thrombocytes

Meaning: Thrombocytes are platelets, small cell fragments without a nucleus that pinch off from large bone marrow cells called megakaryocytes. They circulate in the blood and take part in hemostasis, the stopping of bleeding. When a vessel wall is damaged, platelets adhere to exposed collagen, become activated, release signals that recruit more platelets, and aggregate into a platelet plug. Activated platelets also support the clotting cascade, in which the enzyme thrombin (made from prothrombin) converts soluble fibrinogen into insoluble fibrin strands. The fibrin mesh reinforces the plug into a stable clot. Thrombopoietin stimulates megakaryocytes to produce platelets.

On the MCAT: Expect questions that sort the formed elements of blood: red blood cells carry O₂, leukocytes defend against pathogens, and platelets clot. A passage may describe a low platelet count or a missing clotting factor and ask you to predict prolonged bleeding. Other questions ask which blood components lack a nucleus (platelets and mature mammalian red blood cells), or trace blood cells back to hematopoietic stem cells in bone marrow.

Common confusion: Platelets are not whole cells, and they are not the same thing as fibrin or the clotting factors. Platelets build the initial plug; the cascade of clotting factors, many made in the liver, produces fibrin that strengthens it. Platelets are also not white blood cells, even though both come from marrow.