SKIN SERIES PART ONE: WHAT IS THE SKIN AND HOW DOES IT WORK?
Three weeks ago, I really badly burned my arm from steam, opening an oven door. It blistered, the skin was thick and bruised and my skin was raw. Three weeks later, the skin is smooth and soft, and the only trace of the burn is a slight difference in colour. In three weeks, my skin had completely re-built itself whilst fighting off infection successfully. And so, this blog post is the first in series dedicated to skin, the largest organ in our bodies. It’s a beautiful thing and you deserve to know what it is, how it works and how to care for it.
So, let’s start with the structure.
The skin structure is broken down into three layers; the epidermis, the top layer; thedermis; and then the underlying subcutaneous (fatty) layer.
The epidermis is around 0.1mm thick and its primary function is to form a barrier, preventing invasion from harmful microorganisms, chemical substances, and allergens. It minimises trans-epidermal water loss (the rate at which water evaporates from the skin), helping to protect against dehydration. It doesn’t have its own blood supply, and so because of this, only the bottom two layers of the epidermis actively create new cells. As these cells move to the top of the epidermis, the desmosomes (link between cells) weaken and they break. This causes the cells to flake off. This process, known as desquamation, is the skin “self-cleansing”- removing old, damaged, dirty or infected cells.
The dermis is a tough, supportive connective tissue matrix, composed of a mix of fibroblast cells, collagen, elastin and then extra-cellular matrix. Its thickness varies, and is between 0.6mm (eg on eyelids) to 3mm thick (back, palms and soles of your feet). The dermis is where specialised structures lie, such as the sweat glands, sebaceous glands, and a variety of different nerve endings. Because of this, the dermis has a much richer blood supply than the epidermis as it helps to provide these structures with the ‘raw materials’ they need to work. The dermis also contains lymph vessels which are part of the body’s lymphatic drainage system, which is responsible for removing interstitial fluid(fluid that surrounds cells) from tissues, toxins, waste and other unwanted materials. Thesebaceous glands produce an oily secretion known as sebum, which flows down the hair follicles onto the skin’s surface. This combines with sweat to form the ‘acid mantle’ on the skin’s surface- the acidic pH (between 4.5-5.5) discourages microbial growth on the skin surface. Cosmetic formulations should be formulated with a pH that respects this. Sebum is also a natural emollient, helping to soften and smooth the skin and keep hair from becoming brittle, reduce trans-epidermal water loss and help to ‘water-proof’ the surface of the skin.
The subcutaneous layer is composed largely of adipose tissue– where a globule of storage fat is in the centre of cells. It contains the main blood vessels and nerves that branch into the dermis. It is an effective heat insulator, food store and shock absorber and we need it. Women naturally have and need a bigger fat store than men, with women typically having 25-30% and men around 12-15% body fat.
So what do these three amazing layers all work together to do?
- Controls water loss from the body– the body is 65-70% water, and all chemical reactions are done in solution. Each reaction in the body needs to be done at the correct concentration. The skin helps regulate hydration levels through trans-epidermal water loss.
- Protects the body from harmful radiation from the sun– melanin is produced by specialised cells (melanocytes) within the skin (N.B.: everyone has the same number of melanocytes, they just vary in activity). Melanin is a pigment that gives skin its colour. Melanin works to protect the body from harmful radiation by providing a protective cap over the nuclei of actively dividing cells in the lower layer of the epidermis. This protects the vulnerable exposed DNA from UV damage. The amount of melanin in the skin can be temporarily increased, as when the skin is exposed to sun, the melanin already present darkens. After prolonged exposure (2-3 days) the melanocytes produce more melanin giving the skin a ‘tan’. Please note that this is essentially the skin reacting to stress, and it saddens me that this is glorified. Also melanin only has an SPF of around 7-8, which shows how important it is to wear sun cream/protective layers and minimise sun exposure, no matter what your natural skin colour.
- Controls the entry of foreign materials– this includes cosmetic products. The skin is a continuous protective layer over our bodies, and this intact barrier acts as a first line of defence along with the previously mentioned ‘acid mantle’. In-between the layers of ‘brick-like’ cells, are glycol-lipids, and these fatty substances help prevent the entry of water soluble substances. Things can penetrate into the skin if they are less than 500 molecular weight, lipophilic (fat-soluble) and non-polar. (It is important to note that the molecular weight of collagen is far too big to penetrate into the skin, and so its inclusion into cosmetic products is very questionable). However, Vitamins A, E, K are fat soluble and absorbed by the skin.
- Prevent entry of microorganisms– the epidermis contains a type of cell called Langerhans cells and these present antigens that activate the immune response in the body. This works by the Langerhans cells taking ‘cell markers’ from the invading species, to pass onto immuno-response cells. In the dermis lie another type of defensive cell known as a Macrophages (‘big-eater’). These are the second line of defence, and work by engulfing invading species. This releases the cell-markers, igniting the immune response system. Eosinophils destroy bacteria and particularly fungi. b-Lymphocytes are a type of cell that produce antibodies. Pretty badass huh?
- Cushion against bumps and shocks– the subcutaneous fatty layer protects us when we bump or fall, providing cushioning and shock-absorption, protecting the vital organs. (That ‘back-fat’ you hate? Protecting your kidneys. Don’t hate the fat).
- Regulate heat loss– the body works best at a temperature between 36.5-37.5°C, and this needs to be maintained. The skin does this through two main mechanisms; through sweating and through adjustment of the blood flow. When the body temperature rises, the blood vessels dilate, increasing blood flow to the skin surface (why you might look red when you’re warm), and sweat glands increase in activity. This produces an evaporative cooling effect that is highly efficient. When you are cold, the opposite happens, blood vessels constrict keeping the warm blood in your body’s core around your vital organs and sweat gland activity decreases.
- Provides sensory reception– the skin has an excellent nerve supply, and several different types of receptors that have different uses. Ruffini’s end organs detect sustained pressure, Messiner’s corpuscle are responsive to change in texture and slow vibration, whereas Pacinian corpuscles detect deep pressure and fast vibrations. Merkel’s discs detect sustained touch and pressure and free nerve endings respond to pain. Thermoreceptors do not have specialised endings. Nerve endings responding to cold (10-40°C) are located in the superficial dermis, and those responding to heat (32-48°C) are located deeper down. Beyond these temperatures, you will feel pain.
- Acts as interface between body and outside- just like how animals have fur with individual spots and markings, we as humans have skin that is individual to us (I have a mole on the palm of my hand for instance), and is a big part in how we recognise each other.
- Transmits social and sexual signals to others- we all blush when we see someone we fancy (unless you’re a cool superhuman in which case, I’m jealous). Pheromones in our apocrine sweat are ‘airborne hormones’ that help transmit sexual signals to others. See, sweat is sexy as hell.
And so that concludes part one of the skin series. I hope you are starting to realise now just how amazing your skin is, have learnt something (I really tried not to be boring, but I’m a geek and I’m sorry if I ramble) and look forward to learning more about the great big bunch of cells that wraps up our organs in one massive burrito.
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