Page 2 of 9
PA13.1-3 | Hematopoiesis & Blood Specimen Basics — Part 1
CLINICAL SCENARIO
It is 7 AM in the haematology OPD. A 9-year-old girl with massive splenomegaly sits in front of you, her mother clutching a stack of old CBC reports. The haemoglobin reads 4.2 g/dL. Before you can plan anything — transfusion, chelation, a bone marrow biopsy — you need to answer a deceptively simple question: where is this child making her red blood cells? Not in her bone marrow, it turns out. Her spleen has taken over. That fact changes everything — the diagnosis, the risk, the surgical calculus. It all starts with understanding haematopoiesis.
WHY THIS MATTERS
Haematology is not an abstract science — it is daily clinical practice. Every ward round includes a CBC. Every pre-operative assessment includes a coagulation screen. Every anaemic patient needs their marrow interrogated. When you understand how blood is made and how to collect it correctly, you stop being a passive CBC-orderer and start being a diagnostician. In Pathology Year 2, this module is your entry point to the entire haematology block — H1 (this module) feeds directly into H2 (approach to anaemia), H3–H5 (specific anaemias), and H6 (haemostasis). Get this right and the rest unlocks cleanly.
RECALL
Recall from Year-1 Physiology (PY3.11) that blood consists of a cellular component — erythrocytes, leukocytes, and platelets — suspended in plasma. You studied erythrocyte structure (biconcave disc, no nucleus, packed with haemoglobin) and the oxygen-dissociation curve. From Anatomy (AN2.1), you know the gross structure of the sternum, iliac crest, and vertebrae — the sites from which bone marrow is sampled. From Biochemistry (BI8), you know haemoglobin as a tetrameric protein with four haem groups, each binding one O₂. Now Pathology layers the question: how does the body produce these cells, regulate their numbers, and respond when the system breaks down?
The Bone Marrow Factory: Stem Cell Hierarchy
The bone marrow is the primary blood-forming organ from birth onward. Think of it as a factory with a strict hierarchy of workers — each level more specialised than the last.
Haematopoiesis (from Greek haima 'blood' + poiēsis 'making') is the continuous process by which all blood cells are produced from a common ancestor.
At the apex of the hierarchy sits the haematopoietic stem cell (HSC) — a rare, self-renewing cell that makes up only 0.01% of marrow cells. Two properties define it:
• Self-renewal — it can divide and produce daughter HSCs, maintaining the pool for life
• Multipotency — it can commit to any blood cell lineage
The HSC gives rise to two great progenitor families:
- Common myeloid progenitor (CMP) — the ancestor of red cells, platelets, neutrophils, monocytes, eosinophils, and basophils
- Common lymphoid progenitor (CLP) — the ancestor of T-lymphocytes, B-lymphocytes, and NK cells
This bifurcation is called lineage commitment — the moment a progenitor cell locks into one developmental path and can no longer turn back. It is driven by transcription factors (master regulatory proteins) and cytokines (signalling molecules from the marrow microenvironment).
Know these key growth factors:
• Erythropoietin (EPO) — produced by peritubular cells of the kidney; drives erythropoiesis; rises in hypoxia
• Thrombopoietin (TPO) — produced by liver and kidney; drives megakaryopoiesis (platelet production)
• G-CSF (granulocyte colony-stimulating factor) — drives neutrophil production; used clinically to mobilise HSCs
• SCF (stem cell factor) — maintains HSC survival and proliferation
Hematopoiesis: From Stem Cell to Mature Blood Cells
SELF-CHECK
A patient with chronic kidney disease (CKD stage 5) develops progressive anaemia despite normal iron stores. Which growth factor is most directly deficient in this patient?
A. A. Thrombopoietin (TPO)
B. B. Erythropoietin (EPO)
C. C. G-CSF (granulocyte colony-stimulating factor)
D. D. Stem cell factor (SCF)
Reveal Answer
Answer: B. B. Erythropoietin (EPO)
Correct — EPO is produced by peritubular interstitial cells of the kidney. In CKD, functioning renal parenchyma is lost, EPO secretion falls, and erythropoiesis is suppressed. This is the anaemia of chronic kidney disease — normocytic, normochromic, with low reticulocyte count. Treatment: recombinant EPO (epoetin alfa). Note: TPO (platelet production), G-CSF (neutrophil production), and SCF (HSC maintenance) are unaffected by renal failure.
