The precise route serving as the pathway for extrusion of products containe
d in platelet storage organelles following cell activation has been controv
ersial for many years. Some workers believe that the granules and dense bod
ies fuse together to form secretion vacuoles that move to the exposed cell
surface, fuse with it and extrude contained products to the outside. Others
suggest that the organelles fuse with channels of the open canalicular sys
tem (OCS) and deposit their contents which are subsequently secreted to the
exterior. The controversy has been difficult to resolve because nearly all
workers have used platelets in suspension to arrive at diverse conclusions
. The present study has used ethylenediamine tetracetic acid (EDTA)-treated
platelets and isometric clot retraction as a model system, and either poly
lysine or tannic acid to define the pathway used for secretion. Results ind
icate that secretion takes place in EDTA platelets under isometric tension
by fusion of granules to OCS channels and passage of products through these
conduits to the cell exterior. There is no evidence to suggest the organel
les form secretion vacuoles or extrude their products by fusion with the in
side of the exposed surface membrane.