This 'more than back to basics' refresher will enable you to better understand the effects of the creams you use and have a greater understanding of dry skin physiology.
Skin is a physical barrier to the environment. It is the alteration of the skin barrier defence systems and actual damage to this barrier that causes dryness and dermatitis when the skin is exposed to water, soaps, gloves, chemicals and harsh weather conditions.
The repair of the damage by creams is related to the physical and chemical interactions of the ingredients with the skin barrier defence systems. Skin is difficult to penetrate therefore an extensive knowledge of skin structure and function is necessary before understanding if a cream can make a difference.
Skin Physiology: Structure and Function Basics
Skin has two main structural layersthe epidermis and the dermis (Figure 1).
The epidermis is the outer layer of skin, which serves as the physical and chemical barrier to the interior body and exterior environment.
The dermis is the deeper layer providing the structural support of the skin.
Epidermis and Stratum Corneum:
The Structure and Growth of Skin
The epidermis consists of stacked layers of cells in transition.
Protein bridges called desmosomes connect the cells.
The bottom layers of cells adjacent to the dermis are the basal cells, which reproduce. These are anchored to the dermal epidermal junction by hemidesmosomes.
As the cells mature, they move towards the outer layer of skin leading to terminal differentiation of the cells. During the process of maturation, the physiology, chemical composition, shape and orientation of the cells change.
When the cells reach the top layer of skinthe stratum corneumthe cells are called corneoctyes and are no longer viable. Corneocytes lack a nucleus and cellular structures.
Corneocytes are flat, hexagonal-shaped cells filled with water-retaining keratin proteins surrounded by a protein envelope and lipids. The cellular shape and the orientation of the keratin proteins add strength to the stratum corneum. There are 10-30 layers of stacked corneocytes.
The thicker skin on the palms and soles has the most layers of stacked corneocytes.
The cells remain connected to each other by protein bridges called desmosomes.
Stacked bilayers of lipids surround the cells in the extracellular space. The resulting structure is the natural physical and water-retaining barrier of the skin.
The Breakdown of Skin
During the process of maturation, the viable cells moving towards the stratum corneum begin to clump proteins into granules.
These granules are present in the granular cell layer of the skin and are filled with a protein called fillagrin.
Fillagrin becomes complexed with keratin proteins in the granular cells. This complex protects fillagrin from proteolytic breakdown.
As the degenerating cells move towards the outer layer of the skin, enzymes break down the keratin- fillagrin complex.
Fillagrin is on the outside of the corneoctyes and water-retaining keratin remains inside the corneoctyes of the stratum corneum.
When the moisture content of the skin is decreased, specific proteolytic enzymes in the stratum corneum are triggered to further break down fillagrin into free amino acids.
Natural Moisturising Factors:
The Natural Retention of Water in the Skin
The free amino acids, along with other physiological chemicals such as lactic acid, urea and salts, are present in the stratum corneum.
Together these chemicals are called "natural moisturising factors" and are responsible for keeping the skin moist and pliable by attracting and holding watera property called hygroscopic.
The water content of the stratum corneum is normally about 30%.
The proteolytic breakdown of fillagrin to amino acids only happens when the skin is dry to control the osmotic pressure of the skin and the amount of water it holds.
There is less need for breakdown of fillagrin in humid weather than in dry weather.
Generation of Natural Moisturising Factors in the Skin:
Maturation and differentiation of epidermal cells in the middle layer of skin
Fillagrin protein clumps into granules in the granular cell layer
Fillagrin becomes complexed to keratin protein to protect from proteolytic degradation
Cells lose their normal cell contents and become protein envelopes filled with keratin/fillagrin proteins called corneocytes
Corneocytes move towards the very outer layer skin
Enzymes break down keratin/fillagrin complex to fillagrin and keratin
Decreased water content stratum corneum
Proteolytic enzymes activated
Fillagrin degraded into individual amino acids
Amino acids plus other chemicals form Natural Moisturising Factors in stratum corneum
Holds water to rehydrate stratum corneum
(The Shedding of Skin Cells)
Desquamation is another important factor in keeping the skin smooth.
Desquamation is the enzymatic process of dissolving the desmosomes, the protein connections between corneocytes, and the eventual shedding of these cells.
Opposite to the production of amino acids from proteolytic degradation of fillagrin proteins, the proteolytic enzymes responsible for desquamation function in the presence of a well-hydrated stratum corneum. These enzymes are located within the cell.
In the absence of water, the cells do not desquamate normally and the result is thickened, dry, rough, scaly skin.
There is a normal physiological balance in the production of corneoctyes and shedding. Increased production of corneocytes (skin diseases such as psoriasis) or decreased shedding (skin diseases like ichthyosis) results in the accumulation of cells on the skin surface and dry, rough skin.
