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Schematic diagram of erythrocyte development and maturation in the bone marrow (Meyer DJ and Harvey JW, Veterinary Laboratory Medicine, Interpretation and Diagnosis, 2nd Edition, Philadelphia, WB Saunders, 1998)

Normal erythropoiesis involves several stages of red blood cell maturation. Progenitor stem cells differentiate into rubriblasts that are large cells with a large, round nucleus; one to two prominent nucleoli; and a high nucleus to cytoplasm ratio. As rubriblasts continue to develop, their nucleus becomes more pyknotic and eventually is extruded[1].

Bone marrow aspirate demonstrating erythropoiesis. Rubriblast (arrow), prorubricytes (arrowhead), and various stages of rubricytes (nucleated erythroid precursors) are present. (Dog, bone marrow, Wright-Leishman stain).

One of the normal stages of erythroid development is the metarubricyte. Frequently referred to as a nucleated red blood cell (nRBC), these immature erythrocytes are the last developmental stage to contain a defined, intact nucleus and immediately precede the appearance of polychromatophilic erythrocytes (reticulocytes). Metarubricytes are approximately the size of a small mature lymphocyte, just a little larger than a mature red blood cell. The pyknotic nucleus is small, condensed and resembles an ink dot when viewed microscopically. Nucleoli are not usually visible and there are few if any light to clear spaces apparent in the nucleus. The cell contains a moderate amount of cytoplasm which can appear anywhere from a medium blue to orange with Wright’s stain.

Metarubricyte (top left) and polychromatophilic erythrocyte (arrow) in regenerative anemia. A monocyte (top right) also is present (Dog, blood, Wright-Leishman stain).

Appropriate Metarubricytosis

In a healthy animal that is not anemic, metarubricytes are a rare occurrence (usually less than one metarubricyte per one hundred leukocytes. Metarubricytosis is not necessarily an abnormal finding, especially in diseases or conditions resulting in intensely regenerative anemias and hypoxia. In such instances, the appearance of nucleated red blood cells should only be mildly elevated and should accompany a marked reticulocytosis, indicating that the bone marrow is reponding to a sudden increased demand for erythrocyte production. Additional situations in which metarubricytosis may be expected include acute splenic trauma, post splenectomy, bone fractures, or following acute blood loss. Intense excitement, particularly in domestic cats, can result in splenic contraction. Since the spleen is a site of maturation of erythrocytes, metarubricytes may be released into the circulation following marked splenic contraction. This hematologic finding, however, should be transient and numbers of metarubricytes should not be extremely high.

Table 1. Physiologic conditions that may consistently accompany mild metarubricytosis.

Markedly regenerative anemia
Acute splenic trauma
Post spelenectomy
Marked splenic contraction (excitement, especially in cats)
Acute blood loss with sudden hypoxia
Bone fractures (with release of nRBC from hematopoietic sites)

Inappropriate Metarubricytosis

Metarubricytosis in the absence of significant polychromasia is an inappropriate hematologic response and may indicate a guarded prognosis for the patient.

Table 2. Differential diagnoses for metarubricytosis in the absence of polychromasia.

Lead toxicosis
Myeloproliferative diseases
Hypoxia and bone marrow necrosis
Extramedullary hematopoiesis
Erythremic myelosis
Familial macrocytosis and dyshematopoiesis in Poodles
Neoplasia metastatic to the bone marrow cavity
Sepsis or endotoxemia Feline leukemia virus infection in cats
Neoplasia, especially hemangiosarcoma

Lead Toxicosis

Lead toxicosis is seen more often in the spring and early summer. Frequently, the onset of clinical signs is more insidious as lead poisoning usually involves repeated ingestion over a period of time before toxicosis is apparent. A lead concentration of greater than 0.4 parts per million is considered toxic. Lead toxicosis is a polysystemic disease, which can induce nonregenerative anemia, proteinuria, neurologic signs, and gastrointestinal disturbances. Neurotoxic effects usually bringing about behavioral changes such as depression, seizures, and loss of vision. Gastrointestinal signs of lead toxicosis may include anorexia, vomiting, and colic.

The hematopoietic toxicity occurs as a result of enzymatic inhibition and damage to the barrier between the blood and bone marrow. Increased lead concentrations will directly inhibit certain enzymes including heme synthetase, aminolevulinic acid dehydrogenase, ferrochelatase, 5’ nucleotidase, and coproporphyrinogenase. Enzymatic inhibition results in an increase in the concentration of heme precursors (protoporphyrins), metarubricytosis, basophilic stippling (particularly in dogs), and erythrocytes that are more susceptible to physical damage. Damage to the blood-bone marrow barrier allows more metarubricytes to escape than would otherwise be released.

