melanosome

Melanosome

A melanosome is a membrane-bound organelle within pigment-producing cells, including melanocytes in the skin and hair follicles and retinal pigment epithelium in the eye. It is responsible for the synthesis, storage, and transport of melanin, the pigment that determines skin and hair color and protects tissues from UV damage. Melanosomes are lysosome-related organelles derived from the endosomal pathway and progress through four morphologically distinct stages (I to IV). Stage I are precursor organelles with an electron-dense vacuole; Stage II develop a fibrillar matrix; Stage III begin melanin deposition; Stage IV are heavily pigmented.

Enzymes such as tyrosinase (TYR), tyrosinase-related protein 1 (TYRP1) and DCT are housed in melanosomes and drive melanin synthesis from tyrosine. Melanin exists as two main forms, eumelanin and pheomelanin; the type and amount produced depend on melanogenic enzymes and substrate availability, influencing color and UV protection.

Transport: Melanosomes are generated near the nucleus and are moved along microtubules toward dendritic tips by motor proteins, guided by Rab27a and its effector melanophilin (SLAC2-a) and myosin-Va. Once near keratinocytes, melanosomes can be transferred to surrounding cells either by exocytosis or cytocrine transfer, contributing to skin and hair pigmentation.

Clinical relevance: Abnormal melanosome function underlies several human disorders. Oculocutaneous albinism results from defects in TYR or related genes, while Hermansky-Pudlak and Griscelli syndromes involve defects in melanosome biogenesis and trafficking.