CHAPTER 1: Anatomy & Physiology of Hematopoiesis

History of Hematopoiesis

• 5th century BC – Empedocles articulates that “blood is life.”
• Polibus (son-in-law of Hippocrates) proposes four‐humor theory (blood, phlegm, black bile, yellow bile).
• 16th century – Servetus recognises systemic vs. pulmonary circulation (burned for heresy).
• 1628 – William Harvey publishes De Motu Cordis; modern circulation theory begins.
• 1650-1700 – Swammerdam (first view of RBCs), Malpighi (capillary networks).
• 1800s – Neumann (1868) proves marrow origin of erythrocytes; modern hematopoiesis science starts.

Phylogeny & Evolutionary Themes

• Packaging Hb inside cells avoids plasma oncotic overload; allows high \mathrm{O2} transport. • Renewal rate of red cells ∝ metabolic rate; e.g., turtle vs. pygmy shrew. • Environmental \mathrm{pO2} regulates erythropoiesis: eels & Daphnia produce Hb only under hypoxia.
• Molecular “oxygen sensors” = HIF-1α/β pathway.

Marrow Anatomy

• Red-marrow volume: child \approx 1000–1400\;g; adult \approx 1200–1500\;g despite 5-fold body-weight difference.
• Sinusoidal network lined by endothelial cells; reticular fibroblastoid–VCAM-1 lattice anchors HSCs via VLA-4 → fibronectin.
• Niches
– Endosteal/osteoblastic: quiescence & self-renewal (SDF-1, osteopontin, Jagged-Notch).
– Perivascular/sinusoidal: activation, mobilisation (CXCL12, angiopoietin-1).
• Extramedullary hematopoiesis: liver, spleen, lymph nodes, adrenals, cartilage, adipose, paravertebral gutters, kidney under stress (myeloid metaplasia).

Cellular Hierarchy

• Long-term repopulating HSC (LTR-HSC)
– Mouse phenotype: CD34− CD150+ CD48− CD41− Flt3− CD49b^lo.
– Human: CD34+ CD38− CD45RA− CD90+ CD49f+.
• Intermediate: STR-HSC → Multipotent progenitor →
– Common lymphoid progenitor (CLP).
– Common myeloid progenitor (CMP).
• CMP →
– Granulocyte-monocyte progenitor (GMP).
– Megakaryocyte-erythroid progenitor (MEP).
• Committed units:
– BFU-E → CFU-E → proerythroblast.
– CFU-G, CFU-M, CFU-GEMM, CFU-Meg, CFU-Eo, etc.

Key Hematopoietic Growth-Factor Families

• Cytokine receptor class I (WSXWS, JAK–STAT): IL-2,3,4,5,6,7,9,11,12,13,15,21; GM-CSF; G-CSF; EPO; TPO; LIF; OSM; CNTF; CT-1.
• Tyrosine-kinase ligands: SCF ↔ KIT; FLT3-L ↔ FLT3; M-CSF ↔ FMS.
• IL-1 family (IL-1α/β, IL-18): early-acting synergy.
• Chemokines: SDF-1/CXCL12 ↔ CXCR4 (homing, mobilisation); MIP-1α (quiescence).
• TNF family: TNF-α, Fas-L, TRAIL (apoptosis, dendritic activation).
• Interferons: IFN-α/β/ω (antiviral), IFN-γ (Th1 polarisation).
• TGF-β superfamily: TGF-β1 (strong negative regulator), BMP-4 (embryonic HSC induction), GDFs.
• Emerging stem-cell regulators:
– Wnt/β-catenin (self-renewal ↑; constitutive activation → aplasia).
– Notch-Jagged/Delta (HSC expansion; critical in T-cell lineage).
– Hedgehog (Shh–Smo–Gli; links to BMP-4).
– Angiopoietin-like 2/3 (×20–30 LTR-HSC expansion).

Lineage-Specific vs. Multilineage HGFs

• Predominantly lineage-specific:
– G-CSF → neutrophils.
– M-CSF → monocytes/macrophages.
– IL-5 → eosinophils.
– EPO → erythrocytes.
– TPO → megakaryocytes/platelets.
• Multilineage: IL-3, GM-CSF (act on CFU-GEMM, CMP).
• Early acting: SCF, FLT3-L (trigger HSC cycling with IL-3/IL-6, etc.).
• Synergists: IL-1, IL-6, IL-11 (“burst-promoting activity”).
• Negative regulators: TGF-β, SDF-1, MIP-1α.

