Morphological characteristics of bovine platelets activated with platelet activating factor or thrombin

The morphological alterations induced by the activation of bovine platelet rich plasma suspensions with the inflammatory mediator, platelet activating factor (PAF), and following the activation of washed bovine platelet suspensions with thrombin are described. The unstimulated bovine platelet exhibits a smooth oval or discoid shape and granules are randomly dispersed throughout the cytoplasm. The initial activation response to PAF is the development of irregularly shaped cells, the migration of granules to the periphery of the cell and the appearance of large pseudopodia devoid of membrane organelles. As activation continues and large platelet aggregates form, two zones of irregularly shaped, discrete platelets are evident within each large aggregate: an outer zone in which the cells are devoid of granules and an inner zone in which many of the platelets exhibit the typical ultrastructural features of a non-activated cell. In washed platelet preparations activated with thrombin, virtually all platelets undergo shape change and yet many cells retain their alpha granules. In addition, discrete irregularly shaped agranular platelets are also found. The distinctive morphological alterations observed in activated bovine platelets are likely associated with the absence of an open canalicular system, characteristic of many other types of mammalian platelets, and with the ability of the cytoplasmic microtubule coil to reorganise into a linear array following thrombin activation. It is postulated that the bovine platelet has evolved as a cell that can respond to various stimuli, for example inflammatory mediators, by releasing active metabolites from its granular stores without forming platelet-platelet bridges that can serve as a foci for thrombus formation. In this manner the bovine platelet can effectively function as an inflammatory cell without acting as a potent thrombogenic agent.[PUBLICATION ABSTRACT]