Normal Blood & Hematopoiesis
Learning objectives
- LO 1Name every normal blood cell and how much of the white count each one makes up.
- LO 2Explain how one stem cell splits into a myeloid branch and a lymphoid branch.
- LO 3Walk each cell line from blast to mature cell and recognize every stage by its key features.
MAPBig-picture map of hematopoiesis
Memory tool — the family tree
The stem cell is the grandparent who can make anyone. Myeloid and lymphoid stem cells are two parents, each able to make only their side of the family. CFUs are the specialists that make one kind of child. Going down the tree, cells lose options and divisions but gain function.
LO 1Normal blood cells and their frequency
Q1. What is blood, as a tissue?
Blood is a highly specialized connective tissue: cells suspended in a liquid matrix (plasma). Its job is transport: O₂ and nutrients to tissues, CO₂ and waste away, cells, hormones and chemical mediators wherever needed, and heat for temperature regulation.
Q2. If you spin a tube of anticoagulated blood, what do you see?
Plasma in one breath
- 90% water, 10% solutes
- Albumin = 50–60% of plasma protein; made by the liver; keeps oncotic pressure; carries hydrophobic cargo
- Globulins (α, β, γ); γ = immunoglobulins from plasma cells
- Fibrinogen ≈7% → becomes fibrin
- Also complement, lipoproteins, glucose, amino acids, hormones, electrolytes, gases
CheckWhy is the buffy coat so thin and why does it sit in the middle?
Memory tool
Plasma − fibrinogen = serum. Let blood clot, fibrinogen is used up, the leftover fluid is serum.
Q3. What are the formed elements, and how do you sort the WBCs?
| Family | Members | Defining feature |
|---|---|---|
| Erythrocytes | RBCs | No nucleus; packed with hemoglobin |
| Platelets | Thrombocytes | Small packets of megakaryocyte cytoplasm; no nucleus |
| Granulocytes | N Neutrophil · E Eosinophil · B Basophil | Specific granules + segmented (lobed) nucleus |
| Agranulocytes ("mononuclear") | L Lymphocyte · M Monocyte | No specific granules (still have a few non-specific azurophilic ones); round or indented nucleus |
Memory tool
Granulocytes are the "BEN" cells: Basophil, Eosinophil, Neutrophil. Lymphocytes and monocytes are agranular.
Q4. What are the normal CBC values?
| Parameter | Women | Men | Shortcut |
|---|---|---|---|
| Leukocytes (WBC) | 4,500 – 11,000 /mm³ | "4.5 to 11" | |
| Hematocrit | 36 – 48% | 40 – 54% | Hct ≈ 3 × Hb |
| Hemoglobin | 12.1 – 15.1 g/dL | 13.8 – 17.2 g/dL | Men higher (testosterone → EPO) |
| Platelets | 150,000 – 450,000 /mm³ | "150 to 450" | |
| Reticulocytes | ≈1% of RBCs | Marrow's RBC output gauge | |
Q5. How common is each WBC? (the differential count)
Differential count = share of each leukocyte type, as % of total WBCs or an absolute number per volume.
Memory tool — most to least common
Never Let Monkeys Eat Bananas → Neutrophil > Lymphocyte > Monocyte > Eosinophil > Basophil.
In-house exam
Know the order and the numbers (60–70 / 28 / 5 / 2–4 / 0.5) and the one-word job of each: neutrophil → bacteria; lymphocyte → humoral and cellular immunity; monocyte → becomes phagocytic macrophage; eosinophil → parasites; basophil → inflammation. Also: diapedesis = migration of leukocytes between endothelial cells into connective tissue.
Q6. How do we see these cells? (Romanovsky stains)
Smears are treated with an alcohol-based Romanovsky-type stain (Wright's, Giemsa), which fixes and stains at the same time and gives four patterns:
| Pattern | Color | Where you see it |
|---|---|---|
| Basophilic | Blue | Basophil granules; ribosome-rich cytoplasm (lymphocytes, young precursors) |
| Azurophilic | Purple | Non-specific (primary) granules — present in all leukocytes |
| Eosinophilic | Orange / pink-red | Eosinophil granules; hemoglobin (RBCs) |
| Neutrophilic | Salmon pink | Neutrophil specific granules |
Q7. How do I recognize each mature cell, and what does it do?
