Extracellular vesicles (EVs) are lipid bilayer nanoparticles involved in cell-cell communication that are released into the extracellular space by all cell types. The cargo of EVs includes proteins, lipids, nucleic acids, and metabolites reflecting their cell of origin. EVs have recently been isolated directly from solid tissues, and this may provide insights into how EVs mediate communication between cells in vivo. Even though EVs have been isolated from tissues, their point of origin when they are in the interstitial space has been uncertain.
In this study, University of Gothenburg researchers performed three-dimensional (3D) reconstruction using transmission electron tomography of metastatic and normal liver tissues with a focus on the presence of EVs in the interstitium. After chemical fixation of the samples and subsequent embedding of tissue pieces in resin, ultrathin slices (300 nm) were cut and imaged on a 120 ekV transmission electron microscopy as a tilt series (a series of subsequent images tilted at different angles). These were then computationally illustrated in a 3D manner to reconstruct the imaged tissue volume. The researchers identified the cells delimiting the interstitial space in both types of tissues, and small distinct spherical structures with a diameter of 30-200 nm were identified between the cells. These round structures appeared to be more abundant in metastatic tissue compared to normal tissue. Thee researchers suggest that the observed spherical structures in the interstitium of the metastatic and non-metastatic liver represent EVs. This work thus provides the first 3D visualization of EVs in human tissue.
Schematic illustration of the steps involved in the acquisition of the
3D reconstructions by electron tomography of tissues
Metastatic liver tissue and macroscopically normal liver tissue were collected from the liver of a patient with uveal melanoma liver metastases. Samples were fixed, dehydrated, and embedded in resin. Sections of 300 nm thickness were obtained using a microtome. During the electron tomography data acquisition, the specimen holder was gradually tilted inside the microscope around an axis perpendicular to the electron beam, and subsequent frames were then acquired at multiple angles (+65° and −65°). Finally, a 3D reconstruction (tomogram) of the tissue was computed from the acquired images using the IMOD program.