ERYTHROPOIESIS IN MOUSE OMENTAL MILKY SPOTS INDUCED BY ERYTHROPOIETIN- LIGHT AND ELECTRON-MICROSCOPIC STUDY

The mouse omentum contains omental milky spots. They are abundantly pr esent particularly in the omental fat band. In normal mice, the milky spots are composed of abundant lymphocytes/plasma cells with macrophag es, granulocytes and various stromal cells. Unlike the lymph node, the y show occasional neutrophilic myelopoiesis, though neither erythropoi esis nor megakaryopoiesis is present. We investigated the haematopoiet ic ability of the milky spots in ddY mice by administering intraperito neal injections of erythropoietin (EPO), 500 units/body/day for 7 cons ecutive days. The omental fat bands were removed the day after the las t EPO injection, and the milky spots were examined by light and electr on microscopy. Small clusters of erythroblasts appeared in the milky s pots in mice injected with EPO. In these clusters, erythroblasts in va rious maturation stages, dividing erythroblasts, denucleating erythrob lasts and reticulocytes were seen by electron microscopy. These findin gs suggest that such clusters represent erythropoietic foci. The prese nce of erythropoietic foci in the milky spots was confirmed in all the mice injected with EPO. Megakaryocytes did not appear in the milky sp ots. These findings suggest that the milky spots have a latent erythro poietic ability, as well as active neutrophilic myelopoiesis.