metarubricyte

Metarubricyte, also known as the orthochromatic erythroblast, is the final nucleated stage of erythroblast development in erythropoiesis. It forms in the bone marrow as erythroid precursors mature and hemoglobin synthesis advances. During this stage the cytoplasm becomes more eosinophilic as hemoglobin content increases, and the nucleus becomes highly condensed (pyknotic).

Morphologically, a metarubricyte is relatively small and exhibits a dark, condensed nucleus with a predominantly pink to reddish cytoplasm. Nuclear extrusion then occurs, yielding a reticulocyte, which enters the peripheral blood. Reticulocytes mature into erythrocytes within a day or two.

In normal physiology, metarubricytes are largely confined to the bone marrow, and only very rarely appear in circulating blood. The presence of nucleated erythroid cells, including metarubricytes, in peripheral blood can indicate stress erythropoiesis or marrow pathology, such as severe anemia, marrow infiltration, hemolysis, or megaloblastic processes. Their detection can be clinically significant, reflecting an accelerated or disrupted erythroid maturation pathway.

Metarubricyte is part of standard hematology terminology and is typically identified on Wright-Giemsa–stained bone marrow smears or peripheral blood smears as the last nucleated erythroblast before nucleus extrusion and reticulocyte formation.