Anucleated red blood cell (NRBC), also known byseveral other names, is ared blood cell that contains acell nucleus. Almost allvertebrate organisms havehemoglobin-containing cells in their blood, and with the exception ofmammals, all of these red blood cells are nucleated.[1] In mammals, NRBCs occur in normal development asprecursors to mature red blood cells inerythropoiesis, the process by which the body produces red blood cells.
NRBCs are normally found in thebone marrow of humans of all ages and in the blood offetuses andnewborn infants.[2][3] After infancy, RBCs normally contain a nucleus only during the very early stages of the cell's life, and the nucleus is ejected as a normal part ofcellular differentiation before the cell is released into the bloodstream. The presence of circulating NRBCs in adults occurs in situations of hematopoietic stress such as severe infection, massive hemorrhage, marrow infiltration, or extramedullary hematopoiesis.[4] That is, if NRBCs are identified on an adult'scomplete blood count orperipheral blood smear, it suggests that there is a very high demand for thebone marrow to produce RBCs, and immature RBCs are being released into circulation. Possiblepathologic causes includeanemia,myelofibrosis,thalassemia,miliary tuberculosis,cancers involving bone marrow (myelomas,leukemias,lymphomas), and in chronichypoxemia.[5]
Several names are used for nucleated RBCs—erythroblast,normoblast, andmegaloblast—with one minor variation inword sense.[6][7][8][9] The namenormoblast always refers to normal, healthy cells that are the immediate precursors of normal, healthy, mature (anucleate) RBCs. The namemegaloblast always refers to abnormally developed precursors. Often the nameerythroblast is usedsynonymously withnormoblast, but at other times it is considered ahypernym. In the lattersense, there are two types of erythroblasts: normoblasts as cells that develop as expected, and megaloblasts as unusually large erythroblasts that are associated with illness.
There are four stages in the normal development of a normoblast.
| Illustration | Description | Image |
|---|---|---|
 | Pronormoblast | |
 | Basophilic normoblast |  |
 | Polychromatic normoblast (alsopolychromatophilic) |  |
 | Orthochromatic normoblast (alsoorthochromatophilic) |  |
A megaloblast is an unusually large erythroblast that can be associated withvitamin B12 deficiency (caused bypernicious anemia ordietary insufficiency),folic acid deficiency, or both (suchanemias are collectively calledmegaloblastic anemias). This kind of anemia leads to macrocytes (abnormally large red cells) and the condition calledmacrocytosis. The cause of this cellular gigantism is an impairment inDNA replication that delays nuclear maturation andcell division. BecauseRNA andcytoplasmic elements are synthesized at a constant rate despite the cells' impaired DNA synthesis, the cells show nuclear-cytoplasmic asynchrony.
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