Metarubricytosis and slight basophilic stippling of erythrocytes in the blood of a dog with lead toxicosis. The remaining erythrocytes appear hypochromic because lead inhibits the synthesis of hemoglobin

Myeloproliferative Diseases

Neoplasia of the hematopoietic system may involve one or several cell lines. Lymphoproliferative disorders tend to be seen more frequently than myeloproliferative disorders. In myeloid leukemias, there are abnormal rates of growth and changes in the cell morphologies. The growth rate of normal, healthy cells in the bone marrow is often decreased as the abnormal cells proliferate, resulting in myelophthisis. This often results in a nonregenerative anemia because less marrow space is available for normal hematopoiesis. As myelophthisis progresses, an increased number of immature cells are released into circulation, resulting in peripheral metarubricytosis without signs of regeneration (polychromasia).

Hypoxia and Bone Marrow Necrosis

Bone marrow is dependent upon oxygen for energy for cellular maintenance and production. Any condition or disease that causes a decrease in the oxygen supply to the bone marrow can result in damage that may lead to release of immature red blood cells into the circulation. When bone marrow does not receive adequate oxygen, the microenvironment of various cells and vessels can be damaged. This subsequently allows elements not normally seen outside of marrow in any significant concentration (for example, evidence of immature erythroid cells) to escape through sinusoidal epithelium that is no longer intact. These cells can then be detected in the peripheral blood.

Erythremic myelosis with erythroblast, metarubricytes, and megaloblastoid change (arrow) in the bone marrow of a cat (Cat, bone marrow, Wright-Leishman stain).
Erythroleukemia with a mixture of erythroblasts (dark blue cytoplasm) and myeloblasts (light blue cytoplasm) (Cat, bone marrow, Wright-Leishman stain).

If severe hypoxia occurs, there can be secondary bone marrow necrosis. Bone marrow necrosis has also been observed with various drugs (at toxic levels), disseminated intravascular coagulation (DIC), sepsis and viral infections such as feline panleukopenia. Myelofibrosis is a frequent sequella of bone marrow necrosis. It can also occur after an episode of intense inflammation (myelitis) and, in some instances, may be part of a myeloproliferative disease process.

Extramedullary Hematopoiesis

In embryonic tissues, hematopoiesis begins in the yolk sac and then occurs in the spleen, liver, thymus and lymph nodes. By the last third of gestation, hematopoiesis of cell lines other than lymphocytes occurs in the bone marrow. In the adult, extramedullary hematopoiesis (EMH) can be stimulated by an overwhelming demand for blood cells or a loss of medullary tissue due to infiltrative disease (myelophthisis). Common organs for EMH include spleen, liver, and lung. Extramedullary hematopoiesis often is seen in chronic forms of anemia and blood dyscrasias such as feline leukemia. If the body experiences hypoxia or hypercapnea, extramedullary hematopoiesis may be stimulated to compensate for these blood gas abnormalities.

Erythremic Myelosis and Erythroleukemia

Erythremic myelosis is a myeloproliferative disorder that selectively involves only the erythroid cell line, while erythroleukemia involves both the myeloid and erythroid lineages. According to the updated World Health Organization classification, they are each a subtype of acute erythroid leukemia, that corresponds to the French-American-British (FAB) category acute myelogenous leukemia M6. It is a condition that primarily affects cats infected with feline leukemia virus subgroup C, a mutation of the infective form of the virus. While the pathogenesis of erythremic myelosis has yet to be determined, differentiation of the erythroid cell line apparently arrests at the metarubricyte stage of development.

Affected cats may present with clinical signs of severe anemia, their hematocrit values often decreased to 12-15%. A complete blood count will reveal a distinct absence of reticulocytes and a marked metarubricytosis. In some cases, peripheral blood smears also have, earlier erythrocyte precursors. The bone marrow is variably affected, but typically has a noticeable erythroid predominance with a variable number of blasts.

If a cat presents with a severe anemia, testing for feline leukemia virus would be warranted. As lymphoma associated with feline leukemia virus can also cause anemia as a result of bone marrow infiltration, biopsies of bone marrow may be necessary to distinguish between it and erythremic myelosis. At this time, there is no cure for erythremic myelosis or erythroleukemia and the long-term prognosis is grave.


When performing diagnostic tests, the presence of nucleated red blood cells on a complete blood count is not necessarily cause for concern. Metarubricytosis in conjunction with polychromasia indicates a regenerative anemia and provides evidence that the bone marrow is responding to the demand for erythrocyte production. However, metarubricytosis in the absence of polychromasia is an abnormal hematologic finding that warrants further investigation.


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