The most immature erythrocyte stage in normal maturation is called the rubriblast.

• A. Rubricyte
• B. Reticulocyte
• C. Rubriblast
• D. Metarubricyte

Answer: (C)

In erythropoiesis, the earliest recognizable precursor in the bone marrow is the rubriblast. This cell is the most immature stage because it is large, with a big nucleus and finely dispersed chromatin, and it contains abundant RNA that gives a deep basophilic (blue) cytoplasm. As maturation proceeds, the nucleus condenses and the cytoplasm changes as hemoglobin begins to be produced, moving the cell through the stages rubricyte, metarubricyte, and ultimately to the reticulocyte before becoming a mature erythrocyte. The rubricyte is a later stage with more condensed chromatin and less RNA than the rubriblast, the metarubricyte is even later and closer to enucleation, and the reticulocyte is released into circulation after the nucleus has been extruded. Therefore, the rubriblast is the most immature erythrocyte stage in normal maturation.