Erythrocyte RBC
Most numerous cell · 120 days- Biconcave disc, 8 µm × 2 µm; no nucleus, no mitochondria, no ribosomes → no protein synthesis
- Squeezes through 3–4 µm capillaries; withstands 5× serum osmotic pressure
- Energy = anaerobic glycolysis
- Removed by splenic macrophages → Fe + bilirubin
Neutrophil (PMN) 60–70%
6 h in blood · 1–4 days in tissue- 2–5 lobes; pale salmon-pink cytoplasm
- Drumstick in ≈3% of female PMNs = inactive X (Barr body)
- Kills bacteria and fungi; leads acute inflammation
Eosinophil 2–4%
Lives in GI, respiratory, lower GU tissue- Bilobed nucleus + large orange-red granules
- Kills parasites (helminths); "damage control" in allergy
Basophil 0.5%
Rarest WBC- Dark blue-purple granules hide a 2–3-lobed nucleus
- IgE on surface → histamine, heparin release; inflammation, recruits eosinophils
Lymphocyte 28%
Days to years- Small (8–9 µm ≈ RBC); round dark nucleus, thin rim of blue cytoplasm
- Activated: up to 16 µm
- Only WBC that recirculates, can still divide, lives years
Monocyte 5%
40 h in blood · months in tissue- Largest WBC; eccentric kidney/horseshoe nucleus, not lobed; 2–3 nucleoli
- Grey-blue "frosted glass" cytoplasm, vacuoles
- Becomes macrophage; does not re-enter blood
Platelet 150–450k
10 days · ⅓ stored in spleen- 2–3 µm fragment; no nucleus but has mitochondria
- Pale hyalomere rim + purple granulomere center
- Clump on smears; stick at once to subendothelial collagen
Deep dive · Erythrocyte structure
- Membrane skeleton: spectrin + actin lattice, attached to the membrane by ankyrin → keeps the biconcave shape, resists shear
- Membrane proteins: glycophorin (transmembrane glycoprotein), band 3 (anion channel, Cl⁻/HCO₃⁻); carbohydrate groups set the ABO blood type and their negative charge prevents clumping
- Hemoglobin: 4 heme groups (iron = O₂-binding site, porphyrin ring) + globin tetramer. Adults: HbA ≈97%, HbA2 2–3%, HbF <1%
- Reticulocyte: new RBC with residual rRNA for 1–2 days; looks polychromatophilic (bluish) on Wright stain; supravital stain (brilliant cresyl blue) shows the reticulum
Step 1
- Spectrin / ankyrin / band 3 defect → hereditary spherocytosis (no central pallor, ↑ MCHC, splenomegaly).
- No mitochondria → total dependence on glycolysis → pyruvate kinase deficiency causes hemolysis. 10% of glucose goes to the HMP shunt → G6PD deficiency.
- Neonatal RBC lifespan is only 60–90 days.
Deep dive · Neutrophil granules and killing
| Azurophilic (primary, non-specific) | Specific (secondary) | |
|---|---|---|
| Stain | Reddish-purple | Salmon pink |
| Share | 10–20% of granules | The majority; tiny, near the limit of light microscopy |
| Contents | Myeloperoxidase, defensins, bactericidal permeability-increasing protein, acid hydrolases, proteases | Lactoferrin, NADPH oxidase, lysozyme, B12-binding protein, alkaline phosphatase |
| Nature | Lysosomes; large, dense, peroxidase + | Small, pale, peroxidase − |
| Made in | Promyelocyte | Myelocyte |
Getting to the infection
- Margination: rolling (selectins) and sticking (integrins)
- Diapedesis between endothelial cells
- Chemotaxis toward chemokines from inflamed tissue
Killing the microbe
- Recognition via receptors
- Phagocytosis → phagosome; much better if microbe is opsonized (antibody or complement)
- O₂-dependent: respiratory burst → O₂⁻ → H₂O₂ → MPO makes HOCl (bleach)
- O₂-independent: acidified phagosome + granule enzymes
- Few mitochondria, abundant glycogen → works in low-O₂ infected tissue
- Pools: 50% circulating, 50% marginated (stuck to endothelium, esp. lung) + a marrow reserve 10× the blood amount
Step 1
- NADPH oxidase defect → chronic granulomatous disease. MPO deficiency → mostly benign, ↑ Candida.
- Neutrophil chemotactic agents: C5a, IL-8, LTB4, 5-HETE, kallikrein, PAF, N-formylmethionine.
