Melanosome

Cell organelles in melanocytes that are rich in tyrosinase and originate from the vesicles of the Golgi apparatus; they develop into melanin granules. They contain melanin, which is synthesized here from tyrosine via dopa by the action of tyrosinase.

Melanosomes are transported from the center to the tip of the melanocyte dendrites via bidirectional transport. This occurs along microtubules via the motor proteins kinesin (anterograde) and dynein (retrograde). They are anchored to the actin filament at the tip by binding to the myosin-5a (Myo5a) protein via the linker proteins Rab 27a and melanophilin (Mlph).

If any component of the three-part complex (MyoVa, Rab 27a, Mlph) is defective, melanosome transport is impaired, leading to a perinuclear accumulation of melanosomes, which manifests as hypopigmentation of the skin and silvery-gray hair.

In all three types of Griscelli syndrome, there are defects in one of these transport proteins, resulting in silvery-gray hair, pale skin with a persistent tendency to tan when exposed to sunlight, and eye changes due to reduced pigmentation. 

Macromelanosomes are abnormally large, melanin-rich pigment organelles found in melanocytes and keratinocytes. They result from disturbances in melanosome biogenesis, particularly from abnormal enlargement or fusion of melanosomal structures. Macromelanosomes have traditionally been described in café-au-lait spots associated with neurofibromatosis type 1, but they also occur in other pigmentation disorders (Jimbow K et al. 1982). 

1. Jimbow K et al. (1982) The nature and significance of macromelanosomes in pigmented skin lesions: their morphological characteristics, specificity of occurrence, and possible mechanisms of formation. Am J Dermatopathol 4:413-420.