Haematopoiesis

Definition and Importance

Haematopoiesis is the process by which all blood cellular components are formed. This process occurs during embryonic development and continues throughout adulthood to maintain and replenish the blood system. Understanding haematopoiesis is crucial for diagnosing and treating blood disorders and cancers, as well as for research into tissue stem cells, ageing, and oncogenesis.

• Haematopoietic stem cells (HSCs): Multipotent stem cells that give rise to all blood cell types.

• Clinical relevance: Disruptions in haematopoiesis can lead to anaemia, leukaemia, and other blood disorders.

Waves of Haematopoiesis

Blood development in vertebrates involves two main waves: the primitive wave and the definitive wave.

• Primitive wave: Occurs in the extra-embryonic yolk sac, producing erythrocytes and macrophages. These progenitors are not pluripotent and lack renewal capacity.

• Definitive wave: Occurs later in development, first in the blood islands, then in the liver and bone marrow. Involves multipotent HSCs capable of generating all adult blood lineages.

Example: In humans, the liver becomes the main haematopoietic organ during fetal development, with bone marrow taking over after birth.

Pathways of Haematopoiesis

Haematopoiesis follows a hierarchical pathway from multipotent HSCs to committed progenitors and finally to mature blood cells.

• Myeloid lineage: Produces erythrocytes, megakaryocytes (platelets), granulocytes (neutrophils, eosinophils, basophils), and monocytes/macrophages.

• Lymphoid lineage: Produces T lymphocytes, B lymphocytes, and natural killer (NK) cells.

Haematocytes (Blood Cells)

Blood contains three main types of cells:

• Red blood cells (erythrocytes): Transport oxygen and carbon dioxide.

• White blood cells (leukocytes): Immune defense; includes lymphocytes, monocytes, and granulocytes (neutrophils, eosinophils, basophils).

• Platelets (thrombocytes): Involved in blood clotting.

Cell Staining

Staining techniques, such as Giemsa stain, are used to visualize nuclear and cytoplasmic material in blood cells. This is essential for differentiating cell types and diagnosing blood disorders.

• Giemsa stain: Differentiates nuclear and cytoplasmic material.

• Hydrochloric acid treatment: Hydrolyzes cytoplasm for better stain uptake.

Stem Cell Theory of Haematopoiesis

Stem cells are undifferentiated cells capable of self-renewal and differentiation into specialized cell types via asymmetric division.

• Progenitor cells: Partially specialized, can produce multiple cell types.

• Committed stem cells: Respond to specific growth factors and mature into end-stage cells.

Hematopoietic Stem Cells (HSCs)

HSCs are defined by their ability to self-renew and differentiate into all blood cell lineages. They are rare, occurring at a frequency of 1:5000 in bone marrow.

• Function: Generate non-self-renewing populations of differentiated cells.

• Surface phenotype: c-kit+Sca-1+Lin-Thy1lo

Stem Cell Niches in Bone Marrow

Stem cell niches are specialized microenvironments in the bone marrow that regulate HSC dormancy, self-renewal, and differentiation.

• Perivascular/vascular niches: Near blood vessels, maintain HSCs.

• Endosteal niches: Along the inner bone surface, associated with quiescent HSCs.

• Key regulatory cells: Mesenchymal stem cells, osteoblasts, endothelial cells.

Obesity-Driven Disruption of Haematopoiesis

Obesity alters the bone marrow environment, leading to increased adipocyte numbers and a shift towards a white adipose profile, which can compromise immune function and haematopoiesis.

• Public health impact: Increased risk of infectious diseases.

• Prevention: Exercise and weight control may help maintain healthy haematopoiesis.

Erythropoiesis

Red Blood Cells (RBCs) or Erythrocytes

RBCs are specialized, anucleate cells filled with haemoglobin, responsible for oxygen and carbon dioxide transport.

• Lifespan: ~120 days.

• Shape: Biconcave disc, ~7.5 μm diameter, 2.6 μm thick at rim.

• Composition: 40% lipid, 10% carbohydrate, 50% protein (membrane).

• Cytoskeleton: Spectrin, actin, band 3, protein 4.1, ankyrin.

Erythropoiesis Process

Erythropoiesis is the process of generating RBCs from stem cells in the bone marrow. It is tightly regulated to maintain constant RBC mass.

• Stages: Proerythroblast → Basophilic erythroblast → Polychromatophilic erythroblast → Orthochromatophilic erythroblast → Reticulocyte → Erythrocyte.

• Regulation: Erythropoietin (EPO), iron, vitamins (B12, folate), and hormones (androgens, thyroxine).

Reticulocytes

Reticulocytes are immature RBCs released from the bone marrow, containing residual RNA. Their count is a marker of bone marrow activity.

• Normal range: 1-2% of RBCs.

• Clinical use: Elevated in increased RBC production (e.g., bleeding); low in production failure.

Factors Affecting Erythropoiesis

• Erythropoietin (EPO): Produced in the kidney, stimulates RBC production.

• Iron: Essential for haemoglobin synthesis.

• Vitamins B12 and folate: Required for DNA synthesis; deficiency leads to megaloblastic anaemia.

• Copper and pyridoxine: Important for iron incorporation into haem; deficiency causes sideroblastic anaemia.

RBC Metabolism

RBCs rely on anaerobic glycolysis for ATP production, as they lack mitochondria.

• Key pathways:

  • Glycolysis (ATP and lactate/pyruvate production)

  • Rapoport-Luebering shunt (produces 2,3-DPG for oxygen release)

  • Pentose Phosphate Pathway (generates NADPH for antioxidant defense)

Anaemia

Classification and Diagnosis

Anaemia is defined as a reduction in haemoglobin (Hb), haematocrit (HCT), or RBC count. It can be classified by cell size (microcytic, normocytic, macrocytic) or by underlying cause.

• Symptoms: Lethargy, weakness, tiredness, shortness of breath, reduced exercise tolerance.

• Diagnostic thresholds: Hb < 13.5 g/dL (males), < 12 g/dL (females), < 11 g/dL (children).

Red Cell Indices

• MCV (Mean Corpuscular Volume): Average RBC size; distinguishes microcytic, normocytic, macrocytic anaemia.

• MCH (Mean Corpuscular Haemoglobin): Average amount of haemoglobin per RBC.

• MCHC (Mean Corpuscular Haemoglobin Concentration): Haemoglobin concentration in RBCs.

• RDW (Red Cell Distribution Width): Variation in RBC size.

Types of Anaemia

• Microcytic: Low MCV; often due to iron deficiency or thalassaemia.

• Normocytic: Normal MCV; can be due to acute blood loss, chronic disease, or renal failure.

• Macrocytic: High MCV; often due to B12 or folate deficiency, liver disease, or bone marrow disorders.

Causes of Anaemia

• Decreased production: Bone marrow failure, nutritional deficiencies, chronic disease.

• Increased destruction: Haemolytic anaemia (intrinsic or extrinsic causes).

• Blood loss: Acute or chronic haemorrhage.

Haemolytic Anaemia

Results from increased destruction of RBCs, which can be intravascular (within blood vessels) or extravascular (in spleen/liver).

• Intrinsic causes: Membrane defects, enzyme deficiencies (e.g., G6PD, pyruvate kinase), haemoglobinopathies (e.g., thalassaemia, sickle cell).

• Extrinsic causes: Immune-mediated destruction, infections, toxins.

Summary Table: Morphology of Red Blood Cells

Additional info: For further reading, see Hoffbrand's Essential Haematology, Chapters 1 and 2.