Well here is the post that will help you answer all about those questions that pop in your head when your trying to decide which moisturizer your going to buy….Why is my skin so dry? How does skin hydrate itself naturally? How does putting on some moisturizer change the game? What kind of ingredients should I be looking for in a moisturizer ? I will try to keep things easy to read and short ….. Lets see if I can stick to that promise 😉
Firstly we need to take a look at the structure of the human skin and soooo ..back to the biology textbooks folks!
Structure of the human skin (Source: Wikimedia commons)
As you maybe aware, the skin is the largest organ in the human body. Its primary functions include providing a protective barrier against microorganisms, toxic substances and UV radiation found in the environment. It also plays an important role in regulating body temperature and preventing moisture loss depending on environmental conditions. The skin consists of 3 main layers; the epidermis, dermis and the fat layer/subcutaneous layer as shown in the picture above. When it comes to answering questions on skin hydration, the most important layer would be the outermost layer which is in direct contact with the external environment i.e the epidermis.
Taking a closer look at the epidermis we see that it can be further subdivided into 4 layers, from bottom to top:
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum corneum
Now lets try and answer some questions !
How does the skin produce new cells and shed old ones?
The primary cells found in the epidermis are keratinocytes. New column shaped keratinocytes are formed by the division of cells found in the stratum basale. These newly formed cells push their way up into the stratum spinosum where they change to a polygonal shape and start producing keratin. These cells are held together by ‘bridges’ called desmosomes and hence have a prickly appearance under a microscope. As more cells are produced in the stratum basale, these prickly cells are further pushed to the next layer, the stratum granulosum. In this layer the cells flatten and lose their nuclei and become granular cells. As these granular cells rich in keratin migrate to the stratum corneum, they secrete lipids and proteins which together with the desmosomes help the stratum corneum to be an efficient barrier to water loss as well as water entry. Once the cells reach the stratum corneum, they are referred to as corneocytes. As these new cells are pushed into the stratum corneum the older cells are shed by a process called desquamation. During desquamation, the desmosomes are broken down by various enzymes thereby allowing the old skin cells to ‘slough off’. This whole process of cells migrating from the the stratum basale all the way to the stratum corneum takes about 3 weeks.
How does the skin keep itself hydrated naturally?
Water is primarily stored in the dermis, where the moisture content is as high as 80% and one of the functions of the dermis is to provide hydration to the epidermis. As I stated earlier, when the granular cells migrate to the stratum corneum they secrete lipids (ceramides, fatty acids etc) which are found surrounding the corneocytes. They arrange themselves into a bilayer and act as a barrier, selectively allowing things to go in and out of the skin. They are hydrophobic and this water repelling nature makes the skin water proof. Additionally they also do not allow water to escape from the dermis and epidermis. This is nature’s way to prevent water loss from the body to the environment through evaporation. The amount of moisture ideally present in the stratum corneum is only 20%. This moisture from the environment is held on with the help of a group of hygroscopic (water grabbing) components referred to as Natural Moisturizing Factor (NMF) found in the corneocytes. The NMF is composed primarily of amino acids and also a mixture of salts like urea and lactates. Once the NMF grabs onto moisture and becomes hydrated, they interact with keratin fibers present in the stratum corneum thereby increasing the elasticity of the skin. Well now you know who is responsible for that healthy and supple looking skin! The skin is highly dynamic and as you may have guessed the production of NMF is related to humidity in the environment thereby regulating the moisture content of the stratum corneum depending on environmental conditions.
How does the skin get dry?
So as I explained above, the hydration of the stratum corneum is due to the moisture it draws from within (from the dermis) and due to the presence of NMF.
- During the dry winter months and when the body is exposed to low humidity conditions water gets lost from the stratum corneum by diffusion. Under dry air conditions a water vapor gradient between the moisture rich skin and drier air gets established,thereby causing water loss from the epidermis. This moisture loss is expressed in terms of transepidermal water loss (TEWL).
- If you go back to my explanation of how the old skin cells are replaced by new ones you’ll see how the process called desquamation is important for releasing the old cells from the skin surface. When the TEWL is very high, the stratum corneum becomes very dry and when this happens the enzymes responsible for breaking down the desmosomes cannot function. The result of this chain of events is the appearance of dry flaky sheets of skin which cannot detach since the cells are all still attached to each other.
- Age can also play a role. It has been shown that there is a sharp decline in the production of lipids which surround the corneocytes with age, thereby increasing our susceptibility to dry skin as we grow older.
- As strange as it may sound too much exposure of the skin to water can also dry it out. The NMF is highly water soluble and excessive contact with water especially hot water can wash away the NMF thereby disturbing the moisture controlling mechanism of the skin.
- Excessive exposure to UVB radiation can reduce the skin’s ability to produce NMF and also disturb the bilayer arrangement of the lipids surrounding the corneocytes.
- Another major culprit responsible for drying skin is soaps and cleansers. Although they have their benefits, prolonged use of harsh cleansers can strip the skin of NMF and disrupt the functioning of the lipids in the stratum corneum.
What is a moisturizer? How does it work?
Source: Wikimedia Commons
A moisturizer is any agent used to balance the effects of harsh environmental conditions and the stripping away of the naturally occurring lipids and NMF from the skin. Most light weight moisturizers and lotions are oil in water emulsions which means they are largely made up of water with oil or lipids dispersed within. This system is held together by emulsifiers which make sure that the oil droplets don’t separate out from the water since in nature oil and water do not mix. Thicker moisturizers are water in oil emulsions which means the main component is oil with water droplets dispersed and are again held together by emulsifiers. So what are the moisture enhancing components in a moisturizer? well, all moisturizer contain either all or a combination of occlusives, humectants and emollients.
- Occlusives: They function by forming a hydrophobic film on the skin and thereby significantly reducing TEWL. They thus act by mimicking the lipids found around the corneocytes. Commonly used occlusives include mineral oil, lanolin, beeswax etc. If present in a high concentration they do contribute to a greasy feeling on the skin after application.
- Humectants: They mimic the NMF. They function by helping the Stratum Corneum grab onto moisture from the external environment and also helps draw more moisture to the epidermis from the dermal layer. Due to this latter function if used alone they can be drying since too much water drawn from the dermis to the surface will result in greater TEWL. Hence occlusives are almost always used along with humectants. Some humectants commonly found in skin moisturizers are glycerin, panthenol, lactic acid, propylene glycol etc
- Emollients: They are mainly lipids and oils which help to soften and smoothen the skin. They achieve the smoothening effect on dry skin by acting as a filler between the flakes of dry skin. Some fatty acid derived emollients also help improve the production of the NMF and lipid activity in the stratum corneum. Examples of emollients : dimethicone, isostearyl alcohol, jojoba oil, castor oil etc.
I hope this post gives some insight into the wonderful mechanisms through which our skin regenerates itself, keeps itself moisturized , when to give it some moisturizer love and what happens when you pamper your skin with moisturizers !
Until next time ciao!
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- Factors Which Influence the Water Content of the Stratum Corneum.The Journal of Investigative Dermatology (1952) 18, 433–440.
- Controlling the hydration of the skin though the application of occluding barrier creams. J. R. Soc. Interface 6 March 2013 vol. 10 no. 80.
- Skin Moisturization. Edited by James J. Leyden, Anthony V. Rawlings. CRC Press, Feb 26, 2002.
- Understanding the Role of Natural Moisturizing Factor in Skin Hydration. Practical Dermatology, July 2012.
- Dry Skin and Moisturizers: Chemistry and Function. Marie Loden, Howard I. Maibach. CRC Press, Dec 6, 1999.