually result from an allergic reaction.
  • Tinea versicolor: A benign fungal skin infection creates pale areas of low pigmentation on the skin.
  • Viral exantham: Many viral infections can cause a red rash affecting large areas of the skin. This is especially common in children.
  • Shingles (herpes zoster): Caused by the chickenpox virus, shingles is a painful rash on one side of the body. A new adult vaccine can prevent shingles in most people.
  • Scabies: Tiny mites that burrow into the skin cause scabies. An intensely itchy rash in the webs of fingers, wrists, elbows, and buttocks is typical of scabies.
  • Ringworm: A fungal skin infection (also called tinea). The characteristic rings it creates are not due to worms.
  • Skin biopsy: A piece of skin is removed and examined under a microscope to identify a skin condition.
  • Skin testing (allergy testing): Extracts of common substances (such as pollen) are applied to the skin, and any allergic reactions are observed.
  • Tuberculosis skin test (purified protein derivative or PPD): Proteins from the tuberculosis (TB) bacteria are injected under the skin. In someone who’s had TB, the skin becomes firm.
  • Corticosteroids (steroids): Medicines that reduce immune system activity may improve dermatitis. Topical steroids are most often used.
  • Antibiotics: Medicines that can kill the bacteria causing cellulitis and other skin infections.
  • Antiviral drugs: Medicines can suppress the activity of the herpes virus, reducing symptoms.
  • Antifungal drugs: Topical creams can cure most fungal skin infections. Occasionally, oral medicines may be needed.
  • Antihistamines: Oral or topical medicines can block histamine, a substance that causes itching.
  • Skin surgery: Most skin cancers must be removed by surgery.
  • Immune modulators: Various drugs can modify the activity of the immune system, improving psoriasis or other forms of dermatitis.
  • Skin moisturizers (emollients): Dry skin is more ly to become irritated and itchy. Moisturizers can reduce symptoms of many skin conditions.


WebMD Feature: “Exercise Your Body — and Your Skin.” “Skin Anatomy,” “Skin Diseases.”

MedicineNet: “Boils (Skin Abscesses).”

American Osteopathic College of Dermatology: “Actinic Keratosis.”

EMedicine: “Skin Cancer: Sqamous Cell Carcinoma.” Seborrheic Dermatitis: What It Is and How to Treat It.”

WebMD Medical Reference: “Skin Testing for Allergies.”

MedicineNet: “Tuberculosis Skin Test (PPD Skin Test).”

© 2019 WebMD, LLC. All rights reserved.


Skin biology and structure

Skin biology and structure

The skin is the human body’s largest organ, with a range of functions that support survival.

A view through the microscope reveals the layered structure of the skin, and the many smaller elements within these layers that help the skin to perform its mainly protective role.

The skin has two main layers, the epidermis and dermis. Below these is a layer of subcutaneous (‘under the skin’) fat.

The epidermis

The outer surface of the skin is the epidermis, which itself contains several layers — the basal cell layer, the spinous cell layer, the granular cell layer, and the stratum corneum. The cells in the epidermis are called keratinocytes.

The deepest layer of the epidermis is the basal cell layer. Here cells are continually dividing to produce plump new skin cells (millions daily). These cells move towards the skin surface, pushed upward by the dividing cells below them.

Blood vessels in the dermis — which is below the basal cell layer — supply nutrients to support this active growth of new skin cells. As the basal cells move upwards and away from their blood supply, their cell content and shape change, as follows.

Cells above the basal cell layer become more irregular in shape and form the spinous layer. Above this, cells move into the granular layer. Being distant from the blood supply in the dermis, the cells begin to flatten and die and accumulate a substance called keratin. Keratin is a protein that is also found in hair and nails.

The stratum corneum (‘horny layer’) is the top layer of the epidermis — it is the layer of the skin that we see from the outside. Cells here are flat and scale- (‘squamous’) in shape. These cells are dead, contain a lot of keratin and are arranged in overlapping layers that impart a tough and waterproof character to the skin’s surface.

Dead skin cells are continually shed from the skin’s surface. This is balanced by the dividing cells in the basal cell layer to produce a state of constant renewal. Also in the basal cell layer are cells called melanocytes that produce melanin.

Melanin is a pigment that is absorbed into the dividing skin cells to help protect them against damage from sunlight (ultraviolet light). The amount of melanin in your skin is determined by your genes and by how much exposure to sunlight you have.

