Your epidermis varies in thickness from 0.05 mm on the eyelids to 0.8 ± 1.5 mm on the soles and palms.
It’s amazing to think that your skin is jam-packed full of millions of cells and is still only about 1-2 pages of paper thick! The epidermis acts as a renewable protective guardian for your body, with complete keratinocyte turnover about every 28 days (although this process can reach up to forty days in older skin). Consisting of 4 horizontal layers (with a 5th on hands and feet), each layer represents differing stages of cell activity and keratin maturation.
There are 4 main cell types in your epidermis:
- the keratinocytes (who synthesise the water insoluble and fibrous protein keratin)
- the melanocytes (pigment producing cells)
- the langerhans (cells involved in the epidermal immune system) and
- the merkel cell (a sensory receptor).
Merkel cells make up less than 1% of all epidermal cells but have the important function of helping us to sense light touch. Merkel cells form a disk along the deepest edge of the epidermis, where they connect to nerve endings in the dermis.
The melanocytes comprise about 5-10% of epidermal cells and are responsible for your skin colour.
Did you know that even though huge variations of skin colour exist around the world, we all have about the same number of melanocytes present? It is the amount and size of the melanosomes produced and melanin distributed that gives us such huge differences from person to person!
Melanocytes insert granules of melanin, a brown pigment, into specialised cellular transporters called melanosomes. The melanocyte itself resides solely in the stratum basale and will transfer the melanosomes using its long dendritic, almost octopus-like ‘arms’ to keratinocytes that are sitting further up in the spinosum and granular layers.
Melanin helps form a photo-protective shield designed to minimise the often catastrophic damaging effects of UV exposure. Skin has developed two defensive barriers in response to sun over exposure – the top layer of your skin will thicken (the leathery look will develop over time!) and the antagonised melanocyte activity results in more melanin distribution (darker tanned skin, can lead to disfunctions showing up as pigment patches).
A compromised skin will often show signs of unwanted and unsightly pigmentation (excess melanin that is unevenly distributed). The possible reasons for this excess are complex and varied, although sun exposure is the dominant cause.
Although, a consistent, individualised and well planned dermal treatment program with your skin care therapist will achieve improvements!
The melanocyte is a complex cell, so I’ll be adding extra information about this little guy in a separate document, so we’ll have a clearer picture of what is going on with the way internal and external triggers affect your pigment production. I really want to concentrate on your epidermis today, so let’s continue!
The first and deepest layer of the epidermis, stratum basale or basal layer, sits firmly anchored at the basement membrane to the dermis (the sub epidermal junction) with some special attachment connectors called hemidesmosomes that can be thought of like tiny ‘press studs’.
These multi-unit protein hemidesmosomes and their counterpart desmosomes are little understood, yet it is known they play a crucial role in the dynamics of cell adhesion, signalling pathways and also mechanical resistance. Gap junctions are intercellular channels composed of special connexin proteins and are involved with mediating cell-to-cell communication and maintaining the homeostasis of the skin.
The basal layer has a single layer of cuboidal shaped keratinocytes (fibrous protein filled cells) sitting on the basement membrane, with every 30 or so keratinocytes interspersed with one pigment forming dendritic (octopus arms!) melanocyte. Between 90-95% of your epidermal cells are keratinocytes. The skins self-renewing capabilities mean there is a small group of permanent stem cell or ‘mother’ keratinocytes that undergo constant cell division to replace the ‘dead’ skin cells that are shed continuously from the uppermost stratum corneum layer.
The mother cells are programmed to remain at the basement membrane and continue their endless task of division, whilst the newly formed daughter cells are programmed to be sent upwards and onwards to mature through the different layers of the skin. As they mature and ascend some fundamental structural and compositional changes occur – these cell changes are what make your skin the way it is, and yes – you can influence this hugely with your diet, sun exposure and product application.
As a non-vascular region, the epidermis is completely reliant on the underlying capillaries in the dermis for diffusion of essential nutrients in and the excretion of toxic waste products out, all via the basement membrane. Only the cells closest to the basement membrane receive these nutrients – the creation of healthy cells is very dependent on what goes IN at this point!
Your skin relies heavily on nourishment delivered from the vitamins, proteins, minerals, lipids and water in your daily diet.
Moving into the second layer, the stratum spinosum, keratinocytes begins to flatten out as more keratin begins to form within it. Spinosum layer is between 8-10 cell layers thick with special attachments, desmosomes, forming between the cells. As mentioned previously, the desmosomes are specialised connectors forming strong cell to cell adhesion to help resist mechanical stressors. Also often found floating about in this layer are the Langerhans cells, which play a role in both immune regulation (skin tolerance) and immune stimulation (zero tolerance for any invading pathogen activates your immune response). This cell can pass freely between the epidermis and dermis, so that it can alert the immune system as needed by processing and presenting an invading pathogen to other immune cells in your lymphatic system.
The third layer is stratum granulosum, or granular layer, composed of 3-6 layers of cells busy producing keratin and lamellar bodies even as their nucleus starts degenerating. Keratin is fibrous protein that gives skin resilience, strength and toughness. You may know that keratin makes up your hair and nails! Lamellar bodies form the precursors for the lipid components in your skin – it is these particular lipids that create your all-important protective lipid bilayer barrier. Lamellar bodies also secrete protective antimicrobial peptides and the proteases (enzymes that break down proteins and peptides) that orchestrate the regularity of desquamation, or natural exfoliation of your ‘dead’ skin cells.
The correct functioning of the Lamellar bodies is a determining factor for the integrity of your lipid barrier – it is known that stress hormones negatively interfere with the production of Lamellar bodies, which is one of the reasons why stress really takes its toll on your skin.
The stratum lucidum is present only in the palms and soles of the feet. It is 3-5 layers of extremely flattened, translucent cells that form an extra protective barrier where we really need it!
So after discovering 4 distinct layers we have found ourselves at the top end of the epidermis for the final layer of maturation from keratinocyte to corneocyte – the keratinocyte has been on a very specialised
evolutionary journey and now is at the end of the trip!
The top, outermost ‘horny’ layer of the epidermis is the stratum corneum, or SC.
This is 25-30 layers of flattened, hard corneocytes that have lost all their organelles and are completely surrounded by Natural Moisturising Factors and epidermal lipids (e.g. ceramides, fatty acids) that act as a binding cement between the cells and help minimise and regulate the trans-epidermal water loss (TEWL) that keeps vital moisture components in the skin. This combination of corneocytes with interspersed epidermal lipids is akin to the roof of a house, with the tightly packed ‘roof tiles’ forming our vital, waterproof and selectively semipermeable moisture barrier.
The stratum corneum is a top priority for any serious corneotherapist or skin therapist, as it is the true protective layer of the skin – understanding the signs and symptoms of distress displayed at the SC is really what helps us determine how, when and what we can do to best help rebalance it. Without a structurally and functionally intact SC your skin will literally start falling apart!
Think of a roof that has lost or broken tiles – the building underneath is adversely affected and none of the residents inside can behave normally.