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Histology · Blood and hematopoiesis

Hematopoiesis and bone marrow !

  • Process

Blood cells live from hours to months, so the bone marrow keeps making new ones from a small pool of stem cells.

Updated 2026-11-01

What it is

Hematopoiesis is the continuous production of blood cells from a hematopoietic stem cell. In the embryo it moves from the yolk sac to the liver and spleen and finally to the bone marrow; in adults it happens mainly in the red marrow of flat bones and the ends of long bones. Two branches, myeloid and lymphoid, produce all the formed elements under the control of growth factors such as erythropoietin and thrombopoietin.

Structure

  • Red marrow: cords of hematopoietic cells between wide sinusoidal capillaries, with fat cells scattered between them.
  • Megakaryocytes are the giant cells of the marrow (50–100 µm) with a large lobed nucleus.
  • Yellow marrow is mostly fat and replaces red marrow in long-bone shafts with age.

Step by step

  1. A self-renewing hematopoietic stem cell divides; one daughter stays a stem cell, the other commits.
  2. The committed progenitor becomes either a myeloid or a lymphoid stem cell.
  3. Growth factors (EPO, TPO, colony-stimulating factors, interleukins) push each progenitor down one line.
  4. Blasts mature: the cell shrinks, the chromatin condenses and the cytoplasm fills with haemoglobin or granules.
  5. Red cell precursors expel the nucleus; the reticulocyte enters blood and matures in a day or two.
  6. Megakaryocytes stay in the marrow and shed platelets into the sinusoids; lymphocytes finish maturing in lymphoid organs.

Clinical notes

  • Bone marrow aspirate and biopsy (usually from the iliac crest) assess cellularity and the ratio of lines.
  • Chronic kidney disease lowers erythropoietin and causes a normocytic anaemia.
  • Aplasia, fibrosis or leukaemic infiltration of the marrow lowers all blood counts (pancytopenia).

Test yourself

Think of the answer, then tap the question to see it.

In what order does hematopoiesis move between organs before birth?

From the yolk sac to the fetal liver (with the spleen) and finally to the bone marrow, which takes over before birth.

Which stem cell line gives rise to platelets?

The myeloid line: megakaryocytes derived from myeloid progenitors shed platelets into the marrow sinusoids under thrombopoietin.

Why does chronic kidney disease cause anaemia?

The kidney is the main source of erythropoietin; with less of it, the marrow makes fewer red cells.

Where in an adult is red marrow usually sampled, and what is assessed?

Usually the posterior iliac crest; the sample shows cellularity, the balance of cell lines and abnormal or infiltrating cells.

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: 18.2 Production of the Formed Elements - Anatomy and Physiology 2e | OpenStax (OpenStax, Rice University, 2022); 18.4 Leukocytes and Platelets - Anatomy and Physiology 2e | OpenStax (OpenStax, Rice University, 2022); Bone Marrow Disease | Bone Marrow | MedlinePlus (U.S. National Library of Medicine, 2024).

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Representative model, for educational purposes only.