Hematopoiesis: From the Embryo to Adult

Hematopoiesis: Definition and Historical Milestones

• Definition of Hematopoiesis: Hematopoiesis is a continuous, regulated process of blood cell production. It is a complex system that encompasses:     * Cell renewal: The ability of stem cells to replicate themselves to maintain the population.     * Proliferation: The rapid multiplication of cells.     * Differentiation: The process by which a cell becomes specialized for a specific function.     * Maturation: The stages through which a cell passes from its earliest precursor form to its fully functional, mature state.
• Key Discoveries in Hematology:     * $1642$: Van Leeuwenhoek provides the first notes on cells observed in the blood.     * $1842$: Donne discovers platelets.     * $1846$: Gulliver differentiates lymphocytes from granulocytes based on their size.     * $1874$: Malassez develops the hemacytometer for counting white blood cells (WBCs).     * $1875$: Hayem defines the methods used for counting platelets.     * $1879$: Ehrlich utilizes aniline dyes to stain white blood cells, allowing for better visualization and categorization.

Blood Composition and Reference Values

• Centrifuged Whole Blood Components:     * Plasma ($55\%$ of total volume): Consists of Water ($91.5\%$), Proteins ($7\%$), and Other Solutes ($1.5\%$).     * Buffy Coat: A thin layer containing White Blood Cells (WBCs) and Platelets. WBCs include:         * Granulocytes: Neutrophils, Eosinophils, and Basophils.         * Mononuclear Cells: Lymphocytes and Monocytes.     * Red Blood Cells (RBCs): The bottom layer containing erythrocytes.
• Normal Adult Peripheral Blood Values (Percentage and Absolute counts per $mm^3$):     * Neutrophilic Band: $2-6\%$ ($100-650/mm^3$).     * Segmented Neutrophil: $50-70\%$ ($2400-7500/mm^3$).     * Eosinophil (Segmented): $0-4\%$ ($0-450/mm^3$).     * Basophil (Segmented): $0-2\%$ ($0-200/mm^3$).     * Lymphocyte: $20-44\%$ ($1200-3400/mm^3$).     * Monocyte: $2-9\%$ ($100-900/mm^3$).
• Adult WBC Reference Range: $4.5 \times 10^9/dm^3$ to $11.5 \times 10^9/dm^3$.

Ontogeny and Hematopoietic Stem Cells (HSC)

• Ontogeny (Developmental Timeline):     * Fetus: Hematopoiesis begins in the Yolk Sac within the first month. By the third month, the Liver and Spleen become the primary sites. Around the seventh month, the Bone Marrow begins to take over.     * Adult: Hematopoiesis is restricted to the Axial Skeleton (pelvis, sternum, ribs, vertebrae, skull) and the proximal ends of distal long bones (humerus, femur).
• Stem Cell Experiments (Till and McCullock, 1961):     * They irradiated mice to destroy bone marrow cells.     * Injected fresh bone marrow intravenously.     * Observed colonies of cells on the spleen surface, leading to the term CFU-S (Colony Forming Unit – Spleen).
• Hematopoietic Hierarchy:     * HSC (Pluripotent Hematopoietic Stem Cell): Capable of self-renewal and self-replication.     * MPP (Multipotent Progenitor): Differentiates into common progenitors.     * CMP (Common Myeloid Progenitor): Gives rise to:         * MEP (Megakaryocyte-Erythroid Progenitor): Produces Erythrocytes and Platelets.         * GMP (Granulocyte-Monocyte Progenitor): Produces Neutrophils, Eosinophils, Basophils, and Monocytes.     * CLP (Common Lymphoid Progenitor): Gives rise to B-lymphocytes, T-lymphocytes, and NK cells.

Cytokines and Growth Factors

• General Properties: Glycoproteins that regulate proliferation, differentiation, and maturation. They act synergistically, decrease the transit time from $G_0$ to $G_1$, and are effective at very low concentrations.
• Major Growth Factors and Interleukins:     * Epo (Erythropoietin): Produced in kidneys/liver; stimulates erythroid progenitors to form erythrocytes.     * G-CSF (Granulocyte CSF): Produced by macrophages, endothelial cells; stimulates granulocyte production.     * GM-CSF (Granulocyte-Macrophage CSF): Stimulates progenitors for neutrophils, eosinophils, and monocytes.     * Tpo (Thrombopoietin): Produced in the liver; stimulates megakaryocyte and platelet formation.     * SCF (Stem Cell Factor/c-kit ligand): Essential for HSC maintenance.     * IL-3 (Multi-CSF): Supports multilineage hematopoietic growth.     * IL-5: Specific for B-cell development and eosinophil differentiation/activity.     * IL-7: Essential for T- and B-cell growth and lymphoid differentiation.     * IL-11: Involved in megakaryocyte maturation and osteoblast differentiation.

