Skin Anatomy & Melanocyte Biology

1. Architecture of the Skin

Skin is the largest organ of the body (~1.5–2 m², ~16% of body mass) and is organised into three structural layers:

• Epidermis — stratified squamous epithelium, ~50–100 μm thick (thicker on palms/soles), avascular, renewed continuously by basal keratinocytes. Contains melanocytes, Langerhans cells, Merkel cells.
• Dermis — collagen-rich connective tissue with vasculature, lymphatics, nerves, hair follicles, eccrine and sebaceous glands. Divided into papillary (superficial, loose) and reticular (deep, dense) dermis.
• Subcutis (hypodermis) — adipose-rich connective tissue, thermal insulation, energy reservoir, mechanical cushioning.

The basement membrane (dermal-epidermal junction) is the critical anatomical boundary for melanoma staging: melanoma in situ sits entirely above it; invasive melanoma crosses it, and the depth into the dermis (Breslow thickness) is the dominant prognostic variable.

2. The Epidermis Layer-by-Layer

The epidermis is composed of four (in glabrous skin: five) layers, named for the keratinocyte differentiation programme that traverses them from base to surface:

The keratinocyte transit time from basal layer to desquamation is approximately 28 days in normal skin — far shorter than melanocyte renewal, which is orders of magnitude slower. Melanocytes are therefore long-lived cells that accumulate UV-induced mutations across decades while exposed neighbours turn over monthly.

3. Melanocytes — Neural-Crest Origin

Melanocytes derive from the neural crest: the migratory population of cells that delaminate from the dorsal neural tube during embryogenesis (week 3–4 in human). The neural crest gives rise also to:

• Peripheral nervous system — sensory neurons (DRG), autonomic ganglia, Schwann cells.
• Adrenal medulla chromaffin cells.
• Craniofacial skeletal mesenchyme (in cranial neural crest).
• Enteric nervous system.
• Melanocytes of skin, hair, uvea, choroid, leptomeninges, inner-ear stria vascularis.

Melanoblasts — melanocyte precursors — migrate dorsolaterally between ectoderm and somites, arriving at the epidermis around week 8–10. The transcription-factor cascade SOX10 → PAX3 → MITF is master-regulatory; MITF(microphthalmia-associated transcription factor) is the lineage-defining transcription factor of melanocytes and is later amplified or activated in ~10–20% of melanomas.

4. Melanocyte Morphology & Density

In adult skin, melanocytes are positioned on the basement membrane in the stratum basale at a frequency of approximately 1 melanocyte per 5–10 basal keratinocytes. Density is similar across all human ancestries; the reason different populations have different skin colour is not the number of melanocytes but the type and amount of melanin they produce and how it is packaged and delivered to keratinocytes.

• Cell body. Round to oval, ~10–15 μm, in the basal layer; contains a pale nucleus and abundant melanosomes at varying maturation stages.
• Dendrites. 30–50 fine cytoplasmic processes extending up between keratinocytes through the spinous layer; on average each melanocyte contacts ~30–40 keratinocytes — the epidermal melanin unit.
• Melanosomes. Lysosome-related organelles produced from early endosomes; mature through stages I–IV, with melanin polymer accumulating at stages III–IV.
• Markers. S100, SOX10, MITF, melan-A/MART-1, gp100/HMB45, tyrosinase.

5. The Epidermal Melanin Unit

The functional cooperative unit of skin pigmentation, defined by Fitzpatrick (1963), is the epidermal melanin unit: one melanocyte and the ~30–40 keratinocytes it contacts via dendrites. Within this unit:

1. The melanocyte synthesises melanin within stage IV melanosomes.
2. Stage-IV melanosomes are trafficked along dendrites by myosin-Va / Rab27a / melanophilin (the “tripartite complex”; mutations cause Griscelli syndrome types 1–3).
3. Melanosomes are transferred to keratinocytes via cytophagocytosis and/or filopodial shedding (mechanism still debated).
4. Inside the keratinocyte, melanosomes form a supranuclear cap — positioned between the nucleus and the surface, shielding nuclear DNA from incoming UV photons.
5. As keratinocytes mature and ascend, melanosomes are processed and degraded by lysosomal proteases.

