Page "Haematopoiesis" Paragraph 12

In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands.

As development progresses, blood formation occurs in the spleen, liver and lymph nodes.

When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism.

However, maturation, activation, and some proliferation of lymphoid cells occurs in secondary lymphoid organs ( spleen, thymus, and lymph nodes ).

In children, haematopoiesis occurs in the marrow of the long bones such as the femur and tibia.

In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum.