Hibernoma-like brown fat in the bone marrow: report of a unique case.

We report on a bone-marrow biopsy of a 61-year-old female patient that was performed because of the clinical suspicion of a myeloproliferative disease. The trephine biopsy showed morphological features that were consistent with an essential thrombocythaemia (ET). The diagnosis of a myeloproliferative disease could be corroborated by demonstration of the V617F mutation of JAK2. Besides the histological features of ET, the marrow showed a peculiar infiltrate that consisted of multivacuolated cells that were immunohistochemically identified as brown adipose tissue with a hibernoma-like picture. To the best of our knowledge, this is the first report on brown adipose tissue in the bone marrow.