melanosomes

Melanosomes are pigment-containing, membrane-bound organelles found in melanocytes, as well as in the retinal pigment epithelium. They synthesize, store, and traffic melanin—the pigment responsible for skin, hair, and eye color—and contribute to protection against ultraviolet radiation by absorbing and dissipating UV light.

Melanosome formation and maturation occur within the endosomal system and proceed through four maturation stages. Stage I melanosomes are early, with minimal melanin; Stage II features a developing internal matrix; Stage III shows substantial melanin deposition; Stage IV contains densely pigmented, fully mature melanosomes. Melanin synthesis within melanosomes relies on enzymes such as tyrosinase, TYRP1, and DCT (DOPAchrome tautomerase). The type and amount of melanin produced determine color variation, with eumelanin giving brown-to-black tones and pheomelanin contributing red-to-yellow hues.

In skin and hair, melanosomes are transported to the periphery of melanocytes along cytoskeletal tracks and are subsequently transferred to neighboring keratinocytes. This transfer helps position melanin over the nucleus of keratinocytes, providing a shield against DNA damage from UV exposure. In the eye, melanosomes in the retinal pigment epithelium absorb excess light and protect photoreceptors from photooxidative stress.

Clinical relevance includes disorders of melanosome biology. Albinism often results from defects in melanin-synthesis enzymes or their trafficking to melanosomes. Hermansky-Pudlak syndrome and related conditions involve defects in the biogenesis or function of lysosome-related organelles, including melanosomes, leading to hypopigmentation and other systemic symptoms. Melanosomes also serve as a model for studying organelle biogenesis and pigmentary variation.