|MadSci Network: Immunology|
Bone marrow is a mesh of interconecting cells called stromal cells, along with collagen fibres and fat cells. You could think of it as a big sponge. Blood vessels called blood sinuses are there too - the escape route for cells that are made in the bone marrow. People think that one cell in the bone marrow can change itself and make all types of blood cells that you see. This is called the pluripotent stem cell. The pluripotent stem cell can become a neutrophil, a lymphocyte, a red blood cell or a platelet or one of several other cell types depending on the signals in the bone marrow. These are all blood cells that perfrom special functions and are important if not essential to stay healthy. The stem cells then commit to becoming one type of cell or another and they divide several times so that one stem cell will make many of the final cell type. The most common type of blood cell is the red blood cell or ethryocyte. These cells are made in the bone marrow from a reticulocyte and when they are ready to leave, they squeeze through the blood sinuses with special molecules called adhesion molecules. Adhesion molecules let cells stick and release themselves to blood vessels (as well as to each other). When the cell has squeezed through, it gets into the blood. All the other blood cells do the same thing, except the platelet which isn't really a cell at all but a fragment of a cells. The precursor (the cell which makes the platelet) doesn't leave the bone marrow but stays there until it dies, making platelets. Hope that answers your question.
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