Gestational development of water and non-electrolyte permeability of human syncytiotrophoblast plasma membranes

In order to establish a gestational profile for placental transcellular permeabilities to water, urea and mannitol, syncytiotrophoblast microvillous (MVM) and basal membrane (BM) vesicles were isolated from human placentae obtained from 16 weeks of gestation to term. Using stop-flow/light-scattering techniques the rate of change in vesicle volume in response to an osmotic challenge was measured and osmotic water permeabilities (Pf) and solute permeabilities (Ps) calculated. Membrane fluidity was assessed by steady-state DPH anisotropy. Permeability of MVM to water and solutes increased by 20-30 per cent in mid-pregnancy and declined again after the 36th week of gestation. In BM, this pattern was apparent only for water permeability; solute permeabilities were not significantly altered. MVM cholesterol content was approx two-fold higher and membrane fluidity lower compared to BM. Cholesterol content in BM, but not in MVM, increased during the late third trimester. Membrane fluidity did not change consistently during gestational development. We conclude that syncytiotrophoblast plasma membranes exhibit small but significant changes in passive permeability to water and non-electrolytes from 16 weeks of gestation to term. It is suggested that an increased water permeability of the syncytiotrophoblast plasma membranes might contribute substantially to the gestational increase in water exchange across the human placenta observed in vivo.