Infantile mesenchymal hamartoma of the liver. Histologic and ultrastructural observations

Mesenchymal hamartoma of the liver is a benign lesion that appears as a large abdominal mass in the first 2 yr of life. This study describes the gross and histologic features of such a tumor in a 14 mth old girl which was removed by right hepatolobectomy. Ultrastructurally, the hamartoma was composed of well differentiated ductal structures surrounded by loose mesenchyme containing active appearing fibroblasts. Some suggestions as regards the histopathogenesis are offered. There is little morphologic evidence to suggest that these tumors are derived or related to blood vessels or lymphatics. The most appealing hypothesis is that this lesion arises from the connective tissue of the portal tracts at some point in their development. Bile ducts are, therefore, not at all unexpected points of origin if this theory is correct. The extent of connective tissue proliferation and location in the liver would explain the differences that exist in the gross appearance and presentation of the tumor. A similar type of hypothesis has been suggested for the vascular tumors of the liver in infancy and childhood, where the blood vessels of the portal tract with accompanying bile ducts proliferate and eventually coalesce to form a solitary mass or multiple nodules. The islands of hepatocytes are there by entrapment and probably do not represent an integral part of the hamartomatous process. An alternative theory is that the hepatocytes are there by an aberrant inductive process between the derivatives of the primitive hepatic diverticulum as it extends into the septum transversum. In this way, the hepatocytes would represent integral elements of the mesenchymal hamartoma.