Comprehensive Study Notes on Hemopoiesis

Overview of Hemopoiesis

  • Definition: Hemopoiesis (or Hematopoiesis) is the process of blood cell production and development.
  • Cell Replacement: Mature blood cells have relatively short life spans and must be replaced continuously from precursors.
  • Adult Sites of Formation:
    • Red Bone Marrow: In adults, this is the primary site for the formation of erythrocytes, granulocytes, monocytes, and platelets.
    • Specific Terminology:
      • Erythropoiesis: Origin and maturation of erythrocytes (red blood cells).
      • Granulopoiesis: Origin and maturation of granulocytes.
      • Monocytopoiesis: Origin and maturation of monocytes.
      • Thrombocytopoiesis: Origin and maturation of platelets (thrombocytes).
    • Lymphopoiesis: The development of lymphocytes; occurs in the bone marrow and lymphoid organs (to which precursor cells migrate from the marrow).
  • Regulatory Factors: Growth and differentiation are promoted by:
    • Erythropoietin (EPO): A glycoprotein produced in the kidneys essential for erythrocyte production.
    • Colony Stimulating Factors (CSF).
    • Cytokines.
    • Growth Factors.

Hemopoiesis During Embryonic Development and Aging

  • Initiation: Hemopoiesis begins in early embryonic development and progresses through three distinct phases:
    1. First or Yolk-Sac Phase: Blood cells arise in the yolk sac mesoderm of the early embryo.
    2. Second or Hepatic Phase: Occurs primarily in the developing liver during the second trimester (weeks 13−2613-26), with the spleen playing a minor role.
    3. Third or Bone Marrow Phase: Occurs in the third trimester (week 2727 to birth). Skeletal elements ossify, and bone marrow develops in medullary cavities, becoming the major hemopoietic organ.
  • Post-Birth: Hemopoiesis is restricted to the red bone marrow and certain lymphatic tissues.
  • Trimester Definitions:
    • 1st Trimester: Weeks 0−120-12.
    • 2nd Trimester: Weeks 13−2613-26.
    • 3rd Trimester: Week 2727 to birth.

Stem Cells and Lineages

  • Pluripotent Hemopoietic Stem Cells (PHSC): All blood cells arise from this single type of stem cell in the bone marrow.
    • These cells divide slowly to maintain their own population.
    • They give rise to two major cell lineages of progenitor cells: Myeloid and Lymphoid.
  • Myeloid Lineage: Includes precursor cells (blasts) for erythropoiesis, thrombopoiesis, granulopoiesis, and monocytopoiesis. These cells are generally involved in innate immunity.
  • Lymphoid Lineage: Forms B lymphocytes, T lymphocytes, and natural killer (NK) cells.
  • Colony-Forming Units (CFUs): Progenitor cells for blood cells are often called CFUs because they give rise to colonies of only one specific cell type.

Bone Marrow Structure and Function

  • Location: Found in the medullary canals of long bones and small cavities of cancellous (spongy) bone.
  • Types of Bone Marrow:
    1. Red Bone Marrow: Active in hemopoiesis. Its color comes from an abundance of blood and hemopoietic cells.
    2. Yellow Bone Marrow: Inactive in hemopoiesis; contains predominantly adipose (fat) cells.
  • Marrow Transformation:
    • In newborns, all bone marrow is red and active.
    • As a child grows, most marrow gradually changes to the yellow variety.
    • Reversion: Under conditions such as severe bleeding or hypoxia, yellow marrow can revert back to red marrow to increase blood cell production.
  • Composition of Red Bone Marrow:
    • Stroma: A meshwork of specialized fibroblastic cells called stromal cells (reticular or adventitial cells) and reticular fibers. It provides structural support and contains macrophages.
    • Hemopoietic Cords/Islands: Clusters of developing blood cells.
    • Sinusoidal Capillaries: Discontinuous endothelium through which mature cells enter circulation.
  • Phagocytic Role: Red marrow is a site where older or defective erythrocytes undergo phagocytosis by macrophages. Macrophages reprocess heme-bound iron for delivery to new differentiating erythrocytes.

Blood Cell Release into Circulation

  • Mechanism of Entry: Mature cells enter the bloodstream by passing through the discontinuous sinusoidal endothelium.
    • Leukocytes: Cross the wall by their own motility (diapedesis).
    • Erythrocytes: Non-motile; they are pushed through the wall by a pressure gradient across the endothelium.
    • Platelets: Released from the tips of thin processes (proplatelets) extended by megakaryocytes through apertures in the sinusoid wall.

