Study Notes on Hemopoiesis (Hematopoiesis)

HEMOPOIESIS

Definition and Overview

  • Hemopoiesis (also known as Hematopoiesis) is the biological process through which mature blood cells are produced.

    • Mature blood cells possess a relatively short lifespan and thus require consistent replenishment with new cells that develop from precursors during hemopoiesis.

  • In adults, the following blood cells originate from the red bone marrow:

    • Erythrocytes (red blood cells)

    • Granulocytes (a type of white blood cell)

    • Monocytes (another type of white blood cell)

    • Platelets (cell fragments involved in clotting)

  • The specific processes of blood cell formation are:

    • Erythropoiesis: Formation of erythrocytes

    • Granulopoiesis: Formation of granulocytes

    • Monocytopoiesis: Formation of monocytes

    • Thrombocytopoiesis: Formation of platelets

  • Additionally, lymphocytes stem from:

    • Lymphopoiesis: Development of lymphocytes in red bone marrow and lymphoid organs (where precursor cells migrate from marrow).

  • Key regulators of these developmental processes include:

    • Erythropoietin (EPO)

    • Colony Stimulating Factors (CSF)

    • Cytokines

    • Growth factors

Hemopoiesis During Development and Aging

  • Initiation: Hemopoiesis commences during early embryonic development. The process consists of several phases:

    1. Yolk-Sac Phase: Initial blood cell production occurs in the yolk sac mesoderm during early embryonic development.

    2. Hepatic Phase: During the second trimester, the developing liver becomes the primary site of hematopoiesis, with the spleen playing a minor role.

    3. Bone Marrow Phase: By the third trimester, skeletal elements ossify, and bone marrow forms within the medullary cavities; it becomes the principal hemopoietic organ.

  • Following birth, hemopoiesis is restricted to red bone marrow and some lymphatic tissues, similar to adult conditions.

Stem Cells and Lineages

  • All blood cells arise from a unique type of pluripotent hemopoietic stem cell located in the bone marrow.

    • Pluripotent Hemopoietic Stem Cells:

    • Divide slowly to maintain their own population.

    • Give rise to two significant progenitor cell lineages:

      • Myeloid Stem Cells

      • Lymphoid Stem Cells

  • Myeloid Lineage: Leads to the precursor cells (blasts) for:

    • Erythropoiesis

    • Thrombopoiesis

    • Granulopoiesis

    • Monocytopoiesis

  • Lymphoid Lineage: Forms:

    • B lymphocytes

    • T lymphocytes

    • Natural killer cells

  • Progenitor cells for blood cells are referred to as Colony-Forming Units (CFUs) as they generate colonies of a specific cell type.

Bone Marrow Structure

  • Bone marrow is located in:

    • Medullary canals of long bones

    • Small cavities of cancellous (spongy) bone

  • Types of bone marrow:

    1. Red Bone Marrow:

    • Active in hemopoiesis.

    • Appears red due to the abundance of blood and hemopoietic cells.

    1. Yellow Bone Marrow:

    • Not active in hemopoiesis; predominantly contains adipose cells.

  • Developmental Changes:

    • In newborns, all bone marrow is red and engaged in blood cell production.

    • With growth, much of the marrow gradually transitions to yellow marrow.

    • Under certain conditions (e.g., severe bleeding or hypoxia), yellow marrow can revert to red.

Features of Red Bone Marrow

  • Active in hemopoiesis and contains multiple components:

    • Reticular Connective Tissue Stroma: A supportive meshwork of specialized fibroblastic cells (stromal cells) and delicate reticular fibers.

    • Hemopoietic Cords or Islands: Aggregates of cells involved in blood cell formation.

    • Sinusoidal Capillaries: Specialized blood vessels facilitating the exchange of cells.

  • The stroma also serves as a site for the phagocytosis of older or defective erythrocytes by macrophages, which recycle heme-bound iron for erythrocyte production.

Detailed Examination of Hemopoiesis

  • Erythropoiesis (red blood cell formation):

    • Significant cellular changes occur during this process:

    • Decrease in cell and nuclear volume; nucleoli diminish and disappear.

    • Increased chromatin density leading to a pyknotic appearance of the nucleus, which is ultimately extruded from the cell.

    • Decrease in the number of polyribosomes (resulting in reduced basophilia) and a concurrent increase in hemoglobin.

    • Organelles, including mitochondria, gradually disappear.

  • Erythropoietin (EPO), produced by the kidneys, is a critical glycoprotein stimulating mRNA production for hemoglobin synthesis, essential for erythrocyte production.

Processes and Stages of Erythropoiesis

  • Erythropoiesis involves 3 to 5 cell divisions between the progenitor cell stage and the actual release of functional erythrocytes into circulation.

