Platelets Blood platelets , or thrombocytes, are not true cells, but rather cytoplasmic fragments of a large cell in the bone marrow, the megakaryocyte. The central portion of a platelet stains purple with Wright's stain and is referred to as the granulomere. The peripheral portion stains clear and is called the hyalomere. The large cell seen is a monocyte. Platelet activation occurs when injury to the vessel wall exposes sub-endothelial components, especially collagen.
Platelets adhere to the damaged area and become cohesive to other platelets.
This aggregation leads to the formation of a platelet plug, which prevents further blood loss and allows the repair process to begin.