Human Skin: Structure, Functions, and Interesting Facts
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.
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
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 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 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.
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. These cells are eventually lost at the surface of the skin. Each new cell layer produced by the stratum basal 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.
Melanocytes and Langerhans 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. 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 epidermis contains other 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.
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
The dermis consists of connective tissue surrounding multiple structures. Collagen and elastin fibers are abundant in the connective 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
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 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 turn 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. 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 structure and is 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 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
I am a student. I would like to describe skin to my friends. Can you give me suggestions about what to say to them?
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.Helpful 8
© 2012 Linda Crampton