Structure of Platelets
Platelets have a unique structure that aids in their function. They are typically 2-3 micrometers in diameter and are composed of a plasma membrane, a dense tubular system, and numerous granules containing clotting factors and other proteins. Despite lacking a nucleus, platelets contain organelles such as
mitochondria and a cytoskeleton composed of
microtubules and
actin filaments.
Formation of Platelets
The formation of platelets, known as
thrombopoiesis, begins in the bone marrow with the differentiation of hematopoietic stem cells into megakaryocytes. These large cells extend cytoplasmic projections into the blood vessels of the marrow, where shear forces from blood flow help fragment these projections into platelets.
Role in Hemostasis
Platelets are central to the process of hemostasis. Upon vascular injury, they adhere to exposed
collagen and other subendothelial structures. This adhesion is mediated by
Glycoprotein Ib on the platelet surface and
von Willebrand factor present in the blood. Following adhesion, platelets become activated, changing shape and releasing granule contents that further promote aggregation and clotting. They form a temporary plug and work in concert with the
coagulation cascade to stabilize the clot.
Microscopic Appearance
Under a light microscope, platelets appear as small, irregularly shaped cell fragments that are often clustered. In a stained blood smear, they can be identified by their basophilic granules and absence of a nucleus. Electron microscopy reveals more detailed structures, including the open canalicular system and dense bodies. Laboratory Evaluation
Platelet counts and function can be evaluated through various laboratory tests. A complete blood count (CBC) provides the number of platelets, while platelet function tests like
platelet aggregometry assess their ability to clump together. These evaluations are essential in diagnosing and managing disorders related to platelet function and number.
