Summary
The ultrastructure of platelet aggregation in vivo in response to bleeding brought about by transection of small mesenteric vessels
in rats and guinea pigs has been studied. Platelets aggregate, degranulate and separating
membranes disappear in parallel with fibrin appearance which is first seen at several
loci after 30 seconds of bleeding. About 40 per cent of the electron opaque granules,
some of which contain platelet factor 3 have disappeared after one minute of bleeding
while the electron lucent granules increase by 70 per cent suggesting that some of
them may be empty vesicles. Most of the platelet aggregates of the random type disappear
leaving clumped red blood cells entrapped by a network of fibrin fibers which emanate
from the remains of platelet aggregates of the rosette type to maintain hemostasis.
