How well do you know your skin ?
Have you heard of a cleverly designed protective lining that insulates, helps maintain external and internal water balance and is vital to the survival of the human species ?
You guessed it- it’s our skin.
This enormous organ that covers almost 1.6 m2-1.8 m2 area of your body surface is anything but skin deep. Thinnest on the eyelids (0.5mm) and thickest on the palms and soles (5mm), this mega-organ is serious about the role it plays in keeping us healthy and happy. So what makes up this ingenious invention of nature ?
Quick Skin Function Fact Sheet
- Ultimate protection against harmful germs, pollutants in the environment, solar radiation, mechanical injuries and chemical stressors. In fact, it is our first line of defense.
- Remarkable water regulation properties. Prevents water loss.
- Temperature regulation (thermoregulation) through sweat evaporation, blood vessel functions (vasomotor), through insulation via body hair, and finally heat conservation through fat.
- Sensorial communication through a vast network of nerves and mini-sensory cells dispersed within the skin layers such as Merkel cells.
- Helps the body to produce antibodies by presenting foreign antigens through skin cells known as Langerhans. Also helps to prime your immune cells (T-lymphocytes or white blood cells) against future attacks. The skin is associated with the skin-associated lymphoid tissue known as SALT. These are important immune functions.
- A nutritional role, with cutaneous vitamin D (Previtamin D3) synthesis when exposed to therapeutic doses of sunlight. Hence the term, ‘the sunshine vitamin’. Studies indicate that this function decreases with increasing age.
- Warning system. Many internal diseases externalize themselves onto the skin. So it’s a good idea to get a check-up if you notice unusual lumps, bumps or rashes on your skin.
- Socio-sexual communication and individuality. Our appearance, the way we look, smell and feel, adds individuality to our persona. Every culture from thousands of years has used the skin to enhance esthetic appearance, create tribal or racial distinctiveness by using various techniques like cosmetics, piercing, tattoos, and other novel methods.
- Psychological aspect. The skin plays a huge role in our psychological well-being. Keeping it healthy is vital for completing the holistic health cycle. Studies indicate that the quality of life is seriously affected when one is suffering from skin disease. Numerous studies indicate that a healthy complexion boosts confidence and helps one maintain positive social interactions.
- Displays our beautifully individual and unique skin tones. It conveys our skin color to the world through a pigment called melanin. The more diversity in skin tones- the more beautiful I say!
- Your human fingerprints are your true signature.
- Your scalp hair is very important in keeping your head warm but it also acts as a physical barrier to toxins and solar radiation. It enhances/customizes your overall physical appearance.
- Nails are important in precision movements of the fingers, they protect the tips of your fingers and toes and can be decorated to enhance physical appearance.
What is a Keratinocyte or Corneocyte ?
Drum-roll please. These are the most abundant skin cells of the epidermis and mostly found in the uppermost layer of the skin (95%). There are many types of keratinocytes each with various functions throughout the epidermis. The outmost layer, which incidentally is the only layer we can see with our naked eye, consists of mature keratin- rich corneocytes, known as horny cells. Keratin is the toughest protein found in nature and makes up most of the horny cell. This is a dead skin cell arranged in a tight brick wall fashion, coated with waxy fatty proteins and plays a vital role in trans-epidermal water loss (TEWL).Our human existence depends on this one highly important function of the mature keratinocyte. Keratinization is a synchronized process by which the entire epidermis undergoes multiple changes to form the water-proofing topmost layer of the skin. This layer is known as the stratified corneum and is formed of about 10-35 layers of tightly packed keratinocytes. As the immature keratinocytes make their way up, they accumulate increasing amounts of tough keratin, become coated in fats, shed their nucleus and become flatter, eventually flaking off as squames and becoming a major part of household dust. This is the evolution of the corneocyte as we know it!
What is the Cell Cycle ?
After understanding the evolution of the keratinocyte, let’s try and find out how long does all of this really take in real-time. The whole point of keratinization is the manufacturing of a tough outer protective layer. The cell cycle is the time it takes for the daughter cells in the basal layer of the epidermis to reach complete maturation or keratinization. It usually takes about 26-42 days in average for this process to happen in real time. In simple terms, every month you are greeted by a vibrant and fresh top layer of skin. In case of diseases such as psoriasis, the cell cycle is shortened and the skin becomes thick and scaly. Aging lengthens the skin cycle. What does this mean? As we age, the capacity of our skin to make new skin decreases. Cell cycle promotors such a fruit acids and retinol speed up the cycle and encourage keratinization.
Whatever you put on your skin gets absorbed into the bloodstream – Myth or Truth ?
