TEM image, 10,000X. Thrombocytes (platelets) are biconvex discs; the shape is well seen on their transverse sections. In case of other, random sections #1, #2, the shape of platelets is differs from typical.
Platelets are surrounded by the plasma membrane with an external layer of glycocalyx. The central part of cytoplasm or granulomere #1, #2 contains mitochondria #1, #2, some elements of Golgi apparatus, and granules of several types:

• alpha granules #1, #2, #3 are the largest ones, they contain different proteins, including blood clotting factors
• dense granules #1, #2 are smaller and less numerous, they contain serotonin and histamine that are absorbed from the blood
• lysosomal granules #1, #2 are the finest ones, they contain hydrolytic enzymes

• an open canalicular system, which participates in the absorption and elimination of various substances (not present in the image)
• circumferential microtubules beneath the plasma membrane; they belong to the cytoskeleton (not present in the image)

SEM image, 16,000X. If endothelium is damaged, platelets are activated; their discoid shape turns into a rounded one #1, #2; then they form long processes #1, #2, #3 (filopodia), anchor the damaged vascular wall, join one another to form aggregates, and constitute a primary blood clot (platelet plug).

The thrombocytic activation is accompanied by a release of granule contents; it leads to a further development of blood coagulation with the formation of secondary blood clot (fibrin mesh).