SN5
SHORT NOTE
5M
★ High-yield
Describe the structure of bone marrow and the sites of haemopoiesis.
General Features
Bone marrow is the soft tissue filling the medullary cavities and the spaces of cancellous bone, and is the site of haemopoiesis after birth.
| Type | Appearance | Distribution |
|---|---|---|
| Red marrow | Haemopoietically active | All bones at birth; in the adult confined to the vertebrae, sternum, ribs, skull, pelvis and proximal ends of the femur and humerus |
| Yellow marrow | Largely adipose; inactive | Replaces red marrow in the long bones from about 5 years; can revert to red marrow when demand rises |
Structure
- A framework of reticular fibres and reticular cells
- Sinusoids with a discontinuous endothelium, through which mature cells enter the circulation
- Haemopoietic cords between the sinusoids, containing the developing cells and adipocytes
- Megakaryocytes lie against the sinusoidal wall and shed platelets directly into the lumen
- Cells enter the blood by squeezing through the endothelium, not between cells, and only mature cells can deform enough — the marrow–blood barrier
Sites of Haemopoiesis Through Life
Cell Lines
| Line | Sequence | Recognition |
|---|---|---|
| Erythroid | Proerythroblast → basophilic → polychromatic → orthochromatic erythroblast → reticulocyte → erythrocyte | Nucleus condenses and is extruded; cytoplasm changes blue to pink |
| Granulocytic | Myeloblast → promyelocyte → myelocyte → metamyelocyte → band → segmented | Nucleus indents then segments; specific granules appear at the myelocyte stage |
| Megakaryocytic | Megakaryoblast → megakaryocyte → platelets | The largest marrow cell; huge lobulated polyploid nucleus |
| Lymphoid | Lymphoblast → lymphocyte | Matures further in thymus or peripheral lymphoid tissue |
- The normal myeloid:erythroid ratio is about 3:1, and its alteration is one of the first things assessed on a marrow smear
Applied Aspects
- Bone marrow aspiration is performed from the posterior superior iliac spine in adults, or the sternum at the level of the second intercostal space; the tibia is used in infants
- Trephine biopsy gives architecture and cellularity, which an aspirate cannot; essential in aplastic anaemia and myelofibrosis, where aspiration yields a "dry tap"
- Aplastic anaemia — a hypocellular marrow replaced by fat, with pancytopenia
- Leukaemia — the marrow is packed with blasts that crowd out normal haemopoiesis, causing anaemia, infection and bleeding together
- Marrow is the most radiosensitive tissue in the body after the gonads, and its suppression limits the dose in both radiotherapy and chemotherapy
- Thalassaemia major — marrow expansion causes the "hair-on-end" skull and the characteristic facies
All questions in Systemic Histology
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- SN5Describe the structure of bone marrow and the sites of haemopoiesis. ★
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