Question

Largest cell of erythrocytic series is

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

Hint:

During erythropoiesis, three main trends occur:
1. Cell size decreases (Rubriblast is the largest, mature RBC is the smallest).
2. Nucleus becomes smaller and eventually gets extruded.
3. Cytoplasm shifts from deeply basophilic (blue) to acidophilic (pink/red) as hemoglobin accumulates.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Erythropoiesis is the developmental process through which pluripotent hematopoietic stem cells differentiate into mature red blood cells (erythrocytes).
As cells progress through the erythrocytic series, they undergo progressive reduction in cell volume, condensation of chromatin, and eventual extrusion of the nucleus.

Step 2: Detailed Explanation:
Let us review the sequential stages of the erythrocytic series from the earliest precursor to the mature cell, focusing on cell size:
1. Rubriblast (Pronormoblast):
This is the earliest morphologically recognizable precursor in erythropoiesis.
It is a large cell, measuring approximately $14$ to $19\ \mu\text{m}$ in diameter, with a large, round nucleus containing fine chromatin and distinct nucleoli, and a deeply basophilic cytoplasm.
Therefore, it is the largest cell in this lineage.
2. Prorubricyte (Basophilic Normoblast):
Slightly smaller than the rubriblast ($12$ to $17\ \mu\text{m}$), with no visible nucleoli.
3. Rubricyte (Polychromatophilic Normoblast):
Even smaller ($10$ to $15\ \mu\text{m}$), characterized by a muddy-gray cytoplasm due to the accumulation of hemoglobin.
4. Metarubricyte (Orthochromic Normoblast):
Slightly smaller ($8$ to $12\ \mu\text{m}$) with a highly condensed, pyknotic nucleus.
5. Reticulocyte:
Anucleated immature erythrocyte ($7$ to $10\ \mu\text{m}$), slightly larger than the mature erythrocyte.
6. Erythrocyte:
The mature, biconcave disc-shaped cell, which is the smallest in this developmental line.
*Note regarding Megakaryocyte:* While a megakaryocyte is a massive bone marrow cell ($50$ to $150\ \mu\text{m}$), it belongs to the thrombocytic (platelet) lineage, not the erythrocytic series.

Step 3: Final Answer:
The largest cell of the erythrocytic series is the Rubriblast.