The placenta: a site of platelet production?

Stripping of cytoplasm from circulating megakaryocytes (MKs) in the pulmonary capillary bed implicates the lungs as an important site for platelet production in normal physiology. The placenta has been proposed as an alternative location for intrauterine platelet production as fetal lungs are essentially non-functioning. To investigate this further, circulating MKs from an umbilical artery and umbilical vein of 10 human placentae were isolated using a modified whole blood filtration technique. Pretreatment of blood, which may be deleterious to MKs, was not required. Following May Grunwald Giemsa staining of filters, MKs were identified by their morphology and classified into 4 types based on subjective assessment of the amount of cytoplasm present. Blood from umbilical arteries and veins contained a mean of 37.2 MKs/0.5 ml (median 37.0; range 17-80) and 23.9 MKs/0.5 ml (median 18.5; range 5-67) respectively (p < 0.01). 43% of arterial MKs and 14% of venous MKs possessed copious cytoplasm (p < 0.01). These results demonstrate a significant reduction of MK numbers across the placenta. Removal of MK cytoplasm at this site is consistent with the hypothesis that the placenta functions as a platelet forming organ during intrauterine life, the lungs taking over this role after birth.