- Glucocorticoids → demargination → ↑ neutrophil count without infection; ↓ eosinophils and lymphocytes.
- Toxic granulation and Döhle bodies = reactive neutrophils in infection. Leukoerythroblastic reaction = left shift + immature RBCs → marrow infiltration (myelofibrosis, metastasis).
Deep dive · Eosinophil and basophil granules
Eosinophil specific granule
- Externum (outer, less dense): peroxidase, hydrolytic lysosomal enzymes
- Internum (crystalloid core):
- Major basic protein: most abundant; anti-parasitic; gives the orange color
- Eosinophil cationic protein: most potent tissue-damaging protein
- Eosinophil-derived neurotoxin
- Anti-inflammatory: phagocytoses Ag–Ab complexes; neutralizes mast cell / basophil mediators
- Drawn in by eosinophil chemotactic factor from mast cells and basophils
Basophil (vs mast cell)
- Preformed: histamine, proteases, heparin (causes the basophilia), ECF
- Made on demand after activation: leukotrienes
- IgE held on surface receptors → antigen cross-links → release
- Mast cells: similar granules but round/oval nucleus, live in tissue, more but smaller granules
Step 1
- Causes of eosinophilia — PACMAN Eats: Parasites, Asthma, Chronic adrenal insufficiency, Myeloproliferative disorders, Allergic processes, Neoplasia (Hodgkin lymphoma), Eosinophilic granulomatosis with polyangiitis.
- Basophilia is uncommon → think CML.
- Mast cell: c-KIT mutation → mastocytosis (↑ tryptase). Vancomycin, opioids, radiocontrast → IgE-independent degranulation. Cromolyn blocks degranulation. Mepolizumab blocks IL-5.
Deep dive · Platelets and the six steps of hemostasis
Platelet anatomy
- Hyalomere: outer, light blue; actin–myosin + marginal bundle of microtubules (keeps the disc)
- Granulomere: central purple; granules ≈20% of volume
- Dense: serotonin, ADP, Ca²⁺
- Alpha: PF4, vWF, fibrinogen, PDGF
- Lambda: lysosomal hydrolases
- Canalicular system (releases granules); dense tubular system
- Membrane glycoproteins (integrins) bind vWF and fibrinogen
Clot formation in order
- Reflex vasoconstriction
- Adhesion to subendothelial collagen via vWF (seconds)
- Activation & release of mediators; disc → sphere with pseudopods; thromboxane made
- Aggregation via fibrinogen → primary plug (primary hemostasis)
- Fibrin from thrombin → firm plug over hours (secondary hemostasis)
- Clot retraction (actin, myosin, ATP)
Step 1
vWF receptor = GpIb (defect: Bernard-Soulier). Fibrinogen receptor = GpIIb/IIIa (defect: Glanzmann). Dense granules = CASH (Ca²⁺, ADP, Serotonin, Histamine). Splenomegaly → more platelets pooled → thrombocytopenia.
Q8. Which properties are shared, and which are unique?
| Property | Who has it | Who doesn't |
|---|---|---|
| Diapedesis | All leukocytes (↑↑ in inflammation) | RBCs and platelets stay in vessels |
| Phagocytosis | Granulocytes and monocytes | Lymphocytes, RBCs |
| Re-enter blood from tissue | Lymphocytes only | All others (monocytes never return) |
| Divide after leaving marrow | Lymphocytes (in response to antigen) | All others |
| Cytokine/chemotactic receptors | Most blood cells | — |
Trap
"Most common WBC" = neutrophil, but "most common cell in blood" = RBC. "Largest WBC" = monocyte. "Largest cell in the marrow" = megakaryocyte (not a blood cell). "Rarest WBC" = basophil.
Q9. A count is abnormal. What does it point to?
Step 1 — which cell is up?
- ↑ Neutrophils + bands (left shift) → bacterial infection; steroids; stress
- ↑ Lymphocytes → viral infection, pertussis, CLL
- ↑ Monocytes → chronic infection (TB), recovery phase
- ↑ Eosinophils → PACMAN Eats
- ↑ Basophils → CML
ApplyLO 1 application questions
A1A child from an endemic area has abdominal pain. WBC 12,500 with 18% of a cell that has a bilobed nucleus and large orange-red granules. Which granule protein does this cell rely on for its main defense role?
A2A febrile patient with lobar pneumonia has WBC 18,000 with many neutrophils whose nucleus is a single unsegmented U shape. What are they and what do they mean?
