Ivan Dečkov, Jozef Záhumenský, Ľudovít Danihel, Katarína Kubišová, Ivan Varga
Providing a short and brief overview of the development of blood elements during the ontogenesis of a human is not
an easy task. It is partially due to the fact that during prenatal development, the anatomical location of haematopoiesis
(haemocytopoiesis) changes several times, from extraembryonic tissues into the body of an embryo. Haematopoiesis
first takes place in the extraembryonic splanchnic mesoderm within the wall of the yolk sac from the 17th day after
fertilization, when the blood islands start to appear. Primitive haemopoietic stem cells of the yolk sac are the precursors
for nucleated erythrocytes (erythroblasts), and probably also megakaryocytes and primitive macrophages. The
wall of the yolk sac serves as a hemopoietic organ until the end of the second month after fertilization. From then, haemopoiesis
starts to relocate into the body of foetus itself - with the liver and the red pulp of the spleen to mention
those most important. And then, to a minor extent, even the thymus and a connective tissue around the aorta (AGM
– aortic, gonad and mesonephric region) serve as next hemopoietic organs. During the 4th week of the prenatal development,
the liver primordium is colonized with hemopoietic stem cells. Only after the 10th week of prenatal development,
these stem cells colonize the bone marrow. However, the bone marrow takes over the function of the hemopoietic
organ from the liver only at the very end of pregnancy. The red bone marrow subsequently becomes the sole organ
of haematopoiesis in postnatal period in humans. Under the pathological conditions such as anaemia, myeloproliferative
syndromes, chronic myeloid leukaemia, return of haematopoiesis into the liver and spleen (extramedullary haemopoiesis)
can be often observed.