Summary
Mammalian platelets and nonmammalian thrombocytes, although they perform a similar
function, differ in morphology and in their response to thrombogenic agents. The present
experiments extend the comparison between the two cell types to include the immature
form of thrombocytes found in early chick embryos. Agents known to induce, or to inhibit,
aggregation of platelets were tested for their effect on thrombocytes which were isolated
from chick embryos of five days incubation age and which had been suspended in a tris-buffered
saline solution containing EDTA. The degree of aggregation of thrombocytes was determined
by phase microscopy after treatment with thrombin, trypsin, adenosine 5′-diphosphate
(ADP), calcium chloride, or the sulfhydryl inhibitor N-ethylmaleimide. Approximately
450 trials were performed using 102 samples of blood, each sample being obtained by
the pooling of the blood of five to six dozen embryos. The results demonstrate that
thrombin and trypsin, in the presence of calcium ions, induce significant aggregation
compared to that of untreated controls. Thrombin, in the absence of calcium ions,
is ineffective in inducing aggregation, as is calcium chloride alone, or ADP in the
presence or absence of calcium ions. Pretreatment of the thrombocytes with N-ethylmaleimide
inhibited aggregation by thrombin in the presence of calcium chloride. It may be concluded
that embryonic thrombocytes resemble platelets in their response to all the aggregating
agents tested, with the exception of ADP, but resemble the thrombocytes of adult chickens
in their failure to respond to ADP.
