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Human Skin: Structure, Functions, and Interesting Facts

Linda Crampton is an experienced teacher with a first-class honors degree in biology. She writes about the scientific basis of disease.

An Impressive and Vital Organ

The skin is an impressive organ that has vital functions. Skin acts as an enclosure that stops water from entering the body, reduces the loss of water, and protects the body from infection. It also helps to regulate body temperature, produces a vitamin D precursor, protects us from damage by ultraviolet light, and detects information in the environment. In addition, the skin contains cells that belong to the immune system and resident bacteria that help us in a variety of ways.

Although skin prevents the entry of water and many other substances into the body, it isn't a complete barrier between the body and the outside world. This is why some medicines can be absorbed through the skin, which is beneficial for us, and why some chemicals in cosmetics can also be absorbed through the skin, which may harm the body. In addition, some of our skin pores allow water to leave the body during perspiration. This process helps us to maintain a constant body temperature.

The skin is an amazing organ that has vital functions throughout our lives.

The skin is an amazing organ that has vital functions throughout our lives.

A tissue is a group of similar cells working together. An organ is a structure containing multiple tissues and performing a specific function (or sometimes several functions). The skin is the largest organ of the body when we consider both the interior and the surface of the body. The liver is the largest organ inside the body.

Structure of the Skin: An Overview

Although many people may think of the skin as simply a thin covering on top of our body, it actually contains multiple types of cells. The skin consists of two layers—the outer, thinner epidermis and the inner, thicker dermis. Underneath the dermis is the hypodermis, also called the subcutaneous layer, which is where fat is stored. The hypodermis isn't considered to be part of the skin, although the bases of the hair follicles and sweat glands may extend into it.

The Epidermis

The most abundant cells in the epidermis are the keratinocytes, which are arranged in layers. The keratinocytes in the upper part of the epidermis contain a protein called keratin, which makes the epidermis strong and waterproof. Cells called melanocytes produce a protective pigment named melanin. In addition, Merkel cells, which detect light touches to the skin, and Langerhans cells, which are part of the immune system, are located in the epidermis.

The Dermis

The dermis contains collagen and elastin fibers, hair follicles, sebaceous glands, the coiled sections of the sweat glands, blood and lymph vessels, nerves, sensory receptors, and protective cells from the immune system. The sebaceous glands produce an oily substance called sebum.

Resident Bacteria on the Surface of the Skin

It may be surprising to learn that bacteria are an important part of our skin. The bacteria that make their home there are known as resident bacteria, as opposed to temporary visitors, which are known as transient bacteria.

Resident bacteria are generally harmless or even helpful. They produce acidic wastes. The bacterial wastes and the lactic acid in our sweat cause the skin surface to have a low pH of around 4 to 5. This pH is fine for the normal bacteria that we carry around but is too low for many harmful bacteria and fungi. Our bacteria population therefore helps to protect us from injury by other microbes. The bacteria may also boost the activity of the immune system in the skin and fight pathogens (microbes that cause disease) in other ways.

The epidermis over most of the body is composed of four layers. The stratum lucidum is present only in thick skin, especially the skin found on the soles of the feet and on the palms.

The Five Layers of the Epidermis

The five layers of the epidermis are shown in the illustration above and are described below.

  • The stratum basale is the deepest layer of the epidermis. It consists of a single layer of cells. The cells divide to replace the skin cells that are shed.
  • The cells of the stratum spinosum are linked to each other by structures called desmosomes. Desmosomes enable cells to adhere strongly to one another. Filaments made of keratin extend from a desmosome and produce a spiny or prickly appearance. The stratum basale and the stratum spinosum are sometimes grouped together and known as the stratum germinativum.
  • The cells of the stratum granulosum contain granules made of a substance called keratohyalin. The granules produces a grainy appearance.
  • The stratum lucidum is a clear layer that contains dead cells. It’s found in the thick skin of the palms and on the soles of the feet.
  • The stratum corneum forms the surface of the skin and contains multiple layers of flattened cells. The cells have no organelles and are gradually shed from the body. Researchers have discovered that the stratum corneum has important barrier functions.
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Epidermal Structure

Keratinocytes and Keratin in the Epidermis

The keratinocytes are the most abundant cell type in the epidermis. The cells in the stratum basale divide to make the keratinocytes. The cells are eventually lost at the surface of the skin. Each new cell layer produced by the stratum basale pushes the preceding layer nearer to the skin's surface. It takes about a month for a specific layer to reach the surface of the skin.

