Why your skin’s natural exfoliation process is crucial for healthy skin
How to design a skincare routine to help reset epidermal turnover time
Skin is unique amongst all human organs because every day it physically defends against external damage, sensitization and environmental aggressors. If damaging pollution particles come into contact with skin, its barrier function acts to prevent them from penetrating deeply. If harmful bacteria are picked up, skin’s acidic acid mantle creates an environment too hostile for them to survive. If abrasion or friction cause damage to skin, its cell turnover process ensures the breach is only temporary.
A healthy cell turnover rate in the epidermis is crucial to a youthful, vibrant, even and hydrated complexion. Cell turnover naturally slows with age, a change which can be compensated for with the correct modifications to skin care and diet. If a natural decline in cellular turnover is left unchecked, skin quickly turns dull, uneven, rough, dehydrated, dry, flaky and patchy. This progression ultimately causes the skin to look mature beyond its years. However, if as a person’s cellular turnover slows, they make alterations which help re-speed epidermal cell turnover, skin may look younger, fresher and renewed for longer.
What is the epidermis?
Human skin is defined by its many layers, of which the first set of layers is defined by three individual sections, the epidermis, the dermis and the hypodermis. The epidermis is the top layer of skin, the dermis the middle layer and the hypodermis the base layer. Each performs different functions and each is home to a different biology. For example, the epidermis is mainly composed of keratinocytes, skin cells made from keratin which give structure, resilience, and defence, however, the dermis, is where underlying collagen and elastin networks live, here they provide a scaffolding for the epidermis to sit upon.
The epidermis itself is also defined by distinct layers with each being home to a different skin cell life cycle. There are four stages or layers of the epidermis;
- Stratum corneum
- Stratum granulosum
- Stratum spinosum
- Stratum basale
Together these four layers provide a tough defence against external assault. They prevent skin penetration of bacteria and viruses, they help protect against UV radiation and they prevent hydration from escaping. The epidermis protects what is held most deeply within skin i.e. collagen, elastin, and water in addition to preventing the infiltration of external allergens, irritants and sensitizers.
In order to fulfil these purposes to the same ability throughout a person’s lifetime, layers of the epidermis must be continually renewed. If skin did not have the ability to renew itself, abrasion, assault by pollution or contact with irritants would quickly cause the skin to age, break down and disintegrate. Skin can only remain a strong defence if old, used epidermal skin cells are replenished with healthy new skin cells.
For more on the 28 day cell rejuvenation and exfoliation process
What is epidermal turnover?
Epidermal turnover, cellular turnover or cell renewal are all phrases used to describe skin’s natural process of exfoliation. An event is scientifically known as desquamation. The four layers of the human epidermis are characterised by different life stages of skin cells, which in the epidermis are either known as keratinocytes or corneocytes (differentiated or ‘dead’ keratinocytes). Skin cells in the epidermis can be characterised by being either;
- A part of the stratum corneum
- A differentiating skin cell
- A proliferating skin cell1
The stratum corneum is the top layer of the epidermis and is made up from dead, differentiated keratinocytes. When keratinocytes near the stratum granulosum: stratum corneum junction, they complete their differentiation into corneocytes. As opposed to keratinocytes, corneocytes are skin cells able to pack extremely tightly together. When keratinocytes terminally differentiate into corneocytes, they lose their nucleus and internal contents. This allows corneocytes to become tightly packed bundles of keratin able to form what is often described as a cornified envelope2. The cornified envelope is responsible for the stratum corneum’s barrier function.
Before skin cells become corneocytes in the stratum corneum, they exist in the stratum granulosum and stratum spinosum as differentiating keratinocytes. In the stratum spinosum, skin cells are called prickle cells and are full of pre-keratin fibres. Cells residing in the stratum spinosum travel through an average of 8 to 10 cell layers after which they move into the stratum granulosum. As skin cells move upwards, they also change in biology or differentiate. In the stratum granulosum, skin cells are defined as becoming flattened and by the initiation of losing their nuclei and internal contents. During this release, a lipid layer is created helping to adhere differentiated skin cells together. A component of skins waterproofing.
The bottom layer of the epidermis, defined as the stratum basale, is where skin cells begin life. Within the stratum basale is found a type of cell called a stem cell. Stem cells are ever-green cells, they may multiply or proliferate infinitely or at least for the lifetime of the mammal they reside within. The continual renewal or creation of skin cells from epidermal stem cells allow the epidermis to replace over a thousand times during the average person’s lifetime3. Stem cells can divide without limit. When a stem cell divides, its daughter cell may proliferate into a stem cell, or a keratinocyte.