Cluster of Designation (CD) Nomenclature

• Origin: Proposed at the $1^{st}$ International Workshop and Conference on Human Leukocyte Differentiation Antigens (HLDA) in $1982$.
• Definition: A cluster of monoclonal antibodies that recognize a specific molecule expressed on a cell surface or intracellular protein. Nomenclature now managed by the Human Cell Differentiation Molecules (HCDM) council.
• Key CD Markers:     * HSC/Stem Cell: $CD34^+$, $CD117^+$, $CD90^+$, $CD38^-$, $CD33^-$, $lin^-$ ($CD34$ is also on endothelial cells).     * T Cells: $CD2$, $CD3$ (signal transduction), $CD4$ (Helper T/Monocytes), $CD8$ (Cytotoxic T).     * B Cells: $CD19$, $CD20$, $CD21$ (EBV receptor), $CD23$.     * Granulocytes: $CD33$, $CD66b$.     * Monocytes: $CD14$ (LPS receptor).     * NK Cells: $CD16$, $CD56$.     * Erythrocytes: $CD235a$.     * Platelets: $CD41$, $CD61$, $CD62$.

Erythropoiesis: The Red Cell Series

• Erythron Definition: The total collection of all stages of erythrocytes throughout the body (Bone Marrow, spleen, liver).
• Maturation Trends: Cell diameter decreases, nuclear diameter decreases, chromatin becomes increasingly condensed/clumped, and the $N:C$ ratio decreases.
• Nomenclature Systems:     * Normoblastic: Pronormoblast -> Basophilic Normoblast -> Polychromatic Normoblast -> Orthochromatic Normoblast -> Reticulocyte -> Erythrocyte.     * Rubriblastic: Rubriblast -> Prorubricyte -> Rubricyte -> Metarubricyte -> Reticulocyte -> Erythrocyte.     * Erythroblastic: Proerythroblast -> Basophilic Erythroblast -> Polychromic Erythroblast -> Orthochromic Erythroblast -> Reticulocyte -> Erythrocyte.
• Morphological Stages:     1. Pronormoblast (Rubriblast): Size $14-24\,\mu m$; $N:C$ ratio $8:1$ to $6:1$; Royal blue cytoplasm; $0-2$ nucleoli.     2. Basophilic Normoblast (Prorubricyte): Size $12-17\,\mu m$; $N:C$ ratio $6:1$ to $4:1$; Dark blue cytoplasm; Indistinct nucleoli.     3. Polychromatophilic Normoblast (Rubricyte): Size $10-15\,\mu m$; $N:C$ ratio $4:1$ to $2:1$; Bluish-pink cytoplasm (first sign of hemoglobin).     4. Orthochromatic Normoblast (Metarubricyte): Size $8-12\,\mu m$; $N:C$ ratio $1:1$ to $1:2$; Pyknotic (solid blue-purple) nucleus; Pinkish cytoplasm.     5. Reticulocyte (Polychromatophilic Erthyrocyte): Size $7-10\,\mu m$; Nucleus extruded; Clear gray-blue color; RNA remains (visualized with New Methylene Blue stain).     6. Mature Erythrocyte: Size $7-8\,\mu m$; Biconcave disc; Pink; No nucleus or RNA.

Myelopoiesis: The Granulocytic Series

• Common Progression: Myeloblast -> Promyelocyte -> Myelocyte -> Metamyelocyte -> Band -> Segmented Granulocyte.
• Neutrophil Stages:     * Myeloblast: Size $15-20\,\mu m$; $N:C$ ratio $7:1$ to $5:1$; No granules; $1-3$ nucleoli.     * Promyelocyte: Size $12-24\,\mu m$; Primary (azurophilic) granules appear. Contains myeloperoxidase, defensins, acid hydrolases.     * Myelocyte: Size $10-18\,\mu m$; "Dawn of neutrophilia"; Secondary (specific) granules appear. Last stage of mitosis.     * Metamyelocyte: Size $10-18\,\mu m$; Nucleus is kidney-bean shaped/indented; Secondary granules abundant; Tertiary granules begin.     * Band: Size $10-16\,\mu m$; Nucleus is C or U shaped (horseshoe) with uniform thickness. Found in Bone Marrow ($10-35\%$ ) and peripheral blood.     * Segmented Neutrophil (PMN): Size $10-16\,\mu m$; $2-5$ lobes connected by thin filaments.
• Neutrophil Granule Contents:     * Primary: Myeloperoxidase, Cathepsins, Defensins.     * Secondary: Lactoferrin, Collagenase, Gelatinase, $B_2$-microglobulin.     * Tertiary: Lysozyme, Acetyltransferase.     * Secretory: $CD11b/CD18$, Alkaline phosphatase.
• Eosinophil Characteristics: IL-3, IL-5, and GM-CSF are primary growth factors. Granules contain Major Basic Protein (MBP) in the core and Eosinophil Cationic Protein in the matrix. Functions include defense against parasitic infections and antigen presentation.
• Basophil Characteristics: Least numerous ($<1\%$ ). IL-3 and IL-25 are key cytokines. Granules contain high concentrations of Histamine and Heparan (chondroitin sulfates). Express IgE receptors.

