DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
Coloured scanning electron micrograph (SEM) of a megakaryocyte. Megakaryocytes are derived from hematopoietic stem cell precursor cells in the bone marrow. During megakaryocyte maturation the cell grows in size and replicates its DNA without cytokinesis, a process called endomitosis. As a result, the nucleus of the megakaryocyte may contain up to 32 copies of the normal complement of DNA in a human cell. Megakaryocytes are responsible for the production of blood thrombocytes (platelets), which are necessary for normal blood clotting. Platelet production begins with the extension of large pseudopodia from the megakaryocyte cell surface that form thin tube-like cytoplasmic extensions (with bulbous thickenings) called proplatelets. As the proplatelet development continues small platelets can be seen forming along the proplatelet processes. Each megakaryocyte produces and releases hundreds of platelets into the circulatory system.
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