Erythropoiesis: From Stem Cell to Red Blood Cell
Erythropoiesis — the production of red blood cells — is the most prolific lineage: the marrow generates 2 million red cells per second in a healthy adult. You need to know the maturation sequence because each stage has a distinct morphology visible on a smear.
The erythroid maturation sequence (from earliest to most mature, all within the bone marrow):
| Stage | Size | Nucleus | Cytoplasm | Visible in normal PBF? |
|---|---|---|---|---|
| Proerythroblast | Large (~20 µm) | Large, pale, prominent nucleoli | Deep blue (RNA-rich) | No |
| Basophilic erythroblast | Smaller | Condensing, no nucleoli | Dark blue | No |
| Polychromatic erythroblast | Smaller | More condensed | Blue-pink (haemoglobin accumulating) | No |
| Orthochromatic erythroblast | Smaller | Dense, eccentric, about to be extruded | Pink | No |
| Reticulocyte | Slightly larger than RBC | None — nucleus extruded | Pink with RNA mesh visible on supravital stain | Yes (0.5–2.5%) |
| Mature erythrocyte | 7–8 µm, biconcave disc | None | Pale centre (1/3 diameter) | Yes |
Key concepts to lock in:
- Haemoglobin synthesis begins at the polychromatic stage — watch the cytoplasm change from blue (RNA) to pink (Hb) across the table.
- Nuclear extrusion occurs at the orthochromatic stage — the nucleus is pushed out and phagocytosed by marrow macrophages. This is why mature RBCs have no nucleus.
- Reticulocytes are the quality-control metric of erythropoiesis. A high reticulocyte count means the marrow is working hard (haemolysis, bleeding recovery). A low count in anaemia means the marrow is failing (aplasia, EPO deficiency).
- From EPO stimulus to mature RBC takes approximately 7 days.
On a peripheral blood film (PBF) in health, you will see only mature erythrocytes and occasional reticulocytes. If you see nucleated red cells (NRBCs — orthochromatic or polychromatic erythroblasts in the peripheral blood), something has severely stressed the marrow: severe haemolytic anaemia, marrow infiltration, or — in neonates — physiological.
Erythroid Maturation Series and Reticulocyte Count Clinical Interpretation
CLINICAL PEARL
Pearl: Reticulocyte count interprets your haemoglobin result.
A Hb of 7 g/dL with reticulocyte count 15% tells a very different story from Hb 7 g/dL with reticulocyte count 0.2%.
- High retics + low Hb = marrow is responding briskly → haemolytic anaemia or acute blood loss (the factory is running flat-out)
- Low retics + low Hb = marrow is failing to respond → EPO deficiency (CKD), aplastic anaemia, marrow infiltration, or iron/B12/folate deficiency (the factory is shut down or lacks raw materials)
Always order a reticulocyte count alongside your CBC in any unexplained anaemia. It costs nothing and answers the key fork: Is the marrow trying?
Myelopoiesis & Lymphopoiesis: The Other Lineages
While erythropoiesis dominates the haematology curriculum, you must hold a working map of the entire myeloid and lymphoid output.
Myelopoiesis — production of granulocytes and monocytes from the CMP:
- Neutrophil series: Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band (stab) cell → Segmented neutrophil
- The myelocyte is the last stage that can divide; from metamyelocyte onwards, the cell only matures.
- You'll encounter the left shift in clinical practice: when bands and metamyelocytes appear in peripheral blood (normally absent), it signals overwhelming bacterial infection driving emergency marrow release.
- Eosinophils and basophils follow analogous series; eosinophilia in allergic disease and parasitic infection is the most common clinical scenario you'll encounter in your OPD posting.
- Monocytes mature in marrow and circulate for ~72 hours before emigrating into tissues as macrophages — the phagocytes of chronic inflammation and granuloma formation.
Megakaryopoiesis (platelet production):
• Megakaryocytes are giant polyploid marrow cells (~50–100 µm) that shed their cytoplasm as platelets through thrombopoiesis — cytoplasmic fragmentation, not cell division.
• One megakaryocyte produces 1,000–5,000 platelets.
• Driven by thrombopoietin (TPO).
Lymphopoiesis:
• CLP gives rise to pre-B cells (mature in marrow), pre-T cells (mature in thymus), and NK cell precursors.
• B-cell maturation in marrow → peripheral blood → lymph nodes → plasma cells (antibody factories)
• T-cell maturation in thymus → peripheral blood → tissue-resident memory cells
Myeloid Cell Series Development on Leishman-Stained Blood Smear (100×)