What is the most immature erythrocyte that can be identified in bone marrow?

• A. Prorubricyte
• B. Erythrocytoblast
• C. Multipotent stem cell
• D. Rubriblast

Answer: (D)

In bone marrow, the earliest identifiable erythroid precursor is the rubriblast, also called the pronormoblast. This cell is large with a high nucleus-to-cytoplasm ratio, a round nucleus that may show nucleoli, and a cytoplasm that stains very basophilic due to abundant RNA. These features reflect an early, committed erythroid stage primed to synthesize hemoglobin.

As erythropoiesis progresses, the next stage is prorubricyte (basophilic normoblast), which is smaller, with a more condensed nucleus and beginning hemoglobin production causing a shift in cytoplasmic staining. The multipotent stem cell sits earlier in hematopoiesis and isn’t specifically an erythroid precursor you’d identify morphologically as an erythrocyte lineage cell in marrow. Erythrocytoblasts can refer to erythroid precursors in general, but rubriblast is the earliest distinct erythroid precursor that can be identified in bone marrow.