Melanocyte
Epidermis · Basal layer Neural crest 1 per 10 basal cells
Neural-crest-derived dendritic cell resident in the stratum basale that synthesises melanin in melanosomes and transfers them along its dendrites to surrounding keratinocytes, where the pigment caps nuclei and absorbs ultraviolet radiation.
Detail
- Density
- About one melanocyte per ten basal keratinocytes; each supplies 30-40 keratinocytes as an epidermal melanin unit
- Skin colour
- Melanocyte number is the same across all skin colours; difference lies in melanosome size, number, dispersion and rate of degradation, and in the eumelanin to phaeomelanin ratio
- Synthesis
- Tyrosinase oxidises tyrosine to DOPA and then dopaquinone, the committed step to eumelanin (brown-black, photoprotective) or phaeomelanin (red-yellow, poorly protective)
- Control
- Alpha-MSH acting on MC1R raises eumelanin; loss-of-function MC1R variants give red hair, poor tanning and high melanoma risk
When it goes wrong
Autoimmune destruction of melanocytes (vitiligo)
Sharply demarcated depigmented macules that Wood's lamp accentuates; associated with autoimmune thyroid disease, pernicious anaemia and type 1 diabetes, so screen for them
Tyrosinase deficiency (oculocutaneous albinism type 1)
Normal melanocyte numbers but no pigment: nystagmus, reduced visual acuity, foveal hypoplasia and very high skin cancer risk from childhood
Malignant transformation of melanocytes (melanoma)
Breslow thickness in millimetres is the single strongest prognostic factor and drives excision margins and sentinel node biopsy; ulceration and mitotic rate add to it
High ACTH driving MC1R (Addison disease, ectopic ACTH)
Generalised hyperpigmentation, worst in palmar creases, buccal mucosa and scars
Practise this structure
16 questions in the bank tagged Skin.
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