People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

Both biological and mechanical signals are known to influence cell proliferation. However, biological signals are mostly studied in two-dimensions (2D) and the interplay between these different pathways is largely unstudied. Here, we investigated the influence of the cell culture environment on the response to bFGF, a widely studied and important proliferation growth factor. We observed that human mesenchymal stromal cells (hMSCs), but not fibroblasts, lose the ability to respond to soluble or covalently bound bFGF when cultured on microfibrillar substrates. This behavior correlated with a downregulation of FGF receptor 1 (FGFR1) expression of hMSCs on microfibrillar substrates. Inhibition of actomyosin or the MRTF/SRF pathway decreased FGFR1 expression in hMSCs, fibroblasts and MG63 cells. To our knowledge, this is the first time FGFR1 expression is shown to be regulated through a mechanosensitive pathway in hMSCs. These results add to the sparse literature on FGFR1 regulation and potentially aid designing tissue engineering constructs that better control cell proliferation.

Electrospun (ESP) scaffolds are a promising type of tissue engineering constructs for large defects with limited depth. To form new functional tissue, the scaffolds need to be infiltrated with cells, which will deposit extracellular matrix. However, due to dense fiber packing and small pores, cell and tissue infiltration of ESP scaffolds is limited. Here, we combine two established methods, increasing fiber diameter and co-spinning sacrificial fibers, to create a porous ESP scaffold that allows robust tissue infiltration. Full cell infiltration across 2 mm thick scaffolds is seen 3 weeks after subcutaneous implantation in rats. After 6 weeks, the ESP scaffolds are almost fully filled with de novo tissue. Cell infiltration and tissue formation in vivo in this thickness has not been previously achieved. In addition, we propose a novel method for in vitro cell seeding to improve cell infiltration and a model to study 3D migration through a fibrous mesh. This easy approach to facilitate cell infiltration further improves previous efforts and could greatly aid tissue engineering approaches utilizing ESP scaffolds. Statement of significanceElectrospinning creates highly porous scaffolds with nano-to micrometer sized fibers and are a promising candidate for a variety of tissue engineering applications. However, smaller fibers also create small pores which are difficult for cells to penetrate, restricting cells to the top layers of the scaffolds. Here, we have improved the cell infiltration by optimizing fiber diameter and by co-spinning a sacrificial polymer. We developed novel culture technique that can be used to improve cell seeding and to study cytokine driven 3D migration through fibrous meshes. After subcutaneous implantation, infiltration of tissue and cells was observed up to throughout up to 2 mm thick scaffolds. This depth of infiltration in vivo had not yet been reported for electrospun scaffolds. The scaffolds we present here can be used for in vitro studies of migration, and for tissue engineering in defects with a large surface area and limited depth. IntroductionElectrospun (ESP) scaffolds are highly porous and consist of nano-or micrometer sized fibers of natural or synthetic polymers, mimicking the fibrous composition of tissue extra cellular matrix (ECM) [1][2][3] . ESP scaffolds provide more mechanical support than hydrogels and are more flexible than scaffolds produced by additive manufacturing, making them interesting for tissue engineering approaches [4] . Large ESP mats are easily produced but are often limited to a thickness of several mm due to delamination and charge distribution. This makes ESP scaffolds particularly interesting for defects with a large surface area, but limited depth. This includes skin patches [5] , corneal repair [6] , cartilage regeneration [7] , vascular grafts [8] and nerve guides [9] , among others. However, due to dense fiber packing and small pores, deep cell infiltration in ESP scaffolds remains a challenge [10, 11] . To create new fully functional tissue, ESP...

Controlling basic fibroblast growth factor (bFGF) signaling is important for both tissueengineering purposes, controlling proliferation and differentiation potential, and for cancer biology, influencing tumor progression and metastasis. Here, we observed that human mesenchymal stromal cells (hMSCs) no longer responded to soluble or covalently bound bFGF when cultured on microfibrillar substrates, while fibroblasts did. This correlated with a downregulation of FGF receptor 1 (FGFR1) expression of hMSCs on microfibrillar substrates, compared to hMSCs on conventional tissue culture plastic (TCP). hMSCs also expressed less SRF on ESP scaffolds, compared to TCP, while fibroblasts maintained high FGFR1 and SRF expression. Inhibition of actin-myosin tension or the MRTF/SRF pathway decreased FGFR1 expression in hMSCs, fibroblasts and MG63 osteosarcoma cells. This downregulation was functional, as hMSCs became irresponsive to bFGF in the presence of MRTF/SRF inhibitor. Together, our data show that hMSCs, but not fibroblasts, are irresponsive to bFGF when cultured on microfibrillar susbtrates by downregulation of FGFR1 through the MRTF/SRF pathway. This is the first time FGFR1 expression has been shown to be mechanosensitive and adds to the sparse literature on FGFR1 regulation. These results could open up new targets for cancer treatments and could aid designing tissue engineering constructs that better control cell proliferation.