Mesenchymal hamartoma of the liver; Clinicopathological, immunohistochemical and flow cytometric study of two cases.

Mesenchymal hamartoma (MH) is a rare liver lesion of infancy. Due to its rapid increase in size, it is often misdiagnosed clinically as a malignant tumour or as a hepatic cyst because of its cystic appearance. We present the clinicopathological, immunohistochemical and flow cytometric features of two cases, involving an 11-month-old boy and a 13-month-old girl. In both cases, the histological appearance and the immunohistochemical findings were identical. Bile ducts and blood vessels showed the expected immunohistochemical profile, whereas the mesenchymal component showed immunoreactivity not only for vimentin but also for muscular markers. Flow cytometry disclosed an aneuploid population in one case, thus favouring the interpretation of MH as a neoplastic lesion. Unlike the characteristically continuous and rapid growth of MH before or shortly after birth, these two cases showed low proliferative and apoptotic indexes and a high immunohistochemical expression of bcl-2 protein. This prompted us to hypothesize that MH might undergo a brief initial proliferative phase, but the cells would later become 'immortalized' by bcl-2 overexpression.