What are the sites for haemopoiesis?

The first site for haemopoiesis is our fetal yolk sacs, which we use for the first two months of gestation.

Thereafter, a population of stem cells seeds (common precusors of both endothelial and haemopoietic cells) seeds the liver, spleen and bone marrow. These organs can then take over the job of creating blood cells as the yolk sac disappears.

• From two to seven months of fetal (i.e. intrauterine) development, the liver and spleen function as haemopoietic organs.
• From five (intrauterine) months through to the end of infancy, the bone marrow of almost all bones does the job.

Thereafter, many of the bones gradually stop producing blood cells (their marrows become filled with adipose tissue) until by adulthood the only bones doing it are those of the axial skeleton (skull, vertebrae, pelvis, sternum, ribs) and the proximal femur.

However, although we obviously don't ever get our yolk sacs back, the remaining dormant haemopoietic tissues can relive their childhoods and return to producing blood cells if needed. In some conditions where the bone marrow is progressively obliterated, for instance, the spleen steps in and takes over the job. This is called extramedullary haemopoiesis.