What is Erythropoiesis? Describe the various stages in the development of RBC. Mention the factors needed for erythropoiesis.

What is Erythropoiesis?

Erythropoiesis is the biological process by which red blood cells (RBCs, or erythrocytes) are produced in the body. It primarily occurs in the bone marrow of adults (and in the liver/spleen during fetal development). This process replaces old RBCs, which have a lifespan of about 120 days, with around 200-300 billion new RBCs produced daily in a healthy adult to maintain oxygen transport.

Stages in the Development of RBCs

RBC development starts from hematopoietic stem cells in the bone marrow and progresses through several stages, taking about 7 days to complete. Here's a concise overview of the key stages:

  1. Proerythroblast: The earliest committed cell; large with a high nucleus-to-cytoplasm ratio. It begins hemoglobin synthesis.

  2. Basophilic Erythroblast: Nucleus shrinks; cytoplasm stains blue due to ribosomes. Hemoglobin production ramps up.

  3. Polychromatophilic Erythroblast: Hemoglobin accumulation gives a mixed (polychromatic) stain. Cell size decreases.

  4. Orthochromatic Erythroblast (Normoblast): Nucleus is ejected; cell appears reddish due to high hemoglobin content. This stage represents about 10-20% of bone marrow erythroid cells in healthy individuals.

  5. Reticulocyte: Immature RBC released into blood; contains residual RNA (reticulum). It matures in 1-2 days, making up 0.5-1.5% of circulating RBCs normally.

  6. Mature Erythrocyte (RBC): Biconcave disc shape, no nucleus, fully hemoglobin-packed for oxygen carrying. Diameter: ~7-8 micrometers; total count in blood: 4.5-6 million per microliter in adults.

Factors Needed for Erythropoiesis

Several hormones, nutrients, and cellular factors are essential. Deficiencies can lead to anemia (e.g., low RBC count below 4 million/μL).

  • Hormones: Erythropoietin (EPO), produced by kidneys in response to low oxygen (hypoxia). Normal EPO levels: 5-25 mU/mL; it stimulates stem cell differentiation.

  • Nutrients: Iron (for hemoglobin; daily need: 1-2 mg), Vitamin B12 (for DNA synthesis; deficiency affects ~1-2% of older adults), Folic acid (for cell division; requirement: 400 μg/day).

  • Other Factors: Stem cell factor (SCF), interleukins (e.g., IL-3), and a healthy bone marrow environment. Hypoxia or blood loss can increase EPO by 100-1000 times to boost production.