MECHANISMS OF ALPHA-FETOPROTEIN TRANSFER IN THE PERFUSED HUMAN PLACENTAL COTYLEDON FROM UNCOMPLICATED PREGNANCY

We investigated the mechanisms of alphafetoprotein (AFP) transfer acro ss the human placenta by correlating measurements of AFP transfer with cytochemical localization of AFP, Placental cotyledons were dually pe rfused in vitro with either the fetal or maternal perfusate containing umbilical cord plasma as a source of AFP, Steady state AFP clearance, corrected for release of endogenous AFP, was 0.973+/-0.292 mu l/min p er gram in the fetal to maternal direction (n = 10), significantly hig her (P < 0.02) than that in the maternal to fetal direction (n = 5; 0. 022+/-0.013 mu l/min per gram), Clearance of a similarly sized protein , horseradish peroxidase was also asymmetric but clearance of the smal l tracer creatinine was not. Using a monoclonal antibody, we localized AFP to fibrinoid deposits in regions of villi with discontinuities of the syncytiotrophoblast, to cytotrophoblast cells in these deposits, to syncytiotrophoblast on some villi, and to trophoblast cells in the decidua, We conclude that AFP transfer in the placenta is asymmetric a nd that there are two available pathways for AFP transfer: (a) from th e fetal circulation into the villous core and across fibrinoid deposit s at discontinuities in the villous syncytiotrophoblast to enter the m aternal circulation; and (b) AFP present in the decidua could enter ve ssels that traverse the basal plate.