Morphometric assessment of the oxygen diffusion conductance in placentae from pregnancies complicated by intra-uterine growth restriction.

The morphometric oxygen diffusive conductance (D(p)) of the placenta provides a measure of the efficiency of oxygen transfer between the mother and the developing fetus. Any change in the D(p)may point towards possible adaptation in the light of altered oxygen transfer. Placentae from normal (n=40) and small for gestational age SGA (n=24) pregnancies were analysed using stereological techniques. Each placenta was uniform randomly sampled and tissue samples processed to wax infiltration and embedding using conventional histological preparatory methods. A combination of stereological techniques and physiological constants were used to estimate the partial conductances across the five major tissue compartments involved in oxygen transfer. There was a significant reduction in both fetal birthweight and placental weight in the SGA group when compared with controls. A decrease in both chorionic (S(cv)) and fetal capillary (S(fc)) surface area was also observed in SGA placentae when compared with controls (P>0.001). Villous membrane harmonic thickness (T(vm)) was reduced in the SGA placentae (2.33 microm) when compared with controls (2.67 microm P=0.019). This resulted in a reduction in the minimum D(p)in SGA placentae when compared with controls (P=0.023). Adjusting for fetal weight resulted in no difference in the specific diffusive conductance. Changes in T(vm)in SGA placentae combined with changes in basic surface areas were insufficient to maintain overall D(p)values comparable with control placentae.