Embryology (ontogenesis)
Early embryogenesis begins in the yolk sac. Definitive hematopoietic cells are derived from stem cells in the AGM (aorto-gonado-mesonephros) region, populating the liver and spleen and eventually migrating to the bone marrow.
- Yolk sac: appear by day 10, continues up to 2 months (hemocytoblasts)
- Liver and spleen: appear by week 5, overlap with the yolk sac, continue up to 7 months
- Bone-marrow: appear by week 11, overlap with the liver and spleen, most important from 6 months
- The liver and spleen can continue until 2 weeks after birth
Before childhood, all bone marrow is hemopoietic; after childhood, only the central skeleton and proximal femur and humerus remain hematopoietic ("red-marrow"), while the rest is replaced with adipose tissue ("yellow-marrow").
In the setting of severe or prolonged blood-loss, yellow marrow can transform back to red marrow.