Serial block‐face scanning electron microscopy reveals novel intercellular connections in human term placental microvasculature

Abstract

The placental microvasculature is a conduit for fetal blood allowing solute exchange between the mother and the fetus. Serial block‐face scanning electron microscopy (SBF SEM) allows ultrastructure to be viewed in three dimensions and provides a new perspective on placental anatomy. This study used SBF SEM to study endothelial cells within the human placental microvasculature from uncomplicated pregnancies. Term human placental villi were aldehyde‐fixed and processed for imaging by SBF SEM. Manual segmentation was carried out on a terminal villous capillary and an intermediate villous arteriole and venule. Twenty‐seven SBF SEM stacks from terminal villi were analysed using stereological approaches to determine the volumes of microvascular components and the proportions of pericyte coverage. SBF SEM analysis of capillary endothelial cells revealed the presence of interendothelial protrusions (IEPs) originating from the donor cell at the endothelial junction and forming deep thin projections up to 7 μm into the adjacent endothelial cells. IEP density was estimated to be in the order of 35 million cm –3 placental tissue. Pericytes cover 15% of the fetal capillary surface area in terminal villi. In comparison, the cytotrophoblast covered 24% of the syncytiotrophoblast basal membrane. A trans‐endothelial channel was observed in a region of the vasculo‐syncytial capillary. Pericyte coverage was extensive in both arteriole and venule. Three‐dimensional imaging of the placental microvasculature identified novel ultrastructural features and provided an insight into factors that may influence capillary permeability and placental function. We hypothesise that the IEPs may allow mechanosensing between adjacent endothelial cells to assist in the maintenance of vessel integrity. The numbers of endothelial junctions, the presence of trans‐endothelial channels and the extent of pericyte coverage all provide an insight into the factors determining capillary permeability.

3D-Reconstructed Retinal Pigment Epithelial Cells Provide Insights into the Anatomy of the Outer Retina

“…However, results obtained using this technique have been shown to be accurate, highly reproducible, and insightful. For instance, similar approaches revealed initial counts and packing geometry of lipofuscin, melanolipofuscin and melanosomes in foveal RPE cells [ 14 ], impaired OS phagocytosis by RPE in enhanced S-cone syndrome patients [ 13 ], the arrangement of nascent rod OS disk membranes [ 15 ], the architecture of the basal RPE labyrinth [ 16 ] as well as novel organisational differences in the developing cornea [ 17 ], amongst other discoveries [ 15 , 18 , 19 , 20 ]. Where possible, we combined manual segmentation of RPE cells with partial automated segmentation of photoreceptor OS, which was possible due to their comparatively electron-dense nature.…”

3D Visualisation of trans-syncytial nanopores provides a pathway for paracellular diffusion across the human placental syncytiotrophoblast

“…Trans-endothelial pores have been reported in mouse endothelial cells where they may play a role in capillary permeability [29]. We have also reported an example of a large pore in human placental endothelium [30].…”

Placental blood flow sensing and regulation in fetal growth restriction

“…This triggers kinase phosphorylation, leading to NO production. Three dimensional electron microscopy images of human placental villi have recently demonstrated inter-endothelial protrusions originating at the endothelial junction and projecting deeply into adjacent ECs [ 35 ]. Determining whether these trans -EC connections facilitate mechanosensing is an exciting prospect.…”