A3A woman starts high-dose prednisone and two days later her WBC rises from 7,000 to 13,000 with no fever. Mechanism?
A4Which WBC can leave the blood, enter tissue, and come back through lymphatics?
A5A cirrhotic patient with portal hypertension has platelets of 70,000 with a normal marrow. Why?
LO 2Myeloid vs lymphoid split
Q1. Why does the body need hematopoiesis at all?
Hematopoiesis is the production of blood cells. Every blood cell has a finite lifespan (neutrophil ≈6 h in blood, platelet ≈10 days, RBC ≈120 days), so cells must be replaced continuously, under the control of growth factors.
Q2. What is the pluripotent hematopoietic stem cell (PHSC)?
| Feature | What to know |
|---|---|
| Potential | Pluripotent ("hemocytoblast"): can become any blood cell, lymphocytes included. Myeloid and lymphoid stem cells are multipotent and self-renewing. |
| Morphology | No distinguishing features — cannot be identified by looking at it |
| How it was proven | Inject marrow cells into lethally irradiated mice → donor cells form spleen colonies and rescue the animal |
| Markers | CD34+ and c-kit (receptor for stem cell factor) by flow cytometry |
| Frequency | ≈0.01% of nucleated marrow cells; only ≈5% of normal stem cells are needed to repopulate the whole marrow |
| Activity | Mostly not dividing (quiescent) → protects them from depletion. Daily output comes from committed progenitors dividing. |
| Where else | Small numbers in blood; large numbers in umbilical cord blood |
CheckIf stem cells mostly don't divide, where do billions of new cells per day come from?
Step 1
CD34 is how transplant centers count and collect stem cells. Donors receive G-CSF (filgrastim) to mobilize CD34+ cells into blood. CD34 also marks blasts in acute leukemia.
Q3. Which cells come from which branch?
| Feature | Myeloid | Lymphoid |
|---|---|---|
| Mature products | Erythrocytes, megakaryocytes → platelets, granulocytes (N, E, B), monocytes → macrophages | T lymphocytes (cellular immunity), B lymphocytes (humoral immunity) → plasma cells, NK cells |
| Where they mature | Bone marrow | Start in marrow; B cells mature in marrow, T cells mature in thymus; then live in lymphoid organs |
| Committed progenitors | BFU-E / CFU-E, CFU-Meg, CFU-GM, CFU-Eo, CFU-Bas | Lymphoid progenitor → lymphoblast |
| Main growth factors | EPO, TPO, G-CSF, GM-CSF, M-CSF, IL-3, IL-5 | Interleukins (e.g., IL-7), thymic signals |
| Neoplasms | AML, CML, myeloproliferative neoplasms, MDS | ALL, CLL, lymphomas, plasma cell myeloma |
CheckSort: plasma cell, eosinophil, macrophage, NK cell, platelet, T cell.
Memory tool
Lymphoid = the "BTNK club" (B, T, NK + plasma cells). Myeloid = everyone else in blood.
Trap
Monocytes are myeloid, even though they are "mononuclear" agranulocytes like lymphocytes. Neutrophils and monocytes are siblings from CFU-GM. "Agranulocyte" is a smear word (LO 1); "lymphoid" is a lineage word (LO 2).
Q4. What tells each progenitor what to become? (growth factors)
- Bone marrow stromal cells make stem cell factor, which stem cells need to survive. No growth factor → apoptosis.
- Growth factors bind specific receptors and control proliferation, differentiation and activation. They act endocrine (EPO from kidney) or paracrine (local marrow signals).
- Committed progenitors are named for the colony they form in culture: CFU = colony-forming unit.
| Process | Committed progenitor | Growth factor | Product · time |
|---|---|---|---|
| Erythropoiesis | BFU-E (needs high EPO for extensive proliferation) → CFU-E (responds to low EPO, divides faster) | EPO: ≈90% kidney peritubular interstitial cells, ≈10% liver | RBC · ≈7 days |
| Granulopoiesis | CFU-GM (neutrophils), CFU-Eo, CFU-Bas | GM-CSF (also G-CSF; IL-5 for eosinophils) | N, E, B · ≈11 days |
| Monopoiesis | CFU-GM (shared with neutrophils) | GM-CSF, M-CSF | Monocyte → macrophage |
| Thrombopoiesis | CFU-Meg | TPO: liver (and marrow, kidney, other tissues) | Megakaryocyte → platelets |
| Lymphopoiesis | Lymphoid progenitor | Interleukins | B, T, NK cells |
Step 1 — growth factors as drugs
- Epoetin / darbepoetin (EPO; signals through JAK2/STAT) → anemia of CKD, chemo anemia. Watch: hypertension, thrombosis; abuse by athletes.
