Skin Science
Glossary
The words our articles keep leaning on, in plain English. Not because the biology is simple, but because it is followable — and a customer who understands what a barrier is can tell a real claim from a decorative one.
The barrier itself
Almost everything Aphora makes is aimed at one structure, roughly as thick as a sheet of kitchen foil.
Stratum corneum
The horny layer
The outermost layer of skin — the only part you actually touch.
Fifteen to twenty layers of flat, dead cells set in a lipid mortar, and around a hundredth of a millimetre thick. It is what holds water in and keeps irritants out. Every topical product meets it first, and most never go further.
Understanding the skin barrierCorneocyte
A flat, dead skin cell — one of the tiles in the barrier's roof.
It begins life deeper in the epidermis as a living keratinocyte, rises upward over several weeks, flattens, fills with keratin and loses its nucleus. Corneocytes are not debris; they are the structural element of the barrier, and they are held together by the lipids between them.
Lipid matrix
The lamellar lipids
The mortar between the tiles.
Ceramides, cholesterol and free fatty acids arranged in organised sheets around each corneocyte. This is where barrier function actually lives — damage the matrix and the wall leaks regardless of how intact the cells are.
What barrier-first actually meansThe 1:1:1 ratio
The proportion of ceramides to cholesterol to free fatty acids the barrier uses to build itself.
The stratum corneum assembles its lipid sheets at roughly equal molar parts of the three. Supply them out of proportion and repair slows rather than speeds up — which is why a formula heavy on one lipid class is not automatically better than a balanced one.
Lamellar bodies
Odland bodies
The skin's own delivery service for barrier lipids.
Small packages inside the cells of the upper epidermis that carry lipids and enzymes upward and release them into the spaces between corneocytes. When the barrier is compromised, production increases — the skin trying to repair itself from the inside.
What barrier-first actually meansAcid mantle
The thin, slightly acidic film sitting on the surface of healthy skin.
Around pH 4.5 to 5.5. It is not decoration: the enzymes that build the lipid matrix work within that range, and so does most of the microbiome. Products that push the surface alkaline, or far below it, interfere with both.
Epidermis
The outer compartment of skin, about as thick as paper.
It has no blood supply of its own and renews continuously from its base. The stratum corneum is its topmost layer — the finished product of everything happening beneath it.
How skin renews itself
Renewal is a process, not an event — and the whole argument about acids is an argument about how hard to push it.
Desquamation
The continuous, invisible shedding of the outermost dead cells.
On healthy skin it happens a few cells at a time and you never notice. It slows with age and particularly after menopause, which is why cells linger, the surface roughens, and skin stops catching light the way it once did. Every exfoliating acid ever made is an attempt to influence this one process.
Why mandelic is the strongest acid we useKeratinocyte
The main living cell of the epidermis.
Born at the base, it migrates upward while filling with keratin, and ends its working life as a corneocyte at the surface. Roughly ninety per cent of epidermal cells are keratinocytes at some stage of that journey.
Cell turnover
The full journey from new cell to shed cell.
Around four weeks in young skin, and measurably longer with age. Slower turnover is a real part of why mature skin looks duller — which is why renewal support addresses something that has genuinely changed, rather than a cosmetic preference.
Filaggrin
The protein that bundles keratin together — and then becomes something else.
As a cell reaches the surface, filaggrin breaks down into the water-binding compounds that make up Natural Moisturising Factor. So the same protein does two jobs: structure first, hydration second.
Water, oil and how skin holds both
Hydration is not one thing. These four words are routinely used as synonyms and mean quite different things.
Transepidermal water loss
TEWL
Water evaporating outward through the skin.
It always happens to some degree. It rises when the barrier is compromised, and it can be measured — which makes it one of the few genuinely objective tests of whether a barrier claim is true. Falling TEWL over weeks is what barrier repair actually looks like on an instrument.
Natural Moisturising Factor
NMF
The skin's own built-in humectant, held inside the surface cells.
A mixture of amino acids, urea, lactic acid and PCA produced when filaggrin breaks down. It keeps corneocytes pliable from within. NMF declines with age, sun exposure and harsh cleansing — which is why some dryness does not respond to moisturiser applied on top.
Humectant
An ingredient that draws water in and binds it.
Glycerin, hyaluronic acid, tremella polysaccharide, betaine, panthenol. Humectants add water to the system but do not hold it there on their own — in dry air without an occlusive above them, they can pull moisture from the skin rather than into it.
Emollient
An ingredient that fills the gaps and softens the surface.
