The influence of mode of fixation on morphometrical data derived from terminal villi in the human placenta at term: a comparison of immersion and perfusion fixation.

The placenta is a highly vascular organ which is subjected to a variable degree of compression during delivery. Partial collapse of the fetal capillary network inevitably results, and this has significant effect upon the maternofetal diffusion distance. Fixation via a dual perfusion system operating at physiological pressures can redistend the vascular tree to its antenatal state. Morphometrical analysis provides a volume fraction for the fetal capillaries within the terminal villi of 38.4 per cent, a figure which approximates closely to values obtained from in situ biopsy of the placenta. The harmonic mean diffusion distance was found to be 3.6 micron. Perfusion fixation resulted in a lower coefficient of variation for many parameters compared with traditional immersion fixation. By negating the unpredictable influence of the placental transfusion, it is considered that perfusion fixation will make true interplacental variations easier to detect.