7. Hemopoiesis: A Complete Guide to Blood Cell Formation and Haemoglobin Synthesis

PROCESS OF HEMOPOIESIS: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future healthcare professionals!

As a pharmacy educator with years of experience teaching human anatomy and physiology, I have always emphasized that hemopoiesis is one of the most remarkable processes in the human body. Every single day, your body produces 250 billion new red blood cells, 20 billion white blood cells, and 25 billion platelets—all from a single population of stem cells in your bone marrow. This continuous, precisely regulated process is essential for life.

In this comprehensive guide, I will take you on a journey through the process of hemopoiesis—the formation of blood cells. We will explore how erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets) are produced, how haemoglobin is synthesized, and what factors regulate these processes. By the end of this article, you will have a solid understanding of one of the most fascinating and clinically important topics in anatomy and physiology. Let us begin.

WHAT IS HEMOPOIESIS?

Hemopoiesis (also called haematopoiesis) is the process of forming blood cells—RBCs, WBCs, and platelets. The word comes from the Greek words “haima” (blood) and “poiesis” (to make).

Every day, the human body must replace approximately:

• 250 billion (25 × 10¹⁰) new RBCs
• 20 billion (20 × 10⁹) new WBCs
• 25 billion (25 × 10⁹) new platelets

These cells are produced in haematopoietic tissues, primarily in the bone marrow, but also in the spleen, thymus, and lymph nodes during certain stages of development or in response to disease.

THE HIERARCHY OF HEMOPOIESIS

Hemopoiesis follows a hierarchical pathway:

1. Pluripotent Stem Cells: The most primitive cells in the bone marrow. They can differentiate into any blood cell type.
2. Multipotent Progenitor Cells: More committed cells that can differentiate into a limited range of cell types.
3. Colony-Forming Units (CFUs): Committed progenitor cells that respond to specific growth factors.
4. Precursor Cells: Cells that are committed to becoming a specific cell type (e.g., proerythroblasts, myeloblasts).
5. Mature Cells: Fully differentiated blood cells (e.g., erythrocytes, neutrophils).

PART 1: ERYTHROPOIESIS (FORMATION OF RBCs)

Erythropoiesis is the process of red blood cell formation. It takes approximately 7-10 days from stem cell to mature RBC.

Steps of Erythropoiesis

1. Pluripotent Stem Cell Division: Pluripotent stem cells divide to yield multipotent myeloid progenitor cells.
2. Colony Formation: In the presence of growth factors, these cells develop into erythrocyte colony-forming cells (CFU-E).
3. Proerythroblast Formation: Erythropoietin (EPO)—a hormone produced by the kidneys—along with other growth factors promotes the production of proerythroblasts (also called pronormoblasts).
4. Series of Cell Divisions: Proerythroblasts give rise to smaller daughter cells through successive divisions:
   • Basophilic Erythroblasts (early normoblasts)
   • Polychromatic Erythroblasts (intermediate normoblasts)
   • Orthochromatic Erythroblasts (late normoblasts)
5. Reticulocyte Formation: Cells become reticulocytes as their nuclei are lost.
6. Maturation: Reticulocytes are released into circulation and mature into erythrocytes in 1-2 days.

Regulation of Erythropoiesis

Erythropoiesis is regulated by erythropoietin (EPO), a hormone produced by the kidneys (90%) and liver (10%). EPO is released in response to hypoxia (low oxygen levels) and stimulates the bone marrow to produce more RBCs.

Factor	Effect on Erythropoiesis
Erythropoietin (EPO)	Stimulates RBC production
Hypoxia (low O₂)	Triggers EPO release
Iron, Vitamin B₁₂, Folate	Essential for RBC maturation
Androgens (testosterone)	Stimulate EPO production

PART 2: LEUCOPOIESIS (FORMATION OF WBCs)

Leucopoiesis is the process of white blood cell formation. All different kinds of WBCs are derived from haematopoietic stem cells in the bone marrow.

Origin of Different WBCs

WBC Type	Precursor Cell	Colony-Forming Unit
Neutrophils	Myeloblasts	Granulocyte colony-forming cells
Monocytes	Monoblasts	Monocyte colony-forming cells
Eosinophils	Eosinophil precursors	Eosinophil colony-forming cells
Basophils	Basophil precursors	Basophil colony-forming cells
Lymphocytes	Lymphoblasts	Lymphocyte colony-forming cells

Lymphocyte Development

Lymphocytes develop from bone marrow stem cells in a distinct pathway:

1. Bone marrow stem cells → Lymphoblasts
2. Lymphoblasts → Prolymphocytes (through cell divisions)
3. Prolymphocytes → Mature Lymphocytes

T-lymphocytes mature in the thymus gland, while B-lymphocytes mature in the bone marrow.

PART 3: THROMBOPOIESIS (FORMATION OF PLATELETS)

Thrombopoiesis is the process of platelet formation. Platelets are produced from megakaryocytes in the bone marrow.

Mechanism of Thrombopoiesis

1. Megakaryoblasts (15-20µm diameter) are the precursor cells of platelets.
2. Megakaryoblasts mature into megakaryocytes—giant cells with large irregular nuclei. They are polyploid cells (contain multiple copies of DNA).
3. Megakaryocyte cytoplasm contains mitochondria, rough endoplasmic reticulum, and Golgi complex.
4. Thrombopoietin (TPO)—a hormone secreted by the liver, kidneys, and skeletal muscles—controls the maturation of megakaryocytes and platelet production.
5. Platelets are formed by the fragmentation of megakaryocyte cytoplasm and are released into the bloodstream.