How is epidermal turnover affected with age?
Skin cells move through the 4 layers of the epidermis sequentially, ultimately maturing into stratum corneum corneocyte cells which allow the skin to effectively form a barrier against the environment. Due to constant environmental assault, the stratum corneum must be replaced continually. This is the purpose of epidermal cell turnover. The time taken for the epidermis to turnover i.e. to replace itself is 30 days4. Cellular turnover rates are also affected by age with young adults having an individual skin cell transit time of 30 days too i.e. the time it takes one skin cell to traverse from stratum basale to desquamate from the stratum corneum. This rate slows gradually with age, drastically decreasing after age 505
Why is epidermal turnover important?
Epidermal skin cell turnover rates which are relatively quick i.e. in the order of a youthful transit time of 20 days allow the skin to create a well-packed stratum corneum and to therefore remain healthy. When epidermal turnover is optimal, skin is able to;
- Retain hydration
- Prevent dull, flaky patches of skin
- Prevent irritation and therefore inflammation
- Create an even skin tone
- Prevent sensitivity
As skin cells proliferate and differentiate they change in shape and biology allowing them to perform their ultimate and vital function within the stratum corneum. Tightly packed and glued together by a lipid based matrix, corneocytes in the stratum corneum create an effective shield and barrier. As this layer is predominantly oil based, water, being unable to mix with oil is locked into skin, therefore enabling skin to remain hydrated.
When skin is youthful it turns over, desquamates or exfoliates relatively evenly. With a smaller amount of time taken to turnover, any difference in epidermal turnover rate dependent on areas of skin i.e. cheeks or forehead, are insignificant. As desquamation rates slow, relative differences become accentuated and visible. When skin turns over in some areas quicker or slower than in others, the integrity of the stratum corneum is breached. Instead of forming aligned layers, there are now breaks. Areas which allow or impair skins ability to retain hydration and prevent trans-epidermal water low (TEWL). This therefore results in dehydration and ultimately in dry, flaky patches of skin. When epidermal turnover rates are healthy, skin’s surface is continually renewed, the stratum corneum is evenly layered and a person’s complexion is therefore even, smooth and radiant.
Every day skin defends against known irritants, allergens and sensitizers in addition to bacteria and viruses. An effective stratum corneum barrier and pH balanced acid mantle allows both functions to perform optimally. However, if such ingredients are able to penetrate deeply into the epidermis, skin’s immune function is activated and inflammation begins. Inflammation is healthy in small quantities, it is ageing and damaging if ongoing. When skin’s stratum corneum is penetrable, irritants, allergens and other such ingredients are often able to absorb deeply into skin, therefore chronically activating inflammation. When the stratum corneum is turned over at a healthy rate, an effective and intact stratum corneum barrier prevents chronic and ageing inflammation.
An even skin tone is a sign of youth. A dull, rough uneven skin tone is a marker of an aged skin type. An efficient epidermal turnover rate allows skin cells of the relative same age to migrate into visible areas of the stratum corneum simultaneously. This function promotes an even skin tone. If areas of skin are affected by a slowed epidermal turnover rate, older areas of skin cells develop, creating a skin tone which is dull, patchy and rough. A healthy epidermal turnover rate promotes a clear, even skin tone.
Exfoliation will help to increase blood circulation to boost oxygen and nutrients to the skin cells, remove congestion such as blackheads and blocked pores and enhance the absorption and penetration of your moisturiser.
As skin cell turnover rates slow and stratum corneum integrity is negatively impacted, environmental aggressors such as particulate matter adhered to toxic pollutants such as dioxins are able to penetrate further into skin. They are also able to reside within skin for longer before being shed or desquamated. The increased ability of environmental matter to penetrate skin increases the sensitivity of skin. When epidermal turnover rates are healthy, sensitivity or the provocation of sensitivity is minimal.
Which factors affect epidermal turnover?
A person’s rate or speed of epidermal turnover may decrease with age however this process is not completely inevitable. With the right alterations to diet and skin care epidermal turnover rates can be perturbed towards a youthful rate of renewal.
Epidermal turnover rates may be affected by;
- Physical exfoliation
- Chemical exfoliation
- Vitamin D
- Calcium gradient
Skin can naturally exfoliate. Skin can also be manually exfoliated. The most basic method effective in speeding a person’s epidermal cell turnover rate is to integrate a daily physical exfoliant. When skin’s ability to desquamate slows, physical exfoliants may intervene.