The supranuclear cap is the proximate basis of melanin photoprotection: it absorbs ~50–75% of incident UV in lightly pigmented skin and >95% in darkly pigmented skin, and quenches the reactive species that escape that absorption.

6. Melanin Biosynthesis — the Tyrosinase Pathway

Melanin biosynthesis (melanogenesis) takes place inside melanosomes and begins from the amino acid L-tyrosine. The rate-limiting enzyme is tyrosinase (TYR), a copper-containing oxidase that catalyses the first two steps:

From dopaquinone the pathway forks. The fork is decided largely by local cysteine availability:

• Eumelanin branch (low cysteine). Dopaquinone → cyclodopa → dopachrome → (DCT/Tyrp2) → DHICA → (Tyrp1) → eumelanin polymer (brown/black).
• Pheomelanin branch (high cysteine). Dopaquinone + cysteine → cysteinyldopa → benzothiazine → pheomelanin polymer (yellow/red).

Three core melanogenic enzymes — TYR, TYRP1, DCT (TYRP2) — are all melanocyte-specific and are transcriptional targets of MITF. Albinism (oculocutaneous albinism types 1, 2, 3, 4) results from loss-of-function mutations in TYR, OCA2, TYRP1, and SLC45A2 respectively.

7. Eumelanin vs Pheomelanin — the Photoprotective Asymmetry

The two melanin polymers are not just different in colour:

In phototype I/II skin (red/blond hair, freckled), the pheomelanin/eumelanin ratio is high; UVA radiation excites pheomelanin to generate reactive oxygen species inside the melanosome, leading to oxidative DNA damage independently of the direct UV-pyrimidine photochemistry covered in Part III. Mouse experiments (Mitra et al., Nature 2012) showed that pheomelanin alone, in the absence of UV, is sufficient to drive melanoma in BRAF- mutant mice — a striking demonstration that the pigment is not innocent.

8. MC1R Signalling — the Switch

MC1R (melanocortin-1 receptor) is the G_s-coupled GPCR on melanocytes that decides whether melanin synthesis follows the eumelanin or pheomelanin branch. It is activated by α-melanocyte-stimulating hormone (α-MSH), which is in turn released by keratinocytes after UV-induced p53 activation. The cascade is:

MC1R is a highly polymorphic gene with >200 variants, of which several common “R variants” (R142H, R151C, R160W, D294H) are loss-of-function:

• Reduced cAMP response to α-MSH.
• Default to pheomelanin synthesis.
• Phenotype: red hair, fair skin, freckles, poor tanning response.
• Independently of pigmentation, MC1R loss-of-function impairs DNA-repair signalling and increases melanoma risk approximately 2–4×.

The agouti signalling protein (ASIP) is an inverse agonist of MC1R; in red- haired humans, MC1R variants make pigmentation MC1R-independent. The MC1R cascade is also why post-UV tanning takes hours-to-days — it is a transcriptional, not an immediate, response.

9. Photoprotection vs Photodamage — the Net Balance

The melanocyte has been described as “a cell asked to fight a war it cannot win”: it must absorb UV photons whose energy is sufficient to damage DNA, and do so without being damaged itself. Three quantitative observations frame the clinical biology:

1. Eumelanin is genuinely photoprotective. Cumulative lifetime melanoma incidence in Fitzpatrick VI is <1% of that in Fitzpatrick I/II at the same UV exposure.
2. Pheomelanin is genuinely pro-mutagenic. Even in the absence of UV, pheomelanin generates ROS that damage DNA inside the melanocyte (Mitra et al., Nature 2012).
3. Melanocytes are long-lived. Unlike keratinocytes (which desquamate in ~28 days), melanocytes can persist for decades, accumulating UV-induced mutations across the life course. This is the cellular basis for the extreme mutation burden of cutaneous melanoma.

The course will return to this balance repeatedly: in Part III we trace the pyrimidine-dimer chemistry of UVB and the oxidative damage of UVA; in Part IV we see how that damage maps onto BRAF/NRAS/NF1 driver mutations; and in Part VIII we examine which sun-protection interventions actually shift the balance back.