Mostly plant oils and lipids. They smooth the spaces between corneocytes, which is what makes skin feel supple rather than merely wet.
Occlusive
An ingredient that slows evaporation by sitting on top.
The seal on the system. A humectant without an occlusive is a bucket with no lid — which is precisely why an acid step is followed by oil and then cream rather than left bare.
Sebum
The skin's own oil.
Produced by sebaceous glands, and a major part of the surface film that holds water in. Production falls sharply after menopause, which is the structural reason so much menopausal dryness does not resolve with water-based products alone.
Menopausal skin: what changes and what helpsAcids and pH
The terms that decide whether an exfoliating product is a useful tool or a slow stressor.
Alpha hydroxy acid
AHA
A family of water-soluble acids that loosen the bonds holding dead cells to the surface.
Glycolic, lactic, citric, malic, tartaric and mandelic. They differ enormously in molecule size, and size governs how deep and how fast each one travels — which is most of what separates a gentle acid from an aggressive one.
Why mandelic is the strongest acid we useFree-acid value
How much of an acid in a formula is actually active.
Governed by the relationship between the formula's pH and the acid itself. Two serums can carry the same percentage on the front of the bottle and differ several times over in what the skin experiences. This, not the percentage, is the real dose.
pH
How acidic or alkaline something is, on a scale of 0 to 14.
Below 7 is acidic, above is alkaline, and each step is a tenfold change rather than a small one. Healthy skin sits around 4.5 to 5.5. Traditional soap sits near 9 or 10, which is why it can leave skin tight for hours afterwards.
Keratolytic
Describes anything that loosens dead cells from the surface.
Acids are keratolytic. So, in a blunter way, are scrubs. The distinction that matters is not whether something exfoliates but how selectively, how deeply, and what the barrier is left holding afterwards.
Defence, tone and the microbiome
The barrier is not only a wall. It is also an immune organ and a habitat.
Skin microbiome
The community of bacteria and fungi living on the surface.
Largely beneficial, and adapted to the acid mantle's pH. A disrupted microbiome shows up as reactivity, breakouts and slower healing — which is why harsh cleansing can make the problems it was meant to solve worse.
Antimicrobial peptides
AMPs
The skin's own built-in defence molecules.
Cathelicidins and beta-defensins among others, produced by keratinocytes and released alongside barrier lipids. Because they travel in the same delivery packages, a compromised lipid barrier also means weakened innate immunity — the two decline together.
Postbiotic
The useful by-products of fermentation, rather than the live organisms themselves.
Peptides, acids and polysaccharides left behind when bacteria ferment a substrate. Stable in a formula in a way live cultures are not, and used to support the microbiome rather than add to it.
Melanocyte
The cell that produces pigment.
It passes melanin to surrounding keratinocytes. Its behaviour becomes less regular through the perimenopausal years, which is why patchy tone and sun spots often appear in this chapter even where sun habits have not changed.
Post-inflammatory hyperpigmentation
PIH
The dark mark left behind after inflammation settles.
A spot, a scratch or an over-aggressive treatment can each leave one, and it often outlasts the original event by months. Risk is higher in deeper skin tones, which is why the choice of acid matters more than the strength of it.
Free radical
An unstable molecule that damages what it touches.
Generated by UV, pollution and ordinary metabolism. Antioxidants neutralise them by donating what the unstable molecule is missing — which is the entire job of a daytime antioxidant layer beneath sunscreen.
Aphora's own terms
Three phrases used throughout the range. Each describes a method, not a mood.
Barrier-first
The rule that every formulation choice is judged by whether it supports the barrier or compromises it.
A criterion rather than a slogan, and a testable one — reduced water loss, improved tolerance, fewer reactions. Where a brand uses the phrase without those measures behind it, it is atmosphere.
What barrier-first actually meansThe 3 R's
Repair, Replenish, Regenerate — the framework the range follows.
R1 repairs the lipid matrix. R2 replenishes with humectants, nutrients and antioxidants. R3 supports renewal without inflammation. Every Aphora product sits at one or more of the three.
The 3 R's of menopausal skinCeramide-like botanical lipids
Plant lipids that behave like the skin's own ceramides without being ceramides.
Olive squalane, camellia, prickly pear and murumuru among others. The phrasing is deliberate and we hold to it: Aphora contains no true ceramide ingredient, and saying otherwise would be a claim the formula cannot support.
Also Available
The ingredients, named just as openly
This page covers the biology. Its companion covers what is actually in the bottles — every ingredient in every Aphora formula, with its botanical source, geographic origin and extraction method.
Ingredients Glossary