The more melanin pigment present, the darker the colour of your skin.

The epidermis also contains dendritic (Langerhans) cells, which are part of the immune system and help protect the body from foreign substances.

The dermis

Below the epidermis is the layer called the dermis. The top layer of the dermis — the one directly below the epidermis — has many ridges called papillae. On the fingertips, the skin’s surface follows this pattern of ridges to create our individual fingerprints. So the ridges are not on the outermost layer of skin, as it might appear.

The dermis contains a variable amount of fat, and also collagen and elastin fibres which provide strength and flexibility to the skin. In an older person the elastin fibres fragment and much of the skin’s elastic quality is lost. This, along with the loss of subcutaneous fat, results in wrinkles.

When the skin is exposed to sunlight, modified cholesterol in the dermis produces vitamin D, which helps the body to absorb calcium for healthy bones.

Here are some of the other structures within the dermis that enhance the skin’s function.

  • Blood vessels supply nutrients to the dividing cells in the basal layer and remove any waste products. They also help maintain body temperature by dilating and carrying more blood when the body needs to lose heat from its surface; they narrow and carry less blood when the body needs to limit the amount of heat lost at its surface.
  • Specialised nerves in the dermis detect heat, cold, pain, pressure and touch and relay this information to the brain. In this way the body senses changes in the environment that may potentially harm the body.
  • Hair follicles are embedded in the dermis and occur all over the body, except on the soles, palms and lips. Each hair follicle has a layer of cells at its base that continually divides, pushing overlying cells upwards inside the follicle. These cells become keratinised and die, the cells in the epidermis, but here form the hair shaft that is visible above the skin. The colour of the hair is determined by the amount and type of melanin in the outer layer of the hair shaft.
  • A sebaceous (‘oil’) gland opens into each hair follicle and produces sebum, a lubricant for the hair and skin that helps repel water, damaging chemicals and microorganisms (‘germs’).
  • Attached to each hair follicle are small erector pili muscle fibres. These muscle fibres contract in cold weather and sometimes in fright — this pulls the hair up which pulls on the skin with the result being ‘goosebumps’.
  • Sweat glands occur on all skin areas — each person has more than 2 million. When the body needs to lose heat these glands produce sweat (a mix of water, salts and some waste material such as urea). Sweat moves to the skin’s surface via the sweat duct, and evaporation of this water from the skin has a cooling effect on the body.

The skin varies in thickness and the number of hair follicles, sebaceous glands and sweat glands in different areas of the body. The thickest skin is on the soles of the feet and the palms of the hands. A large number of hair follicles are on the top of the head.

Subcutaneous fat

The innermost layer of the skin is the layer of subcutaneous fat, and its thickness varies in different regions of the body. The fat stored in this layer represents an energy source for the body and helps to insulate the body against changes in the outside temperature.

Functions of the skin

As you can see, there are many different structures within the skin. Together these structures impart many protective properties to the skin that help avoid damage to the body from outside influences. In this way, the skin:

  • protects the body from water loss and from injury due to bumps, chemicals, sunlight or microorganisms (‘germs’);
  • helps to control body temperature;
  • is a sensor to inform the brain of changes in the immediate environment; and
  • synthesises vitamin D.


1. Merck Manual – Consumer version. Structure and function of the skin. (accessed Jul 2015).
2. DermNet NZ. Structure of the epidermis (updated 10 Nov 2014). (accessed Jul 2015).


How does skin work?

Skin biology and structure

Created: September 28, 2009; Last Update: April 11, 2019; Next update: 2022.

Even at its thickest point, our skin is only a few millimeters thick. But it is still our heaviest and largest organ, making up about one seventh of our body weight: Depending on your height and body mass, it weighs between 3.5 and 10 kilograms (7.5 and 22 pounds) and has a surface area of 1.5 to 2 square meters. This goes to show how important skin is for your body and metabolism.

Skin has a lot of different functions. It is a stable but flexible outer covering that acts as barrier, protecting your body from harmful things in the outside world such as moisture, the cold and sun rays, as well as germs and toxic substances.

Just looking at someone’s skin can already tell you a lot – for instance, about their age and health. Changes in skin color or structure can be a sign of a medical condition. For example, people with too few red blood cells in their blood may look pale, and people who have hepatitis have yellowish skin.

