Melanin vs. Keratin: Distinct Roles in Skin, Hair & Eye Biology

Melanin and keratin are both crucial biopolymers in human anatomy, but they serve fundamentally different functions.

Melanin: The Photoprotective Pigment

Nature & Synthesis

• Type: Heteropolymer derived from tyrosine via melanogenesis in melanosomes.
• Cells: Produced by melanocytes (neural crest origin) in basal epidermis, hair follicles, iris, choroid.
• Types: Eumelanin (brown/black, photoprotective), Pheomelanin (red/yellow, less protective, pro-oxidant).

Functions

• UV radiation shielding: Absorbs/scatters UVR, preventing DNA damage, photoaging, skin cancer.
• Color determination: Ratio and distribution of eumelanin/pheomelanin define skin, hair, eye color.
• Reactive oxygen species (ROS) scavenging: Eumelanin quenches free radicals.
• Thermoregulation: Influences heat absorption/reflection.

Keratin: The Structural Scaffold

Nature & Synthesis

• Type: Fibrous structural protein (α-keratin in mammals; β-keratin in reptiles/birds).
• Cells: Produced by keratinocytes (epidermal differentiation); assembled into intermediate filaments.
• Types: Soft (skin), hard (hair, nails, hoof, horn) – differing cysteine content & disulfide crosslinking.

Functions

• Mechanical barrier: Toughness, abrasion resistance, tensile strength.
• Waterproofing: Stratum corneum lipids + keratin prevent desiccation.
• Appendage structure: Hair shaft (cortex/cuticle), nail plate, corneal epithelium.
• Cellular integrity: Keratin cytoskeleton maintains cell shape, distributes mechanical stress.

Key Distinctions

Property	Melanin	Keratin
Chemical class	Pigment (indole/quinone polymer)	Protein (fibrous, α-helical)
Primary role	Photoprotection, coloration	Structural, barrier
Location	Melanosomes → transferred to keratinocytes	Keratinocyte cytoplasm, cornified layer
Genetic control	MC1R, TYR, OCA2, SLC24A5, etc.	KRT gene family (54 functional genes)
Pathology	Albinism, melasma, melanoma	Ichthyosis, epidermolysis bullosa, pachyonychia

Interdependence

Melanin is packaged in melanosomes and transferred to keratinocytes, where it forms a "supranuclear cap" protecting nuclear DNA. Keratinocytes thus serve as both structural matrix and melanin distributors.