Keratinocytes make a chemical called keratin. Keratin is a fibrous protein that forms hair and nails as well as being present in skin cells. It makes the skin tough and contributes to its ability to block water movement through the skin. By the time a layer of keratinocytes reaches the surface of the epidermis, the cells have a flattened, hexagonal shape and their keratin is fully formed.

In the stratum corneum, the keratinocytes die, although their tough keratin still protects the skin. Eventually, the dead cells fall off. This loss is usually balanced by the production of new cells deeper in the epidermis. The cells that leave the body make up a large part of household dust.

Researchers estimate that we lose 30,000 to 40,000 skin cells each minute, or 500 million cells per day.

Langerhans cells in the epidermis during an infection, with a stain added to make the dark granules in the cells clearly visible

Langerhans cells in the epidermis during an infection, with a stain added to make the dark granules in the cells clearly visible

Melanocytes, Langerhans Cells, and Merkel Cells


Keratinocytes aren't the only type of cell in the epidermis. Melanocytes are found in the bottom layer of the epidermis. These cells make melanin, a pigment that gives color to the skin. Melanin absorbs ultraviolet light, preventing it from damaging the body. It's important to realize that the pigment doesn't completely protect us from UV light. An additional form of protection is needed when we are exposed to sunlight.

Langerhans Cells

The epidermis also contains Langerhans cells. These cells are classified as a type of dendritic cell because they have extensions called dendrites at some point in their life. They are part of the immune system, but it's not completely clear how they function. Their biology is an active area of research. They are named after Paul Langerhans (1847–1888), a German doctor.

Merkel Cells

Merkel cells are located at the base of the epidermis. They lie close to nerve endings and are sensitive to light touch. They produce hormones, but the function of these chemicals is unknown. As is the case for Langerhans cells, more research is necessary. The cells are named after Friedrich Sigmund Merkel (1845–1919), a German anatomist.

The epidermis also contains chemicals, including lipids and antimicrobial peptides (short chains of amino acids that fight pathogens). Nutrients for the epidermal cells are supplied by the blood vessels in the dermis, which also remove waste substances made by the cells.

Ultraviolet light from the sun is needed for the skin to make vitamin D, but too much UV radiation can injure the skin.

Ultraviolet light from the sun is needed for the skin to make vitamin D, but too much UV radiation can injure the skin.

The Epidermis and Vitamin D Production

The process of vitamin D production in the body is a multistep process. The basic steps are as follows.

  • A chemical in the epidermis called 7-dehydrocholesterol is struck by ultraviolet light from the sun.
  • The 7-dehydrocholesterol is converted into an inactive form of vitamin D called cholecalciferol.
  • The cholecalciferol is converted into calcidiol in the liver.
  • The calcidiol is converted into calcitriol in the kidneys. Calcitriol is the active form of vitamin D.

Vitamin D is necessary for the absorption of calcium in the small intestine. The calcium is sent to the bones and keeps them strong. The vitamin may also boost the activity of the immune system.

Facts About the Dermis

Connective Tissue

The dermis consists of connective tissue surrounding multiple structures. Collagen and elastin fibers are abundant in the tissue. These proteins provide firmness, flexibility, and elasticity, enabling the dermis to act as a supporting layer for the skin.

The thinner, upper layer of the dermis is known as the papillary dermis. The collagen and elastin fibres are loosely arranged here. The papillary dermis forms projections called papillae that extend into the epidermis. The thicker reticular dermis below the papillary layer contains fibres in a tighter arrangement.

