Introduction

The blood platelet is unique in that it appears as a very simple cell with a clear cytoplasm, no nucleus, and a variety of organelles. These organelles contain various components which serve in metabolic and secretory processes. In response to a variety of stimuli, platelets liberate granule constituents in the outer medium. Yet, platelets cannot be considered a secretory cell, since they do not synthesize the secreted components. The purpose of this chapter is to describe the different platelet organelles, to analyse the role of each of them and to describe the function of the different granule constituents.

The different organelles, including few mitochondria, are dispersed randomly in the cytoplasm together with skeletal components (microtubules and actin filaments), a large amount of glycogen as a source of energy, and a complex membranous system. The latter consists of two prominent structures: the open canalicular system, which allows connections between the cytosol and the surrounding medium, and the dense tubular system (DTS), which stores important metabolic enzymes. Mitochondria and DTS are involved in metabolic processes, whereas specific granules (dense granules, alpha granules, and lysosomes) are involved in platelet secretion, the so-called ‘platelet release reaction’. In other words, mitochondria and DTS work in concert to provide the metabolic energy and control the cytosolic calcium required for secretion of the different granule constituents.

Dense granules (also termed dense bodies, dense core granules, and δ-granules)

These are so-called because they are both heavy and electron dense.