Erythropoiesis in mouse omental milky spots induced by erythropoietin: light and electron microscopic study.

The mouse omentum contains omental milky spots. They are abundantly present particularly in the omental fat band. In normal mice, the milky spots are composed of abundant lymphocytes/plasma cells with macrophages, granulocytes and various stromal cells. Unlike the lymph node, they show occasional neutrophilic myelopoiesis, though neither erythropoiesis nor megakaryopoiesis is present. We investigated the haematopoietic ability of the milky spots in ddY mice by administering intraperitoneal injections of erythropoietin (EPO), 500 units/body/day for 7 consecutive days. The omental fat bands were removed the day after the last EPO injection, and the milky spots were examined by light and electron microscopy. Small clusters of erythroblasts appeared in the milky spots in mice injected with EPO. In these clusters, erythroblasts in various maturation stages, dividing erythroblasts, denucleating erythroblasts and reticulocytes were seen by electron microscopy. These findings suggest that such clusters represent erythropoietic foci. The presence of erythropoietic foci in the milky spots was confirmed in all the mice injected with EPO. Megakaryocytes did not appear in the milky spots. These findings suggest that the milky spots have a latent erythropoietic ability, as well as active neutrophilic myelopoiesis.