Site Of Haemopoiesis
In the first few weeks of
gestation the yolk sac is a transient site of haemopoiesis. However, definitive
haemopoiesis derives from a population of stem cells first observed on the AGM
(aorta‐gonads‐mesonephros) region. These common precursors of endothelial and
haemopoietic cells (haemangioblasts) are believed to seed the liver, spleen and
bone marrow. From 6 weeks until 6–7 months of fetal life, the liver and spleen
are the major haemopoietic organs and continue to produce blood cells until
about 2 weeks after birth (Table 1.1; see Fig. 7.1b).
The placenta also
contributes to fetal haemopoiesis. The bone marrow is the most important site
from 6–7 months of fetal life. During normal childhood and adult life the
marrow is the only source of new blood cells. The developing cells are situated
outside the bone marrow sinuses; mature cells are released into the sinus
spaces, the marrow microcirculation and so into the general circulation.
In infancy all the bone marrow is haemopoietic but during childhood there
is progressive fatty replacement of marrow throughout the long bones so that in
adult life haemopoietic marrow is confined to the central skeleton and proximal
ends of the femurs and humeri (Table 1.1). Even in these haemopoietic areas,
approximately 50% of the marrow consists of fat (Fig. 1.1). The remaining fatty
marrow is capable of reversion to haemopoiesis and in many diseases there is
also expansion of haemopoiesis down the long bones. Moreover, the liver and
spleen can resume their fetal haemopoietic role (‘extramedullary
haemopoiesis’).
