TESTING
In vivo assessments of hair greying treatments
Kunyanatt Chalothorn, Fabrice Perin – DermaProof Asia
Hair colour has long been a symbol of youth and health. The progressive loss of pigmentation in the hair shaft (hair greying or canities) is one of the most obvious signs of ageing, being not only an aesthetic concern but also a social and psychological challenge for many individuals. These perceptions can result in decreased
self-esteem, social withdrawal, reduced professional opportunities, and more generally in a decline in the quality of life. It has been reported that worldwide 6–23% of people have 50% grey hair by 50 years of age. Onset of greying typically begins in the mid-
30s for Caucasians, the late-30s for Asians, and the mid-40s for Africans. When the onset occurs earlier, canities are referred as premature. In men, grey hair typically begins at the temples and sideburns, then spreads to the vertex and lastly to the occiput whereas in women greying develops at the boundaries of the scalp and moves towards the vertex.1 With the increasing life expectancy, cosmetic
demands for maintaining a youthful appearance are also rising. Despite this, research on canities has remained relatively limited. The purpose of this article is therefore to shed light on this subject.
Etiology of grey hair Keratin, the main constituent of the hair, is colourless. The colour of hair comes from melanin pigments granules located in the cortex. As for the skin, melanin is produced by melanocytes within organelles called melanosomes that are transferred to keratinocytes of the hair shaft. The ratio of black-brown eumelanin to yellow-reddish pheomelanin, the quantity, and the distribution of the pigments determine the final hair colour. Hair follicle structure is divided by the
‘line of Auber’, which separates the lower undifferentiated cell region from the upper differentiated region that forms the inner sheath and hair (Figure 1). Cells move from the matrix to the upper
bulb, elongate vertically, and some still show mitotic activity, though insufficient for significant hair growth. In pigmented hair, the pigmentary unit is a black structure at the dermal papilla tip above Auber’s line, containing melanogenically- active melanocytes. Below the line, only unpigmented and
undifferentiated melanocyte stem cells are typically found. In grey hair, the pigmentary
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unit becomes less defined, with fewer, rounded melanocytes and lightly pigmented oligodendritic melanocytes detectable below Auber’s line. Normal melanogenesis of human hair is
cyclical and occurs only during anagen III-VI phases when the hair follicle generates a hair shaft. Without a good coordination between the proliferation and differentiation of keratinocytes and melanocytes during this period of the hair cycle, a normal pigmentation cannot be achieved. Melanosomes loaded with melanin
pigments are transferred from melanocytes into keratinocytes of the hair shaft during these phases. As a consequence, it means that greying happens during the anagen phase (and can only be reversed in this phase too). Hair progressively appears grey because
there is less and less melanin in the hair shaft keratinocytes. There are several possible causes for this melanin decrease. Firstly, this pigment loss results from a reduction in the number of melanogenically active melanocytes and a depletion of melanocyte stem cells in grey anagen hair follicles. Secondly, greying may also involve
defective melanosomal transfer to cortical keratinocytes. Melanosomes may also be removed by autophagy. Thirdly, the decrease
Sebaceous gland Inner root sheath Outer root sheath
Dermal sheath Melanocytes Critical line of Auber
Hair bulb matrix
Dermal papilla
Figure 1: Pigmentary unit in relation to the dermal papillae and the line of Auber
of melanin synthesis is due to a decrease in tyrosinase activity. Indeed, true grey hair shows reduced tyrosinase activity, while white hair bulbs show none.2 A large body of evidence supports the free
radical theory as a major cause of greying. Indeed, oxidative damage is common to the above processes. Melanogenesis generates high oxidative stress via the hydroxylation of tyrosine
November 2024 PERSONAL CARE
Hairshaft Arrector muscle
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