3.1 Hematopoiesis, RBC Disorders & Anemias

Hematopoiesis: Sites, Stem Cells, and Lineages

Hematopoiesis is the ordered production of blood cells from multipotent hematopoietic stem cells (HSCs). Embryologically, blood formation begins in the yolk sac, shifts to the fetal liver and spleen, and after birth resides primarily in bone marrow. In adults, active marrow is concentrated in vertebrae, pelvis, sternum, ribs, and proximal long bones; extramedullary hematopoiesis in liver or spleen reappears when marrow is destroyed, fibrotic, or insufficient (myelophthisis, severe hemolytic anemias, myeloproliferative neoplasms).

HSCs sit at the apex of a hierarchy. They self-renew and give rise to common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs). CMPs generate erythrocytes, megakaryocytes/platelets, neutrophils, monocytes/macrophages, eosinophils, and basophils. CLPs generate B cells, T cells, and NK cells. Lineage commitment is controlled by transcription-factor networks (for example, GATA-1 for erythroid/megakaryocytic programs; PU.1 for myeloid programs) and by growth-factor niches in marrow stroma.

Erythropoietin Physiology

Erythropoietin is produced mainly by renal peritubular interstitial fibroblasts. When local oxygen tension falls, hypoxia-inducible factor (HIF) is less hydroxylated by prolyl hydroxylases, escapes VHL-mediated degradation, and transcriptionally upregulates EPO. EPO binds a receptor on CFU-E and late erythroid progenitors, activating JAK2–STAT5 survival signaling and reducing apoptosis. Without adequate EPO (chronic kidney disease), the marrow underproduces red cells despite iron stores that may be normal, yielding a normocytic, hypoproliferative anemia with a low reticulocyte count.

Reticulocytes are young anucleate red cells released after nuclear extrusion. An appropriate marrow response to anemia raises the reticulocyte count (or reticulocyte index after correction for anemia severity). A low reticulocyte response in anemia signals underproduction; a high response signals blood loss or hemolysis.

---

Anemia Framework: MCV First, Then Mechanism

CBSE-style questions almost always reward a disciplined sequence: quantify anemia → classify by mean corpuscular volume (MCV) → assess marrow response with reticulocytes → refine with iron studies, hemolysis labs, and peripheral smear. Red cell distribution width (RDW) rises when red cells are heterogeneous in size (for example, iron deficiency or mixed deficiencies) and is often lower/normal in uniform thalassemic microcytosis.

Microcytic Anemias — Compare Mechanisms

Iron deficiency anemia reflects depleted iron stores needed for heme synthesis. Ferritin is low, serum iron is low, total iron-binding capacity (TIBC) is high (transferrin upregulated), and transferrin saturation is low. RDW is often high. Clinical clues include chronic blood loss (GI, menstrual), malabsorption, or increased demand. Smear may show hypochromic microcytes and pencil cells; chronic severe deficiency can add thrombocytosis from reactive cytokine effects.

Anemia of chronic disease (anemia of inflammation) is driven by inflammatory cytokines, especially IL-6, which increase hepatic hepcidin. Hepcidin binds ferroportin on enterocytes and macrophages, internalizing the iron exporter and trapping iron in stores. Result: low serum iron, low TIBC, normal or high ferritin, and a hypoproliferative anemia that is often normocytic early and microcytic later. The mechanism is iron sequestration, not true store depletion.

Thalassemias are quantitative defects in globin-chain synthesis. Alpha-thalassemia (chromosome 16 deletions) and beta-thalassemia (chromosome 11 point mutations/splicing defects) create imbalance: excess unmatched chains precipitate, damage membranes, and cause ineffective erythropoiesis plus variable hemolysis. MCV is disproportionately low relative to the degree of anemia; RBC count is often normal or high; RDW may be relatively normal in trait. Beta-thalassemia major/intermedia shows elevated hemoglobin A2 (and often F) on electrophoresis; severe alpha-chain loss yields hemoglobin H (β4) disease or hemoglobin Bart's (γ4) hydrops fetalis.

Sideroblastic anemia reflects defective heme synthesis within mitochondria. Iron enters erythroblasts but cannot be incorporated into protoporphyrin, forming ring sideroblasts (iron-laden mitochondria ringing the nucleus on Prussian blue stain). Causes include congenital ALA synthase defects, vitamin B6 (pyridoxine) deficiency, isoniazid (interferes with B6), alcohol, copper deficiency, and lead poisoning (inhibits ferrochelatase and ALA dehydratase). Iron studies often show high ferritin and high serum iron with low TIBC—opposite of iron deficiency.

Normocytic Patterns

Early iron deficiency, acute blood loss (before volume equilibration and reticulocytosis fully declare), pure hemolysis, chronic kidney disease, aplastic anemia, and marrow replacement all sit here until secondary features appear. Aplastic anemia is pancytopenia with a hypocellular marrow from immune attack, drugs, radiation, viruses (parvovirus B19 on erythroid precursors can be pure red cell aplasia), or inherited marrow-failure syndromes. Myelophthisic anemia from space-occupying lesions can release teardrop cells and immature myeloid/erythroid precursors (leukoerythroblastic picture).

