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Megakaryocyte: Immature

The megakaryocyte lineage is the cell line responsible for the production of platelets found in the peripheral blood. Unlike the other bone marrow lineages that decrease in size as they mature, the megakaryocyte starts smaller and increases in size as it matures.

The megakaryocyte begins as a mononuclear cell that has the same physical size and nuclear/cytoplasmic proportions as a lymphoblast. Eventually the megakaryocyte matures into a multinucleated giant cell with vast amounts of cytoplasm. As this cell matures, it can actually increase to more than ten times the size of other nucleated cells found in the bone marrow.

In the early stages of development, the cytoplasm of a megkaryocyte is basophilic without any obvious platelet granules. The cytoplasm will be darker near the edges of the cell and may have a "foamy" look in the golgi area adjacent to the nucleus. Cytoplasmic granule development is not usually noticeable until the cell's cytoplasm color begins to lighten.

Notice the sizes of the early-intermediate stage megakaryocytes (red arrows) in comparison to the background bone marrow cells present in the two images to the right. The megakaryocyte nucleus makes up the largest part of the cell at this early stage. Notice the increasing lobulation as the cell increases in size and how the cytoplasm becomes more foamy and slightly more granular as well.