- Filgrastim (G-CSF) → chemo neutropenia, stem cell mobilization. Sargramostim (GM-CSF) → marrow recovery.
- Romiplostim, eltrombopag (TPO receptor agonists) → ITP.
- Hypoxia → ↑ EPO → secondary polycythemia; EPO-secreting tumors (renal cell carcinoma, HCC, hemangioblastoma) → inappropriate polycythemia.
In-house exam
Know: renal insufficiency → ↓ EPO → anemia; EPO stimulates erythroid progenitor proliferation/differentiation and inhibits apoptosis; rhEPO cut transfusions in dialysis by 70–80%; rhGM-CSF helps leukopenic patients (BMT, cancer therapy, AIDS, aplastic anemia); TPO agonists treat ITP.
Q5. Where is blood made over a lifetime?
| Stage | Site | Key detail |
|---|---|---|
| Primordial | Embryonic yolk sac | Large, nucleated RBCs |
| Hepato-spleno-thymic | Liver, spleen, thymus | Dominant in the 2nd trimester |
| Medullo-lymphatic | Bone marrow + lymphatics | Dominant in the 3rd trimester; the only site at term and after |
Memory tool
"Young Liver Synthesizes Blood" → Yolk sac → Liver → Spleen → Bone marrow.
High yield
Prolonged, severe demand in a child or adult can restart blood production in the fetal sites → extramedullary hematopoiesis in the liver and spleen → hepatosplenomegaly. Step 1 settings: β-thalassemia major ("crew-cut" skull, chipmunk facies), severe hemolysis, primary myelofibrosis and CML (massive spleen).
Q6. Where is blood made inside the adult skeleton?
| Red marrow | Yellow marrow | |
|---|---|---|
| Activity | Hematopoietically active | Inactive: mostly fat + connective tissue |
| Where (adult) | Ribs, sternum, pelvis (also vertebrae, skull) + proximal quarter of humerus and femur | Shafts of long bones |
| How much | All marrow at birth; ≈50% of marrow in adults | Increases with age |
| Switching | Interconvertible as demand changes | |
- Marrow sampling: young children → tibia; adults → pelvis (iliac crest) or sternum
- Marrow functions: make blood cells · destroy old RBCs · store iron (from Hb breakdown) · central organ for B-cell development
Q7. What does the marrow look like up close? ("seeds in the soil")
CheckWhy do megakaryocytes sit on the sinus wall while granulocyte precursors sit far away?
Analogy — seeds in the soil
Stem cells are seeds; they only grow in the right soil, the Hematopoietic Inductive Microenvironment (HIM): stromal cells + reticular fibers (scaffold), adhesion molecules (let stem cells "home" back to the marrow, which is why IV-infused transplant cells find their way) and growth factors (stem cell factor). Different lineages grow in different "beds" of the garden.
Q8. What changes as a cell moves down the tree?
| Trend | Stem cell end | Mature end |
|---|---|---|
| Mitotic activity | Self-renewal → heavy proliferation (progenitors) | Non-mitotic |
| Growth factor sensitivity | Broad (SCF, IL-3, GM-CSF) | Lineage-specific (EPO, TPO, G-CSF) |
| Functional capacity | None (only potential) | Full function |
| Morphology | Big, blast-like, fine chromatin, nucleoli; stem cells all look alike | Specialized shape (LO 3) |
Q9. How does the tree help me localize disease?
High yield — count the lineages
- All lines low (pancytopenia) → stem cell / whole marrow. Hypocellular fatty marrow = aplastic anemia.
- Myeloid lines high → myeloid stem cell (myeloproliferative neoplasm, CML).
- One line abnormal → its progenitor, its growth factor, or the periphery (CKD → only RBCs fall).
- Leukemia = clone blocked at one stage; the blocked branch names it (AML vs ALL).
In-house exam — classic item style
"Based on the abnormal CBC finding, which hematopoietic cell is most likely to show a quantitative abnormality in the marrow?" Match the abnormal count to its precursor: ↑ platelets → megakaryocyte; ↑ RBC → proerythroblast line; ↑ granulocytes → myelocyte line.
