MORPHOLOGICAL-CHARACTERISTICS OF BOVINE PLATELETS ACTIVATED WITH PLATELET-ACTIVATING-FACTOR OR THROMBIN

The morphological alterations induced by the activation of bovine plat elet 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 bo vine platelet exhibits a smooth oval or discoid shape and granules are randomly dispersed throughout the cytoplasm. The initial activation r esponse to PAF is the development of irregularly shaped cells, the mig ration of granules to the periphery of the cell and the appearance of large pseudopodia devoid of membrane organelles. As activation continu es 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 w hich many of the platelets exhibit the typical ultrastructural feature s of a non-activated cell. In washed platelet preparations activated w ith thrombin, virtually all platelets undergo shape change and yet man y cells retain their alpha granules. In addition, discrete irregularly shaped agranular platelets are also found. The distinctive morphologi cal alterations observed in activated bovine platelets are likely asso ciated with the absence of an open canalicular system, characteristic of many other types of mammalian platelets, and with the ability of th e cytoplasmic microtubule coil to reorganise into a linear array follo wing thrombin activation. It is postulated that the bovine platelet ha s evolved as a cell that can respond to various stimuli, for example i nflammatory mediators, by releasing active metabolites from its granul ar 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 poten t thrombogenic agent.