The influence of oxygen and tumor necrosis factor-α on the cellular kinetics of term placental villous explants in culture

Explanted placental fragments may provide a more physiological in vitro model of component cell function than single cell isolates. We have characterized these fragments for cell turnover and have monitored responses from 14 normal placentas under conditions of exogenous TNFalpha and atypical oxygen concentrations (3% and 17%), conditions associated with abnormal pregnancy and an aberrant in utero environment. Explants were assessed for apoptotic morphology, immunolocalization of Mib-1 (a proliferation marker), caspase 3 activity (an apoptosis promoter), lactate dehydrogenase (a necrosis marker), and human chorionic gonadotrophin [hCG, a marker of cytotrophoblast (CT) differentiation]. Consistent with a reduction in hCG, explants under 17% O-2 (with and without TNFa) showed a progressive degeneration of syncytiotrophoblast (ST) (days 0-2) followed by a restoration of hCG (days 4-8) localized to newly differentiated but not syncytialized CTs. In 3% O-2, hCG showed the same initial decline but failed to. recover thereafter. Proliferation dropped significantly in 17% O-2 but was restored and exaggerated sixfold in 3% O-2. All reductions in hCG were associated with cell death and caspase-3. Early apoptosis was linked with syncytial loss; later apoptosis (days 8-11) was localized to the non-ST. Prolonged exposure to TNFalpha (days 4-11) increased ST apoptosis and necrosis but 3% O-2 had no significant effect. These findings show that placental explants can accommodate many aspects of CT proliferation, differentiation, and ST apoptosis in culture. TNFalpha enhanced ST decline but 3% oxygen (compared with 17%) was associated with reduced CT differentiation and a strong shift towards proliferation. These outcomes may reflect previous morphological changes in compromised pregnancies and confirm a possible role for oxygen and TNFalpha in aberrant trophoblast turnover.