Tcherner Elad et al. present a novel approach to electrospin substrates for in this case endothelial cells directly on TEM supports in their manuscript “An engineered platform to study the influence of extracellular matrix nanotopography on cell ultrastructure”.

This approach has indeed a lot of potential and could provide a powerful tool. The characterisation of cell/substrate interaction at the nanoscale, thanks to the resolution of electron microscopy and in 3D, as is possible in cryo-EM, could tell us a lot about details of this interaction that has been elusive so far.
Looking at cell adhesion at the nanoscale is highly relevant, because many cell adhesive and mechanotransductive processes are determined at this scale, e.g., regarding integrin clustering, integrin adhesion complex maturation and cytoskeletal remodelling, both in the of extracellular matrix and engineered biomaterial substrates. Very recent findings, such as Zhang et al. (2023, doi: 10.1038/s41556-023-01238-1) and Chighizola et al. (2022, doi: 10.1186/s12951-022-01585-5), demonstrate how strongly the nanotopography regulates mechanotransductive processes and how relevant it is to better understand these processes at the nanoscale level, in particular e.g. in cancer (Ashworth and Cox, 2024, doi: 10.1038/s41568-024-00704-8).

Indeed, the authors mention that “This new class of cryo-ET cell culture supports can be used to investigate the organization of the cell-ECM adhesions that un derlie the mechanosensitivity of endothelial cells to changes in ECM topography”. Considering this potential with respect to the dissection of cell adhesion at the nanoscale, it is unfortunate that in the cryo-tomogram of Fig. 5 no such interaction sites between electrospun fibres and the cell have actually been visualised. This is my main critique of this manuscript. It would be good if the authors would show such a site, this would further strengthen the message of this manuscript.

Minor remarks:
Some of the structures indicated as filopodia in Fig. 4B I wouldn’t necessarily define as such. They seem more like cell protuberances (sprouting behaviour?) along the electrospun aligned fibres, at the tip of this protuberances you can guess the filopodia (although the resolution and focus is not ideal in this image for this). However, the ones indicated in Fig. 4E are instead seem filopodia.