Bone Marrow Morphology and Hemopoiesis Study Notes

HEMOPOIESIS: OVERVIEW AND DEVELOPMENTAL ORIGINS

  • Hemopoiesis (derived from Greek: haimahaima, blood + poiesispoiesis, a making) refers to the process of continuous replacement of mature blood cells, which have short lifespans, by precursors.
  • Throughout human development, the site of hemopoiesis shifts:
    • Early Embryo: Blood cells arise primarily in the yolk sac.
    • Second Trimester: Hemopoiesis occurs mainly in the developing liver, with the spleen playing a minor role.
    • Third Trimester: As skeletal elements ossify, bone marrow develops within medullary cavities and becomes the major hemopoietic organ.
    • Childhood and Adulthood: Erythrocytes, granulocytes, monocytes, and platelets continue to form from stem cells located in the bone marrow.
  • The maturation processes for specific cell types are named as follows:
    • Erythropoiesis: Origin and maturation of red blood cells (Greek: erythroserythros, red + poiesispoiesis).
    • Granulopoiesis: Maturation of granulocytes.
    • Monocytopoiesis: Maturation of monocytes.
    • Thrombocytopoiesis: Formation of platelets.
    • Lymphopoiesis: Development of lymphocytes, occurring in both the bone marrow and lymphoid organs after precursor migration.

STEM CELLS, GROWTH FACTORS, AND DIFFERENTIATION

  • Stem Cells: Defined as pluripotent cells capable of asymmetric division and self-renewal.
    • One daughter cell becomes an irreversibly committed progenitor cell.
    • The other daughter cell remains as part of a small pool of slowly dividing stem cells to maintain the population.
  • Experimental Identification and Study:
    • Fluorescence-activated cell-sorting (FACS): Uses fluorescence-labeled antibodies to mark surface antigens, allowing isolation of stem cells.
    • In Vivo Techniques: Injecting donor bone marrow into irradiated mice (whose own hemopoietic cells are destroyed) results in the formation of hemopoietic colonies in the marrow and spleen. This research is the foundation for clinical bone marrow transplants used to treat lethal disorders.
    • In Vitro Techniques: Uses semisolid tissue culture media with substances from marrow stromal cells to study cytokines that promote growth and differentiation.
  • Pluripotent Hemopoietic Stem Cells: All blood cells originate from a single type of pluripotent stem cell in the bone marrow.
    • These cells are rare and proliferate slowly.
    • They give rise to two major restricted lineages (progenitor cells):
      1. Lymphoid Lineage: Leads to lymphocytes; these progenitors migrate to the thymus, lymph nodes, spleen, and other lymphoid structures to differentiate.
      2. Myeloid Lineage: (Greek: myelosmyelos, marrow) Develops in the marrow into granulocytes, monocytes, erythrocytes, and megakaryocytes.

PROGENITOR AND PRECURSOR CELL DYNAMICS

  • Progenitor Cells/Colony-Forming Units (CFUs): These cells give rise to colonies of only one cell type when cultured or injected. The four major CFUs are:
    • Erythroid Lineage: Erythrocytes.
    • Thrombocytic Lineage: Megakaryocytes (for platelet formation).
    • Granulocyte-monocyte Lineage: Three types of granulocytes and monocytes.
    • Lymphoid Lineage: B lymphocytes, T lymphocytes, and natural killer (NK) cells.
  • Precursor Cells (Blasts): Progenitor cell lineages lead to precursor cells that gradually develop the morphologic characteristics of mature cells.
  • Distinguishing Characteristics:
    • Stem and progenitor cells cannot be morphologically distinguished; they both resemble large lymphocytes.
    • Stem cells divide slowly; progenitor and precursor cells divide rapidly to produce high volumes of mature cells.
  • Hemopoietic Microenvironment (Niche): Hemopoiesis requires a specific niche consisting of local cells and extracellular matrix (ECM) providing endocrine, paracrine, and juxtacrine factors.
  • Growth Factors (CSFs/Cytokines): These are glycoproteins that stimulate cell proliferation, differentiation, and maturation. Genes for these factors have been cloned for clinical use in treating hemopoietic disorders.

BONE MARROW STRUCTURE AND MICROENVIRONMENT

  • Types of Bone Marrow:
    • Red Bone Marrow: Active in blood cell production. Its color comes from an abundance of blood and hemopoietic cells. In newborns, all marrow is red.
    • Yellow Bone Marrow: Inactive marrow filled with adipocytes. Usually replaces red marrow as a child grows. Under severe conditions like hypoxia or bleeding, yellow marrow can revert to red.
  • Stroma Structure:
    • Consists of a reticular connective tissue stroma (Greek: stromastroma, bed).
    • Includes specialized fibroblastic cells called stromal cells (also called reticular or adventitial cells) and a web of reticular fibers.
    • The matrix contains collagen type II, proteoglycans, fibronectin, and laminin.
  • Cellular Components of the Niche: Includes stroma, osteoblasts, and megakaryocytes.
  • Sinusoidal Capillaries: Run between hemopoietic cords and possess a discontinuous endothelium, allowing newly formed blood cells and platelets to enter the general circulation.
  • Phagocytosis: The red marrow is a site for the removal of old or defective erythrocytes. Macrophages reprocess heme-bound iron for delivery back to differentiating red blood cells (RBCs).

