Strengths and limitations
Imaging of thick tissue (i.e., no need to make thin sections) allowed imaging of nearly intact 3D morphology of placental villi. This supported accurate identification of knots, bridges and sprouts, and allowed assessment of the intact 3D organization of the villous tree and intact microvascular networks. Classical histopathology is only able to give a single plane 2D representation of these complex 3D structures (Figures 1A,1B) and therefore MPM offers a better alternative. Imaging of even larger fields of view by image stitching (2 X 2,5 mm2) is possible as demonstrated in Video S1, although it is more time consuming (3 hours).
MPM penetration depth of unprocessed placenta is limited to approximately 200 μm. Consequently, images of small (and frequently incomplete) networks cause risk for sampling bias. This could be addressed by optically clearing the tissue40, a process that homogenizes the tissue’s refraction index, to allow imaging down to some millimetres. Combining image stitching and optical clearing would increase the imaged volume many fold. Larger coverage would reduce sampling bias and improve image quantification. However, image acquisition time would increase correspondingly.
Limited sample size only allowed to generate preliminary results, but our goal was to demonstrate the method and its potential. Future efforts should focus on investigating regional variation within the same placenta (sampling bias) and variation between placentas with larger sample size. Additionally, obtaining a complete overview of clinical information (including doppler ultrasound findings41is crucial for future research, so that observations can be correlated accordingly.