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Blood |
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Blood cell formation or hemopoiesis already starts at the second week of embryological development when blood islands containing primitive erythroblasts become visible in the yolk sac and body stalk. By the sixth week of development, primitive erythroblasts also appear in the liver primordial and two weeks later granular leukocytes and megakaryocytes start to appear in the liver sinusoids. Somewhat later, the spleen also becomes site for hemopoiesis. During the fourth month of development, hemopoiesis starts declining in the liver and spleen and moves to the red bone marrow that becomes the predominant site for hemopoiesis postnatal.
All blood cellular elements develop from stem cells in the bone marrow and go through a complicated maturation process before released into the circulation. The development of red blood cells and platelets are known as erythropoiesis and thrombopoiesis respectively, while white cell development is normally divided into granulopoiesis, monopoiesis and lymphopoiesis. The presence of precursors in the systemic circulation is rare and usually indicates serious pathology. The one exception is the appearance of the reticulocytes, immediately preceding mature red blood cells. These cells are often used as an indicator of red bone marrow activity. The only cell we would expect you to know and recognize in a bone marrow smear is the megakaryocyte. This is a large cell up to 100?m in diameter. The irregular multilobar nucleus contains dispersed chromatin and does not display any nucleoli. The cytoplasm is extensive and contains fine basophilic granules. |
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