PROCESS OF HEMOPOIESIS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
Every day, our body replaces old or dead blood cells by producing new ones. Approximately, the following number of new cells are formed daily: 250 billion (25 × 10¹¹) new RBCs, 20 billion (20 × 10⁹) new WBCs, and 25 billion (25 × 10⁹) new platelets. The process of forming blood cells (RBCs, WBCs, and platelets) is known as Haemopoiesis or Haematopoiesis. The tissues that perform this function are called Haematopoietic Tissues.
Dpharmguru’s exam insights:
In my years of teaching haematology, I have observed that students often confuse the terms haemopoiesis, erythropoiesis, leucopoiesis, and thrombopoiesis. Remember: Haemopoiesis is the general term for blood cell formation. Erythropoiesis is RBC formation, leucopoiesis is WBC formation, and thrombopoiesis is platelet formation. This distinction is frequently tested in exams!
1. ERYTHROPOIESIS (FORMATION OF RED BLOOD CELLS)
The process of RBC formation is known as Erythropoiesis. This process mainly occurs in the red bone marrow and involves the following stages:
Steps of Erythropoiesis
- Stem Cell Stage: The process begins with pluripotent stem cells, which have the ability to form different types of blood cells. These divide to form multipotent myeloid progenitor cells.
- Formation of Colony-Forming Units: Under the influence of growth factors, these progenitor cells develop into erythrocyte colony-forming units (CFU-E).
- Action of Erythropoietin (EPO): The hormone erythropoietin, along with other growth factors, stimulates these CFU-E cells to produce proerythroblasts (also called pronormoblasts).
- Formation of Erythroblasts: The proerythroblasts undergo several cell divisions, forming smaller and more mature cells in sequence: Basophilic erythroblasts (early normoblasts), Polychromatic erythroblasts (intermediate normoblasts), Orthochromatic erythroblasts (late normoblasts).
- Formation of Reticulocytes: As the cells mature, their nuclei are lost, and they become reticulocytes.
- Maturation into RBCs: These reticulocytes are released into the blood circulation, where they mature into fully functional erythrocytes (RBCs) within 1–2 days.
Dpharmguru’s exam insights:
Erythropoietin (EPO) is a hormone produced mainly by the kidneys that stimulates RBC production. A common exam question is: “Which hormone stimulates erythropoiesis?” The answer is erythropoietin. Also, reticulocytes are immature RBCs—they are an important marker of bone marrow activity!
2. LEUCOPOIESIS (FORMATION OF WHITE BLOOD CELLS)
Leucopoiesis refers to the formation of white blood cells (WBCs). All WBCs develop from haematopoietic stem cells located in the bone marrow.
- Myeloblasts and Monoblasts arise from colony-forming cells and act as precursor cells for neutrophils and monocytes, respectively.
- Eosinophils and Basophils develop from eosinophil and basophil colony-forming cells.
- Lymphocytes also arise from bone marrow stem cells. Lymphoblasts are the immature cells that divide several times to form prolymphocytes, which later mature into lymphocytes.
Thus, all different types of WBCs — granulocytes, monocytes, and lymphocytes — are produced through the process of leucopoiesis.
Dpharmguru’s exam insights:
Remember: All blood cells originate from pluripotent stem cells in the bone marrow. Myeloblasts give rise to granulocytes, monoblasts give rise to monocytes, and lymphoblasts give rise to lymphocytes. This is a commonly tested concept in haematology exams!
3. THROMBOPOIESIS (FORMATION OF PLATELETS)
Thrombopoiesis is the process of platelet formation. It begins with large precursor cells called megakaryoblasts (15–20 µm in diameter).
Steps of Thrombopoiesis
- Megakaryoblast Formation: These cells originate from haematopoietic stem cells and mature into megakaryocytes, which are giant cells with large, irregular nuclei containing excess DNA (polyploid cells).
- Cell Structure: The cytoplasm of megakaryocytes contains many mitochondria, rough endoplasmic reticulum, and Golgi complex.
- Maturation: Megakaryocytes mature inside the bone marrow.
- Hormonal Control: The liver, kidneys, and skeletal muscles produce a hormone called Thrombopoietin (TPO). This hormone regulates megakaryocyte maturation and platelet production.
