Increased neonatal platelet deposition on subendothelium under flow conditions: The role of plasma von Willebrand factor

In vitro platelet function of umbilical cord blood and neonatal peripheral vein blood from full-term newborns was compared with that of adults. Citrat ed whole blood was subjected to shear stress (1300 s(-1)) on subendothelial extracellular matrix (ECM)-coated wells in a cone and plate(let) analyzer. Adhered platelets on the ECM were quantitated by image analyzer. Both umbi lical cord and neonatal peripheral blood platelets demonstrated more extens ive adhesion than adult platelets, and similar aggregate formation on ECM. The ability of neonatal platelets to form aggregates on ECM was confirmed b y scanning electron microscopy. Similar activation of neonatal and adult pl atelets after subjection to shear stress, in the suspension phase, was esta blished by flow cytometry, which showed an increase in fibrinogen binding a nd a decrease in glycoprotein To expression on platelet membrane. The diffe rence in adhesion rates between neonatal and adult platelets was preserved even when the hematocrit level of the neonatal blood was adjusted to that o f adults. Reconstitution of neonatal or adult platelet-rich plasma with aut ologous or heterologous red packed cells yielded no change in adhesion and aggregation. When von Willebrand factor-covered plates were used to prevent deposition of plasma von Willebrand factor on the surface, no difference i n platelet adhesion was seen between neonatal and adult blood. In conventio nal aggregometry assay, the response to ristocetin of washed platelets of e ither neonatal or adult source was higher on addition of plasma from neonat es than from adults. Our data suggest that the extensive neonatal platelet deposition on ECM is mediated by plasma von Willebrand factor, which is kno wn to be more multimerized and, therefore, more active in neonates than in adults. This mechanism may provide balanced primary hemostasis in neonates despite the platelet hyporeactivity to agonists without application of shea r stress.