Light and electron microscope study of splenoportal milky spots in New Zealand black mice: comparison between splenoportal milky spots and aberrant spleens.

The omentum contains peculiar lymphoid tissues termed omental milky spots. In mice, similar milky spots (splenoportal milky spots) are present in splenoportal fat bands developing along the splenic artery. We found that New Zealand Black (NZB) mice, which are known to develop spontaneous autoimmune diseases, have well developed splenoportal milky spots. However, little is known about these milky spots. Thus we investigated splenoportal fat bands in NZB mice by light and electron microscopy. Splenoportal fat bands contained sporadic aberrant spleens as well as abundant milky spots. In addition, transitional forms between splenoportal milky spots and aberrant spleens, although sporadic, were present in the fat bands. Splenoportal milky spots were supplied with offshoots from the splenic artery and were composed of abundant lymphocytes with macrophages, plasma cells, granulocytes, megakaryocytes and various stromal cells. In addition, they showed active neutrophilic myelopoiesis and probable megakaryopoiesis. Aberrant spleens were also supplied by branches from the splenic artery. They showed active granulopoiesis, megakaryopoiesis, and erythropoiesis. The transitional forms resembled splenoportal milky spots in structure, but the former showed extramedullary haematopoiesis of three cell lineages. The morphological transition from aberrant spleens, via transitional forms, to splenoportal milky spots seems to indicate that splenoportal milky spots represent splenoid lymphoid tissues.