Electron tomography of vesicles

Combining array tomography with electron tomography provides insights into leakiness of the blood-brain barrier in mouse cortex

Like other volume electron microscopy approaches, automated tape-collecting ultramicrotomy (ATUM) enables imaging of serial sections deposited on thick plastic tapes by scanning electron microscopy (SEM). ATUM is unique in enabling hierarchical imaging and thus efficient screening for target structures, as needed for correlative light and electron microscopy. However, SEM of sections on tape can only access the section surface, thereby limiting the axial resolution to the typical size of cellular vesicles with an order of magnitude lower than the acquired xy resolution. In contrast, serial-section electron tomography (ET), a transmission electron microscopy-based approach, yields isotropic voxels at full EM resolution, but requires deposition of sections on electron-stable thin and fragile films, thus making screening of large section libraries difficult and prone to section loss. To combine the strength of both approaches, we developed 'ATUM-Tomo, a hybrid method, where sections are first reversibly attached to plastic tape via a dissolvable coating, and after screening detached and transferred to the ET-compatible thin films. As a proof-of-principle, we applied correlative ATUM-Tomo to study ultrastructural features of blood-brain barrier (BBB) leakiness around microthrombi in a mouse model of traumatic brain injury. Microthrombi and associated sites of BBB leakiness were identified by confocal imaging of injected fluorescent and electron-dense nanoparticles, then relocalized by ATUM-SEM, and finally interrogated by correlative ATUM-Tomo. Overall, our new ATUM-Tomo approach will substantially advance ultrastructural analysis of biological phenomena that require cell- and tissue-level contextualization of the finest subcellular textures.

Pericytes on placental capillaries in terminal villi preferentially cover endothelial junctions in regions furthest away from the trophoblast

Introduction

Pericytes are a common feature in the placental microvasculature but their roles are not well understood. Pericytes may provide physical or endocrine support for endothelium and in some tissues mediate vasoconstriction.

Methods

This study uses serial block-face scanning electron microscopy (SBFSEM) to generate three-dimensional (3D) reconstructions of placental pericytes of the terminal villi and transmission electron microscopy (TEM) to study pericyte endothelial cell interactions. The proportion of endothelial cell junctions covered by pericytes was determined.

Results

The detailed 3D models of placental pericytes show pericyte structure at a new level of detail. Placental pericytes have many fingers extending from the cell body which can span multiple capillary branches. The proportion of endothelial cell-cell junctions covered by pericytes was significantly higher than pericyte coverage of capillary endothelium as a whole (endothelium: 14%, junctions: 43%, p < 0.0001). However, the proportion of endothelial cell-cell junctions covered by pericytes in regions adjacent to trophoblast was reduced compared to regions >3 μm away from trophoblast (27% vs 62% respectively, p < 0.001). No junctional complexes were observed connecting pericytes and endothelial cells but there were regions of cell membrane with features suggestive of intercellular adhesions.

Discussion

These data suggest that the localisation of pericytes on the villous capillary is not random but organised in relation to both endothelial junctions and the location of adjacent trophoblast. This further suggests that pericyte coverage may favour capillary permeability in regions that are most important for exchange, but limit capillary permeability in other regions.

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Three-dimensional imaging highlights the structure of placental pericytes.

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Placental pericytes preferentially cover endothelial junctions.

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The proportion of covered junctions decreased in regions adjacent to trophoblasts.

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The localisation of placental pericytes suggests endothelial coverage is non-random.

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Junction coverage may alter capillary permeability in key regions of exchange.