There is a popular rumor circulating around town that whatever you apply on your skin gets absorbed. You might have heard another rumor that states that 60% of what you put on your skin gets absorbed into your bloodstream. Now, here’s the thing, it just doesn’t happen that way. If this were the case, we would all be walking sacs of toxins and wouldn’t last that long on earth! The main function of the skin (mostly the epidermis) is keeping toxic stuff from being absorbed. The problem occurs when the toxins are of a very small molecular weight allowing easy penetration. Only substances of a particular size can pass through the impenetrable barrier and go past the epidermis and into the blood stream. Anything more than 1000 Daltons won’t pass through. Pharmaceutical drugs and certain cosmetic companies formulate products so that percutaneous absorption is more effective. But quantifiable measurements of exactly how much is being absorbed into the skin, is not an easy task as there are multiple factors at play. Absorption through the skin (percutaneous) is dependent on many factors such as health of the skin, dehydration, disease, climate, vehicles, and molecular size of the particles being applied. That being said, small amounts of substances might get in through various routes. The disconcerting fact here is that although the skin is resistant to most chemicals, toxic substances (an unknown amount) can still seep in. Therefore, it’s better to be safe and stick to products that do not contain harmful chemicals unless it is a drug intended for treatment, for example steroids.
Dry, damaged, diseased and sun damaged skin will greatly impair the barrier making it easier for toxins to seep in and enter the bloodstream.
- Toxins can make their way in through these routes:
- Through the micro-channels in between the cells (intercellular)
- Directly through the cell walls through permeation (intracellular)
- Through surface openings such as sweat and oil glands (transappendageal)
Three Multidimensional Layers – one unified function
The main function of our skin is protective. What is really happening inside that the eye cannot see so that this main function is fulfilled?
On the Front Lines- The Epidermis
The epidermis is the warrior of our skin tales and is about 100 um thick. This tough outer layer is further subdivided into 5 sub-layers that work synchronically to keep the barrier renewed and intact throughout our life. Let’s take an upside down peek at what’s happening skin deep. Main functions of the epidermis are water-proofing and protective. Studies point out that almost 20% of topically applied substances of certain molecular size under the right conditions will remain in the epidermis. Hence, our epidermis also functions a reservoir of substances. An interesting fact when considering topical applications to the skin.
Layers of the Epidermis
- Dermo-epidermal Junction or Basement Membrane. Is the thin line that separates the epidermis from the deeper dermis and connects both through a network of anchoring fibers.
- Basal cell layer or Germinating Layer. This is where all the action happens. This innermost layer is responsible for maintaining the topmost layer of our skin by continually multiplying and pushing daughter cells upward. Stem cells in this layer slowly climb their way to the top to form mature skin cells known as horny cells. Scattered throughout this layer are the pigment cells (melanocytes) that are responsible for phenotypic skin tone and help in protection from solar radiation. Interesting cells of neuro-ectodermal origin known as Merkel cells can also be found here and it is thought that they help in tactile communication.
- Spiny layer. Well, as the name suggests, the cells here are mechanically tougher and are connected to each other via a spiny filamentous tension-reducing structure known as the desmosome complex. Desmosomes are formed of proteins and adhesive molecules that help bind cells together, allow flexibility of the cells and promote inter-cellular communication. Small granules (lamellar) containing ceramides, fatty acids, cholesterol, and enzymes are introduced here and serve an important function. These granules are indicative of the first signs of mature keratinocyte formation. It has been observed through histological studies that these granules expel their contents as they move upwards releasing all the rich ceramides and fats onto the cell surface, water-proofing, zip-locking and fortifying the barrier. This is one of the factors that makes the skin soft, supple and most importantly, water-proof.
- Granular layer. This is a grainy gritty layer of the epidermis rich in protein filaments known as filaggrin. These filaments further strengthen and fortify the cells, making them highly resilient. Numerous lipo-proteins in this layer join forces and coat the skin cells. Granules also secrete enzymes that will dissolve the nucleus of the cells facilitating keratinization.
- Horny layer or Stratified Corneum (SC) 10 um thick. This is the heroic layer of our skin. Germs, toxins, solar radiation, pollution, dust and dirt, it keeps it all out. This layer consists of several layers (10-35 stacked layers) of mature keratinocytes that have completed the cell cycle. The tight brick wall arrangement ensures a strong infrastructure whereas the lipid bilayer ensures water-locking abilities. This brick and mortar arrangement is unique as it allows for water-proofing of the epidermis and plays a vital role in TEWL (trans-epidermal water loss). NMF (natural moisturizing factors) are water loving molecules that are exclusively found in the stratum corneum.
