The Skin Ledger

Module 00 · Before everything else

A map of the organ, layer by layer.

Two square metres, roughly four kilograms — the largest organ you own, and the only one everyone can see. Before grading a single ingredient, the Ledger maps the territory: six layers, four kinds of resident, and the living processes that every product claim eventually has to answer to. Descend, click, and run the machinery.

The skin at steady state: the conveyor turning slowly, the barrier holding, the glands secreting, the vessels idling. Click any layer or structure to read it — or run one of the processes to watch the organ work.

Cross-section · not to scaleStratum corneum · 0 – 20 µm
020µm100µm300µm3mm

Click a layer to jump to its entry; click the follicle, sweat gland, or nerve to read the structure. Depths are real; proportions are not.

Stratum corneum

0 – 20 µm

The wall. Dead, and doing the most important job in the whole organ.

  • 15–20 layers of flattened, dead corneocytes — the 'bricks' — bound by a lipid 'mortar' of ceramides, cholesterol, and fatty acids in a 3:1:1 ratio.
  • Thinner than a sheet of printer paper, yet it is the entire difference between you and dehydration: it holds ~64% of your body's water inside.
  • It maintains a pH of ~4.7–5.5 — the acid mantle. Most bar soaps sit at pH 9–10, which is why 'squeaky clean' means 'chemically stripped'.
  • TEWL — transepidermal water loss — is the measurable rate at which water escapes through it. Cold, dry air (a Norwegian winter, say) can double it.

What reaches this depth

  • SPFSunscreen is designed to sit here and go no deeper — a film, not a treatment.
  • CERCeramide moisturisers replace the mortar directly at this depth.
  • AHA/BHAExfoliating acids loosen corneocyte adhesion in this layer.

Living epidermis

20 – 100 µm

The factory. Every cell here is on a 28-day conveyor belt toward the surface.

  • Keratinocytes are born at the bottom, and spend ~28 days migrating upward — flattening, filling with keratin, and finally dying on schedule to join the wall above. Retinoids work by speeding and normalising exactly this conveyor.
  • The stratum granulosum is where cells assemble their lipid cargo and their 'cornified envelope' — the packaging step before becoming a brick.
  • Langerhans cells patrol here: the immune system's border guards, sampling everything that gets through the wall.
  • There are no blood vessels anywhere in the epidermis. Every cell is fed by diffusion from the dermis below — which is why the dermal border is folded into ridges to maximise contact area.

What reaches this depth

  • RETRetinoids bind nuclear receptors in these keratinocytes and rewrite the turnover schedule.
  • NIANiacinamide boosts the lipid-synthesis machinery of the granular layer.
  • VIT-CVitamin C neutralises UV-generated radicals throughout the living layers.

Basal layer & melanocytes

~100 µm

The birthplace — and the paint shop. One row of cells decides your color.

  • A single row of stem-like keratinocytes dividing for life. Every brick in the wall above was born here.
  • Melanocytes sit in this row — roughly 1 for every 10 basal cells — and everyone on earth has about the same number. Skin color differences come from how much melanin they make and how it is packaged, not how many cells you have.
  • Melanin is handed to neighbouring keratinocytes in packets called melanosomes, which stack above each nucleus like tiny parasols — literally shading the DNA below.
  • Tyrosinase is the rate-limiting enzyme of the whole pigment pathway — which is why melasma treatments (azelaic acid, tranexamic acid) all converge on it.

What reaches this depth

  • AZAAzelaic acid selectively calms hyperactive melanocytes at this depth.
  • TXATranexamic acid interrupts the keratinocyte-to-melanocyte signalling that drives melasma.
  • UV-BUVB penetrates exactly this far — deep enough to hit dividing DNA. That is the sunburn and the mutation risk.

Papillary dermis

100 – 300 µm

The interface. Finger-like ridges that feed, sense, and grip the factory above.

  • The dermal papillae — the wavy ridges — push capillary loops and nerve endings as close to the epidermis as possible. On fingertips, these ridges are literally your fingerprint.
  • Capillary loops here are the flush you see: vasodilation in this layer is blushing, rosacea, and inflammation made visible.
  • Meissner corpuscles — light-touch sensors — live in the papillae. The fingertip has ~150 mechanoreceptors per cm².
  • This is where a 'fine line' begins: the papillary collagen is the first to disorganise under UV.

What reaches this depth

  • UV-AUVA reaches the dermis — through window glass — and generates the radicals that dissolve collagen. The 'A' is for aging.
  • RET*Retinoids don't travel here, but their downstream collagen signal does — dermal remodelling is why they take twelve weeks.

Reticular dermis

0.3 – 3 mm

The mattress. A woven net of collagen and elastin that is 90% of the skin's thickness.

  • Type I collagen bundles woven with elastin fibres: the collagen resists stretching, the elastin snaps it back. Together they are the skin's entire mechanical identity.
  • Fibroblasts — the weavers — sit sparsely in the net, producing and recycling it continuously. After ~25, net production declines roughly 1% per year.
  • MMP enzymes (activated by UV) cut collagen; this layer is where photoaging structurally happens, years before it is visible.
  • Scars are this layer's repair work: fast, dense, disorganised collagen laid in parallel instead of woven — which is why a scar behaves differently from the skin around it, and why keloids are this process refusing to stop.
  • Hair follicles, sebaceous glands, and sweat glands are all epidermal structures that plunge down through this layer — tunnels of surface lining the deep skin.

What reaches this depth

  • µ-needleMicroneedling and fractional lasers deliberately wound this layer to trigger organised re-weaving — clinic territory.
  • HA-fillerInjectable fillers restore volume here. No cream reaches this depth, whatever the label implies.

Hypodermis

3 mm +

The foundation. Fat lobules that cushion, insulate, and quietly run an endocrine organ.

  • Lobules of adipocytes divided by fibrous septa — shock absorber, thermal insulation, and energy reserve in one tissue.
  • It is an active endocrine organ: leptin and adiponectin from this layer talk to the whole body's metabolism.
  • Pacinian corpuscles — deep pressure and vibration sensors the size of a rice grain — sit here, the deepest sensors in the skin.
  • Its thickness varies from ~1 mm on the eyelid to several centimetres elsewhere — the reason the same face ages so differently in different places.

What reaches this depth

  • —Nothing topical reaches here. Anything claiming to 'melt fat' or 'lift' from a jar is graded F in the Codex on anatomy alone.

The residents

Structures that refuse to stay in one layer — tunnels, coils, and wiring that run the full depth of the map.

Now that you know the map —

every claim in the Codex is a claim about one of these layers, and every grade asks the same question: can the molecule actually get there, and what are the trials that say it did?

Open the Codex Read your surface