Preface: Blood Cell Production and Maturation

The bone marrow is the major site of blood cell production (haemopoiesis) by 20 weeks gestation, with an increase in activity during the third trimester of pregnancy.

At birth haemopeietic cells (red marrow) occupy all the bone marrow space. During childhood there is gradual replacement with yellow fatty marrow, until red marrow is confined to the pelvis, cranium, ribs, vertebrae, and sternum in the adult. When there is continuous increased haemopoietic demand, the marrow throughout the body may become red and active once more.

The spleen, liver, and lymphatic tissues are also capable of producing blood cells and are the main sites of extramedullary haemopoiesis in the developing fetus.