Skin: How it works

The skin has three basic layers — the epidermis, the dermis, and the hypodermis.

Epidermis

The epidermis is the outermost layer. It is a waterproof barrier that gives skin its tone. It’s main roles are:

• to make new skin cells
• to give the skin its color
• to protect the body from the external environment

Humans shed around 500 million skin cells each day. In fact, the outermost parts of the epidermis consist of 20–30 layers of dead cells.

The epidermis constantly makes new cells in its lower layers. Over the course of around four weeks, these cells make their way to the surface, become hard, and replace the shedding, dead cells.

Keratinocytes are the most common type of cells within the epidermis. Their job is to act as a barrier against bacteria, parasites, fungi, viruses, heat, ultraviolet (UV) rays, and water loss.

The epidermis contains no blood vessels. The color of the skin comes from a pigment called melanin, which is produced by melanocytes. These are found in the epidermis and protect the skin from UV rays.

The five layers of the epidermis are:

• stratum corneum
• stratum lucidum
• stratum granulosum
• stratum spinosum
• stratum germinativum

A thin sheet of fibers known as the basement membrane divides the epidermis and the dermis.

Dermis

The dermis serves as connective tissue and protects the body from stress and strain. It also gives the skin strength and elasticity. In addition, its main roles are:

• to make sweat and oil
• to provide sensation and blood to the skin
• to grow hair

The reason the dermis can perform these functions is that it houses the hair follicles, blood vessels, and lymphatic vessels. It is home to a number of glands, including sweat glands and sebaceous glands, which produce sebum, an oil that lubricates and waterproofs hair.

The dermis also contains the receptors that detect pressure (mechanoreceptors), pain (nociceptors), and heat (thermoreceptors).

If the dermis stretches a lot, such as during pregnancy, then it can tear. This will show up later as stretch marks.

The dermis further splits into two layers:

Papillary region

The papillary region contains loose connective tissue. It has finger-like projections that push into the epidermis. These projections give the dermis a bumpy surface and are responsible for the patterns of a person’s fingertips.

Reticular region

The reticular region contains dense, irregularly organized connective tissue. Protein fibers in the reticular region give the skin its strength and elasticity.

Subcutaneous tissue

The deepest layer of the skin is the subcutaneous tissue, the hypodermis, or the subcutis. It is not technically part of the skin, but it helps attach the skin to the bones and muscles. Subcutaneous tissue also provides the skin with nerves and blood supply.

The hypodermis contains mostly fat, connective tissue, and elastin, which is an elastic protein that helps tissues return to their normal shape after stretching. The high levels of fat help insulate the body and prevent a person from losing too much heat. The fat layer also acts as protection, padding the bones and muscles.

Some of the many roles of skin include:

• Protecting against pathogens. Langerhans cells in the skin are part of the immune system.
• Storing lipids (fats) and water.
• Creating sensation through nerve endings that detect temperature, pressure, vibration, touch, and injury.
• Controlling water loss by preventing water from escaping by evaporation.
• Providing water resistance by preventing nutrients from being washed from the skin
• Helping with thermoregulation by producing sweat and dilating blood vessels, which helps keep the body cool. “Goosebumps” and blood vessel constriction help people retain heat.

Skin color is a phenotype, which is an observable trait like eye color or height. The color results from different types of a pigment called melanin.

Melanin’s primary role is to protect the skin from damaging UV light from the sun, which can cause skin cancer. When skin is exposed to UV light, melanocytes start producing melanin, creating a suntan.

People who have more pheomelanin will have paler skin. People who have more eumelanin will have darker skin. Research on evolution shows that historically, populations closer to the equator evolved to have darker skin for better protection again the sun’s UV rays. On the other hand, people in colder climates evolved lighter skin to better maintain Vitamin D.

In general, females have lighter skin than males. This may be because women need more calcium during pregnancy and while breastfeeding. Vitamin D is produced when the skin is exposed to the sun and is important for absorbing calcium.

As with any other organ of the body, the skin is susceptible to certain diseases. These include:

• Atopic dermatitis: Also known as eczema, this is an inflammatory skin disease characterized by dry, red, itchy patches of skin.
• Acne: This is perhaps the most common skin disorder. It occurs when hair follicles become clogged with dead skin cells and oil.
• Melanoma: A type of skin cancer caused by exposure to excess sunlight.
• Rosacea: A common rash found in middle-aged people. They have a tendency to flush and have small red bumps on the center of the face.
• Psoriasis: This is an auto-inflammatory skin disease. It causes red, flaky patches to appear on the skin.
• Scabies: An itchy skin condition caused by the human scabies mite.
• Shingles: Also called herpes zoster, it is a painful, blistering rash caused by a virus.
• Lichen planus: An itchy non-infectious rash. The bumps have flat shiny tops.

As a person gets older, their skin changes. It becomes thinner and more easily damaged. The epidermis becomes slower at replacing dead skin cells and the process of healing slows. Overall, a person will have less skin, and it becomes less elastic.

Older adults may find that their skin becomes more dry, irritated, and thin. The skin may itch more, bruise, and become infected more easily.

There are a number of reasons why the skin goes through these changes as a person ages. These include environmental, genetic, and cellular factors. Hormonal changes can also impact the skin, as well as exposure to UV rays, which increases the risk of skin cancer.

Recommended skin care for older adults places particular emphasis on moisturizing the skin and keeping it protected from the sun.

The skin is a large, complex organ with a wide range of vital roles, from protecting people from pathogens to helping maintain the right body temperature. People would not be able to live without skin.