Human skin consists of many types of cells organized in three main layers. Click the name of each skin layer in the illustration below to learn about its functions and the cell types it contains.

The epidermis protects the body from the elements, such as wind and ultraviolet (UV) radiation. It also serves as a barrier to keep bacteria and viruses out and water in. The epidermis is, however, a selective barrier, as some gases, such as oxygen, can pass through easily. One other important function of the epidermis is to produce vitamin D (see below). Most cells in the epidermis are keratinocytes. Other cell types include melanocytes—skin cells that produce the pigment melanin—and different kinds of immune cells that defend the body against infection. You’ll learn more about this layer of skin later in the video.

The dermis contains blood vessels, lymph vessels, hair follicles, sweat glands, oil glands, and nerve endings. Its function is to supply the epidermis with oxygen and nutrients from blood and to regulate body temperature. When it’s hot, vessels in the dermis expand, exposing them more to the surface where they can radiate excess heat. When it’s cold, vessels in the dermis constrict, which helps retain heat within the body. In addition, liquid sweat expelled from sweat glands cools the body as it evaporates. Sweating is also a way to get rid of some wastes produced in the body. Oil glands within the dermis secrete lipids that travel through the pores to the surface to lubricate and waterproof the skin.

The hypodermis, which contains fat cells, functions mainly to insulate the body, provide a protective padding around internal organs, and store energy.

The epidermis is the outer layer of skin. It contains several different cell types. Two important ones are keratinocytes and melanocytes.

Keratinocytes and melanocytes.

Click the buttons above to find out what the cells do.

Keratinocytes make up almost all the cells in the epidermis. Their name comes from the fact that they contain the structural protein keratin, which is also found in hair and nails. Keratin provides strength, flexibility, and toughness to the skin. New keratinocytes form at the bottom of the epidermis from dividing stem cells. As they migrate to the top of the epidermis, they increase keratin production and eventually stop dividing. The top layer of the epidermis consists of tightly packed dead keratinocytes that have lost their cell nucleus and other organelles. The skin owes most of its tough, waterproof barrier function to this top layer. These dead cells slough off easily and are replaced every 4 to 6 weeks by new keratinocytes that have migrated from the bottom of the epidermis to the top.

Melanocytes make up only about 1% of the cells in the epidermis. They produce the pigment melanin, which gives skin its color. Melanocytes are not round or flattened like keratinocytes. They instead look more like nerve cells with their long projections. The long projections in melanocytes serve to deliver melanosomes—which are pigment-containing organelles—to keratinocytes. One melanocyte supplies many keratinocytes, in the same way that one nerve cell may interact with a large number of other nerve cells.

Humans are primates—a group of animals that includes monkeys, chimpanzees, and gorillas. All nonhuman primates are covered in thick body hair. Instead, we have mostly very thin hairs on our bodies.

It’s reasonable to assume that the last common ancestor we shared with our closest primate relatives, chimpanzees, was also covered in thick hair and had light skin. Genetic and fossil evidence suggests that this common ancestor existed most recently about 7 million years ago. So, sometime in the past 7 million years, we evolved a mostly hairless body.

The pigment melanin, which gives human skin its color, is produced inside the skin’s melanocytes, by organelles called melanosomes.

Organelles are parts of cells typically surrounded by a membrane that separates their contents from the rest of the cell. The nucleus, mitochondrion, and lysosome are examples of organelles.

There are two primary types of melanin: pheomelanin and eumelanin. Pheomelanin is a reddish-yellow pigment. Eumelanin comes in two varieties, brown and black. The ratios of the different types of melanin contribute to skin color. Skin with mostly pheomelanin appears a light pinkish color. Skin with mostly eumelanin appears brown.

In general, the more eumelanin that is present, the darker a person’s skin color.

Although almost everyone has approximately the same number of melanocytes, the amount and size of the melanosomes and the amount of melanin within them can differ greatly. In general, more eumelanin-producing melanosomes will result in darker shades.

Melanin has many functions. In human skin, melanin’s main function is to absorb and scatter UV radiation. Dark skin, which contains more eumelanin than fair skin, is better protected against UV-induced damage. Darker skin also receives less UV radiation for vitamin D synthesis.

Many genes affect the synthesis, transport, distribution, and degradation of melanin. Studies have shown that differences in some of these genes can have a large impact on skin color in different populations.

Watch the video clip below to learn about one gene in particular.

The MC1R protein sits on the surface of melanocytes and is activated by the melanocyte-signaling hormone (MSH) in the blood. MSH comes in many varieties and its production by the pituitary gland is triggered by many factors.

Melanosomes mature within melanocytes. Melanin-containing melanosomes are then transported through the melanocytes’ projections and into the skin’s keratinocytes. What happens to them there? Find out by clicking the labels in the illustration below.

The keratinocytes closer to the bottom of the epidermis have nuclei and melanosomes, as shown here. But the keratinocytes in the uppermost epidermis are dead cells that lack organelles.

Some melanosomes form a kind of cover, called a supranuclear cap, above the keratinocyte’s nucleus. These caps look like small umbrellas around the nucleus and act to shield the DNA inside from damaging UV rays.

Certain types of UV radiation—UVA and UVB radiation—can penetrate the cells of the epidermis and can cause damage to DNA and other molecules.

Not all melanosomes form supranuclear caps. In fact, many melanosomes are found distributed throughout the cytoplasm of keratinocytes. They form a shield that protects important molecules circulating in blood vessels in the dermis.

The nucleus is the cellular organelle that contains DNA. In humans, DNA is arranged in 23 pairs of chromosomes.

Too much UVB radiation also destroys folate, an important vitamin circulating in blood. Folate deficiency can result in decreased fertility, and in a pregnant woman it can result in severe birth defects.

Folate is a B vitamin present in certain foods. Unlike vitamin D, our bodies cannot synthesize folate. The best sources of folate are leafy green vegetables and fruits. Folate is necessary for DNA synthesis and repair, as well as in reproduction and fetal growth and development.

Radiation given off by the sun is divided into five regions in order of increasing wavelength (and therefore decreasing energy). They are UVC, UVB, UVA, visible light, and infrared.

Different regions of the world receive more or less UV radiation throughout the year. In general, the closer to the equator, the higher and more consistent the radiation level. The reason for this difference is the position of the sun relative to Earth. Areas far from the equator receive less direct sunlight. UV radiation also increases at higher altitudes and decreases in places with a lot of moisture, such as tropical rain forests.

The skin color we are born with is called our constitutive or baseline skin color. Biologists typically measure this constitutive skin color by taking a measurement of the skin under the armpit. Constitutive skin color is determined primarily by our genes. But skin color can change when we tan in response to UV radiation or other environmental factors. The altered color of skin that results from tanning is called facultative skin color. Unlike constitutive skin color, facultative skin color is temporary.