CorpusHuman Anatomy
Lymphatic & ImmunePhysiological process

Haematopoiesis

Also known as: hematopoiesis, haemopoiesis, hemopoiesis, blood cell formation, myelopoiesis, lymphopoiesis

The formation of all blood cells from self-renewing haematopoietic stem cells, which branch into myeloid and lymphoid lineages under the control of growth factors and cytokines.

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Overview

Daily production≈ 5 × 10¹¹ cells (≈ 2 × 10¹¹ red cells)
Red cell maturation≈ 7 days (reticulocyte 1–2 days in blood)
Neutrophil maturation≈ 10–14 days in marrow

All blood cells arise from haematopoietic stem cells (HSCs) — rare (≈ 1 in 10⁴–10⁵ marrow cells), mostly quiescent, CD34⁺ cells that can both self-renew and generate every blood lineage. HSCs give rise to multipotent progenitors and then to two main branches. The common myeloid progenitor produces megakaryocyte–erythroid progenitors (→ erythrocytes and megakaryocytes/platelets) and granulocyte–monocyte progenitors (→ neutrophils and monocytes, which become macrophages and some dendritic cells); eosinophils, basophils and mast cells arise from related myeloid progenitors. The common lymphoid progenitor produces B cells (in the marrow), T-cell precursors (which complete development in the thymus), NK cells and innate lymphoid cells. As cells mature they lose proliferative capacity and acquire lineage-specific features; the earlier "blast" stages are normally confined to the marrow.

Morphological sequences are classic exam material. Erythropoiesis: proerythroblast → basophilic → polychromatic → orthochromatic erythroblast (normoblast) → nucleus extruded → reticulocyte (1–2 days in blood) → erythrocyte, taking about a week. Granulopoiesis: myeloblast → promyelocyte (azurophilic/primary granules) → myelocyte (specific/secondary granules; last stage able to divide) → metamyelocyte → band → segmented neutrophil, taking ≈ 10–14 days. Thrombopoiesis: megakaryoblast → megakaryocyte (nuclear replication without division — endomitosis — up to 64N) → each sheds thousands of platelets into marrow sinusoids. Monopoiesis: monoblast → promonocyte → monocyte.

Location

Embryo/fetus: yolk sac → AGM region → liver (and spleen) → bone marrow. After birth: red bone marrow only (lymphocytes also proliferate in lymphoid organs).

Parts & subdivisions

  1. Haematopoietic stem cell
    Self-renewing multipotent cell (CD34⁺) maintained in marrow niches by CXCL12 and stem-cell factor (KIT ligand).
  2. Common myeloid progenitor
    Gives erythrocytes, platelets, granulocytes, monocytes/macrophages, mast cells and most dendritic cells.
  3. Common lymphoid progenitor
    Gives B cells, T cells, NK cells and innate lymphoid cells.
  4. Erythropoiesis
    Driven by erythropoietin from the kidney in response to hypoxia; needs iron, vitamin B12 and folate.
  5. Granulopoiesis and monopoiesis
    Driven by G-CSF, GM-CSF, M-CSF and IL-3; eosinophils need IL-5.
  6. Thrombopoiesis
    Driven by thrombopoietin (made constitutively by the liver and cleared by platelets).

Anatomy

Development

Primitive haematopoiesis begins in the yolk-sac blood islands in week 3 (large nucleated red cells with embryonic haemoglobins). Definitive HSCs arise from haemogenic endothelium in the aorta–gonad–mesonephros (AGM) region around weeks 4–5 and seed the liver, the main site from about week 6 to the third trimester (with a minor contribution from the spleen). The bone marrow is colonised from late in the first trimester and becomes the dominant site in the third trimester.

Function

  • Maintains blood cell numbers throughout life (hundreds of billions of cells per day).
  • Responds rapidly to demand: bleeding, hypoxia, infection.
  • Generates the cells of both innate and adaptive immunity.
Regulation

Erythropoietin (peritubular interstitial cells of the kidney; hypoxia via HIF), thrombopoietin (liver), G-CSF (neutrophils), GM-CSF, M-CSF, IL-3 (multilineage), IL-5 (eosinophils), IL-7 (lymphoid), stem-cell factor and FLT3 ligand; transcription factors such as GATA1 (erythroid/megakaryocyte), PU.1 and C/EBPα (myeloid), and PAX5 (B cells) fix lineage choice.

Clinical significance

Extramedullary haematopoiesis

When the marrow cannot meet demand (thalassaemia major) or is replaced (myelofibrosis), haematopoiesis resumes in the liver and spleen (hepatosplenomegaly) and paravertebral masses; marrow expansion in thalassaemia widens the skull ("hair-on-end" radiograph) and maxilla (frontal bossing).

Myeloproliferative neoplasms

Clonal stem-cell disorders with excessive production of mature cells: polycythaemia vera (JAK2 V617F in ≈ 95%), essential thrombocythaemia, primary myelofibrosis and CML (BCR–ABL1).

Growth factors as drugs

Recombinant erythropoietin treats the anaemia of chronic kidney disease; G-CSF (filgrastim) shortens chemotherapy-induced neutropenia and mobilises stem cells; thrombopoietin-receptor agonists treat immune thrombocytopenia.

Mnemonics

Sites of fetal haematopoiesis — "Young Liver Synthesises Blood"

Yolk sac (from week 3) → Liver (main fetal site, from ≈ week 6) → Spleen (minor) → Bone marrow (dominant from the third trimester and after birth).

Compiled from standard anatomy and physiology references — see the Lymphatic & Immune System sources. For education only; not medical advice.