VILLOUS CYTOTROPHOBLAST REGULATION OF THE SYNCYTIAL APOPTOTIC CASCADEIN THE HUMAN PLACENTA

Villous trophoblast in the human placenta consists of a population of proliferating stem cells which differentiate and individually fuse int o the syncytiotrophoblast. We studied the apoptotic cascade in this co mplex epithelial layer by immunohistochemical localization of Fas, Fas t, Bcl-2, Mcl-1, pro-caspase-3 and caspase-3, T-cell-restricted intrac ellular antigen-related protein (TIAR), poly(ADP-ribose) polymerase (P ARP), lamin B, topoisomerase II alpha, and transglutaminase II in cryo stat and paraffin-fixed tissue sections from, normal human first-trime ster and term placental villi. The relationship between the apoptotic cascade and syncytial fusion was studied by coincubation of intact vil li with FITC-coupled annexin-V, to detect the phosphatidylserine flip, and propidium iodide, to detect plasma membrane permeability. The fin al events of the apoptotic cascade were studied by the TUNEL reaction and ultrastructural appearance of the trophoblast. The phosphatidylser ine flip was identified in some of the villous cytotrophoblastic cells , but the presence of both Bcl-2 and Mcl-1 proteins presumably prevent ed continuation of the apoptotic cascade. The syncytiotrophoblast demo nstrated heterogeneous findings, suggesting variable progression along the apoptotic cascade. In some areas Bcl-2 and Mcl-1 predominated, wi th preservation of the nuclear proteins PARP, lamin B, and topoisomera se II alpha; in other areas, especially in and around syncytial sprout s, Bcl-2 and Mcl-1 were absent, accompanied by loss of nuclear protein s, presence of phosphatidylserine flip, and TUNEL positivity. These da ta suggest that the apoptotic cascade is initiated in the villous cyto trophoblast, which in turn promotes syncytial fusion. Donation of anti -apoptotic proteins into the syncytium, such as Bcl-2 and Mcl-1, focal ly inhibits further progression along this cascade. Completion of the apoptotic cascade takes place in and around syncytial sprouts, providi ng further evidence that these are the sites of trophoblast shedding i nto the maternal circulation.