Skin: The body's natural barrier

Hello everyone and welcome to this week's blog post. I will be discussing the skin. I will first describe the structure of the skin before explaining its functions. I will then explore how the skin is able to regenerate after being damaged, before finally outlining the 4 different types of burns.

Structure of skin in humans

The skin is human's largest organ by surface area. It is made up of several different parts, which work together to allow it to perform its functions. There are three different layers to the skin. The outer layer (the part which you can see) is called the epidermis. The epidermis itself consists of 5 different layers and contains a variety of cells.

For example, the epidermis contains keratinocytes, which produce keratin. Keratin is one of the most abundant proteins in the skin and makes up a variety of other structures including the hair and nails. In addition, in other mammals, it also forms hooves, claws, horns and scales. Since keratin is a fibrous protein, it helps provide structure to the skin and protect the body from trauma.

The outer layer, called the stratum corneum of the epidermis consists of squamous cells. These are the cells that are exposed to the external environment. The squamous cells are the ones which are constantly shed and replaced. This can happen as new skin cells are produced at the base of the epidermis. Once the cells above them flake off, they move closer to the surface. As such, according to the American Academy of Dermatology Association, the epidermis is regenerated roughly every month.

In addition, the epidermis also contains melanocytes. These cells are responsible for producing melanin. This pigment is responsible for your skin colour, as well as protecting your body from the harmful effects of UV radiation.

A micrograph of the melanocyte cells in the epidermis.

The middle layer of the skin is called the dermis. This layer contains many vital structures, including blood vessels, nerves, lymphatic vessels, sweat glands, receptors and fibroblasts (cells that are a major part of connective tissue).

The dermis also contains hair follicles. These are the structures from which little hairs grow. These hairs are important in regulating our body temperature, as we can use tiny muscles to make these hairs stand upright, thus trapping air next to the skin and insulating the body.

Furthermore, the dermis also contains nerve endings that help us feel things. These receptors send signals to the brain when they are stimulated, allowing the body to detect touch and pain and respond accordingly.

In addition, the dermis also contains lots of collagen. Collagen is the most abundant protein in the skin - according to the Cleveland Clinic, collagen makes up approximately 75-80% of the organ. Collagen is also a fibrous protein, making it incredibly important for providing strength and structure to the skin.

The third layer of skin is called the subcutaneous fat layer. This layer consists of a network of fat and collagen, which are very useful in insulating the body, reducing the energy required for maintaining our high body temperature. Futhermore, by storing fat, this layer also protects muscles and bones from trauma. The subcutaneous fat layer also contains connective tissue that anchors the dermis to the muscles and bones, thus allowing the skin to stay attached to the rest of the body.

A diagram that shows the structure of human skin.

Functions of the skin

The skin performs many functions for the body. I have already mentioned some of these functions, including its role in protecting the body from trauma, shielding the body from UV radiation, allowing us to feel things when we touch them and regulating our body temperature.

Vitamin D3 is also produced by the skin. This process occurs when UV radiation from the sun reacts with 7-dehydrocholesterol to produce a vitamin D precursor in the skin. This chemical is then oxygenated and hydrogenated in the liver and kidneys to form vitamin D. The amount of vitamin D produced depends on the amount of UV radiation absorbed by the skin, so people with darker skin colours are likely to produce less. Interestingly, the location you are on the globe also impacts the amount of vitamin D produced, as it affects the presence of UV radiation from the sun. According to Harvard Health Publishing, those living north of the 37-degree latitude line are not exposed to enough sunlight during the winter months, thus preventing them from producing vitamin D during this period.

A diagram that shows the structure of 7-Dehydrocholesterol.

In addition, the skin acts as a physical barrier between organisms and their environment. This is incredibly useful - for instance, whilst some water is lost through sweating, the skin helps to reduce water loss from the body. This is because the fibrous, structural proteins form a barrier that prevents water from seeping out of cells and out of the body.

This physical barrier is also crucial in supporting the immune system and preventing pathogens from entering the body. Not only does the skin provide a passive barrier to entry, but it also utilises active means to fight infections. For example, the skin contains dendritic cells called Langerhan cells. These cells are antigen presenting cells. This means that they capture pathogens and cut them up into smaller pieces. They then display the pathogens' antigens on their surface. This stimulates other parts of the immune system, such as T cells, to be stimulated and fight the infection.

The skin is also covered in a microbiome, consisting of a variety of bacteria, viruses and other microbes. These microbes can enhance our immune system by producing antimicrobial chemicals, or directly attacking pathogens. For example, our microbiome can contain bacteriophages, which are viruses that attack and destroy bacteria.

How does the skin repair itself?

Sometimes, the skin can get broken, forming a hole for pathogens to enter through. If this occurs, then the body works quickly to repair the damage before too many pathogens enter. Within a few minutes, blood will form a clot to 'plug' the hole and stop the bleeding. This clot then dries to form a scab.

Afterwards, the body's immune system works to fight any pathogens that may have entered through the wound. The area becomes pink and swollen, as blood vessels in the area dilate to allow oxygen and nutrients to flow to the area. This also allows white blood cells to flow to the wound and help fight the infection.

After this stage, the tissues start to repair themselves. First, the damaged tissue gets broken down. Red blood cells also assist with the production of collagen. These fibres help form the structure for new tissues to build around. The wound also fills with granulation tissue, with new skin forming over the top. This tissue pulls inwards as the wound heals, causing a scar to form and the wound to become stronger. This scar tissue is weaker than the surrounding tissue as it doesn't grow back the same as the surrounding area.

A diagram explaining how the skin repairs following injury.

Burns

The skin can also get damaged by burns. There are four different types of burn. The first is a superficial epidermal burn. This type of burn occurs when the epidermis is damaged. It is the least harmful burn, as this layer can usually regenerate. In addition, a superficial dermal burn can also occur, which is when the epidermis and part of the dermis get damaged.

Partial-thickness burns can also occur, where the epidermis and dermis both get damaged. According to the NHS, this type of burn results in red and blotchy skin, with swelling and blisters. Furthermore, a full-thickness burn can also occur, when all three layers of the skin get damaged. This often causes the skin to get completely burnt away, exposing the tissue underneath.