Process of Haemopoiesis

PROCESS OF HAEMOPOIESIS

Haemopoiesis is the process by which blood cells are formed, developed, and matured in the body. It is a continuous process that takes place throughout life to replace old and destroyed blood cells.

Blood cells formed by haemopoiesis are:

• Red blood cells (RBCs)
• White blood cells (WBCs)
• Platelets

DEFINITION

Haemopoiesis is defined as the production of blood cells from haemopoietic stem cells, mainly occurring in the bone marrow.

It includes three major processes:

• Erythropoiesis – formation of red blood cells
• Leucopoiesis – formation of white blood cells
• Thrombopoiesis – formation of platelets

SITES OF HAEMOPOIESIS

During Foetal Life

• 3rd week of intra-uterine life
  Blood cells are formed in the yolk sac
• 3rd month of intra-uterine life
  Liver and spleen become active sites
• 5th month of intra-uterine life onwards
  Bone marrow becomes the main site

After Birth and in Adults

• Bone marrow is the primary site of haemopoiesis
• Active marrow is present mainly in:
• Skull
• Vertebrae
• Ribs
• Sternum
• Pelvic bones
• Upper ends of femur and humerus

BONE MARROW

Red Bone Marrow

• Actively produces blood cells
• Present in all bones up to 20 years of age
• In adults, limited to certain bones
• Responsible for producing RBCs, WBCs, and platelets

Yellow Bone Marrow

• Inactive marrow
• Contains fat cells
• Can convert into red marrow during emergencies such as severe blood loss

HAEMOPOIETIC STEM CELLS

Pluripotent Stem Cells

• All blood cells originate from a single pluripotent stem cell
• These cells have the ability to:
• Self-renew
• Differentiate into different blood cell types
• Some stem cells remain unchanged to maintain future supply

Committed Stem Cells

• Derived from pluripotent stem cells
• Already committed to form specific blood cells
• Examples:
• CFU-E → red blood cells
• CFU-GM → granulocytes and monocytes

REGULATION OF HAEMOPOIESIS

Growth Inducers

• Proteins that stimulate growth and multiplication of stem cells
• Example: Interleukin-3 stimulates several blood cell lines

Differentiation Inducers

• Proteins that help stem cells mature into specific blood cells

External Factors Affecting Haemopoiesis

• Low oxygen levels
• Infections
• Tissue damage and inflammation
  These conditions increase blood cell production according to body needs

CELL RENEWAL IN HAEMOPOIESIS

Progenitor Cells

• Intermediate cells between stem cells and blast cells
• Limited ability to divide

Blast Cells

• Immature precursor cells
• Found only in bone marrow
• Not present in normal blood
• Examples:
• Erythroblast
• Myeloblast
• Megakaryoblast
• Lymphoblast
• Monoblast

ERYTHROPOIESIS (FORMATION OF RED BLOOD CELLS)

Erythropoiesis is the process of formation of red blood cells in the bone marrow.

Stages of Erythropoiesis

Stem cell
→ CFU-E
→ Proerythroblast
→ Early normoblast
→ Intermediate normoblast
→ Late normoblast
→ Reticulocyte
→ Mature erythrocyte

Changes During Erythropoiesis

• Cell size decreases
• Haemoglobin content increases
• Nucleus becomes smaller and disappears
• Reticulocytes enter blood and mature within 1–2 days
• Normal reticulocyte count is less than 1%

Regulation of Erythropoiesis

Role of Oxygen

• Oxygen level in tissues is the main regulator
• Low oxygen (hypoxia) increases RBC production

Erythropoietin (EPO)

• A hormone that stimulates RBC formation
• Mainly produced by kidneys
• Small amount produced by liver
• Released during:
• Anaemia
• Blood loss
• High altitude
• Lung diseases
• Poor circulation

LEUCOPOIESIS (FORMATION OF WHITE BLOOD CELLS)

Leucopoiesis is the process of formation of white blood cells that help protect the body against infections.

Types of White Blood Cell Lineages

• Myelocytic lineage
• Neutrophils
• Eosinophils
• Basophils
• Monocytes
• Lymphocytic lineage
• Lymphocytes
• Plasma cells

Sites of Leucopoiesis

• Bone marrow – granulocytes and monocytes
• Lymphoid organs – lymph nodes, spleen, thymus, tonsils

Regulation of Leucopoiesis

White blood cell production is regulated by:

• Colony-stimulating factors (CSFs)
• Cytokines released during infection and inflammation
• These substances increase WBC production to fight infections

THROMBOPOIESIS (FORMATION OF PLATELETS)

Thrombopoiesis is the process of formation of platelets from large bone marrow cells called megakaryocytes.

Platelets

• Small, disc-shaped cells
• Size: 1–4 micrometers
• Formed by fragmentation of megakaryocytes
• Normal platelet count: 150,000–300,000 per microlitre of blood