Ultrastructural characteristics of freeze-dried human blood platelets

Ultrastructural characteristics of freeze-dried human blood platelets were investigated. Extensive ice crystal formation was found in platelets frozen in the absence of cryoprotective agents. Platelet preservation was much improved by the addition of dimethyl sulfoxide, or dimethyl acetamide, with glucose or 0.25 M sodium glycerophosphate, to platelet-rich plasma prior to the freeze drying process. Platelet membranes and organelles were generally well preserved, and many granules demonstrated eccentric areas of electron density. Microtubules were not clearly identified, but some platelets showed tubule-like structures within platelet hyaloplasm. Pseudopods were often observed at the platelet surface, and often contained membrane-bound granule-like structures. Some pseudopods contained electrondense material which extended between adjacent platelets in interplatelet bridges and which was released to the surrounding medium. Interplatelet bridge formation was not prevented by chelation of calcium by ethylenediamine tetraacetic acid. The observations were consistent with the suggestion that contractile protein is present in interplatelet bridges and serves as a basis for clot retraction.