<P> Megakaryocytopoiesis is haematopoiesis of megakaryocytes . </P> <P> In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands . As development progresses, blood formation occurs in the spleen, liver and lymph nodes . When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism . However, maturation, activation, and some proliferation of lymphoid cells occurs in the spleen, thymus, and lymph nodes . In children, haematopoiesis occurs in the marrow of the long bones such as the femur and tibia . In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum . </P> <P> In some cases, the liver, thymus, and spleen may resume their haematopoietic function, if necessary . This is called extramedullary haematopoiesis . It may cause these organs to increase in size substantially . During fetal development, since bones and thus the bone marrow develop later, the liver functions as the main haematopoetic organ . Therefore, the liver is enlarged during development . </P> <P> In some vertebrates, haematopoiesis can occur wherever there is a loose stroma of connective tissue and slow blood supply, such as the gut, spleen or kidney . </P>

Fetal hematopoiesis occurs in which structure gut spleen bone marrow thymus
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