Erythrocytes are Produced in Thick Membrane-Like Structures in Adult Mouse Blood

Erythrocyte-restricted progenitors are believed to have self-renewal capabilities. However, a recent study did not detect daughter pairs of erythrocyte progenitors, which questions the self-renewal ability of erythrocyte-restricted progenitors. In adult mouse blood, we identified a group of dense-eosin-stained thick membrane-like cellular structures that were from 30 to 400 μm in diameter and called them “erythrocyte sacs”. Erythropoiesis was initiated inside these sacs and terminated when the sac membrane broke to release hemoglobin+ and CD34+ functional erythrocytes. Numerous erythrocytes were produced, enucleated and released from one sac, which explains why the paired daughter cells were not detectable, and why the number of erythrocytes is about 1000 times higher than the sum of other blood cells. After erythrocytes were released, the thick sac membrane became fiber-like and degraded. Our data provide evidence that erythropoiesis occurs inside these erythrocyte sacs. These sacs do not renew and they produce only erythrocytes and no other types of blood cells.