Haemopoiesis (Formation of blood cells)

Haemopoietic cells (those which produce blood) first appear in the yolk sac of the 2-week embryo.
By 8 weeks, blood making has become established in the liver of the embryo, and by 12-16 weeks the liver has become the major site of blood cell formation. It remains an active haemopoietic site until a few weeks before birth. The spleen is also active during this period, particularly in the production of lymphoid cells, and the foetal thymus is a transient site for some lymphocytes.
The highly cellular bone marrow becomes an active blood making site from about 20 weeks gestation and gradually increases its activity until it becomes the major site of production about 10 weeks later.

At birth, active blood making red marrow occupies the entire capacity of the bones and continues to do so for the first 2-3 years after birth.
The red marrow is then very gradually replaced by inactive, fatty, yellow, lymphoid marrow. The latter begins to develop in the shafts of the long bones and continues until, by 20-22 years, red marrow is present only in the upper ends of the femur and humerus and in the flat bones of the sternum, ribs, cranium, pelvis and vertebrae. However, because of the growth in body and bone size that has occurred during this period, the total amount of active red marrow (approximately 1000-1500 g) is nearly identical in the child and the adult.
Adult red marrow has a large reserve capacity for cell production. In childhood and adulthood, it is possible for blood making sites outside marrow, such as the liver, to become active if there is excessive demand as, for example, in severe haemolytic anaemia or following haemorrhage.
In old age, red marrow sites are slowly replaced with yellow, inactive marrow.
Red marrow forms all types of blood cell and is also active in the destruction of red blood cells.
Red marrow is, therefore, one of the largest and most active organs of the human body, approaching the size of the liver in overall mass although as mentioned it is distributed in various parts of the body.
About two-thirds of its mass functions in white cell production (leucopoiesis), and one-third in red cell production (erythropoiesis). However as we have already seen there are approximately 700 times as many red cells as white cells in peripheral blood. This apparent anomaly reflects the shorter life span and hence greater turnover of the white blood cells in comparison with the red blood cells.
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