The Desquamation of Cells
Intact corneocytes in upper level of hydrated stratum corneum
Proteolytic enzymes break protein connections between corneocytes
Skin stays normal without dry scales
The last factor that is necessary in explaining how the natural skin barrier works to keep the skin moist and pliable is the function of the intercellular lipids.
These lipids form stacked bilayers (multilamellae) surrounding the corneocytes in the stratum corneum and incorporate water into this architecture
The lipids are derived from the degradation of cells in the granular layer of skin (similar to the origin of the protein granules). Special lipid structures called lamellar granules are released into the extracellular spaces of the degrading cells.
There is also release of lipids from the former cell membranes.
These released lipids include cholesterol, free fatty acids and sphingolipids.
Ceramide, a type of sphingolipid derived from the lamellar granules, is one of the major lipid components responsible for generating the stacked lipid structures.
These lipids trap water molecules in their hydrophilic (water attracting) region.
The newly formed stacked lipids surrounding the corneocytes provide an impermeable barrier for the passage of water out of the stratum corneum and the prevention of the natural moisturising factors from leaching out of the surface layers of skin.
There are sharp decreases in intercellular lipids after age 40 resulting in more susceptibility to dry skin conditions.
Formation of Intercellular Lipid Layers(figure 1 right)
Lipids in granular cell layer of skin form granules
Degeneration of granular cells releases lipid granules into intercellular spaces
Lipids released from degeneration of cells and lipid granules complex together to form intercellular stacked lipid structures
Lipid layers hold water and surround corneocytes to provide permeability barrier
The intercellular lipids and corneocytes containing proteins and natural moisturizing factors work together to provide an efficient barrier against water loss and water retention to maintain the flexibility of the skin. The protective forces shield the skin from desiccation and environmental assaults.
Irritants and Damage to the Skin Barrier
The major factor responsible for dry, scaly skin and irritant dermatitis can be related to the loss of water from the stratum corneum.
This loss of water is called trans-epidermal water loss (TEWL).
The stratum corneum receives water from the dermis and some from the environment.
Water is the "plasticiser" of the skin making it pliable and smooth.
Numerous external factors can cause TEWL.
The water content of the stratum corneum fluctuates with environmental humidity levels. There is accentuation of dry skin with exposure to cold, wind and low humidity.
Other external factors damage the stratum corneum barrier by denaturing keratin protein, removing natural moisturising factors and interrupting the lipid bilayers.
These factors include solvents, detergents, excessive use of water and soap, and other irritating chemicals.
The severity of the damage is dependent on the type and intensity of exposure to these irritating factors. An irritant is any agent that is capable of producing cell damage if there is exposure for sufficient time and in sufficient concentrations.
It may require repeated and prolonged exposure to see the actual damage.
Anyone exposed to excessive amounts of cleaning agents and water--health care personnel, hairdressers, food service workers, bartenders, and dishwashersmay experience extreme changes in the health and nature of their skin. Water is a weak irritant.
Significant water exposure causes the loss of the soluble natural moisturising factors and some of the protective lipids paradoxically resulting in TEWL.
Persistent hydration of the skin from exposure to water results in:
Penetration of foreign substances and contribute to allergic and irritant contact dermatitis and changes in the normal ecological environment in/on the skin, which can support the overgrowth of pathological organisms on the skin. Many soaps and cleansers are considered mild irritants.
These agents are beneficial surfactants that remove exogenous dirt, bacteria, and skin oils, perspiration and crusts (desquamated skin cells).
Repeated and prolonged exposure to this type of cleanser results in denatured skin proteins, disorganisation of the lipid lamellae layers removal of the protective intercellular lipids, loss of natural moisturising factors and decreased cohesion between cells.
The temperature of the water can increase the irritant capacity of skin cleansers by causing increased absorption of the cleanser with warmer temperatures. There is greater removal of the protective lipids with hotter water.
The end result of prolonged use of water and cleansing agents is alteration of the water-holding capacity of the skin and an increased TEWL. Dry, scaly skin that is less pliable and damaged is the physical result.
There are also endogenous factors that make one more susceptible to damaged skin by external factors.
These factors include having active skin disease such as psoriasis, eczema, inherited dry skin conditions (ichthyosis), a previous history of skin diseases (childhood eczema), sensitive skin and/or older age.
These endogenous factors may exacerbate dryness of the skin and increase ones susceptibility to dermatitis.
In summary, the exposure to irritants with the resulting trans-epidermal water loss compromises the barrier function of the stratum corneum and decreases its ability to protect the skin against environmental influences.
The harsher the cleansers or solvents, and/or prolonged exposure to irritants, the greater removal of protective lipids, proteins, natural moisturising factors and water loss.
With decreased water capacity, there is also loss of function of the normal enzymes to desquamate the corneocytes.
If the water content of the skin is less than 10%, these interacting factors are disturbed and the result is dry, scaly, fissured and less pliable skin.
How moisturisers work: Read this article under cosmetic chemistry section