ApplyLO 2 application questions
A1A sibling donor receives 5 days of injections; stem cells are collected from blood and purified for one surface marker. Which drug and marker?
A2A man on hemodialysis has Hb 8.1, normal MCV and low reticulocytes. Which cell normally makes the missing hormone, and which progenitor responds to low levels of it?
A3A boy with β-thalassemia major has frontal bossing and a liver 6 cm below the costal margin; biopsy shows nucleated red precursors. When was this organ normally the main site of blood formation?
A4Pancytopenia after chloramphenicol. What will the marrow biopsy show?
A5CML (t(9;22)) can turn into a lymphoid blast crisis. What does that tell you about the cell that first carried BCR-ABL?
LO 3Maturation steps and cell ID
Q1. Is there a shortcut so I don't have to memorize every stage cold?
| Feature | Young (blast) | Mature | Why |
|---|---|---|---|
| Size | Big | Smaller (except megakaryocyte, which grows) | Divisions split the cell |
| Nucleus : cytoplasm | High | Low | Cytoplasm fills with product |
| Chromatin | Fine, open | Coarse, condensed, dark | Genes switch off |
| Nucleoli | Visible | Gone | Less ribosome building |
| Cytoplasm | Deep blue | Pink (Hb) or granule color | Blue = ribosomes / RNA |
| Division | Mitotic | Postmitotic | Early stages expand numbers |
Memory tool
"Shrink, condense, lose the blue." As a precursor matures it shrinks, its chromatin condenses, its nucleoli vanish, and its cytoplasm loses the blue ribosomes as it fills with its final product.
Erythropoiesis The RBC line
Q2. What are the erythroid stages, and how do I tell them apart?
EPO acts on BFU-E → CFU-E. Recognizable maturation takes ≈7 days and happens in erythroblastic islands. The first three stages divide; from the normoblast on, no more mitosis.
| Reticulocyte facts | |
|---|---|
| Nucleus | None — extruded nucleus is phagocytosed by the nurse macrophage |
| What it still does | Makes the final 20% of hemoglobin and its cytoskeleton |
| Maturing | Shrinks; loses mitochondria and ribosomes over 1–2 days in blood |
| Stains | Wright: polychromatophilic (bluish). Supravital: blue web of clumped ribosomes |
CheckWhich cell comes right before the polychromatophilic erythroblast? Right after?
Memory tool — order and color
"Please Be Polite, Or Run Early" → Pro → Basophilic → Polychromatophilic → Orthochromatophilic (normoblast) → Reticulocyte → Erythrocyte.
Color story: blue → blue + pink → pink. "Poly-chromato-philic" = loves many colors. "Ortho-chromato-philic" = the correct (RBC) color.
In-house exam
Two erythroid photo items appear in the lecture's own practice set: identify the polychromatophilic erythroblast (clue: polychromatophilia) and "which cell precedes it?" (basophilic erythroblast). Also identify the normoblast: pyknotic, eccentric nucleus in a cell about the size of an RBC. The normoblast is incapable of mitosis.
Step 1 — erythroid line
- Reticulocytes separate anemias: ↑ = marrow responding (hemolysis, bleeding); ↓ = marrow problem (aplasia, iron/B12/folate lack, CKD). Normal ≈0.5–1.5%; correct with the reticulocyte production index.
- Howell-Jolly bodies = nuclear remnants the spleen normally removes → asplenia.
- Basophilic stippling = clumped ribosomes → lead poisoning, thalassemia.
- Megaloblastic anemia (B12/folate): DNA lags while Hb fills → nuclear–cytoplasmic asynchrony.
- Parvovirus B19 infects erythroid progenitors → aplastic crisis in sickle cell or spherocytosis.
- Nucleated RBCs in blood → marrow stress or infiltration (severe hemolysis, myelofibrosis).
Granulopoiesis The granulocyte line
Q3. What are the granulocytic stages, and what is the key feature of each?
GM-CSF acts on CFU-GM, CFU-Eo and CFU-Bas. Maturation takes ≈11 days; neutrophils then wait ≈4 days in the marrow reserve before release. Watch two stories at once: the nucleus (round → indented → horseshoe → lobes) and the granules (none → azurophilic → specific).
| Stage | Amoeboid movement | Granule production | Mitosis |
|---|---|---|---|
| Myeloblast | No | None | Yes |
| Promyelocyte | No | Azurophilic (primary) | Yes |
| Myelocyte | No | Specific (secondary) | Yes |
| Metamyelocyte | Yes | None | No |
| Band → segmented | Yes | None | No |
Memory tools
"My Parents Made Me Be Smart" → Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band → Segmented.
