Metarubricyte: most mature form of nucleated red blood cell (nRBC). Nuclei are small and pyknotic, often eccentric, with condensed (smooth dark) chromatin. Cytoplasm is smooth and uniform and may stain red or purple. Basophilic stippling may be present.

Rubricyte: less mature forms of nRBC. Nuclei are larger and centred with chromatin more clumped than metarubricytes. Cytoplasm may stain purple or dark blue.

Look-alike: nRBC (particularly rubricytes) may resemble lymphocytes. nRBC can be distinguished from lymphocytes based on nuclear chromatin (nRBC have small pyknotic nuclei with condensed or clumped chromatin that stains dark, while lymphocytes have larger nuclei with non-condensed heterogenous chromatin that stains lighter) and cytoplasm (nRBC have smooth uniform cytoplasm staining red to purple to dark blue with ruffled or irregular borders, while lymphocytes have slightly grainy or heterogenous cytoplasm that stains dark purple to blue with smooth round borders). See images at top of page for comparison of nRBC versus lymphocytes.

Clinical relevance: nRBC are most commonly seen in dogs, cats, and camelids with strongly regenerative anemias. Less commonly associated with other conditions such as lead poisoning, abnormal splenic function, bone marrow injury. nRBC are rare in cattle and horses, even with strongly regenerative anemias.