7. PROCESS OF HEMOPOIESIS

Process of Hemopoiesis

Introduction

Hemopoiesis (or Haematopoiesis) is the process of formation, development, and maturation of blood cells.
All types of blood cells — RBCs, WBCs, and platelets — originate from stem cells present mainly in the bone marrow.

The process ensures a continuous supply of new blood cells to replace the old and damaged ones, maintaining a healthy balance within the circulatory system.

Original Notes:

Definition

Hemopoiesis is defined as:

“The process by which blood cells are formed from undifferentiated stem cells.”

Sites of Hemopoiesis

The site of blood cell formation changes during different stages of life:

1. In the Embryo:
   • Occurs first in the yolk sac.
   • Later shifts to the liver and spleen.
2. In the Fetus:
   • Takes place mainly in the liver, spleen, and bone marrow.
3. After Birth (Adult Life):
   • Occurs primarily in the red bone marrow of:
     • Vertebrae
     • Ribs
     • Sternum
     • Skull bones
     • Pelvic bones
     • Ends of long bones (humerus and femur)

Types of Bone Marrow

1. Red Bone Marrow:
   • Actively produces all blood cells.
   • Found in flat and irregular bones.
   • Rich in stem cells.
2. Yellow Bone Marrow:
   • Mainly composed of fat cells.
   • Found in long bones.
   • Can convert into red marrow during conditions like severe blood loss.

Stem Cells and Their Differentiation

All blood cells originate from a single type of pluripotent stem cell in the bone marrow.
These stem cells have the ability to self-renew and differentiate into various specialized cells.

Stages of Differentiation:

1. Pluripotent Stem Cell
   ↓
2. Multipotent Hematopoietic Stem Cell
   ↓
3. Two Major Lineages:
   • Myeloid Stem Cells → Give rise to RBCs, platelets, granulocytes, monocytes.
   • Lymphoid Stem Cells → Give rise to lymphocytes (T and B cells).

Stages of Blood Cell Formation

1. Erythropoiesis (Formation of RBCs)

• Origin: From myeloid stem cells in red bone marrow.
• Stimulated by the hormone erythropoietin (from the kidneys).
• Maturation sequence:
  Proerythroblast → Erythroblast → Reticulocyte → Mature RBC.

Regulation:

• Stimulated by low oxygen levels (hypoxia).
• Controlled by erythropoietin secretion.

2. Leukopoiesis (Formation of WBCs)

• Origin: From myeloid and lymphoid stem cells.
• Granulocytes (neutrophils, eosinophils, basophils) develop in bone marrow.
• Lymphocytes mature in lymphoid organs (thymus, spleen, lymph nodes).

Regulation:

• Controlled by colony-stimulating factors (CSFs) and interleukins.

3. Thrombopoiesis (Formation of Platelets)

• Origin: From megakaryocytes in bone marrow.
• Controlled by the hormone thrombopoietin (from the liver and kidneys).
• Megakaryocytes fragment to form platelets (thrombocytes).

Summary of Blood Cell Formation

Blood Cell Type	Site of Formation	Regulating Factor
RBC (Erythrocyte)	Red bone marrow	Erythropoietin
WBC (Leukocyte)	Bone marrow & lymphoid organs	CSFs, Interleukins
Platelets	Bone marrow (from megakaryocytes)	Thrombopoietin

Factors Affecting Hemopoiesis

• Hormones: Erythropoietin, thrombopoietin, CSFs.
• Nutrients: Iron, Vitamin B₁₂, Folic acid, Proteins.
• Bone marrow health: Radiation or toxins can suppress it.
• Oxygen level: Hypoxia stimulates erythropoiesis.

Disorders Related to Hemopoiesis

1. Anemia: Reduced RBC formation or hemoglobin level.
2. Leukemia: Abnormal increase of immature WBCs.
3. Thrombocytopenia: Low platelet count leading to bleeding.
4. Aplastic Anemia: Bone marrow failure to produce blood cells.