Macromelanosome

Abnormal melanosome maturation; fusion of melanosomal structures; intracellular melanosome transport; organelle homeostasis; neurofibromatosis type 1; neurofibromin; melanosome biogenesis;

A histological term for (pathological) 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.

Morphology:

• Size: often about 1–5 µm, compared to normal melanosomes, which are about 0.3–0.7 µm.
• Shape: roundish to oval, occasionally irregularly shaped.
• Under a light microscope: strikingly large, dark brown to black pigment granules.
• Electron microscopy: large, mostly membrane-bound, highly electron-dense melanosomal structures.
• Location: particularly in basal and suprabasal keratinocytes, as well as in melanocytes.

Neurofibromatosis Type 1 (NF1): Classically found in café-au-lait spots; a historically significant ultrastructural feature

Café-au-lait spots of other etiologies: Also detectable; therefore not specific to NF1

Albinism, particularly oculocutaneous albinism type 2: Abnormally large pigment granules have been described in certain variants

Pigment mosaics and segmental hyperpigmentation: Occasionally detected in hyperpigmented skin areas

Becker’s melanosis and other melanocytic hyperpigmentations: Macromelanosomes have been reported in isolated cases but are not a characteristic diagnostic feature

The mechanism underlying the formation of macromelanosomes is not fully understood. The following are under discussion:

• Abnormal melanosome maturation: disruption of the normal development and size regulation of melanosomal organelles.
• Fusion of melanosomal structures: Formation of oversized pigmented organelles.
• Disruptions in intracellular melanosome transport and degradation: Changes in pigment distribution and organelle homeostasis.
• In neurofibromatosis type 1, a disruption of the intracellular signaling pathways regulated by neurofibromin is discussed as a possible factor influencing melanosome biogenesis.

Macromelanosomes should not be confused with melanophages. Melanophages are melanin-laden macrophages, whereas macromelanosomes are intracellular pigment organelles.

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.
2. Shendrik I et al. (2025) Acral Melanosis with Macromelanosomes (AMM): The First AI-Defined Pathological Entity and Proposal of Provisional Diagnostic Criteria. Cureus 17:e97276.