Skin physiology


Skin physiology

Dr Beth Briden discusses the physiology and functions of our skin

Our skin tells our life story. It shows whether we are healthy, or if we have internal disease; it shows if we have had a life with a lot of outdoor activity and a lot of ultraviolet sun exposure, and it shows our age. This means it’s important to understand the physiology of the skin and what we can do to enhance it.

The skin is a very unique organ with many functions. One of it’s main functions is to protect us from environmental dangers such as extreme heat, cold, ultraviolet light, pollutants, external irritants. It also helps maintain internal hydration, and protects our internal organs. It is very important in temperature regulation, containing sweat glands to  dissipating heat through sweating and helping generate heat by forming “goose bumps” and a shivering reaction. It is also very important in producing vitamin D, which is involved in multiple functions. Researchers have found recently that vitamin D levels are markedly decreasing with the use of sunscreens, so it’s important to make sure that we maintain healthy levels of vitamin D, either with limited sun exposure or oral supplementation.

The skin is the largest organ of the body, weighing nearly 9 lbs. It’s one of the few organs that has the ability to continually renew itself, allowing us to heal well after superficial cuts and scrapes. About every 30 days our epidermis renews itself, with older, dead cells being shed off and replaced by new cells.

The subcutaneous layer
Skin is comprised of three layers: epidermis, dermis and subcutaneous. The deepest layer of subcutaneous tissue is the fat layer. We all have a given number of fat cells , which enlarge and shrink, depending on how much fat we store. The fat layer can store lipophilic hormones—such as oestrogens that can remain in the fatty layer for years and can be released into the body for years.

Our fat layer provides energy storage and stores and releases energy by breaking down fat into triglycerides and fatty acids. This occurs in “starvation” states to release energy in our system as needed. The subcutaneous tissue is helpful in maintaining body heat in winter. Animals that hibernate, like bears, have a thicker fat layer to help them keep their body temperature and maintain their warmth throughout the cold winter months.

As dermatologists and cosmetic surgeons, we try to manipulate the subcutaneous layer to enhance beauty. We currently have technology to tighten the skin by using lasers (ultrasound, radiofrequency, and infrared) that affect the fibrous connective tissue between the fat cells. We can also reduce the fat cells with liposuction to remove fat cells, and newer technology allows us to use LED lights to “break down” fat into triglycerides to be removed in targeted areas. In the case of liposuction, since we do not generate new fat cells, if a patient then regains weight and has lost fat cells in a certain area, the skin becomes lumpy and is not uniform throughout.

The LED photo-modulation stimulates a starvation state and break down fats from the adipose tissue into the triglycerides and fatty acids. Then the triglycerides are circulated in the blood and can be burned off by exercising within 2-3 hours of the procedure. If the triglycerides are not burned off, they are re-deposited back into the fat layer. This treatment allows us to target fat reduction in specific areas such as the thighs or “love handles”.  This technique does not remove fat cells, but shrinks the size, simulating a more realistic physiologic state.

With ageing we see a significant redistribution of the subcutaneous fat on our facial area. When we’re young we have a nice round face. With ageing the muscles relax, they cannot maintain their tightness, the facial bones resorb and get smaller, and the epidermis and dermis thin and the elastic fibres in the dermis lose their recoil ability, and connective tissue and elasticity comes out of the skin. The elastic tissues and collagen gets damaged and glycated so that the skin becomes stiffer and less resilient. Subsequently the subcutaneous fat tends to fall and we get a more triangular-shaped face with jowls and fat on the chin. We need to address these changes if we are going to address the full aspects of ageing. We can correct the volume redistribution that occurs with ageing with surgery, tissue tightening, fillers and liposuction.

The Dermis
The dermis is one of our main structural components of the skin and it is also the thickest layer. It contains our structural proteins, collagen and elastic tissue and ground substance containing acid mucopolysaccharides, which bathe our collagen and elastic fibres. It also contains some of our immune modulators, including Langerhans cells and mast which involved in our body’s immunity. Mast cells release histamine and heparin, which are responsible for allergic reactions and hives.

The dermis contains the blood supply and the pain sensors. The dermis also is continually reproducing itself, but, at a slower rate than the epidermis. Enzymes such as collagenase, elastase and hyaluronidase that are constantly breaking down the ground substance, the hyaluronic acid, the collagen molecules and the elastic fibres so they can be reformed. Our fibroblasts work overtime there and always produce new collagen elastic tissue and ground substance.

80% of the dermis is comprised of the collagen, which is usually Type 1 and Type 4. This can actually become damaged with ultraviolet and glycation forming age products, which are called the advanced glycation end-products. These accumulate and can give skin that yellow sallowness colour.