Skin also plays an important role in regulating your body temperature. It helps prevent dehydration and protects you from the negative effects of too much heat or cold. And it allows your body to feel sensations such as warmth, cold, pressure, itching and pain. Some of these sensations trigger a reflex, automatically pulling your hand back if you accidentally touch a hot stove.

Skin also functions as a large storeroom for the body: The deepest layer of skin can store water, fat and metabolic products. And it produces hormones that are important for the whole body.

If skin is injured, the blood supply to the skin increases in order to deliver various substances to the wound so it is better protected from infections and can heal faster. Later on, new cells are produced to form new skin and blood vessels. Depending on how deep the wound is, it heals with or without a scar.

To be able to do all of these things, skin consists of three different layers: the outer layer (epidermis), the middle layer (dermis) and the deepest layer (subcutis).

Depending on where it is on your body and the demands made on it, your skin varies in thickness.

The thickness of your skin depends on your age and sex too: Older people generally have thinner skin than younger people do, and men generally have thicker skin than women do.

The outermost layer of skin which you can see is called the epidermis. It is mostly made up of cells that produce keratin (keratinocytes). These cells are gradually pushed to the surface of the skin by newer cells, where they harden and then eventually die off. The hardened keratinocytes (corneocytes) are packed closely together and seal the skin off from the outside environment.

The epidermis constantly renews itself: New cells are made in the lower layers of the epidermis. These move to the surface within four weeks. This constant renewal serves to replace the cells that are lost and fall to the ground as tiny flakes of skin when the skin is rubbed.

The cells in the epidermis grow faster in response to pressure or rubbing. The amount of skin flakes that are shed remains the same, though. As a result, the layer of hardened skin on the surface gradually becomes thicker and a callus develops.

The skin does this to protect itself – to better withstand pressure and rubbing.

Only rarely is the balance of new cell production and old cell shedding affected by illness. Examples include infections, autoimmune disorders or genetic diseases that cause increased growth of rough, scaly skin on the entire body. Hardened skin in only one place may be a sign of non-melanoma skin cancer or skin changes that may develop into cancer.

Depending on where it is on the body, the epidermis varies in thickness. For instance, it is only 0.3 millimeters thick on your elbows and the back of your knees, and up to 4 millimeters thick on other parts of your body such as the soles of your feet and palms of your hands.

The epidermis also contains other types of cells with special functions:

  • Melanocytes produce and store a black pigment called melanin. They produce more melanin when your skin is exposed to sunlight, which is why it becomes darker. This protects the skin from the sun’s harmful UV rays.

  • Lymphocytes and Langerhans cells play an important role in fighting germs. They “grab” the germs and take them to the nearest lymph node.

  • Merkel cells are special nerve cells in the skin that enable you to sense pressure.

Under the epidermis, firmly stuck to it, lies the middle layer of skin (the dermis). It is made up of a dense network of tough, elastic collagen fibers.

These make the skin strong and robust, while at the same time stretchy. If skin is stretched a lot – for instance the skin covering a pregnant woman’s belly – the dermis might tear.

The torn dermis can be seen as light lines (stretch marks).

In places, the dermis bulges into the connective tissue that surrounds our muscles and bones and connects them with the skin.

The dermis contains a network of nerve fibers and very small blood vessels called capillaries. Nutrients and oxygen in the blood pass from the capillaries into cells. The other main function of the capillaries is to help your body cool down if it gets too hot. The dermis is also the skin layer that contains the most sensory (feeling) cells and sweat glands.


  • Menche N (Ed). Biologie Anatomie Physiologie. Munich: Urban und Fischer; 2016.
  • Moll I. Duale Reihe Dermatologie. Stuttgart: Thieme; 2016.
  • Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.
  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie.

    Berlin: Springer; 2017.

  • Williams H, Bigby M, Herxheimer A, Naldi L, Rzany B, Dellavalle R et al. Evidence-Based Dermatology. John Wiley & Sons; 2014.

  • IQWiG health information is written with the aim of helpingpeople understand the advantages and disadvantages of the main treatment options and healthcare services.

    Because IQWiG is a German institute, some of the information provided here is specific to theGerman health care system. The suitability of any of the described options in an individualcase can be determined by talking to a doctor. We do not offer individual consultations.

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