Non-syncytial sources of fetal DNA in transcervically recovered cell populations.

We have previously shown that fetal DNA can be detected in swabs and flushings obtained from the lower uterine pole prior to the termination of pregnancy. The presence of syncytiotrophoblast vesicles in transcervically retrieved samples suggested that this distinctive placental tissue was an abundant source of fetal DNA and a valuable resource in prenatal diagnosis strategies. In a more extensive study involving 150 terminations of pregnancy between 7 and 17 weeks gestational age, 29% of transcervically retrieved samples contained visible syncytial vesicles. Flushing of the uterine pole more frequently contained syncytia than direct aspiration (39% compared with 26% of samples) but this difference was not statistically significant. No samples > 14 weeks gestational age contained syncytia. Polymerase chain reaction analysis using Y-sequence specific-nested primers indicated the presence of fetal DNA in the absence of intact syncytial vesicles. We therefore examined samples by in-situ hybridization using Y-specific DNA probes. Positive labelling was observed in syncytial vesicles where present and in clumps of unidentified fetal cells. In addition, high numbers of naked nuclei were labelled in samples devoid of syncytia. These isolated nuclei are possibly derived from disrupted syncytia, and may be an important and hitherto overlooked contributory factor in fetal material which collects at the lower uterine pole.