A combination of mature keratinocytes rich in keratin and water-loving NMF’s combined with intercellular lipid bilayers is what makes the skin soft, hydrated, supple yet exceptionally water-proofed.
Miraculous isn’t it? NMF levels decline which age, hence our skin will become drier with progressing time. The final stage of the skin cycle is desquamation, where the squames (flat dead cells) will detach themselves from the horny cells and flake off, becoming household dust. Exfoliation and scrubbing helps in reducing the thickness of this layer, revealing fresher skin, encouraging penetration of substances along with hastening of the cell cycle.
The dermis, about 3 mm thick is what gives the skin its oomph plump factor and accounts for 90% of our skin thickness. Lending bulk, structure and support, the dermal matrix is composed mainly of collagen and elastin fibers that decline with age.
The dermis also supports various structures like nerves, blood vessels, lymph vessels, sweat glands, oil glands, sensory and tactile receptors and hair follicles which all serve important functions mentioned in section one.
The dermis is responsible for conveying your age to the world. A healthy dermis will be plump, elastic, thick and strong with minimal deformities.Collagen, one of most abundant proteins found in nature is also the most resilient.
We start losing about 1% of collagen protein after the age of 30 annually as the skin stops manufacturing it.
Physically this will translate into wrinkled, thin, fragile and blotched skin.
So what is collagen you ask? Collagen is actually made up of a family of 18 or more complex proteins. Procollagen is the original form of collagen synthesized by cells in the dermis known as fibroblasts. Efficient synthesis of collagen involves a complex series of events that requires the help of ferulic acid, ascorbic acid (vitamin C) and alpha-ketoglutarate. It has been reported that fruit acids, retinol and ascorbic acid will encourage collagen production.
Broadly speaking, what are the main types of collagen found in the dermis?
- Collagen 1 comprises 85% of collagen found in the dermis and provides tensile strength. Let’s think of this collagen as the youth-giving collagen. The more our age, the lesser the collagen. Collagen 1 is also severely damaged by solar radiation. The more the solar radiation, the less the collagen.
- Collagen 3 comprises of about 15% of the collagen found in the dermal matrix. This type of collagen helps support blood vessels and gives compliance to the dermis.
- Collagen 6 plays an important role in gluing the dermis to the epidermis.
- Collagen 5 comprises 2%-5% of the collagen found in dermal matrix and is randomly scattered through the dermis and lends support to smaller structures within the dermis.
The next type of prominent fibers found in the dermis is Elastin. As the name suggests, these fibers give the skin its elastic nature. It is important to note that with increased sun exposure, elastin fibers become distorted, disorganized and finally degraded leading to inelastic non-pliable skin.
Both collagen and elastin fibers are suspended in a thick gel-like substance known as the ground substance made up of mucopolysaccharides (complex sugars) through which nutrients and waste can diffuse to and from other tissue components and blood vessels.
What is this ground substance actually made of? This thick gel is composed of some really important molecules such a glyocosaminoglycans (hyaluronic acid, dermatin sulphate, heparin sulphate and chondroitin sulphate) and glycoproteins. Let’s delve in and see what they do.
- Glyocosaminoglycans, as the name suggests, are complex sugars attached to a core protein. They have the propensity to imbibe water and maintain salt and water balance within the dermis. Hyaluronic acid (HA) plays a key role in cell growth and keeping the cells vital and viable at all times and is also a powerful humectant (a substance that bonds with water and retains it). HA is seen to cling to collagen and elastin fibers in young skin, hence maintaining optimal health and turgor of the dermis. In aged skin, levels of HA are diminished, which can adversely affect the health of collagen and elastin fibers, and lead to diminished thickness of the skin with loss of plumpness.
- Glycoproteins are also sugar and protein complexes that help orient the various cells of the dermis and keep them in place. Some glycoproteins also help in dermal healing after injury or cosmetic procedures. Glycoproteins like fibronectin are vital in order for the dermis to heal.
This final layer acts as a shock absorber, insulator and an efficient energy reserve. It is about 1.5 to 4 mm thick and laden with fat cells (adipose cells), collagen and elastin fibers. As we age, fat cells start migrating and redistributing themselves into other body areas, giving the body a disproportionate appearance.
So there we have it in all its complex glory and multidimensional grandeur. Isn’t the human skin one of the most marvelous inventions of nature?
And I haven’t even touched the surface of the biochemical and physiological processes that take place 24/7 within our skin throughout our lives.
It’s truly important that we take care of our skin by nurturing, preserving and nourishing it as much as possible. It is the mantle that harbors our heart, mind and soul, not to mention all the organs of our body.
Let me know if you also find the skin as intriguing as I do.
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