3D structural quantification of equine placental villi through multiscale correlative imaging

As placental processes occur within the bounds of a three-dimensional (3D) anatomical structure, the quantification of placental structures across different scales is necessary for a proper understanding of placental function and disease (Lewis and Pearson-Farr, Placenta. 2020;102:55-60). Three-dimensional imaging techniques have recently identified and quantified novel structures in human placental villi. In equidae, fetal growth relies on feto-maternal exchanges taking place in placental villi facing the entire uterine endometrium. Villi length increases throughout pregnancy, but branching and internal blood flow have been poorly explored so far. Here, we present a workflow for correlative multiscale 3D imaging combining X-ray Computed Microtomography (microCT) and Serial Block Face Scanning Electron Microscopy (SBF-SEM) applied to villi from sections (∼3 × 3 mm) of the chorioallantois taken from term placentas in horses (n=2),zebras (n=2), donkeys (n=3), and mules (n=5). Although this work is still ongoing and biological conclusions are pending, this workflow, resulting in 3D villi reconstructions, enabled the quantification of the 3D villus structure across multiple orders of magnitude for metrics such as surface area, volume, and branching angle. Not only were the reconstructions able to identify novel nanoscale structures in equine villi, such as stromal macrovesicles previously only observed in human placental villi, but they also allowed the computational modelling of physiological processes such as blood flow and diffusion through realistic tissue geometries. Taken together, the structural data generated from this correlative workflow will help us to better resolve equine placental organization, contribute towards a holistic understanding of equine placental function in health and disease, and identify potential differences between different equid species.