Receptor Architecture & Signalling

• Class I/II cytokine Rs: extracellular fibronectin type III repeats with \text{WSXWS}; lack intrinsic kinase → recruit JAK1/2/3/TYK2.
• Homodimeric Rs (EPO-R, TPO-R/MPL, G-CSF-R) predominantly use JAK2 → STAT5 activation.
• Heterodimeric Rs share signalling chains:
– βc: IL-3, GM-CSF, IL-5.
– gp130: IL-6, IL-11, LIF, OSM.
– γc: IL-2,4,7,9,15,21.
• Receptor tyrosine kinases: KIT, FLT3, FMS → RAS/MAPK, PI3K/AKT, PLCγ.
• Negative feedback: SOCS-1/3, SHP-1/PTPN6 (bind EPOR Y429), SHP-2/PTPN11 (mutated in Noonan/JMML).

Kinetic Compartment Models

• Neutrophil system (steady-state adult):
– Mitotic pool \approx 2.6\times10^{9}\,\text{cells/kg}; transit 66\,h.
– Maturation/storage ≈2.7\times10^{9}\,\text{cells/kg}; 95\,h.
– Circulating vs. marginated pool ≈ 1:1; intravascular half-life 6–8\,h.
• Erythroid maturation: 5 days marrow transit under normal, 1–2 days in stress (skipped divisions → macrocytosis, i-antigen retention).

Molecular Dysfunctions & Mouse Knock-Outs

• G-CSF−/− → neutrophils 20-30 % normal, infection susceptibility.
• GM-CSF−/− → normal counts but alveolar proteinosis (macrophage surfactant-clearance defect).
• EPO−/− or EPOR−/− → embryonic lethality (fetal-liver failure).
• Mpl−/− → platelets 15-20 % normal; reduced HSC competitiveness.
• SOCS-3−/− → embryonic erythrocytosis (EPO hypersensitivity).

Clinical Applications

• rhEPO: anemia of chronic kidney disease, chemo-induced anemia (target \le 12\;g/dL to minimise \mathrm{TE}_{\text{risk}}).
• G-CSF: prophylaxis/mobilisation (\uparrow neutrophils, CD34+ yield); dosing 5\,\mu g/kg q.d. SC typical.
– SCN responders ↓ infections but long-term risk of MDS/AML esp. with high-dose G-CSF & G-CSF-R truncating mutations.
• GM-CSF: pulmonary alveolar proteinosis therapy; adjuvant in dendritic-cell vaccines.
• TPO mimetics (romiplostim, eltrombopag): chronic immune thrombocytopenia; monitor for marrow reticulin.
• Plerixafor (CXCR4 antagonist) + G-CSF: enhanced HSC mobilisation for autologous transplant.

Ethical & Practical Implications

• Over-correction (EPO to Hb > 13.0\,g/dL) → \uparrow cardiovascular/thrombotic risk; FDA black-box warnings.
• G-CSF in healthy donors: bone pain, transient leukocytosis; rare splenic rupture.
• Potential leukemogenic synergy: long-term cytokine pressure + congenital mutations (e.g., SCN).

Key Equations & Numerical References

• Stem-cell symmetric division probability P{\text{self}} must satisfy P{\text{self}} = 0.5 for steady-state pool (Till & McCulloch stochastic model).
• Physiologic [\text{EPO}]–\text{Hb} feedback approximated: \Delta\,[\text{EPO}]\,\propto\,e^{-k\,\mathrm{pO2}}. • Platelet mass feedback on TPO: [\text{TPO}]{\text{plasma}} \approx \frac{\text{constant}}{\text{Platelet\;mass}} (clearance model).

Integrative Connections

• Embryology: runx1, GATA2, Notch, BMP-4 orchestrate endothelial-to-HSC transition in AGM; parallels vasculogenesis.
• Immunology: GM-CSF crucial for dendritic-cell differentiation; CSF cross-talk with Th1/Th17 pathways (IL-12, IL-23).
• Oncology: JAK2 V617F in PV/ET; CSF-R truncations in leukemic evolution; therapeutic CSFs can mask chemo toxicity.
• Gene therapy: HOXB4 over-expression + iPSC‐derived blood offers route to autologous HSC correction (Rag2−/− mouse proof-of-concept).