Erythroblast blood cell. Light micrograph of cells from a sample of bone marrow, including a polychromatic erythroblast (upper left). This one has multiple cell nuclei (dark red, four in total). Erythroblasts are the immediate precursors to mature red blood cells as they develop from stem cells in the bone marrow. There are four erythroblast stages, with this one being the third stage. Erythroblasts undergoing normal development are also called normoblasts.