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Background: Progressive mucinous histiocytosis (PMH) is a benign, non-Langerhans cell histiocytosis with characteristic ultrastructural features that can be used for diagnosis. Once an important tool in dermatologic diagnosis, electron microscopy has been largely replaced by immunohistochemistry and immunofluorescence techniques today. However, electron microscopy occasionally still plays a crucial role in the diagnosis of dermatologic conditions. We report a case of PMH as an example of a dermatologic disorder that requires electron microscopy for its diagnosis. Methods: A 60-year-old woman presented to our clinic with a history of small, sharply demarcated, skin-colored papules ranging from 2 to 5 mm in diameter distributed over the arms, forearms, and dorsal hands. The results of light microscopy, immunohistochemical studies, and clinical examination were inconclusive. Another biopsy for electron microscopy showed the characteristic features of PMH. Conclusion: This case demonstrates that a dermatopathology service still needs to have access to electron microscopy for diagnostic purposes to successfully diagnose a small number of rare conditions. Antecedents: L'histiocytose mucineuse progressive (HMP) est une histiocytose benigne non langerhansienne possedant des caracteristiques ultrastructurelles particulieres qui facilitent le diagnostic. La microscopie electronique, aujourd'hui largement remplacee par l'immunocytochimie et l'immunofluorescence, etait un important outil diagnostique en dermatologie. Cette technique continue a jouer occasionnellement un role important dans le diagnostic des affections dermatologiques. Nous rapportons un cas d'histiocytose mucineuse progressive pour illustrer l'usage de la microscopie electronique dans le diagnostic. Methodes: Une femme âgee de 60 ans, qui s'est presentee a notre clinique, avait de petites papules, nettement demarquees, qui ont la meme couleur que la peau et qui mesurent entre 2 et 5 mm de diametre, distribuees sur le bras, l'avant-bras, et le dos de la main. Les resultats d'une microscopie photonique, des etudes immunohistochimiques, et de l'examen clinique n'etaient pas probants. Une autre biopsie envoyee a la microscopie electronique a revele les caracteristiques d'une HMP. Conclusion: Le present cas prouve que les services de dermatopathologie ont toujours besoin d'avoir acces a la microscopie electronique pour poser les bons diagnostics en presence d'un petit nombre d'affections rares.

Progressive mucinous histiocytosis is a rare, benign, non-Langerhans' cell histiocytosis limited to the skin. Ten cases – all women – in four families and one sporadic case have been described in the literature. The disorder usually begins in childhood and progresses slowly. We report two sporadic cases of adult-onset progressive mucinous histiocytosis in unrelated African-American women, aged 48 and 55 years, respectively, who developed red-brown and flesh-coloured, asymptomatic papules on the face, the arms and the legs without truncal, mucosal or visceral involvement. The lesions showed no spontaneous regression. Both patients lacked associated systemic symptoms, including polyuria, polydipsia or seizures. There was no underlying hyperlipidaemia, paraproteinaemia or lymphoproliferative disease. No family history of similar lesions could be identified. Light microscopy revealed dermal proliferation of spindle-shaped histiocytes with abundant mucin deposition. Electron microscopy demonstrated a high number of myelin figures or zebra bodies in the cytoplasm of histiocytes. On immunohistochemistry, positive staining with macrophage markers – CD68, HAM56 and lysozyme – and factor XIIIa, a transglutaminase present in dermal dendrocytes, and negative staining with Langerhans' cell markers – CD1a and S100 – and CD34, a marker present in dermal dendritic cells derived from uncommitted mesenchymal cells, were observed.

Progressive mucinous histiocytosis is a very rare, benign, non-Langerhans' cell histiocytosis limited to the skin. In total ten patients (all women) in four families and three sporadic cases have been reported. We report here the first published case of a male patient with progressive mucinous histiocytosis. The multiple red papules on the scalp and forearms were asymptomatic and had slowly increased over approximately the past 20 years. The patient's mother had similar lesions. Histological examination revealed nodules in the dermis with histiocytes and mucin deposition. The histiocytes stained positively with CD31 and negative with CD34, CAM 5.2, PGM-1 and factor XIIIa. Ultrastructurally, the histiocytes showed numerous circular myelin bodies and zebra bodies reminiscent of those seen in lysosomal storage diseases. The genetic transmission of hereditary progressive mucinous histiocytosis remains unclear; we assume an autosomal dominant transmission with some hormonal factor that makes hereditary progressive mucinous histiocytosis more likely in women.

Hereditary progressive mucinous histiocytosis is a rare autosomal dominant non-Langerhans cell histiocytosis. We describe a sporadic case of this syndrome in a 64-year-old woman who had multiple dark-red dome-shaped papulonodules located mainly on the back of her hands, forearms and thighs. Light microscopy revealed a circumscribed upper dermal aggregate of ovoid or spindle-shaped histiocytes with abundant mucin deposition. Iron deposits and numerous mast cells were scattered throughout the tumour but giant cells were rare. Electron microscopy revealed a high number of zebra bodies and myeloid bodies in the cytoplasm of the histiocytes. Immunohistochemistry showed positive labelling with alpha-1 antitrypsin, Factor XIIIa and CD68, while CD1a, CD34 and S100 protein were negative. The differential diagnosis of histiocytic syndromes is discussed.

Progressive mucinous histiocytosis is a rare, benign, non-Langerhans cell histiocytosis limited to the skin. A total of ten cases, all women, in four families, and one sporadic case have been described in the English literature. The disorder usually begins in childhood and progresses slowly. We report two sporadic cases of adult-onset progressive mucinous histiocytosis in unrelated African American women, age 48 and 55 years old respectively, who developed red-brown and flesh-colored, asymptomatic papules on the face, arms and legs without truncal, mucosal or visceral involvement. The lesions show no spontaneous regression. Both patients lack associated systemic symptoms including polyuria, polydipsia or seizures. There is no underlying hyperlipidemia, paraproteinemia or lymphoproliferative disease. No family history of similar lesions can be identified. Light microscopy reveals dermal proliferation of spindle-shaped histiocytes with abundant mucin deposition. Electron microscopy demonstrates a high number of myelin-figures or zebra bodies in the cytoplasm of histiocytes. On immunohistochemistry, positive staining with macrophage markers CD68, HAM56 and lysozyme, and Factor XIIIa, a transglutaminase present in dermal dendrocytes, and negative staining with Langerhans cell markers CD1a and S100, and CD34, a marker present in dermal dendritic cells derived from uncommitted mesenchymal cells, are observed.