Macrocytic Anemias — Megaloblastic vs Nonmegaloblastic

Megaloblastic anemia results from impaired DNA synthesis with relatively preserved RNA/protein synthesis, producing nuclear-cytoplasmic asynchrony, large cells, and ineffective hematopoiesis. Vitamin B12 (cobalamin) and folate deficiencies are the classic causes; both impair thymidylate synthesis. Hypersegmented neutrophils (≥5 lobes) are a high-yield smear clue. B12 requires intrinsic factor from gastric parietal cells for terminal ileal absorption; pernicious anemia (autoimmune destruction of parietal cells/IF), terminal ileum disease, strict vegan diet, and metformin or PPI associations appear in vignettes. Folate stores are smaller and depleted faster with poor diet, increased demand, or methotrexate (dihydrofolate reductase inhibition).

Biochemically, both deficiencies raise homocysteine. Only B12 deficiency also raises methylmalonic acid, because methylmalonyl-CoA mutase needs adenosylcobalamin. Neurologically, B12 deficiency damages posterior columns and lateral corticospinal tracts (subacute combined degeneration); folate deficiency does not classically cause this myeloneuropathy. Nonmegaloblastic macrocytosis accompanies alcohol toxicity, liver disease (cholesterol-laden membranes), hypothyroidism, and marked reticulocytosis (reticulocytes are larger than mature RBCs).

---

Hemolysis: Extravascular vs Intravascular Categories

Hemolysis is premature red-cell destruction. Shared laboratory signatures: elevated LDH, elevated indirect (unconjugated) bilirubin, decreased haptoglobin (binds free hemoglobin), and reticulocytosis. Jaundice and pigment gallstones follow chronic indirect hyperbilirubinemia.

Extravascular hemolysis occurs mainly in splenic and hepatic macrophages. Membrane defects, antibody-coated cells, and many enzyme/hemoglobin disorders are cleared here. Splenomegaly and spherocytes are common. Intravascular hemolysis releases hemoglobin directly into plasma: hemoglobinemia, hemoglobinuria, hemosiderinuria, and more profound haptoglobin consumption. Causes include mechanical fragmentation (MAHA), complement-mediated lysis (PNH, ABO incompatibility), severe G6PD oxidative crises, and some toxins.

High-Yield Hemolytic Entities

Hereditary spherocytosis is usually an autosomal dominant defect in red-cell membrane–cytoskeleton proteins (ankyrin, spectrin, band 3). Loss of surface area produces spherocytes that are less deformable and trapped in the spleen. Labs: extravascular hemolysis pattern, increased MCHC, osmotic fragility. Splenectomy reduces hemolysis but raises infection risk from encapsulated organisms.

G6PD deficiency is X-linked. Reduced NADPH generation impairs glutathione regeneration, so oxidative stress (primaquine, sulfa drugs, dapsone, nitrofurantoin, fava beans, infection) denatures hemoglobin into Heinz bodies; bite/blister cells appear after splenic pitting. Episodes are often intravascular and self-limited as older enzyme-poor cells are cleared.

Sickle cell disease (HbS, β6 Glu→Val) polymerizes deoxygenated hemoglobin, distorting cells, causing vaso-occlusion and chronic hemolysis. Triggers include hypoxia, acidosis, dehydration, and infection. Howell-Jolly bodies appear after functional asplenia. Heterozygous sickle trait is usually milder but can show sickling under extreme stress. Compound heterozygotes (HbSC, sickle-β-thalassemia) modify severity.

Autoimmune hemolytic anemia: warm IgG typically causes extravascular spherocytic hemolysis (DAT positive for IgG); cold IgM fixes complement and can cause agglutination and intravascular features (DAT positive for complement).

Microangiopathic hemolytic anemia (MAHA) produces schistocytes from fibrin strand shearing (DIC, TTP, HUS, malignant hypertension, mechanical valves).

Peripheral Smear Shortcuts

Putting Lab Patterns Together

Use one integrated board algorithm: (1) MCV classifies synthesis vs DNA vs mixed problems; (2) reticulocyte count separates hypoproliferative from regenerative anemias; (3) iron studies distinguish iron deficiency, anemia of inflammation, and sideroblastic overload patterns; (4) hemolysis panel (LDH, bilirubin, haptoglobin) plus smear localizes destruction; (5) specialty tests (hemoglobin electrophoresis, DAT, B12/folate/MMA/homocysteine, osmotic fragility, G6PD assay after crisis resolution) confirm mechanism. CBSE favors the physiologic link—hepcidin–ferroportin, EPO–JAK2, membrane protein loss, oxidative NADPH failure, HbS polymerization—over memorizing isolated numbers.