Where is blood produced?

Well at a glance, this looks like a very simple question a primary school student would ask their teachers, and a quick intelligent response to that question would be in the bone marrow. Technically, that is a correct answer. However, blood has not been entirely produced in the bone marrow through out our entire lives. In fact, blood started to be produced in the bone marrows after 6 months of life. So, let's search a little bit further to understand where blood is actually produced.

Hematopoiesis or hemopoiesis is the "expensive medical term" that medical students (and the medical community) use to say the production of blood cells. As stated earlier, bone marrow was not the production site for blood our entire lives.
Our blood factories have changed places as we grow older and can be seen in this table.

Age	Hemopoiesis Site
Fetus (0-2 mo)	yolk sac
Fetus (2-6 mo)	liver and spleen
Fetus (6-9 months)	bone marrow
Baby	bone marrow (generally in every bone)
Adult	specific bone marrows: vertebrae, costae, sternum, skull, sacrum hip bones, and the proximal part of the femur

In the first few weeks of gestation, the yolk sac is the main site for hematopoiesis to occur, with the stem cells found in a region known as the AGM (aorta-gonad-mesonefros) region.
These stem cells along with endothelial stem cells and hemangioblasts migrate to the liver and spleen in the 6th week of gestation until the 6th month, where these organs become the main production site for blood. After the age of 6 months, the bone marrow starts to become the main factory for blood production.

It is interesting to see that in the born baby, every bone virtually produces blood. However, as we grow older, the bone marrow is progressively replaced with lipids or fat, and in adults, only the central bones and parts of the femur are the only bones left, which are able to produce blood.

In certain cases, extramedullary hemopoiesis can occur, where the liver and spleen become active blood producing sites again. Or in other diseases, such as leukemia, the production site enlarges to the central part of the bones.

So, in conclusion we can see that there is an easy way to remember the production site for blood with a simple 6-6-6 rule. A fetus aging below 6 weeks will have blood produced from the yolk sac, between 6 weeks to 6 months, blood is produced in the liver and spleen, and more than 6 months, blood is mainly produced in the bone marrow. In the new born, all bones are able to produce blood, and as we grow older, only specific bones are left with the capability to produce blood.


Reference:


Hoffbrand & Moss. 2011. Essential Haematology, 6th Ed. Wiley-Blackwell.