MATURATION OF ERYTHROCYTES (ERYTHROPOIESIS)

  • Overview: Terminal differentiation involving hemoglobin synthesis, nuclear condensation, and extrusion to form enucleated, biconcave corpuscles. The process takes approximately a week and involves 3−53-5 cell divisions.
  • Role of Erythropoietin (EPO): A glycoprotein growth factor produced in the kidneys; it stimulates mRNA for hemoglobin components and is essential for RBC production.
  • Stages of Erythropoiesis:
    1. Proerythroblast: Large cell with loose, lacy chromatin, visible nucleoli, and basophilic cytoplasm.
    2. Basophilic Erythroblast: Smaller than proerythroblast; features cytoplasmic basophilia due to high numbers of polysomes for hemoglobin synthesis.
    3. Polychromatophilic Erythroblast: Volume reduces, polysomes decrease, and hemoglobin begins to fill the cell, creating both basophilic and acidophilic regions.
    4. Orthochromatophilic Erythroblast (Normoblast): Cytoplasm becomes uniformly acidophilic as basophilia is lost. The nucleus is ejected at the end of this stage and phagocytized by macrophages.
    5. Reticulocyte: Still contains a few polyribosomes. When treated with brilliant cresyl blue, these form a stained network. They enter circulation, representing about 1%1 \% of RBCs, and lose their ribosomes within a few days to become mature erythrocytes.
  • Morphologic Trends:
    • Cell and nuclear volume decrease.
    • Nucleoli disappear.
    • Chromatin density increases until the nucleus is pyknotic.
    • Polyribosomes (basophilia) decrease while hemoglobin (eosinophilic) increases.

MATURATION OF GRANULOCYTES (GRANULOPOIESIS)

  • Cytoplasmic Development: Involves the successive synthesis of two types of granules:
    1. Azurophilic Granules: Formed first; contain lysosomal hydrolases and myeloperoxidase. They are similar across all granulocyte types.
    2. Specific Granules: Formed second; contents differ by cell type, determining the specific properties of neutrophils, eosinophils, and basophils.
  • Stages of Granulopoiesis:
    1. Myeloblast: Most immature recognizable myeloid cell; finely dispersed chromatin and faint nucleoli.
    2. Promyelocyte: Characterized by basophilic cytoplasm and azurophilic granules.
    3. Myelocyte: First visible sign of specific differentiation with the appearance of specific granules.
    4. Metamyelocyte: Specific granules occupy most of the cytoplasm; the nucleus begins to condense and change shape.
    5. Band Cell: Intermediate stage specifically for the neutrophilic line; the nucleus is elongated and non-segmented (U-shaped).
    6. Mature Granulocyte: Development from myeloblast to circulating neutrophil takes 10−14 days10-14 \, \text{days} and involves 55 mitotic divisions.

NEUTROPHIL COMPARTMENTS AND KINETICS

  • Four Functional Compartments:
    1. Granulopoietic Compartment: Active production in the marrow.
    2. Storage Compartment: Mature cells held in the marrow until needed.
    3. Circulating Population: Neutrophils moving freely in the blood.
    4. Margination Population: Neutrophils adhering loosely to the endothelial surface of venules and small veins.
  • Tissue Infiltration: In response to infection/injury, neutrophils leave the microvasculature via diapedesis (migration through cell junctions). They enter a fifth compartment (inflamed connective tissue) where they perform phagocytosis and die by apoptosis within a few days.

MATURATION OF AGRANULOCYTES

  • Monocytes:
    • Monoblast: Identical to the myeloblast.
    • Promonocyte: Large cell (up to 18 μm18 \, \mu m) with basophilic cytoplasm and a large, indented nucleus. Divides twice before becoming a monocyte.
    • Monocyte: Circulates for several hours before entering tissues to mature into macrophages, where they function for several months.
  • Lymphocytes:
    • Progenitor cells originate in the bone marrow.
    • T Lymphocytes: Precursors migrate to the thymus for maturation, then populate peripheral lymphoid organs.
    • B Lymphocytes: Differentiate in the bone marrow and then migrate to peripheral organs (spleen, lymph nodes).
    • Markers: Mature lymphocytes are typically larger than new ones. Specific subsets are identified via immunocytochemical detection of cell surface proteins.

MEDICAL APPLICATIONS AND MYELOPROLIFERATIVE DISORDERS

  • Leukemias: Malignant clones of leukocyte precursors causing a shift in hemopoiesis, resulting in excessive production of some cells and a lack of others (anemia). They can be lymphoblastic (lymphoid tissue) or myelogenous (bone marrow).
  • Diagnosis: Accomplished via bone marrow aspiration, typically performed at the iliac crest using a needle through compact bone. Monoclonal antibodies are used to identify precise cell types for diagnosis.

ORIGIN OF PLATELETS (THROMBOPOIESIS)

  • Process: Platelets (thrombocytes) are membrane-enclosed fragments originating from megakaryocytes, driven by the hormone thrombopoietin.
  • Megakaryoblast: 25−50 μm25-50 \, \mu m in diameter with a large ovoid/kidney-shaped nucleus and basophilic cytoplasm. Undergoes endomitosis (DNA replication without cell division) to become polyploid.
  • Megakaryocyte: Giant cells (up to 150 μmup \, to \, 150 \, \mu m) with large, irregularly lobulated nuclei. Located near sinusoidal capillaries.
  • Platelet Formation:
    • Proplatelets: Long (>100 μm>100 \, \mu m) branching processes extended through the sinusoidal endothelium.
    • Internal Framework: Actin filaments and microtubules transport vesicles and granules to the distal ends of proplatelets.
    • Release: Distal teardrop-shaped enlargements are pinched off into the bloodstream to form platelets.
    • Demarcation Membranes: Invaginations of the plasma membrane that serve as a reservoir for proplatelet elongation.
  • Efficiency: Each megakaryocyte produces several thousand platelets before undergoing apoptosis and being removed by macrophages.