Читать книгу Neonatal Haematology - Barbara J. Bain, Irene Roberts - Страница 12

Haemopoiesis in humans begins in the yolk sac between 2 and 3 weeks post‐conception (Fig. 1.1).^1,2 This is known as primitive haemopoiesis. Studies in other species, particularly in mice, indicate that the predominant cell types produced in the yolk sac are erythroid cells and macrophages.^2,3 While megakaryocytes and lymphoid cells may also be yolk sac‐derived, the current consensus of opinion is that true, long‐lived haemopoietic stem cells (HSC) arise from a region of specialised endothelium (‘haemogenic’ endothelium), which is localised to the ventral wall of the dorsal aorta in a region known as the aorto‐gonado‐mesonephros (AGM).^4–6 In humans, haemopoiesis begins in the AGM at around 5 weeks post‐conception and is known as definitive haemopoiesis.^6–8 Haemopoiesis in the aortic wall is only transient, presumably because this region lacks the necessary physical space and specialised microenvironment to support expansion and differentiation of the HSC and progenitor populations required to meet the needs of the growing fetus.

By 6 weeks post‐conception, HSC and progenitor cells have migrated to the fetal liver,^8,9 which remains the main site of blood cell production throughout fetal life^10,11 and AGM haemopoiesis ceases. The first signs of haemopoiesis in the bone marrow are evident from around 11 weeks post‐conception.^8,12 Although fetal bone marrow is able to give rise to cells of all lineages, it is becoming clear that the predominant cell types produced in the bone marrow are B lymphocytes and their progenitor cells together with granulocytes, monocytes and their progenitors. Although erythropoiesis and megakaryopoiesis take place in the fetal bone marrow from the end of the first trimester, most red blood cell and megakaryocyte production takes place in the fetal liver until shortly before term.⁸ Thus, for preterm infants, the liver is the main haemopoietic organ at and shortly after birth; this is likely to be a contributory factor in a number of disorders, including the haematological abnormalities seen in neonates with Down syndrome (see pages 154–160 and 206).

Fig. 1.1 Ontogeny of human haemopoiesis in embryonic and fetal life. AGM, aorto‐gonado‐mesonephros; pcw, post‐conceptional week. Based on references 1 and 2.