1) YOUR SKIN IS YOUR ARMOUR - skin structure

1.1) Skin structure - Bricks & Mortar

You go about your daily life protected by approximately two square meters of skin¹ - a physical, chemical and immunological armour walling you off from a myriad external enemies. The structure and function of your skin depends on its hydration, pH (acidity) and microbial balance. When all of these factors are harmonious and healthy, the skin can withstand certain amounts of damage, and has amazing self-healing properties. If damage is extensive or prolonged though, these self-healing capacities can be overwhelmed, leading to skin disease and aging.

The first line of defence is your cutaneous microbiome. We will discuss that in more detail shortly. Beneath the microbiome lies a collection of complex and beautiful skin layers - the stratum corneum, the epidermis, and the dermis:¹

1.1a) The stratum corneum - a horny overachiever

Uppermost is a ‘dead’ layer of skin called the stratum corneum. Originating from the Latin, stratum corneum literally means ‘horny layer,’ and while this layer may get rather turned on by a luscious, organic moisturiser, ‘horny’ in this case is actually a reference to the fact that skin cells are hard like horns.

The stratum corneum is made up of:

• Corneocytes (‘bricks’):

  • Corneocytes are the dead versions of keratinocyte skin cells, which are produced deep in the epidermis, and gradually rise up through the skin layers over a period of about 28 days (it takes longer as we get older),² after which time they are shed from the stratum corneum and end up on our clothes, bedding, chairs etc. We generate and lose an estimated 200 million - 1 billion skin cells each day.³

• Intercellular lipid-matrix (‘mortar’):

  • This matrix keeps everything ‘stuck together’. It is made predominantly of:

    • ~ 50% ceramides

    • ~ 25% cholesterol

    • ~ 10-25% free fatty acids (mainly palmitic, stearic, lignoceric, behenic and cerotic acid)^4,3

The cells in the stratum corneum may be dead, but they are far from useless. This layer is a dermal overachiever. It functions as multiple types of barriers:⁴

• Permeability barrier:

  • Cells in the stratum corneum (corneocytes) contain ‘Natural Moisturising Factor (NMF),’ which is full of humectants that draw in water, keeping the skin hydrated and elastic. NMF is produced in response to dehydration - it is a self-repair mechanism. The uppermost cells contain the most NMF, thus excessive dermabrasion of dead cells can lead to lower levels of NMF, moisture loss and a reduced ability to maintain skin hydration.

  • One of the mechanisms by which the stratum corneum maintains consistent acidity is by determining what permeates (gets in and out of) our skin.

• Antimicrobial barrier:

  • Inhibits the growth of pathogenic microbes via:

    • Maintaining acidity

    • Producing small proteins such as antimicrobial peptides (AMPs) which are secreted in sebum and sweat

• Antioxidant barrier:

  • A range of enzymes, vitamins (vitamin C and E), and other antioxidants (such as CoQ10) inhibit oxidation in the skin.

• Immune response barrier:

  • The innate (in-built) and acquired (learned as we grow) immune systems operate in this layer.

  • Dendritic cells and toll-like receptors recognise invaders, and AMPs disrupt the cell membranes of these pathogens.

• Photoprotection barrier:

  • Melanin, proteins, and antioxidants contribute to photoprotection.

  • The thickness of the stratum corneum is also important. Excessive removal of the dead cells in the stratum corneum (for example though acid peels, or dermabrasion) may increase the risk of sun damage.⁴

The importance of maintaining good hydration can not be overstated: “many of the enzymes that catalyze vital SC [stratum corneum] functions are hydrolytic and do not operate efficiently if water content is below a requisite threshold concentration.”⁴ In other words, if there is no water in the tank, everything else starts to go wrong. Correct skin acidity is equally vital. “An elevated skin pH induces a multitude of direct and indirect effects that adversely alter several epidermal barrier functions.”⁴

The superficiality of the stratum corneum renders it vulnerable to damage from a range of insults, including:⁴

• Dry air

• Excessive dermabrasion

• External irritants (including those found in many skincare products)

• Environmental allergens

• Topical medications

• Harsh cleansers, or cleansing too often

• Long, hot showers

1.1b) A few notes on fats - background for understanding the lipid matrix

Fatty acids are the building blocks of fat. They can be:

• Saturated (no double bonds - very stable, and solid at room temperature)

• Monounsaturated (one double bond)

• Polyunsaturated (multiple double bonds - unstable, and liquid at room temperature)

A double bond is inherently less stable than a single bond. When a fat ‘oxidises’ it means that double bonds have been broken by heat, pressure, exposure to air, or chemical interaction. Saturated fats are the most stable fats as they are least prone to oxidation.

Fatty acids can be found on their own as free fatty acids (FFAs), or…

Bound up in specific lipid molecules such as:

• Triglycerides: three fatty acids bound by a glycerol backbone (this is the form we store as energy; the major form of fat in foods such as beef tallow and olive oil)

• Ceramides: a fatty acid attached to a sphingoid backbone (the most abundant structural lipids in the skin barrier)

• Cholesterol: an important structural lipid found in animal cells (it helps regulate membrane fluidity and supports the skin barrier)

• Phospholipids: the main fats in cell membranes (they provide structure and regulate what gets in and out of the cell)

Fats found in nature are a collection of different lipids mixed together (e.g. olive oil is a mix of predominantly monounsaturated, but also polyunsaturated and saturated fats, mostly in the form of triglycerides).

1.1c) The epidermis - the living layer

Beneath the stratum corneum lies the living layers of the epidermis (from the Greek ‘ to sit on top of the skin’) - a dynamic collection of structural, functional and immune cells involved in skin renewal and pathogen recognition:

• Keratinocytes (living bricks)

• Intercellular lipid-matrix (mortar)

• Immune cells (eg Langerhans cells and T cells)

• Commensal microbes

The epidermis is also home to hair follicles, sebaceous glands and sweat glands.

Sebaceous glands secrete sebum (a lipid-rich fluid), which:

• Lubricates and waterproofs the epidermis and hair.

• Nourishes the skin barrier so that it can retain water but keep pathogens out.

• Feeds lipophilic (fat-loving) commensal bacteria.⁵

Sweat glands

• Produce sweat for thermoregulation.

• Carry water and nutrients to the surface microbiome.

• Maintain optimum skin acidity.

• Carry antimicrobial peptides (AMPs) to the surface to control pathogen overgrowth.

1.1d) The dermis - the foundation of skin

The undulating underside of the epidermis is coated in a basement membrane which acts as an anchor, a communication channel, and a selective filter for the dermis below.

The dermis (Greek for ‘skin’) is mostly composed of:

• Structural, elastic and strengthening proteins: like collagen and elastin.

• Fibroblasts: which make the proteins and repair wounds.

• Blood vessels: to provide oxygen and nutrients which diffuse upward, feeding the keratinocytes and allowing for their maturation.

The skin is somewhat like a house erected by an sisyphean builder who continually knocks down their existing walls and replaces them with new ones :

• The dermis is the concrete foundation, containing the plumbing and electrical supply.

• The basement membrane bolts the house to the foundation, but allows access between the house and the foundation.

• The living epidermis are bricklayers who are busily laying bricks.

• The stratum corneum is the finished brick wall (until it gets knocked down and removed to make way for a new one).