The platelet body The platelet consists of: 1. Cytoskeleton or peripheral zone containing the plasma membrane, receptors and the open canalicular system. 2. Sol-Gel zone that is centrally located and contains the cytoplasm and contractile proteins 3. Organelle zone made up of granules, lysosomes, mitochondria, Granules and cytoplasmic organelles The platelet membrane is a phospholipid bi-layer and responds to stimuli. It communicates with the interior via channels called the open canalicular system (OCS). The latter are invaginations of the surface membrane and provide a channel for entry of plasma proteins and exit of platelet chemicals. The membrane also has glycoproteins (Gp) receptors for various proteins such as collagen, von Willebrand factor. The glycoproteins participate in shape change and adhesion. Contractile proteins of the microtubules and microfilaments in the sol-gel zone are responsible for the discoid shape of the platelets. The microtubule lies as an circular coil underneath the membrane. They dissociate into subunits during platelet shape change. Actin is the predominant contractile protein in the cytoplasm. Actin-binding proteins bind actin and Gp1b-IX