The elastic fibres, which provide the recoil and the elasticity of the skin, represent only a small percentage of the dermis. With aging and photo-damage they become shorter and clumpy and lose their ability to elongate and recoil as illustrated with laxity of the skin and solar elastosis.

The ground substance also decreases in the dermis with aging as production decreases. This is where fillers like hyaluronic acid can be added to help plump up and restore volume to the dermis. We can also stimulate production of new ground substance, elastic fibres and collagen by applying topical lotions, such as retinoids, alpha-hydroxy acids and poly-hydroxy acids.

A very effective topical agent, N-acetylglucosamine is a precursor to hyaluronic acid, which is absorbed and plumps the skin. There is also N-acetyl tyrosinamide, which is an acetylated amino acid that has been shown to plump dermal thickness. These can just be applied topically in addition to the injectables to enhance and maintain dermal thickness.

The extracellular matrix of the dermis contains macrophages, which are phagocytic cells that are involved in clearing up pigment or debris. We have our immune system, the lymphocytes, the mast cells and the neutrophils in the dermis, and the fibres that give it the structure, that are constantly renewing themselves.

If you can combine some of the dermal repair plumping agents, studies show that there is a synergistic effect. If you combine retinoids with alpha-hydroxy acids and poly-hydroxy acids you get a much more dramatic affect.

If you combine retinoids and N-acetylglucosamine you will also plump up the dermal matrix much more than by using either agent on their own. Peptides, like the Matrixyl, can combine with retinoids to provide skin maintenance and fitness. Using some of these compounds means that skin doesn’t keep atrophying, because by itself it can’t keep up with the normal production and tends to atrophy and thin with age.

The Epidermis
The top layer of the skin, the epidermis, provides our epidermal/dermal barrier, which protects us from absorbing harmful chemicals. We need to look at the structure of its lipid components to understand how molecules can penetrate. Only small molecules can really penetrate the epidermis. Large molecules, such as high molecular weight hyaluronic acid,  are unable to penetrate the epidermal barrier.

To understand the barrier function of the epidermis is important to look at the stratum corneum, which is the top, dead layer of the epidermis. With ageing, the stratum corneum becomes more compact and thinner. This can make our skin look dry, sallow and leathery. The dead cells of the stratum corneum need to be shed  off naturally and return it to its normal basket-weave pattern, to give us that normal skin barrier.

There are many lipids, including the ceramides, cholesterol and fatty acids, that comprise this barrier. Using harsh products that contain alcohol or dehydrating agents will dry this out, make it become more compact and lose its normal barrier function.

The barrier function of the stratum corneum is very important in maintaining the hydration of the skin. We know that when we lose some of the oils and lipids of our skin barrier, that water can evaporate causing our skin to become very dehydrated, red, cracked and irritated.

We can measure the effectiveness of our barrier function by looking at transepidermal water loss—the water that evaporates from the skin. If the barrier is disrupted and not functioning properly, water will be lost from the skin. There are certain things that can actually improve on trans-epidermal water loss and the barrier function. These are poly-hydroxy acids, bionic acids and ceramides. Sometimes retinoids can irritate and break down the skin barrier a little bit, even though they are good exfoliants.

Our normal cell renewal process is called exuviation. Every 28 days our whole epidermis and stratum corneum renews itself. This does tend to slow down, as the cells get a little bit stickier. 50% of the skin diseases that we know of are due to abnormal keratinisation or exuviation, so if we can normalise the skin barrier and the process of exuviation, we can actually improve many of our skin disorders and restore the tolerability.

Ageing of the epidermis creates a more compact, thickened stratum corneum. In the dermis, photodamage causes solar elastosis—where the elastic fibres clump together and are not uniform. This is frequently called the grenz zone of sun damage or solar elastosis. After some topical treatment like alpha-hydroxy acids you can restore a normal basket-weave pattern, collagen, glycoaminoglycans and normal elastic fibres. The epidermis also becomes thicker and plumper, you maintain the ridges and that normal underlying pattern, and you can actually restore.

Factors in skin function
Internal ageing also has an effect on the skin. The normal process of cell renewal, both in the epidermis and dermis, slows down naturally with ageing. Other external factors such as pollution can damage our skin. The reactive oxygen species and free radicals that form from exposure to ultraviolet light and pollution set up an inflammatory cascade of enzymes such as MMPs, collagenase, elastase and hyaluronidase that damage our skin.

We need to be able to protect ourselves from environmental insults. Sunscreens that contain antioxidants are better than just sunscreens alone. We also need to moisturise the skin barrier and restore some of the natural skin barrier function and natural oils, like ceramides.

Dr Briden is founder of the Advanced Dermatology and Cosmetic Institute in Edina, USA.

Author: bodylanguage

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