Skin

The Skin is the body’s largest organ and a very sophisticated one. It plays a vital role in maintaining harmony within the body by performing various functions essential for the the body's efficient operation and survival. It is not an exageration to say EVERYBODY wants to have healthy, beautiful, and flawless skin. Often, this magnificant organ's importance is overlooked until there is a problem and solutions are being sought. This makes it imperative to have a basic understanding of the skin's structure and purpose to aid in conceptualising the changes happening deep within. Understanding intricacies of the skin enables us to better protect and care for it.

Functions of the Skin

The skin has many functions which are vital for the health and safety of the body, and these include:

• Temperature Regulation for the body by dilating or constricting blood vessels found in the skin. Dilation of the skin's blood vessels, which carry warm blood, cools down the body when it is hot. This phenomenon can be observed on a hot day or during workouts for instance when facial flushing occurs. Constriction of blood vessels on the other hand reduces heat loss to keep the body warm, which can be seen in winter and results in that pale skin effect due to blood vessels constricting and moving away from the surface of the skin.

• Collection of Sensory Information from the surrounding environment (i.e. touch, temperature changes, pain) through touch receptors within the skin. This information is then sent to the brain via a network of signaling pathways.

• Vitamin D production which enables the body to efficiently absorb calcium. Calacium not only maintains the health of your bones but also supports the production of collagen which is the most abundant protein in the body and the body's natural anti-wrinkle solution.

• Banlances Skin Hydration by minimising moisture loss to the external environment.

• Protective Barrier against external assaults like chemicals, pollution, pathogens, and the sun's ultraviolet radiation (UVR). UVR is the main factor implicated in the development of skin cancer, and it also exacerbates some skin disorders like rosacea and acne.

For a visual demonstration of the skin's structure and function please watch Video 2 at the bottom of this web page.

Structure of the Skin

The skin consists of two layers, the top layer is the epidermis and bottom layer is the dermis. These two skin layers sit on top of a fatty layer called the hypodermis (Figure 1).

• Epidermis (Top)

• Dermis (Bottom)

• Hypodermis (Subcutaneous layer)

Epidermis (Top layer)

The epidermis is the top layer of the skin that can be seen and touched. It is the first line of defense against harmful external elements and the is the body's protective barrier. The epidermis is made up of 4 to 5 sub-layers or strata depending on which area of the body it covers (Figure 1) (Video 1). Thicker areas of the body like the soles of the feet and palms have five layers. The layers consist of closely packed cells that are filled with the protein keratin. The epidermis is the skin's first line of defense, often referred to as the skin's protective barrier. It accomplishes this by preventing harmful pathogens like bacteria and viruses from entering your body and causing infections, and it also protects the body from damaging UVR.

The epidermis does not have its own blood supply, it relies on the dermis beneath it with its blood supply to keep it alive. This top layer of the skin can range in thickness from 0.1 mm on the eyelids to 1 mm on the palms and soles of the feet and contains many specialised cells which have distinct functions. These cells' individual functions contribute to the overall function of the skin.

These cells include:

• Keratinocytes (skin cells)

• Melanocytes (pigment producing cells)

• Langerhans cells (immune cells)

• Merkal Cells (touch sensing cells)

Dermis (Bottom layer)

Below the epidermis lays the second layer of the skin known as the dermis. The main role of the dermis is providing support to the epidermis. It ranges between is divided into two layers, the upper region is the papillary dermis and the lower one is the reticular dermis. The dermis houses sweat glands, hair follicle roots, blood capillaries, lymph vessels, immune cells, nerve endings connected to pressure receptors, collagen and elastin fibers (Figure 1) (Video 2).

In the skin, the dermis has the following functions:

• Nourishes the epidermis

Blood capillaries in the dermis play an important role in keeping the epidermal layer alive by transporting nutrients to it.

Blood capillaries can also act as a transporting medium for skin cancer cells facilitating their spread to other parts of the body which can occur in some cases of melanoma. The process of cancer cells spreading from one anatomical location to another is referred to by health professionals as metastasis. Metastasis makes it difficult to eliminate cancer.

• Protection from tearing, and provides cushioning to protect internal organs

The main cell in the dermis is the fibroblast which is responsible for producing elastin and collagen. Collagen is the protein found in abundance (70-80%) in this layer. It is what cushions the skin's surface and gives it strength when it is stretched preventing it from tearing. It is the abundance of collagen in the skin that gives skin its wrinkle-free appearance and is associated with youth. As we age, our skin's collagen reserves become gradually depleted resulting in the development of wrinkles. Elastin fibers in the dermis give skin its elasticity and bounce-back capability. Aging and overexposure to ultraviolet radiation damages elastin in the skin resulting in loss of elasticity which is seen as sagging skin.