Erythropoiesis (Erythrocyte Maturation)

  • Developmental Changes:
    • Cell and nuclear volumes decrease.
    • Nucleoli diminish in size and disappear.
    • Chromatin density increases until the nucleus is pyknotic (small, dark-stained, condensed) and finally extruded.
    • Basophilia decreases (fewer polyribosomes) as hemoglobin (eosinophilic/acidophilic protein) increases.
    • Mitochondria and other organelles gradually disappear.
  • Stages of Maturation:
    1. Proerythroblast: First recognizable precursor; large cell with spherical nucleus, visible nucleoli, and mild basophilia (free ribosomes).
    2. Basophilic Erythroblast: Slightly smaller; condensed nucleus; strong basophilia due to large numbers of polyribosomes synthesizing hemoglobin.
    3. Polychromatophilic Erythroblast: Reduced cell volume; cytoplasm shows both basophilia (ribosomes) and acidophilia (hemoglobin).
    4. Orthochromatophilic Erythroblast (Normoblast): Uniformly acidophilic cytoplasm; nuclear volume condenses further until the pyknotic nucleus is ejected.
    5. Reticulocyte: Anucleate cell containing a few remaining polyribosomes (forming a network when stained with brilliant cresyl blue). They constitute approximately 1%1\% of circulating red blood cells and quickly mature into erythrocytes after entering circulation.
    6. Erythrocyte: Mature, functional oxygen carrier.

Granulopoiesis (Granulocyte Maturation)

  • Stages of Maturation:
    1. Myeloblast: First recognizable cell; euchromatic spherical nucleus with 3−53-5 nucleoli; little cytoplasm.
    2. Promyelocyte: Characterized by basophilic cytoplasm and azurophilic granules (containing lysosomal enzymes and myeloperoxidase).
    3. Myelocyte: First stage where specific granules (neutrophilic, eosinophilic, or basophilic) appear.
    4. Metamyelocyte: Specific granules are fully dispersed; nucleus begins to indent.
    5. Band (Stab) Cell: Intermediate stage specifically for neutrophils; nucleus is elongated but not yet polymorphic.
    6. Mature Granulocyte: Segmented nucleus.
  • Timeline: Total time for a myeloblast to produce mature circulating neutrophils is 10−1410-14 days.

Neutrophil Compartmentalization and Dynamics

  • Anatomical Compartments:
    1. Granulopoietic Compartment: Developing progenitor cells in bone marrow.
    2. Storage (Reserve) Compartment: Marrow buffer system holding mature neutrophils for release when needed.
    3. Circulating Compartment: Neutrophils moving throughout the blood.
    4. Marginating Compartment: Cells temporarily accumulated at the endothelium surface of venules and small veins.
  • Movement between Compartments:
    • Marginating and circulating compartments are roughly equal in size with constant interchange.
    • Diapedesis: Neutrophils enter connective tissues through intercellular junctions of postcapillary venules.
    • Inflamed Connective Tissue: Acts as a fifth terminal compartment where cells reside for a few days before undergoing apoptosis.
  • Clinical Considerations:
    • Neutrophilia: Increased circulating neutrophils. Can be caused by intense muscular activity or epinephrine (moving cells from marginating to circulating compartments) without an actual increase in granulopoiesis.
    • Glucocorticoids (e.g., Cortisone): Increase mitotic activity of precursors, raising blood counts.
    • Bacterial Infection: Increases production and shortens storage duration. Immature forms (band cells, metamyelocytes, or myelocytes) may appear in the blood (shift to the left).

Agranulopoiesis

  • Lymphocyte Maturation:
    • Lymphoblast: Large progenitor cell capable of dividing 2−32-3 times.
    • T Lymphocytes: Progenitors migrate to the thymus to acquire specific surface proteins and then populate peripheral lymphoid organs.
    • B Lymphocytes: Differentiate and mature within the bone marrow before migrating to lymphoid organs.
  • Monocyte Maturation:
    • Monoblast: Progenitor virtually identical to the myeloblast.
    • Promonocyte: Large cell (up to 18 μm18\,\mu m) with basophilic cytoplasm, a slightly indented nucleus, lacy chromatin, and visible nucleoli. They divide twice while developing into monocytes.
    • Fate: Monocytes circulate for several hours, then enter tissues to mature into macrophages or other phagocytic cells, functioning for several months.

Thrombopoiesis (Origin of Platelets)

  • Lineage: Progenitor cells differentiate into Megakaryoblasts under the influence of Thrombopoietin.
  • Megakaryoblast:
    • Size: 25−50 μm25-50\,\mu m.
    • Large ovoid/kidney-shaped nucleus.
    • Endomitosis: The cell undergoes repeated DNA replication without cell division, resulting in a highly polyploid nucleus (8N8N to 64N64N).
  • Megakaryocyte:
    • Giant cells up to 150 μm150\,\mu m in diameter.
    • Large, irregularly lobulated nuclei with coarse chromatin.
    • Cytoplasm contains mitochondria, well-developed RER, and an extensive Golgi apparatus.
  • Platelet Release Process:
    • Proplatelets: Long, branching processes extended by megakaryocytes into microvascular sinusoids.
    • Demarcation Membranes: Numerous invaginations of the plasma membrane in mature megakaryocytes. These act as a membrane reservoir facilitating the elongation of proplatelets.
    • Fragmentation: Platelets are pinched off from the ends of proplatelet processes.