Melanin is a pigment that gives hair its color. It is produced by cells called melanocytes that are found in the hair follicle. The amount and type of melanin determine someone’s hair color. People with dark hair have more eumelanin, while those with lighter hair have more pheomelanin. Understanding melanin in the hair follicle is important for understanding hair color and health.
What are Melanocytes?
Melanocytes are pigment-producing cells located in the basal layer of the epidermis. They produce melanin within organelles called melanosomes. Melanocytes are also found in hair follicles where they inject melanin into the cortical cells that make up the hair shaft.
During the anagen (growth) phase of the hair cycle, melanocyte stem cells at the base of the follicle proliferate and differentiate into mature melanocytes. These melanocytes produce melanin and transfer melanosomes containing melanin pigments into the cortical keratinocytes of the hair shaft.
The melanocytes remain active throughout the anagen phase, continually delivering melanin to the hair as it grows. This results in continuous pigmentation of the hair. When the follicle enters the catagen regression phase, the melanocytes stop producing pigment and start to undergo apoptosis (programmed cell death).
Types of Melanin
There are two main types of melanin produced by melanocytes:
- Eumelanin – A dark brown or black pigment. Higher levels of eumelanin result in darker hair colors like black or brown.
- Pheomelanin – A red or yellow pigment. Higher levels of pheomelanin result in lighter hair colors like blonde or red.
The ratio and concentration of eumelanin and pheomelanin determine someone’s natural hair color. People with mostly eumelanin will have very dark brown or black hair. Those with more pheomelanin will have lighter brown, blond, or red hair.
Melanin Production
Melanin production occurs in 4 main steps:
- Tyrosine oxidation – The amino acid tyrosine is oxidized into dopaquinone with the help of the enzyme tyrosinase.
- Dopaquinone conversion – Dopaquinone is converted to dopachrome and then to 5,6-dihydroxyindole (DHI) or 5,6-dihydroxyindole-2-carboxylic acid (DHICA).
- Polymerization – DHI and DHICA polymerize and oxidize to form eumelanin or pheomelanin polymers.
- Melanosome maturation – The melanin polymers are deposited in melanosomes which are transferred to keratinocytes to pigment hair.
The type of melanin produced is influenced by the local environment. Higher cysteine levels drive pheomelanin production while eumelanin is favored in its absence. Other factors like pH also play a role.
Melanin Distribution in Hair
In hair, melanin is distributed in the cortex of the hair shaft. The cortex contains long, thin cells full of keratin bundles that provide strength. Melanosomes released by melanocytes are uptaken by cortical cells during hair growth.
Melanin is not evenly distributed along the length of a hair strand. Hair also does not have the same concentration or type of melanin across different regions. This results in variations and patterns of pigmentation seen as lighter or darker shades.
Some key facts about melanin distribution in hair:
- Hair bulb near follicle has the most concentrated melanin.
- Melanin content gradually decreases up hair shaft giving proximal ends a lighter color.
- Individual strands do not have the same melanin content throughout their length.
- Different regions of the head can have different melanin amounts and types.
What Controls Melanin Production?
Melanocyte activity and melanin synthesis are controlled by several endogenous and exogenous factors:
Genetics
Genetics play a major role in melanin production. Gene variants influence the structure and regulation of melanogenic enzymes like tyrosinase. They also affect melanosome formation and transportation.
Some key genes involved in human hair pigmentation include:
- MC1R – Melanocortin 1 Receptor – Involved in melanocyte stimulation and eumelanin vs pheomelanin synthesis.
- OCA2 – Oculocutaneous Albinism Type 2 – Encodes the P protein which controls tyrosinase activity.
- TYR – Tyrosinase – Encodes the tyrosinase enzyme that catalyzes melanin synthesis.
- SLC24A5 – Solute Carrier Family 24 Member 5 – Helps regulate calcium required for melanogenesis.
Ultraviolet Radiation
UV exposure stimulates melanin production. Melanocytes have receptors that detect and respond to UV radiation from the sun. This triggers increased melanogenesis and melanosome transfer to keratinocytes.
This natural defense mechanism protects against UV-induced damage of skin and hair by absorbing photons and neutralizing free radicals.
Hormones
Hormones like estrogen, progesterone, and melanocortins bind receptors on melanocytes and influence melanin synthesis:
- Estrogen and progesterone can darken hair during pregnancy.
- MSH and ACTH melanocortins increase eumelanin production.
- Melatonin inhibits melanogenesis while serotonin stimulates it.
Inflammation
Inflammatory mediators like prostaglandins and cytokines modulate melanocyte function and melanogenesis. They alter enzyme expression and activity in melanogenic pathways.
Stress
Stress hormones like cortisol and CRH can reduce melanin synthesis. Stress is linked to accelerated graying and can exacerbate hair loss conditions.
Aging
Melanocyte activity declines naturally with age. The antioxidant defense systems that protect melanocytes also weaken. This causes a gradual decrease in melanin content over time.
Natural Hair Color Changes
Changes in melanin content and composition in the hair follicles lead to natural color changes over a person’s lifetime:
Birth to Early Childhood
Newborns often have very light hair that is not necessarily indicative of their natural hair color. Over the first 2 years of life, the melanocytes mature and hair pigmentation develops into the natural shade.
Childhood to Early Teens
Melanin production is maximal leading to the darkest natural hair color during this time. Hair color is influenced by inherited genetics during this period.
Late Teens to 30s
Melanin content remains relatively stable at this age so minimal color changes occur. Permanent hair color is most effective at this stage of life.
30s and Beyond
Gradual decreases in melanocyte function lead to some graying of hair. The rate of reduction in pigment content determines the extent of graying with age. Premature graying is linked to genetics, vitamin deficiencies, and autoimmunity.
Conditions Affecting Hair Pigmentation
Several medical conditions lead to alterations in melanin content and distribution in hair follicles. These include:
Vitiligo
An autoimmune disease causing melanocyte destruction and depigmented patches on the skin and hair.
Albinism
Genetic disorder characterized by an inability to produce melanin leading to white hair, skin, and eyes.
Piebaldism
Rare congenital disorder causing white patches of skin and hair due to localized absence of melanocytes.
Waardenburg Syndrome
Genetic condition associated with abnormal melanocyte migration and development leading to pigmentary disturbances.
Chediak-Higashi Syndrome
Immunodeficiency disease causing defective melanosome transport resulting in premature graying.
Nutritional Deficiencies
Deficiencies of copper, vitamin B12, selenium, iron or protein can impair melanogenesis and cause hair depigmentation.
| Condition | Cause | Hair Changes |
|---|---|---|
| Vitiligo | Autoimmune destruction of melanocytes | White patches in hair |
| Albinism | Inability to produce melanin | White hair |
| Piebaldism | Absence of melanocytes in patches | White forelock and patches |
| Waardenburg Syndrome | Abnormal melanocyte development | White forelock |
| Chediak-Higashi Syndrome | Defective melanosome transport | Premature graying |
| Nutritional Deficiencies | Impaired melanogenesis | Depigmentation and graying |
Conclusion
In summary, melanin produced by melanocytes in the hair follicle controls hair color. The type, amount, and distribution of eumelanin and pheomelanin melanin pigments determine someone’s natural hair shade. Many genetic, hormonal, and environmental factors regulate melanin synthesis and influence hair pigmentation over a lifetime. An understanding of the melanocytes and melanin within the hair follicle provides insights into the biology of human hair color.