Dpharmguru’s exam insights:
Thrombopoietin (TPO) is the hormone that regulates platelet production. It is produced by the liver, kidneys, and skeletal muscles. A common exam question is: “Which hormone stimulates platelet formation?” The answer is thrombopoietin. Also, megakaryocytes are giant cells in the bone marrow that fragment to form platelets.
4. FORMATION OF HAEMOGLOBIN
Haemoglobin (Hb) is the red pigment found in RBCs that gives blood its colour and helps in oxygen transport. It is a conjugated protein, meaning it is made up of both a protein part (globin) and a non-protein part (heme).
- Heme (4%) – contains iron (Fe²⁺) and porphyrin ring
- Globin (96%) – a protein molecule
Haemoglobin is produced inside immature RBCs during erythropoiesis in the red bone marrow.
Normal Levels of Haemoglobin
| Group | Normal Hb Level (g/dL) |
|---|---|
| Adult males | 14 – 17 |
| Adult females | 12 – 15 |
| Newborn babies | 14.5 – 18.5 |
Haemoglobin levels can vary depending on age, sex, altitude, physical activity, and hormone levels (e.g., adrenaline).
Dpharmguru’s exam insights:
Normal haemoglobin levels are frequently tested in exams. Remember: Adult males: 14–17 g/dL, Adult females: 12–15 g/dL. A level below these ranges indicates anaemia. Also, newborn babies have higher Hb levels because of the need for oxygen transport in the womb.
Structure of Haemoglobin
Haemoglobin molecule is made up of two main parts:
- Heme: A cyclic tetrapyrrole structure made up of four pyrrole rings. Gives RBCs their red colour. Has attached side groups — methyl (M), vinyl (V), and propionate (Pr).
- Globin Chains: Adult haemoglobin (Hb-A) has four globin chains — two alpha (α₁ and α₂) chains and two beta (β₁ and β₂) chains. These form two dimers (α₁β₁ and α₂β₂).
Each haemoglobin molecule can bind four oxygen molecules — one to each heme group.
Dpharmguru’s exam insights:
Each haemoglobin molecule contains four heme groups and can carry four oxygen molecules. This is a frequently tested concept! Also, adult haemoglobin has two alpha and two beta chains. Foetal haemoglobin has two alpha and two gamma chains, which have a higher affinity for oxygen.
Synthesis of Haemoglobin
Haemoglobin is synthesised inside developing RBCs through the following steps:
- Condensation Reaction: In mitochondria, succinyl-CoA combines with glycine to form 5-aminolevulinic acid (ALA).
- Formation of Pyrrole: Succinyl-CoA and glycine react to form a pyrrole ring.
- Formation of Protoporphyrin IX: Four pyrrole rings combine to form protoporphyrin IX.
- Formation of Heme: Protoporphyrin IX binds with iron (Fe²⁺) to form heme.
- Formation of Globin: Ribosomes in immature RBCs synthesize the globin polypeptide chains.
- Assembly of Haemoglobin Chain: Each heme combines with a globin chain to form a haemoglobin subunit. Four such subunits combine loosely to form one haemoglobin molecule (molecular weight ≈ 64,000).
Dpharmguru’s exam insights:
Haemoglobin synthesis is a complex process that requires iron, vitamin B12, and folic acid. Deficiencies in any of these can lead to anaemia. In exams, remember that heme is synthesized in mitochondria, while globin is synthesized in ribosomes. This is a frequently tested distinction!
SUMMARY TABLE: BLOOD CELL FORMATION
| Process | Cell Formed | Key Hormone | Site |
|---|---|---|---|
| Erythropoiesis | Red Blood Cells (RBCs) | Erythropoietin (EPO) | Red Bone Marrow |
| Leucopoiesis | White Blood Cells (WBCs) | Various growth factors | Red Bone Marrow |
| Thrombopoiesis | Platelets | Thrombopoietin (TPO) | Bone Marrow |
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Hoffman, R., Benz, E. J., Silberstein, L. E., et al. (2018). Hematology: Basic Principles and Practice (7th ed.). Elsevier.
- Bain, B. J., Bates, I., & Laffan, M. A. (2017). Practical Haematology (12th ed.). Wiley-Blackwell.
- National Institutes of Health (NIH). (2022). Blood and Haematology Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