Primary granules appear in the Promyelocyte; Secondary (Specific) granules in the myelocyte.
"Meta" means after: once a cell is a metamyelocyte it has stopped dividing and started moving.
Traps
- Metamyelocyte vs band: kidney bean (a dent) vs horseshoe/U (deep curve, even width).
- Promyelocyte vs myelocyte: promyelocyte is bigger with only purple granules; myelocyte is smaller with specific granules (pink, orange or blue).
- Largest cell of granulopoiesis = promyelocyte. Largest cell in the marrow = megakaryocyte.
In-house exam
Practice-set items: name this granulopoiesis cell (myelocyte), identify the kidney-bean cell (metamyelocyte), and "which cell precedes this one?" shown a band (answer: metamyelocyte). Also: "excess band neutrophils in blood may indicate a bacterial infection."
Step 1 — granulocytic line
- Acute promyelocytic leukemia: cells stuck at the promyelocyte, t(15;17) PML-RARα, many Auer rods (fused azurophilic granules), DIC; treat with ATRA.
- Left shift (↑ bands, metamyelocytes) → infection or CML. Leukemoid reaction has high LAP; CML has low LAP and basophilia.
- Hypersegmented neutrophils (≥6 lobes) → B12 or folate deficiency.
Monopoiesis The monocyte line
Q4. How does a monocyte develop, and what does it become?
- Comes from CFU-GM, the same progenitor as the neutrophil
- Mononuclear phagocyte system (MPS) = macrophages, their precursors (incl. monocytes) and specialized descendants (e.g., giant cells)
- Macrophage jobs: phagocytosis (intracellular bacteria, viruses, fungi, protozoa, debris), secretion (IL-1, TNF), antigen presentation to lymphocytes
Step 1
Macrophages by tissue: Kupffer (liver), microglia (CNS), osteoclasts (bone), histiocytes (connective tissue), alveolar macrophages; Langerhans cells (skin dendritic cells). Activated by IFN-γ; present on MHC II; LPS binds CD14 → septic shock. Fuse into giant cells in granulomas (TB, sarcoid).
Thrombopoiesis The platelet line
Q5. How does one cell make thousands of platelets?
TPO regulates the maturation, size and number of megakaryocytes (via CFU-Meg). The trick is endomitosis: the nucleus copies its DNA without the cell dividing, giving one giant polyploid nucleus (2n up to 64n).
- Megakaryocytes sit on the abluminal surface of sinusoids and extend proplatelets through the endothelium
- Some enter the blood and finish making platelets in the lung capillaries (seen in pulmonary cytology)
Trap
Megakaryocytes break the "cells shrink as they mature" rule: they grow. A giant marrow cell with one lobulated nucleus = megakaryocyte; a giant cell with many separate nuclei on bone = osteoclast.
Step 1
Marrow megakaryocytes + low platelets: ↑ megakaryocytes → peripheral destruction (ITP, anti-GpIIb/IIIa); ↓ megakaryocytes → production failure. Essential thrombocythemia → many atypical megakaryocytes (JAK2, CALR, MPL). Liver failure → ↓ TPO.
Lymphopoiesis The lymphocyte line
Q6. What do lymphoid cells look like on the way to maturity?
Step 1
B cells: CD19, CD20, CD21; mature in marrow. T cells: CD3 (CD4 or CD8); mature in thymus; ≈80% of circulating lymphocytes. NK: CD16, CD56; perforin + granzymes; kill cells with ↓ MHC I. Plasma cell: clock-face chromatin, perinuclear halo (Golgi); normally does not circulate; malignant in multiple myeloma. ALL = lymphoblasts, TdT+.
