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Skin: an organ at work
The skin as the largest organ of the integumentary system: the epidermis, dermis and fat layer, what skin does, how it responds to heat, cold, UV light and injury, melanin and skin cancer, and how Australian scientists grow new skin.
Every minute you shed tens of thousands of dead skin cells. Much of the dust in your house is old you. Yet your skin never wears through, because underneath, new cells are dividing all the time to replace them. You grow a completely new outer layer roughly every month.
The body's biggest organ
Skin is an organ: it is built from several tissues (epithelial, connective, muscle, nerve and blood) working together. It is the main organ of the integumentary system, along with hair, nails, and sweat and oil glands. On an adult it covers about 2 square metres and makes up about 16% of body mass, more than any other organ. It varies in thickness from about 0.05 mm on your eyelids to 1.5 mm on the soles of your feet.
Three layers
- Epidermis: the thin outer layer, made of epithelial tissue. Cells at its base divide constantly by mitosis. As new cells push up from below, older cells fill with a tough protein called keratin, die and flatten, and are eventually shed. The top of your skin is a waterproof layer of dead cells. The base of the epidermis also contains cells that make melanin, a brown pigment.
- Dermis: the thicker layer underneath, mostly connective tissue. It contains blood vessels, nerve endings, sweat glands, hair follicles, oil glands and tiny muscles attached to the hairs. There's no blood in the epidermis, which is why a shallow scratch doesn't bleed.
- Fat layer (subcutaneous layer): stores energy, cushions the body and insulates it against heat loss.
What skin does
- Protects against germs, damage, drying out and UV light.
- Controls body temperature, using sweat and blood flow.
- Senses touch, pressure, pain, heat and cold through nerve endings.
- Removes some wastes in sweat (water, salts and a little urea).
- Makes vitamin D when exposed to sunlight, which you need for strong bones.
- Try a hot day, then a cold morning. Which structures change, and how does each change help keep your body at 37 °C?
- Try strong sunlight. What happens in the epidermis?
- Try the paper cut. Which process from topic 3F repairs the skin?
- Try touching a hot pan. Which tissue responds?
Skin and the Sun
Ultraviolet (UV) light from the Sun can damage the DNA in skin cells. Melanin absorbs UV and protects the cells underneath, so when your skin is exposed to UV it makes more melanin and you tan. A tan is not a sign of health: it means damage has already happened.
If a damaged cell escapes apoptosis, it can become cancerous. Australia has one of the highest rates of skin cancer in the world: about two in three Australians will be diagnosed with some form of skin cancer by the age of 70. The most dangerous kind, melanoma, starts in the melanin-making cells and can spread through the body. Most cases are preventable.
In 2007 Clare Oliver, a 26-year-old from Melbourne, died of melanoma that she believed was caused by using solariums (tanning beds, which give out strong UV). In her last weeks she campaigned to have them banned. By 2016 every Australian state and territory had banned commercial solariums, and Australia became one of the first countries in the world to do so.
The Slip, Slop, Slap, Seek, Slide rule still works: slip on clothing, slop on SPF 50+ sunscreen, slap on a broad-brimmed hat, seek shade and slide on sunglasses, whenever the UV index is 3 or above. In much of Australia that's most of the year.
Test yourself
Which layer of skin contains blood vessels, sweat glands and nerve endings?
The dermis holds most of the skin's working structures.
On a cold day, the blood vessels near the skin's surface
Less blood flowing near the surface means less heat lost, so fingers look pale.
Melanin protects skin cells by
The dark pigment absorbs UV before it reaches the cells' DNA.
Why is skin described as an organ rather than a tissue?
An organ is several tissues working together.
The outer surface of the epidermis is made of
Cells are made at the base, then die and flatten as they are pushed up.
Label the parts of the skin.
Which responses happen when you're too hot, and which when you're too cold?
The skin is part of the system. Cells at the base of the divide by mitosis to replace cells lost from the surface. The brown pigment absorbs UV light. The most dangerous type of skin cancer is . Skin makes vitamin in sunlight.
A student goes for a run on a hot day. Explain how two structures in the skin help keep the student's body temperature close to 37 °C.
The sweat glands in the dermis release more sweat onto the skin. As the sweat evaporates it takes heat energy from the skin, cooling the body. Blood vessels near the skin's surface also widen, so more warm blood flows close to the surface, where heat can be lost to the air. Together these stop the body temperature rising far above 37 °C.
Explain why exposure to UV light increases the risk of skin cancer, using what you know about cell division.
UV light damages the DNA in skin cells. Usually a badly damaged cell destroys itself by apoptosis, but if the damage affects the genes that control cell division and the cell escapes, it keeps dividing by mitosis without stopping. Its daughter cells carry the same damage, and they form a tumour, which may be a skin cancer such as melanoma.