Megakaryocyte

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Histological features of the bone marrow.

Megakaryocytes are a special type of bone marrow cells that give rise to thrombocytes (platelets). These cells are large and measure about 50–150 µm in diameter and are usually found near sinusoidal capillaries of the bone marrow. They have complex polyploid nuclei that bear multiple sets of chromosomes and a cytoplasm that contains numerous mitochondria, Golgi apparatus and rough endoplasmic reticulum.

In the bone marrow, hemopoietic stem cells (hemocytoblast) differentiate into common myeloid progenitor cells (myeloid stem cell) which give rise to megakaryocyte/erythrocyte progenitors. These cells in turn differentiate into monopotent megakaryocyte-committed progenitor cells, which differentiate into megakaryoblasts. Under the influence of the growth factor thrombopoietin, megakaryoblasts mature into promegakaryocytes and finally become megakaryocytes. 

Driven by thrombopoietin, megakaryocytes form several thin processes (pseudopodia) called proplatelets. These penetrate the lumens of sinusoidal capillaries. Complete separation of cytoplasmic fragments subsequently occurs at the peripheral regions of proplatelets by the action of microtubules, forming platelets, which are then release into the circulating blood in the sinusoids as small, membrane-bounded, discoid structures without nuclei.

A mature megakaryocyte possesses several demarcation membranes, which are invaginations of the plasma membrane across the cytoplasm. These membranes were initially thought to be partitioning points of platelet release but are now considered to assist in proplatelet elongation.

A single megakaryocyte typically forms two to four thousands platelets and subsequently become apoptotic and are ultimately cleared by macrophages.