Connected to nerve endings in the dermis are pressure receptors like Lamellar and Pacinian corpuscles which detect the mechanical energy from pressure against the skin and transfers that to an electrical signal and sends it through sensory nerves to the brain. The brain processes the information from this signal and orders the body to react accordingly. These pressure receptors in the dermis protect the skin from injury by sending these electrical signals to warn the brain of impending danger from a sharp or hard object for instance. In response, the brain orders the body to move to prevent damage from occurring. This enable the skin to perform one of it's vital functions which is protecting the body's internal organs from harm.

• Wound healing

There are various immune cells in the dermis like resident macrophages and mast cells. Macrophages are deployed after the skin is injured to clear the wound site from invaders (pathogens) by engulfing and digesting them as well as other dead immune cells like neutrophils (Video 3). After clearing the wound site dermal macrophages secrete chemical signals known as chemokines, to summon fibroblasts which commence laying down collagen and elastin to strengthen and remodel the wound .

Mast cells are another type of immune cells which can detect bacteria, viruses, fungi, and parasites. But they are mainly known for their role in allergic reactions associated with hayfever which causes itchy skin, a runny nose, and watery eyes as a result of histamine secretion by mast cells. These cells also produce other hormones known as chemokines to summon other immune cells to the wound site initially when the injury occurs .

Each of these immune cells has a different role in ridding the skin from potential infection which allows the wound healing cycle to proceed normally. Both macrophages and mast cells also secrete various chemokines that signal cells to perform different functions involved in the wound healing cascade. These functions include initiation of the inflammatory process to clean the wound site from invaders or debris, tissue repair by fibroblasts which produce collagen and elastin to strengthen and remodel the wound.

Hypodermis (Subcutaneous layer)

This layer is also called the subcutaneous layer and is predominantly made from fat cells called adipocytes. The hypodermis is not part of the skin structure but it is often mistaken for the third skin layer (Figure 1). It is sandwiched between the dermis above it and the muscle fascia below it. The hypodermis provides the skin with its main structural support, and it has two main functions:

• Protection against physical injury by providing a cushioning effect that protects the body's internal organs from damage that could result from mechanical or physical trauma.

• Thermoregulation, by maintaining the body's temperature at a safe level to shield it from extreme heat or cold.

• Energy reserve. Adipocytes produce fat which is stored in the hypodermis. This process of fat accumulation provides the body with an alternative source of energy to carbohydrates which are the body's primary source of energy.

• Anchors the layers above it, the epidermis and dermis, to the muscle fascia below it.

The skin is a magnificent organ with many sophisticated functions. It maintains the body's safety from harmful external elements in an organized sequential manner with each layer contributing in its own unique way. In the epidermis, specialized cells act as the body's first line of defense against pathogens, UV damage, and skin dehydration to maintain the skin's protective barrier function. In the dermis blood vessels keep the skin alive and healthy by supplying the epidermis with nutrients. Collagen and elastin in the dermis give skin strength and elasticity to protect it from wear and tear, while immune cells like resident macrophages protect the skin from infection by clearing up the wound site when skin injury occurs. Another key role the dermis provides the skin is sensation. Nerve receptors in the dermis enable the body to sense temperature changes, touch, and enable us to recognize pain to save us from danger.

This is a brief overview of the structure and functions of the skin. It is essential to have a basic understanding of our skin as this will not only enable us to protect it, but this knowledge will also enable us to know how certain skincare products affect our skin. It will also help us design a skincare routine suitable for our skin type and with the most appropriate products to address our skin concerns. The skin is a complex organ with numerous components which could not be comprehensively covered in this discussion as they are beyond the scope of this discussion. But if you have any questions about certain skin concern you may want to contact your GP or a dermal clinician.

You may also like to watch the videos below to help you visualize what the fuss is all about :)

References:

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Brown, T. M., & Karthik Krishnamurthy. (2018, December 6). Histology, Dermis. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK535346/

Casabona, G., & Nogueira Teixeira, D. (2017). Microfocused ultrasound in combination with diluted calcium hydroxylapatite for improving skin laxity and the appearance of lines in the neck and décolletage. Journal of Cosmetic Dermatology, 17(1), 66–72. https://doi.org/10.1111/jocd.12475

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Yutskovskaya YA;Kogan EA. (2017). Improved Neocollagenesis and Skin Mechanical Properties After Injection of Diluted Calcium Hydroxylapatite in the Neck and Décolletage:A Pilot Study. Journal of Drugs in Dermatology : JDD, 16(1). https://pubmed.ncbi.nlm.nih.gov/28095536/

‌Zerbinati, N., & Calligaro, A. (2018). Calcium hydroxylapatite treatment of human skin: evidence of collagen turnover through picrosirius red staining and circularly polarized microscopy. Clinical, Cosmetic and Investigational Dermatology, Volume 11, 29–35. https://doi.org/10.2147/ccid.s143015

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