Function and ultrastructure of platelets of neonates: enhanced ristocetin aggregation of neonatal platelets.

We have investigated platelet morphology and function in human maternal-newborn pairs. Fibrinogen concentration and factor-VIII activity in plasma were also determined. Our results showed that, compared to maternal platelets, neonatal platelets were poorly responsive to adenosine diphosphate, adrenaline and collagen. Uptake of labelled serotonin by neonatal and maternal platelets was the same, but release of the radioactivity was reduced in the former. Phagocytic activity of neonatal platelets, demonstrated with latex particles, was similar to that of maternal platelets. Although the ultrastructure of neonatal platelets approximated that of maternal platelets, immature appearing platelets were occasionally found in the neonatal samples. An unanticipated finding was that platelet aggregation induced by ristocetin was more vigorous in neonatal than in maternal platelet-rich plasma samples. Furthermore, neonal -lasma, which had lower fibrinogen and factor-VIII content than maternal plasma, facilitated maternal platelet aggregation by ristocetin and showed a greater ability than maternal plasma to promote ristocetin-induced aggregation of platelets of a patient with von Willebrand's disease. These results indicate that the plasma of neonates contains large quantities of the ristocetin-dependent platelet aggregation factor (RAF), probably more than is in maternal plasma, despite the higher levels of factor-VIII procoagulant activity in the latter. Thus, in the newborn, there is a clear dissociation between factor-VIII clotting activity and the RAF activity.