Q7. What are the "first / last / largest" facts the exam loves?
| Question | Answer |
|---|---|
| First recognizable RBC precursor | Proerythroblast |
| Checkerboard chromatin | Basophilic erythroblast |
| Last erythroid stage that divides | Polychromatophilic erythroblast |
| Erythroid stage that ejects its nucleus | Normoblast |
| First stage with no nucleus | Reticulocyte |
| First azurophilic (primary) granules | Promyelocyte |
| First specific granules · first stage to call N vs E vs B · last to divide | Myelocyte |
| First motile granulocyte · kidney-bean nucleus | Metamyelocyte |
| Largest cell of granulopoiesis | Promyelocyte |
| First identifiable monocyte precursor | Promonocyte |
| Largest cell in marrow · endomitosis | Megakaryocyte |
| Maturation time | RBC line ≈7 days · granulocyte line ≈11 days |
ApplyLO 3 application questions
A1Gum bleeding, oozing IV sites, low fibrinogen, high D-dimer. Marrow full of large cells packed with purple granules and rod-shaped inclusions. Cytogenetics?
A2A child with sickle cell has a sudden Hb drop from 8 to 4 g/dL with reticulocytes 0.1% after a "slapped cheek" rash. What does the virus target?
A3A splenectomy patient's RBCs contain single small dark purple inclusions. What are they?
A4Isolated platelets of 15,000 with normal Hb and WBC; marrow shows increased megakaryocytes. Explanation?
A5Macrocytic anemia; marrow erythroid precursors are oversized with immature, fine chromatin but already pink cytoplasm. Which process is impaired?
IDName that cell
Cell 1 — granulopoiesis
Cell 2 — nucleated cell
Cell 3 — normal blood
Cell 4 — erythropoiesis
Cell 5 — what precedes it?
Cell 6 — erythropoiesis
Cell 7 — what precedes it?
Cell 8 — granulopoiesis
Cell 9 — erythropoiesis
Cell 10 — normal blood
Cell 11 — marrow
Cell 12 — monopoiesis
MCQExam-style questions
1A 64-year-old woman has fatigue and a ruddy complexion. CBC: Hb 12.5 g/dL, Hct 36%, WBC 8,500/mm³, platelets 580,000/mm³. Based on the abnormal finding, which hematopoietic cell is most likely to show a quantitative abnormality in her marrow?
2A 62-year-old man with CKD from long-standing diabetes has fatigue. Hb 9.5 g/dL, Hct 25%, normocytic normochromic RBCs, low reticulocytes. A hormone made by renal interstitial cells is deficient. Which process is most directly impaired?
3A 6-year-old with mild fever has a normal WBC count, but bands make up 12% of the differential. Which description fits the stage immediately before the elevated cell?
4A marrow aspirate from a healthy donor shows a central macrophage surrounded by developing erythroid cells at different stages. What is the central cell's main job?
5In a granulocyte precursor, which is the first stage at which a neutrophil can be distinguished from an eosinophil?
6Which erythroid stage is the last one capable of mitosis?
7Researchers inject purified marrow cells into lethally irradiated mice; the cells form spleen colonies and rescue the animals. The cells have no distinctive morphology. Which marker would identify them?
8A smear shows a cell with a dark blue-purple granule mass that obscures a bilobed nucleus. Which of the following is stored in these granules and responsible for their staining?
9A 3-year-old needs a bone marrow aspirate. Which site is preferred?
10A woman's neutrophils show a small "drumstick" appendage on the nucleus in about 3% of cells. This represents:
11A 30-year-old with pancytopenia has a bone marrow biopsy. Which finding best fits aplastic anemia?
12Which mature cell shares its committed progenitor (CFU-GM) with the neutrophil?
!Numbers that differ between in-house and Step 1
| Item | In-house | Step 1 / board review |
|---|---|---|
| Hematocrit | Women 36–48%, men 40–54% (CBC); spun tube: men 40–50%, women 35–45% | Rule of thumb Hct ≈ 3 × Hb |
| Reticulocytes | ≈1% of RBCs | 0.5–1.5% |
| Platelet lifespan | 10 days | 8–10 days |
| Fetal hematopoiesis | 3 stages: primordial (yolk sac) → hepato-spleno-thymic (2nd trimester) → medullo-lymphatic (3rd trimester) | Yolk sac wk 3–8 · liver wk 6–birth · spleen wk 10–28 · marrow wk 18 onward |
| Eosinophilia causes | Parasites, allergy | PACMAN Eats (older sources use NAACP) |
| Dense granules | Serotonin, ADP, Ca²⁺ | CASH: Ca²⁺, ADP, Serotonin, Histamine |
| TPO source | Liver, bone marrow, other tissues (also listed: liver and kidney) | Liver |
| Band cells | Normal up to 3%; >10% may indicate infection | "Left shift" = ↑ bands/metamyelocytes |