Expansion of haematopoietic stem cells during development
Haematopoietic stem cells (HSCs) arise in the embryo in the aorta-gonad-mesonephros region and the placenta, afterwards colonise the liver (and spleen) and finally the bone marrow, which, from birth on, remains the sole site of haematopoiesis. In mouse, after the fetal phase of rapid HSCs expansion, the number of these stem cells continues to increase after birth in order to sustain the growing blood volume of the developing infant. The number of total circulating reticulocytes seems to indicate that this growth stops around the third week of murine life, well before the whole animal stops to grow. Around the same time, a period of abundant lymphopoiesis ends. Human HSCs seem to show similar patterns during post-natal growth and these could be useful to understand the peculiar features of paediatric leukaemogenesis.