A Muscle and a Sensory Receptor

The hair follicle is a common structure in the dermis. Attached to each follicle is an arrector pili muscle. This muscle causes the hair to become erect when the skin is cold or when we experience strong emotions. The erect hairs produce a "goose bumps" or "goose flesh" appearance on the surface of the skin.

One type of sensory receptor in the dermis is the Pacinian corpuscle. It's classified as a mechanoreceptor and is triggered by touch and pressure. It responds to stimuli such as rough surfaces and vibrations and sends an impulse along the attached sensory neuron. The message is sent to the brain via a sensory nerve, enabling us to detect the sensation. The receptor's name begins with a capital letter because it's named after Filippo Pacini, an Italian anatomist and microbiologist who lived from 1812 to 1883. He discovered the receptor.

The Dermal Layer of the Skin

Glands in the Dermis

Sebaceous Glands

The dermis contains three types of glands—sebaceous glands, eccrine or merocrine glands, and apocrine glands. Sebaceous glands are usually attached to hair follicles. They secrete sebum, an oily substance that contains a mixture of lipids. Sebum lubricates and waterproofs the skin and hair. The greatest amount of sebum is secreted during puberty.

Eccrine Glands

Our skin contains two types of sweat glands, or sudoriferous glands. Eccrine glands are found over most of the body and release sweat directly to the surface of the skin. This sweat is watery and almost odorless. It contains many dissolved chemicals, including water, urea (a waste substance produced from protein metabolism), lactic acid, and sodium chloride.

Apocrine Glands

Apocrine glands are found only in certain areas, such as the armpits. They become active at puberty and release a thick, milky, and fatty liquid into a hair follicle. Certain conditions, such as stress, stimulate the release of liquid from apocrine glands. When the odorless liquid reaches the surface of the skin, bacteria break it down, producing odoriferous compounds. The function of apocrine glands is unknown. It's been suggested that in the past (and perhaps in the present) their secretion contained a pheromone, which is a chemical that attracts the opposite gender.

The Skin's Role in Temperature Regulation

The skin has two ways to regulate body temperature. One method is by changing the diameter of the blood vessels. When blood vessels in the dermis dilate, they allow more blood to flow through them. Heat radiates from this blood, moving up through the skin and into the outside world. The reddening of the skin due to increased blood flow can be seen through the thin epidermis. When the body is cold, the blood vessels constrict, reducing the flow of blood. This causes the skin to turn pale and reduces heat loss.

The second method of heat regulation is by perspiration. Water leaving the eccrine sweat glands absorbs heat from the skin as it changes into a gas and evaporates into the atmosphere. The gaseous water carries heat from the body with it as it escapes, cooling the body down.

A team of researchers has found that our skin may be useful even when it's shed from our body and forms part of the dust in buildings. The researchers have found that a chemical in the discarded skin called squalene absorbs some of the ozone from polluted air.

Our Wonderful Skin

Our skin is an amazing organ and is more complex than some people realize. It protects us from stresses that could hurt our bodies, helps us to detect our environment, and produces important chemicals. We notice changes in our skin's appearance when we're injured or as we age, but many of us don't stop to realize what a marvelous and hard-working structure the organ really is. It has an interesting composition and is much more than a simple barrier between our body and the outside world. The skin is an organ that’s worth studying.


  • Introduction to skin histology from the Southern Illinois School of Medicine
  • Skin structure, functions, and disorders from the Merck Manual
  • Shed skin cells reduce air pollution from the American Chemical Society.
  • Vitamin D and the skin from Oregon State University
  • Information about melanin from the University of Bristol in the UK
  • Skin gland information from the University of Leeds
  • Filippo Pacini: A Determined Observer (abstract) from the National Institutes of Health (NIH)

Questions & Answers

Question: I am a student. I would like to describe skin to my friends. Can you give me suggestions about what to say to them?

Answer: The information that you share with your friends is up to you. I would suggest that you first make sure that you understand the facts about skin very well. Then you need to choose the facts that you think are most important or most interesting and decide what you are going to say about them or how you are going to describe them to your friends.

© 2012 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on October 02, 2018:

Thank you, Jonas. I'm glad the article was helpful.

