Abnormal erythrocyte morphology Circulating nucleated red blood cells 2020-11-28T19:50:48Z The presence of nucleated red blood cells in the peripheral blood circulation. Nucleated red blood cells (NRBCs) are early erythrocyte precursors not present in the peripheral blood of normal adults. Fenestrations in the bone marrow provide a physical filter to the release of the large NRBCs into the circulation, and the rare NRBC that escapes is rapidly cleared from peripheral blood by the spleen. The presence of circulating NRBCs in adults thus reflects extreme increases in erythropoietic activity or failure of the blood filtration mechanisms.