Red marrow is haematopoietic; yellow marrow is fat
Expansion
Distribution
- Fetus: yolk sac, then liver and spleen, then marrow from about 5 months
- Child: red marrow in essentially all bones
- Adult: red marrow confined to the axial skeleton (vertebrae, sternum, ribs, pelvis, skull) and the proximal femur and humerus. The rest converts to yellow (fatty) marrow
Structure: haematopoietic cords between sinusoids, with a stromal framework of reticular cells, adipocytes and macrophages. Megakaryocytes lie adjacent to sinusoids and extend proplatelets directly into the lumen.
Cellularity declines with age, approximated by 100 minus age as a percentage.
Extramedullary haematopoiesis in liver and spleen occurs when marrow fails or demand is extreme, as in myelofibrosis and thalassaemia, producing hepatosplenomegaly and a leucoerythroblastic blood film.
Sampling: aspirate gives cytology, trephine gives architecture and cellularity. A dry tap suggests fibrosis or a packed marrow.
Reticulocytes retain ribosomal RNA, staining with methylene blue, and their count indicates whether the marrow is responding.