Jonas on October 02, 2018:

Thank You! I am doing a research paper on skin and this article was very helpful!

Linda Crampton (author) from British Columbia, Canada on September 11, 2018:

Thanks, Shubhada. I appreciate your comment.

Shubhada on September 11, 2018:

This article is really amazing with a lot about our skin and its facts as well.Really loved it Miss Linda.Tysm.

Linda Crampton (author) from British Columbia, Canada on September 08, 2018:

I'm glad the article was helpful, Kusuma.

Kusuma on September 07, 2018:

It is helpful to my studies

Thank u

Linda Crampton (author) from British Columbia, Canada on March 14, 2013:

Thank you very much for the visit and the comment, Tonipet!

Tonette Fornillos from The City of Generals on March 14, 2013:

So enjoyed reading about the wonders of the skin organ. I'm a bit guilty I haven't been giving my skin the best care. Thanks so much for this, Alicia. You are an eye-opener. Blessings! -Tonette

Linda Crampton (author) from British Columbia, Canada on June 24, 2012:

Thank you for the comment and the vote, girishpuri. I appreciate your visit.

Girish puri from NCR , INDIA on June 24, 2012:

Alicia, this is indeed an addition to my knowledge, voted up

Linda Crampton (author) from British Columbia, Canada on June 24, 2012:

Thank you for the comment, Peter Geekie. It's nice to meet you!

Peter Geekie from Sittingbourne on June 24, 2012:

Dear Alicia C

A very well researched and written article

Thank you

Kind regards Peter

Linda Crampton (author) from British Columbia, Canada on June 19, 2012:

Thank you for the visit and the comment, teaches. It is easy to forget that the skin is classified as an organ!

Dianna Mendez on June 19, 2012:

I always forget that the skin is an organ. We discussed this in class the other day as we talked about organ donation and this popped up. Interesting topic and well covered.

Linda Crampton (author) from British Columbia, Canada on June 19, 2012:

Thank you very much for the comment and for sharing the hub, mary. I appreciate the link in your hub, too! Skin cancer is a very important topic. I'm glad that you wrote about it.

Mary Hyatt from Florida on June 19, 2012:

Wow! You really did a lot of research for this Hub. It is very informative and interesting. I guess we just take our skin for granted until something goes wrong like skin cancer. I voted this Hub UP, etc. and I would like to share with followers and FB. May I link this Hub to the one I just published about Basal Cell Carcinoma? Thanks in advance.

Linda Crampton (author) from British Columbia, Canada on June 18, 2012:

Thank you, drbj. I appreciate your visit, your comment and the vote! The skin certainly is an amazing organ, and it does so much for us.

drbj and sherry from south Florida on June 18, 2012:

The skin is an amazing, remarkable organ, Alicia, and you have done an excellent job bringing all this valuable, interesting information to your readers. Thank you, m'dear, and a large Up!

Linda Crampton (author) from British Columbia, Canada on June 18, 2012:

Hi, whonunuwho. Thanks for commenting. Yes, safe sunlight exposure and an adequate vitamin D supply are very important topics! On the one hand dermatologists tell us that melanin isn't sufficient to protect us from all UV radiation and that we must use sunscreen to protect ourselves from sunburn and skin cancer, and on the other hand researchers say that many of us are deficient in vitamin D, which is turning out to be an extremely important chemical. Deciding whether sunscreen ingredients are safe or not is also a controversial topic!

What I do is use a combination of protective clothing and the safest sunscreen that I can find to protect me from the sun, as well as take a vitamin D supplement every day - but that's another controversial topic. Scientists and nutritionists can't seem to agree on the dose of vitamin D supplement that we should be taking. It needs to be both safe and effective.

whonunuwho from United States on June 18, 2012:

I realize that the skin absorbs sunlight and creates vitamin D that we need for bones and teeth. I wonder if vitamin supplements, milk and milk products, or the sun. itself is best in supplying Vitamin D. Sunlight can be a cause of skin cancer and it concerns me about exposing my skin, more so than when I was younger.

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