    • Recognizable stages in erythropoiesis:

    1. Proerythroblast: Large cell with a spherical nucleus and visible nucleoli; mild basophilia due to free ribosomes.

    2. Basophilic Erythroblast: Smaller cell with a more condensed nucleus; intense basophilia due to extensive polyribosome presence. Hemoglobin synthesis begins altering staining behavior.

    3. Polychromatophilic Erythroblast: Progressive reduction in volume and decrease in polyribosomes; regions of acidophilia (hemoglobin) appear.

    4. Orthochromatophilic Erythroblast (Normoblast): Retains a few polyribosomes, leading to acidophilic cytoplasm as the nucleus is ejected and phagocytosed by macrophages, resulting in reticulocytes.

    5. Reticulocyte: Enters circulation (constituting approximately 1% of red blood cells), quickly matures into erythrocytes by losing remaining polyribosomes.

Granulopoiesis (Granulocyte Formation)

Stages of Granulocyte Maturation

  • Myeloblast Stage: The initial identifiable cell in granulopoiesis with a large, euchromatic, spherical nucleus; matures into promyelocyte.

  • Promyelocyte Stage: Characterized by the presence of basophilic cytoplasm and azurophilic granules containing lysosomal enzymes and myeloperoxidase.

  • Myelocyte Stage: Specific granules for one of the three granulocyte types form.

  • Metamyelocyte Stage: Characterized by distinct specific granules and early nuclear indentation.

  • Band Cell Stage: Intermediate stage leading to mature neutrophilic granulocytes; these are often released prematurely during infections.

  • Total maturation time: 10 to 14 days from myeloblast to mature circulating neutrophils.

Neutrophil Compartments in Bone Marrow

  • Neutrophils exist in multiple anatomically and functionally distinct compartments, reflecting cell numbers:

    1. Granulopoietic Compartment: Developing progenitor cells in bone marrow.

    2. Storage Compartment: Acts as a buffer, releasing mature neutrophils when needed.

    3. Circulating Compartment: Neutrophils present throughout the blood.

    4. Marginating Compartment: Neutrophils temporarily adhere to the endothelium of venules and small veins, providing a means of reserve in case of need.

  • Both marginating and circulating compartments are of approximately equal size, demonstrating constant interchange of cells.

Neutrophil Behavior During Infection

  • During injury or infection, neutrophils migrate through endothelial junctions into connective tissues via diapedesis,
    actively entering inflamed tissues where they function and subsequently undergo apoptosis.

Neutrophil Counts and Their Interpretation

  • Neutrophilia refers to increased circulating neutrophils, which does not always indicate increased granulopoiesis.

    • Factors influencing neutrophilia include intense muscle activity or epinephrine release causing marginating neutrophils to enter circulation without increased granulopoiesis.

    • Glucocorticoids like cortisone enhance mitotic activity in neutrophil precursors, contributing to elevated blood neutrophil counts.

    • During bacterial infections, neutrophilia arises from increased production and a reduced lifespan in the medullary storage compartment, possibly leading to the presence of immature forms like band cells in circulation.

Lymphocyte Development

  • Lymphocyte progenitor cells stem from bone marrow:

    • Some migrate to the thymus for T lymphocyte differentiation, subsequently populating peripheral lymphoid organs.

    • Others remain in the bone marrow, differentiate into B lymphocytes, then migrate to peripheral lymphoid organs.

  • Lymphoblast: The first identifiable progenitor cell in lymphocyte lineage is capable of dividing two or three times to differentiate into lymphocytes.

Monocyte Formation

  • Monoblast: Committed progenitor cell similar to myeloblast morphologically.

  • Promonocyte: A large cell with basophilic cytoplasm and a slightly indented nucleus that divides to form monocytes.

  • Monocytes: Circulate in blood for hours before entering tissues, where they mature into macrophages or other phagocytic cells, functioning for several months.

Platelet Formation

  • Platelets (thrombocytes) are small, membrane-enclosed elements formed through fragmentation of mature megakaryocytes.

  • Megakaryoblast: A precursor that is 25-50 μm in diameter, characterized by basophilic cytoplasm and a large ovoid or kidney-shaped nucleus.

  • Undergoes endomitosis (DNA replication without cell division), becoming polyploid (8N to 64N) as they differentiate into megakaryocytes.

Megakaryocyte Characteristics

  • Megakaryocytes: Large cells (up to 150 μm), with highly lobulated, polyploid nuclei and a well-developed cytoplasm full of organelles.

  • Form platelets through extended processes called proplatelets which penetrate vascular endothelium and become exposed to circulating blood.

  • Demarcation Membranes: Previously thought to facilitate platelet release are now understood to serve as a reservoir for membrane during proplatelet elongation.