Products such as the griffin+row Exfoliate natural exfoliant cloth gently increase stratum corneum or corneocyte shedding thus increasing cell turnover rate to reveal the healthier, smoother skin below.
Skin cells are bound together by ingredients called desmosomes6 and when cells mature through layers of the epidermis, specific enzymes begin to degrade these junctions. Desquamation or cellular turnover is promoted by dissolution of desmosome junctions. Similarly, the use of chemical exfoliants is also active in dissolving the glue which holds skin cells together. Aged and oily skin types both adhere to skin cells for longer than is necessary, therefore slowing epidermal turnover. Products containing alpha-hydroxy acids (AHAs) or beta-hydroxy acids (BHAs) penetrate the top layer of epidermis to speed epidermal turnover.
Vitamin D present as cholecalciferol, or vitamin D3 is synthesised within skin on exposure to UV light. The presence of vitamin D in skin allows for keratinocytes to proliferate and differentiate into coenocytes. Studies show a diet deficient in vitamin D paired with no exposure to sunlight promotes skin scaling, erythema and TEWL, similar to the clinical manifestation of psoriasis7. Therefore in the promotion of a healthy epidermal turnover, skin should be exposed regularly to sunlight, avoiding times of high UV intensity (10am-2pm). A diet rich in vitamin D as found in mushrooms, mackerel and salmon may also prove beneficial.
The presence of vitamin D in skin is also closely linked to skin’s calcium gradient. Starting at a low level in basale layers and increasing throughout the epidermis. When calcium levels are low keratinocytes proliferate quickly but differentiate slowly. When calcium levels are high, keratinocytes predominantly and terminally differentiate into coenocytes therefore forming the stratum corneum and therefore ultimately desquamating8. Falling calcium levels in the epidermis are a hallmark of ageing skin9. The calcium gradient of skin may be negatively affected by water impermeable coverings e.g. clothes or highly occlusive creams for example those containing mineral oil10. Calcium gradient
Sources and References
- Epidermal turnover time. Iizuka H. J Dermatol Sci. 1994 Dec;8(3):215-7.
- The cornified envelope: a model of cell death in the skin. Candi E, Schmidt R, Melino G. Nat Rev Mol Cell Biol. 2005 Apr;6(4):328-40
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Epidermis and Its Renewal by Stem Cells.
- Cengage Learning Australia, titled PROFESSIONAL BEAUTY THERAPY 2nd Edition by Lorraine Nordmann and Andrea Day, 2015. Accessed 10 September 2017.
- Gary L. Grove, PhD, Albert M. Kligman, MD, PhD; Age-associated Changes in Human Epidermal Cell Renewal, Journal of Gerontology, Volume 38, Issue 2, 1 March 1983, Pages 137–142, https://doi.org/10.1093/geronj/38.2.137″ rel=”nofollow”>https://doi.org/10.1093/geronj/38.2.137
- Desmosomes: regulators of cellular signaling and adhesion in epidermal health and disease. Johnson JL, Najor NA, Green KJ. Cold Spring Harb Perspect Med. 2014 Nov 3;4(11):a015297. doi: 10.1101/cshperspect.a015297
- Daniel D. Bikle, Sreekumar Pillai; Vitamin D, Calcium, and Epidermal Differentiation, Endocrine Reviews, Volume 14, Issue 1, 1 February 1993, Pages 3–19, https://doi.org/10.1210/edrv-14-1-3″ rel=”nofollow”>https://doi.org/10.1210/edrv-14-1-3
- Daniel D. Bikle, Sreekumar Pillai; Vitamin D, Calcium, and Epidermal Differentiation, Endocrine Reviews, Volume 14, Issue 1, 1 February 1993, Pages 3–19, https://doi.org/10.1210/edrv-14-1-3″ rel=”nofollow”>https://doi.org/10.1210/edrv-14-1-3
- Skin aging, gene expression and calcium. Rinnerthaler M, Streubel MK, Bischof J, Richter K. Exp Gerontol. 2015 Aug;68:59-65. doi: 10.1016/j.exger.2014.09.015. Epub 2014 Sep 26.
- Origin of the epidermal calcium gradient: regulation by barrier status and role of active vs passive mechanisms. Elias P, Ahn S, Brown B, Crumrine D, Feingold KR J Invest Dermatol. 2002 Dec; 119(6):1269-74.
