Some mucocutaneous bleeding disorders are associated with platelet granule deficiencies that result in platelet function defects. Several studies have found that platelets from neonates have reduced activation responses compared to platelets from adults, but this relative platelet hyporeactivity is not associated with excessive bleeding in healthy infants. Little information is available concerning the ultrastructure of neonatal or fetal human platelets. Using whole mount electron microscopy to count calcium-rich platelet dense granules, we observed that neonatal and fetal platelets obtained from umbilical cord blood contain significantly fewer dense granules compared to cells from children and adults obtained from peripheral blood.It is well established that most plasma coagulation factor concentrations and/or activities are decreased in neonates compared to children and adults. The notable exceptions are clotting factor VIII and von Willebrand factor, which are unchanged and increased, respectively.~(~(1)) Less is known about the function of neonatal platelets, partly due to the lack of standardized methods, until recently, for assessing platelet function~(~(2)) and also due to the large blood volumes required for assays of platelet function. In a study of 47,000 patients, Wiedmeier et al .~(~(3)) reported that platelet volumes remain relatively constant throughout gestation, and show a transient increase (along with platelet number) in neonates a few days following birth. A recent study~(~(4)) also confirmed earlier reports that neonatal platelets have reduced activation responses compared to adults, which contrasts with the apparent hyperactivity of the neonatal hemostatic system when assessed by bleeding time and in vitro assays.~(~(5),~(6)).
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