Human Skin: Structure, Functions, and Some Interesting Facts
An Impressive and Vital Organ
The skin is a very impressive organ that has many 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 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
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 the hypodermis.
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. Keratin makes the epidermis strong and waterproof. Cells called melanocytes, which produce a protective pigment named melanin, are also present in the epidermis. 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 contains collagen and elastin fibers, hair follicles, sebaceous glands, the coiled sections of the sweat glands, blood and lymph vessels, nerves, sensory receptors, and cells from the immune system. The sebaceous glands produce an oily substance called sebum. Most sebaceous glands are connected to a hair follicle.
Attached to each hair follicle is an arrector pili muscle which 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.
Parts of the Skin
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.
Layers of the Epidermis
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.
Keratinocytes and Keratin in the Epidermis
At the base of the epidermis are cells called keratinocytes. These cells divide to make new keratinocytes. The new cells slowly migrate upwards through the epidermis until they reach the top layer, or the stratum corneum. The migration process takes about a month.
As the keratinocytes migrate, they gradually manufacture 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 the migrating keratinocytes reach the surface of the epidermis they 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. 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.
The Epidermis and Vitamin D Production
The process of vitamin D production in the body is a multistep process. The basic steps are 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.
Melanocytes, Langerhans Cells, and Merkel Cells
Melanocytes are found in the bottom layer of the epidermis. These cells make melanin, a pigment which gives color to the skin. The pigment is transported to other epidermal cells. Melanin absorbs ultraviolet light, preventing it from damaging the body. It's important to realize that melanin doesn't completely protect us from UV light, however. An additional form of protection is needed when we are exposed to sunlight.
Langerhans and Merkel Cells
The epidermis also contains Langerhans and Merkel cells. Langerhans 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. Merkel cells are located at the base of the epidermis. They lie close to nerve endings and are sensitive to light touch.
Other Cells and Chemicals
The dermis contains other sensory cells as well as a variety of chemicals. These chemicals include lipids and antimicrobial peptides (short chains of amino acids that fight pathogens). The epidermis doesn't contain blood vessels. Nutrients for the epidermal cells are supplied by the blood vessels in the dermis, which also remove waste substances made by the cells.
Glands in the Dermis
The dermis contains three types of skin 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.
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 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 Dermal Layer of the Skin
Collagen and elastin fibers are abundant in the dermis. These proteins give firmness, flexibility, and elasticity to the dermis, enabling it to act as a supporting layer for the skin. The dermis also contains sensory receptors and nerve endings to detect temperature, touch, pressure, and vibration.
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 through the epidermis, making the skin turn pale and reducing 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.
Body Temperature Regulation
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 found that a chemical in the discarded skin called squalene absorbs some of the ozone from polluted air.
Our skin is an amazing organ. 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 skin really is. It's much more than a simple barrier between our body and the outside world.
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 melanocytes and melanin from the University of Bristol
Skin gland information from the University of Leeds
Questions & Answers
© 2012 Linda Crampton