Our skin is the largest organ in our body, acting as a physical and biological barrier and gatekeeper between our internal and external environment. Our skin keeps us alive by preventing organ dehydration and infection. The structure of our skin gives it the ability to do this, so let’s take a closer look.
The structure of the skin
The visible layer of our skin, the stratum corneum, is 10-20 μm thick, (a strand of hair measures ~70 μm thick). The stratum corneum is a brick and mortar structure, where the bricks represent non-living corneocyte cells composed primarily of cross-linked keratin. The intercellular mortar is a mixture of lipids organised largely in bilayers, (double layers of closely packed molecules).
Underneath the stratum corneum layer is the viable epidermis, which measures 50-100 μm and is avascular (no blood supply). The viable epidermis is made of 4 layers of stratum lucidum where dead keratinocytes form the natural moisturising factors that keep cells hydrated. The stratum granulosum is the site of lamellar granules that help build the protective lipid bilayer for the external stratum corneum. The stratum spinosum is the site of live keratinocytes. The tight junctions formed by the keratinocytes prevent microbes and other foreign substances from passing through this layer. The stratum basale is the site of keratinocyte development and multiplication (proliferation) and where melanocytes reside. Melanocytes are the cells that produce the melanin that determines the colour of our skin.
Deeper still is the dermis, a 1-2 mm thick, highly vascularised tissue. Within the dermis resides the erector pili muscle, sweat glands, nerves, hair follicles and fibroblasts. The dermis is also composed of the extracellular matrix or ECM, (the area between the cells) made up of collagen, elastin, proteoglycans and glycoproteins. Below the dermis is the hypodermis, or subcutaneous tissue where arteries and veins supply and remove blood. The hypodermis is where we find fat and lymphatic vessels.
Collagen & elastin
Changes on the surface of our skin, reflect events that are occurring deeper in the layers. You may be familiar with the structural fibres that make up the extracellular matrix, collagen and elastin. When we stop producing collagen and elastin, or once it starts to degrade, we can observe this change externally as our skin loses firmness and elasticity.
Collagen is the most abundant protein in the human body, making up ~30% of all proteins. There are 19 different types of collagen with the most common and relevant to the skin being Type I and III. Type I collagen is robust structural collagen and the most abundant in our skin (~80-85%). Type III collagen is also structural, but plays an important role in maintaining the expanding tissues in our body, such as the uterus. Type III collagen is also involved in wound healing.
Elastin fibres provide the stretch or elasticity in our skin. They can increase their length by 150% when force is applied.
Our collagen and elastin fibres are produced from fibroblast cells within the dermis. As we age, so do our fibroblast cells as they lose their ability to produce collagen and elastin. Fibroblasts also recruit immune cells to assist with wound healing and scar formation.
Proteoglycans and glycoproteins
Within the extracellular matrix (ECM), are proteoglycans and glycoproteins. Proteoglycans hydrate, stabilise and fill space within the ECM. Proteogylcans have a negative charge enabling them to absorb water from surrounding tissues. Small proteogylcans provide mechanical stability by interacting with collagen fibres. Large proteoglycans are like bristles of a bottlebrush, which fill the gelatinous, interfibrillar space between skin cells. Glycoproteins regulate cell shape, cell mobility and stimulate cell proliferation. Proteoglycans and glycoproteins appear as ingredients in many skin treatments as they are important in maintaining the tone and texture of skin.
Our skin is able to restrict water loss from the body and prevent the entry of microbial pathogens through the skin barrier. Our skin barrier is a two-component system that includes the lipid bilayer and the acid mantle. Some of the lipids within the bilayer exhibit powerful antibacterial and antimicrobial activity. These, along with the acid mantle, which maintains an acidic pH on the skin surface, serve to encourage the growth of normal skin flora and prevent pathogens and unhealthy bacteria from entering the body.
Many factors such as sunlight, humidity, detergents and skincare ingredients can influence the integrity of the skin barrier. If the barrier is disturbed or damaged, water loss may occur from the epidermis (trans-epidermal water loss) and lead to dry skin, itching and loss of elasticity.
Maintaining skin health
There are things that we can do to support the health of our skin. It is much easier to prevent damage, than it is to undo or repair it. The treatments and the skincare we apply to the surface of our skin should support our skin and the natural mechanisms that keep it healthy. This means choosing products that work with our skin’s natural pH and encourage the existence of healthy bacteria. Avoid using harsh chemical ingredients regularly on your skin, especially those that might interfere with your protective skin barrier.
• Bos JD, Meindardi MMHM (2000) The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Exp Dermatol 9: 165-169
• Elias PM (2005) Stratum Corneum Defensive Functions: An integrated view. J Invest Dermatol 125: 183-200