Ultrastructural study on the release reaction of the platelet. Release I.

The ultrastructural changes of human or rabbit platelets in response to release inducers I were studied by using isolated cells from the respective platelet rich plasma. The changes of platelets were more dynamic in ADP than those in epinephrine and endotoxin though common changes occurred in all cases. In the platelets on release reaction, there were well preserved alpha-granules and the disappearance of very dense granules accompanied by simultaneous appearance of round vesicles. The contraction wave of the microtubules rarely occurred and the microfibrils increased in amount in the cytoplasm of platelets. The canalicular system in response to ADP alone is morphologically interpreted as the open canalicular system. On the other hand, epinephrine and endotoxin did not cause obvious morphological changes of the open canalicular system. When the contents of very dense granules alone were discharged leaving the round vesicles behind in the release I, it seems likely that the microfibrils play a more important role than the microtubules. Furthermore, the author suggests that the dense tubular system or third canalicular system may play an essential role for the release I as a secretory pathway in place of the open canalicular system.