PART 4: FORMATION OF HAEMOGLOBIN

Haemoglobin (Hb) is a respiratory pigment present in RBCs, giving them their red colour. It is a conjugated protein composed of:

• Heme (4%): Iron (Fe²⁺) + porphyrin
• Globin (96%): Protein component

Haemoglobin is synthesised within immature erythrocytes during erythropoiesis in the red bone marrow.

Normal Haemoglobin Levels

Category	Normal Hb Level (gm/dL)
Adult Males	14-17 gm/dL
Adult Females	12-15 gm/dL
Newborn Babies	14.5-18.5 gm/dL

Note: Age, sex, altitude, exercise, excitement, and adrenaline level affect Hb levels in the blood.

Structure of Haemoglobin

Heme: A cyclic tetrapyrrole with four pyrrole molecules. It gives the red colour to RBCs. Contains methyl (M), vinyl (V), and propionate (Pr) groups.

Globin Chains: Each adult haemoglobin (Hb-A) molecule consists of 4 globin chains:

• 2 alpha (α₁ and α₂) chains
• 2 beta (β₁ and β₂) chains

These 4 chains form 2 dimers (α₁β₁ and α₂β₂).

Synthesis of Haemoglobin

1. Succinyl-CoA + Glycine: Condense to form 5-aminolevulinic acid in the mitochondrion.
2. Pyrrole Formation: Succinyl-CoA and glycine bind to form a pyrrole molecule.
3. Protoporphyrin IX Formation: 4 pyrrole molecules combine to form protoporphyrin IX.
4. Heme Formation: Protoporphyrin IX combines with iron (Fe²⁺) to form the heme molecule.
5. Globin Synthesis: Each heme molecule combines with globin (a long polypeptide chain synthesised by ribosomes).
6. Haemoglobin Subunit: This forms a subunit of haemoglobin (molecular weight ~16,000).
7. Complete Hb Molecule: 4 of these chains loosely combine to form a complete haemoglobin molecule.

SUMMARY OF HEMOPOIESIS

Process	Cell Type Produced	Key Regulator	Site of Production
Erythropoiesis	RBCs (Erythrocytes)	Erythropoietin (EPO)	Red Bone Marrow
Leucopoiesis	WBCs (Leukocytes)	Various growth factors	Red Bone Marrow
Thrombopoiesis	Platelets (Thrombocytes)	Thrombopoietin (TPO)	Red Bone Marrow

A TEACHER’S PRACTICAL INSIGHTS

Over my years of teaching, I have developed a few key insights about hemopoiesis that I always share with my students:

• “Stem Cells Are the Source”: All blood cells come from a single population of pluripotent stem cells. This is why bone marrow transplants can cure blood disorders—they replace the faulty stem cells with healthy ones.
• Clinical Relevance: Understanding hemopoiesis is essential for understanding anemia, leukemia, thrombocytopenia, and bone marrow disorders.
• Use Mnemonics: “Pro-Bas-Poly-Ortho-Reticulocyte-Erythrocyte” helps remember the stages of erythropoiesis: Proerythroblast → Basophilic → Polychromatic → Orthochromatic → Reticulocyte → Erythrocyte.
• Think About EPO: Erythropoietin is one of the most clinically important hormones. It is used to treat anemia in kidney disease and cancer patients.

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What is hemopoiesis?

Hemopoiesis is the process of forming blood cells—RBCs, WBCs, and platelets—from haematopoietic stem cells in the bone marrow.

2. What is the difference between erythropoiesis, leucopoiesis, and thrombopoiesis?

Erythropoiesis is the formation of RBCs. Leucopoiesis is the formation of WBCs. Thrombopoiesis is the formation of platelets.

3. What hormone regulates erythropoiesis?

Erythropoietin (EPO) is the primary hormone regulating erythropoiesis. It is produced by the kidneys in response to low oxygen levels (hypoxia).

4. What is the function of thrombopoietin?

Thrombopoietin (TPO) is a hormone produced by the liver, kidneys, and skeletal muscles that controls the maturation of megakaryocytes and platelet production.

5. What is the normal haemoglobin level?

The normal haemoglobin level is 14-17 gm/dL for adult males and 12-15 gm/dL for adult females.

6. What is the structure of haemoglobin?

Haemoglobin is a conjugated protein composed of 4% heme (iron + porphyrin) and 96% globin (4 globin chains: 2 alpha and 2 beta chains).

7. What are the stages of erythropoiesis?

The stages of erythropoiesis are: Proerythroblast → Basophilic Erythroblast → Polychromatic Erythroblast → Orthochromatic Erythroblast → Reticulocyte → Erythrocyte.

SUMMARY

Hemopoiesis is the remarkable process of blood cell formation that occurs continuously throughout life. Erythropoiesis produces RBCs under the regulation of erythropoietin (EPO). Leucopoiesis produces WBCs through distinct lineages for each cell type. Thrombopoiesis produces platelets from megakaryocytes under the regulation of thrombopoietin (TPO).

Haemoglobin—the oxygen-carrying protein in RBCs—is synthesised during erythropoiesis. It consists of heme (iron + porphyrin) and globin (4 polypeptide chains).

Understanding hemopoiesis is essential for healthcare professionals because blood disorders—such as anemia, leukemia, and thrombocytopenia—result from disturbances in this process.

As I always tell my students: “Hemopoiesis is the body’s continuous miracle—billions of new cells every day, all from a single source. Understand this process, and you understand the foundation of haematology.”

REFERENCES & 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.
• Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
• Hoffbrand, A. V., & Moss, P. A. H. (2016). Essential Haematology (7th ed.). Wiley-Blackwell.
• National Heart, Lung, and Blood Institute (NHLBI). (2022). Haematology and Blood Disorders Resources. Retrieved from NHLBI Official Website.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.