Introduction

Blood platelets respond to many stimuli by changing shape from their resting, normal discoid form into more rounded structures possessing blebs and pseudopodia1–5. These morphological transitions are generally referred to as the platelet ‘shape change’ (SC). As described earlier in Chapters 2 and 3, platelets are subcellular fragments derived from megakaryocytes and circulate in the blood as small anucleated ovoid discs, with average dimensions of about 3.0 × 0.5 µm and volumes of 6–8 fl. The surfaces of resting platelets are almost featureless except for indentations or pits that represent entrances into the opencanalicular system (OCS) illustrated in a freeze–fracture micrograph (Fig. 22.1). Platelets exhibit significant heterogeneity in both size, density and biochemical composition, with a corresponding heterogeneity in aggregation and adhesion characteristics. Shape-change behaviour is also heterogeneous, such that some platelets will remain discoid while neighbouring cells will have undergone significant morphological changes.

The transformations in platelet morphology associated with the SC represent highly sensitive indices of platelet activation. They are generally considered to be the first measurable physiological responses after activation by specific platelet agonists such as ADP or thrombin, and to begin before significant aggregation occurs. Platelet SC is usually characterized morphologically by spheration and contraction of the cell, cytoskeletal rearrangements, folding or ruffling of the surface membrane, and formation of pseudopodia.