Familial gigantic melanocytosis.

Background: Familial gigantic melanocytosis (FGM) is a rare disorder first described in 1984 and termed "familial melanopathy with gigantic melanocytes". The cause of the disorder is still unknown. Melanocytes in both hyper- and hypopigmented skin seem to be unable to deliver melanin to the surrounding keratinocytes.

Objective: In this study, we report four new cases of FGM. Electron microscopic examination was performed in a trial to shed more light on the underlying defect in this disorder.

Patients And Methods: Patients were examined clinically and biopsies were taken from both hyperpigmented and hypopigmented areas, and divided into two parts; one part was processed for routine microscopic examination with hematoxylin and eosin and Masson Fontana stains. The other portion of the biopsy was fixed in glutraldhyde 3% and processed for electron microscopic (EM) examination.

Results: By light microscopy, the patients' skin showed areas of hyperpigmented basal cells alternating with poorly pigmented areas. Hair follicles in the scalp biopsies showed the same pathology. By EM, pigmented areas showed gigantic melanocytes and heavily pigmented keratinocytes. Nonpigmented areas showed poorly pigmented keratinocytes and fewer, but also gigantic melanocytes.

Conclusions: The raindrop-like hypopigmentation in this disorder can be explained by a failure of melanocytes to deliver melanin to their surrounding keratinocytes. The cause of the presence of heavily pigmented keratinocytes in the hyperpigmented zones could not be determined. There is a strong possibility of a more widespread abnormality affecting not just the melanocytes.