Morphology of the platelet

Abstract

When examining peripheral blood, either with phase-contrast illumination or a fixed film and a Romanowsky stain, the smallest elements seen are platelets. These cells usually range from 1 to 3µm in diameter, although an occasional larger cell up to 5µm may be seen. With higher magnification a central area is seen, termed the granulomere because it contains granules, and a peripheral clearer area called the hyalomere. When wet preparations are examined by phase-contrast the granules move within the cell. If a fixed film has been obtained from a finger prick, rather than from venous anticoagulated blood, clumps of platelets are observed due to the triggering of platelet aggregation following the tissue trauma and glass contact. Artifacts may occur in the platelets’ appearance if the blood is collected into anticoagulants and allowed to stand for any length of time. This is particularly so when the anticoagulant used is ethylene diamine tetra-acetic acid (EDTA) in which the platelets swell on incubation. Studies of recalcification of platelet-rich plasma using phase-contrast microscopy showed a rapid change in shape from disc to a spiny sphere and the early appearance of long thin filaments with aggregation into masses of cells which coalesce and lose the granules, and from these radiate strands of fibrin. This phenomenon was called viscous metamorphosis, but examination by electron microscopy shows that each individual cell retains its integrity, and that the changes observed by the light microscope reflect both the release reaction (v. infra) and the redistribution of the cells’ organelles during platelet aggregation as well as changes in the platelets’ shape.