Monocyte and Macrophage Maturation

• Developmental Stages:     * Monoblast: Size $15-25\,\mu m$; $N:C$ ratio $7:1$ to $4:1$; Eccentric nucleus with thin, thready chromatin.     * Promonocyte: Size $14-20\,\mu m$; Indented/folded nucleus; May have "bleblike" pseudopodia.     * Monocyte: Size $12-18\,\mu m$; "Ground glass" cytoplasm; Brain-like nuclear convolutions.     * Macrophage: Size $25-80\,\mu m$; Professional phagocytes in tissues. Named by location: Kupffer cells (liver), Alveolar macrophages (lungs), Microglia (brain), Osteoclasts (bone).
• Functions: Phagocytosis via Toll-Like Receptors (TLRs) and Opsonin Receptors (Complement and Fc receptors).

Lymphopoiesis and Specialized Cells

• B-Cell Maturation: Pro-B -> Pre-B -> Immature B -> Mature B. Involves RAG expression and TdT expression for gene recombination ($V, D, J$ gene segments). Final stage of differentiation is the Plasma Cell ($9-20\,\mu m$), which produces antibodies.
• T-Cell Maturation: Occurs in the Thymus. Stages include Pro-T, Pre-T, Double Positive ($CD4^+CD8^+$), and Single Positive ($CD4^+$ or $CD8^+$).
• NK Cells: Large granular lymphocytes ($CD56^+CD16^+$). They are peroxidase negative but perforin positive.
• Osteoblasts/Osteoclasts:     * Osteoblast: Up to $30\,\mu m$; Round, eccentric nucleus; Resembles plasma cells; Involved in bone formation.     * Osteoclast: Very large ($>100\,\mu m$); Multinucleated; Involved in bone resorption.
• Mast Cells: Not considered a leukocyte group; move from blood to tissues where they mature under Kit Ligand stimulation to mediate allergic responses.

Hematopoietic Kinetics and Stem Cell Cycle

• Cell Cycle Phases:     * $G_0$: Resting stage.     * $G_1$: Growth and component synthesis.     * $S$: DNA replication.     * $G_2$: Premitotic stage.     * $M$: Mitosis.
• Transit and Storage (Neutrophils):     * Proliferative Pool: $2.1 \times 10^9$ cells/kg.     * Maturation Pool: $5.6 \times 10^9$ cells/kg.     * Transit Time: Myeloblast to Myelocyte takes approximately $6$ days.
• Eosinophil Kinetics:     * Turnover: $2.2 \times 10^8$ cells/kg per day.     * Half-life: $18$ hours in circulation.     * Tissue time: $2-5$ days.
• Platelet Properties: Megakaryoblasts undergo endomitosis (doubling DNA without division), creating polyploid nuclei. One megakaryocyte ($35-160\,\mu m$) produces $2000-7000$ platelets. Platelets live for $7-10$ days; $30\%$ are stored in the spleen.

Case Report: Sickle Cell Disease (SCD) and HSCT

• Patient: $8$-year-old male with SCD and history of ischemic stroke ($8$ months age); splenectomy ($3$ years age).
• Donor: Younger sister ($1$ year $5$ months).
• HSC Collection:     * $1^{st}$ Harvest: Volume $191\,cm^3$; Total nucleated cells $1.34 \times 10^8/kg$; $CD34^+$ cells $4.42 \times 10^6/kg$.     * $2^{nd}$ Harvest: Volume $173\,cm^3$; Total nucleated cells $1.97 \times 10^8/kg$; $CD34^+$ cells $5.06 \times 10^6/kg$.
• Outcome: Successful Hematopoietic Stem Cell Transplant (HSCT). Four months post-transplant, the patient had $85\%$ donor host hematopoietic chimerism, which was sufficient to cure the genetic defect.