This work was supported by the American Heart Association (grant number 15SDG25740035, awarded to J.Z.), the National Institute of Biomedical Imaging and Bioengineering (NIBIB) of the National Institutes of Health (grant number EB007507, awarded to C.C.), and the Alliance for Regenerative Rehabilitation Research &#38; Training (AR3T, grant number 1 P2C HD086843-01, awarded to J.Z.). We would like to acknowledge Prof. Jeanne Stachowiak (The University of Texas at Austin) for her technical advice on confocal microscopy. We are also grateful for discussions with Samuel Mihelic (The University of Texas at Austin), Dr. Alicia Allen (The University of Texas at Austin), Dr. Julie Rytlewski (Adaptive Biotech), and Dr. Leon Bellan (Vanderbilt University) for their insight on the computational analysis of 3D networks. Finally, we thank Dr. Xiaoping Bao (University of California, Berkeley) for his advice on differentiating iPSCs into iPSC-EPs.

<ol>
	<li>Wang, K., Lin, R. -. Z., Melero-Martin, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bioengineering+human+vascular+networks:+trends+and+directions+in+endothelial+and+perivascular+cell+sources.">Bioengineering human vascular networks: trends and directions in endothelial and perivascular cell sources.</a> Cellular and Molecular Life Sciences. , 1-19 (2018).</li><li>Kant, R. J., Coulombe, K. L. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integrated+approaches+to+spatiotemporally+directing+angiogenesis+in+host+and+engineered+tissues.">Integrated approaches to spatiotemporally directing angiogenesis in host and engineered tissues.</a> Acta Biomaterialia. 69, 42-62 (2018).</li><li>Briquez, P. S., Clegg, L. E., Martino, M. M., Mac Gabhann, F., Hubbell, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+principles+for+therapeutic+angiogenic+materials.">Design principles for therapeutic angiogenic materials.</a> Nature Reviews Materials. 1 (1), 1-15 (2016).</li><li>Arnaoutova, I., George, J., Kleinman, H. K., Benton, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+endothelial+cell+tube+formation+assay+on+basement+membrane+turns+20:+State+of+the+science+and+the+art.">The endothelial cell tube formation assay on basement membrane turns 20: State of the science and the art.</a> Angiogenesis. 12 (3), 267-274 (2009).</li><li>Bezenah, J. R., Kong, Y. P., Putnam, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluating+the+potential+of+endothelial+cells+derived+from+human+induced+pluripotent+stem+cells+to+form+microvascular+networks+in+3D+cultures.">Evaluating the potential of endothelial cells derived from human induced pluripotent stem cells to form microvascular networks in 3D cultures.</a> Scientific Reports. 8 (1), 2671 (2018).</li><li>Nakatsu, M. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=VEGF121+and+VEGF165+regulate+blood+vessel+diameter+through+vascular+endothelial+growth+factor+receptor+2+in+an+in+vitro+angiogenesis+model.">VEGF121 and VEGF165 regulate blood vessel diameter through vascular endothelial growth factor receptor 2 in an in vitro angiogenesis model.</a> Laboratory Investigation. 83 (12), 1873-1885 (2003).</li><li>Shamloo, A., Heilshorn, S. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Matrix+density+mediates+polarization+and+lumen+formation+of+endothelial+sprouts+in+VEGF+gradients.">Matrix density mediates polarization and lumen formation of endothelial sprouts in VEGF gradients.</a> Lab on a Chip. 10 (22), 3061 (2010).</li><li>Jeon, J. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+of+3D+functional+microvascular+networks+with+human+mesenchymal+stem+cells+in+microfluidic+systems.">Generation of 3D functional microvascular networks with human mesenchymal stem cells in microfluidic systems.</a> Integrative Biology. 6 (5), 555-563 (2014).</li><li>Takahashi, K., Yamanaka, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Induction+of+Pluripotent+Stem+Cells+from+Mouse+Embryonic+and+Adult+Fibroblast+Cultures+by+Defined+Factors.">Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors.</a> Cell. 126 (4), 663-676 (2006).</li><li>Kusuma, S., Macklin, B., Gerecht, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Derivation+and+network+formation+of+vascular+cells+from+human+pluripotent+stem+cells.">Derivation and network formation of vascular cells from human pluripotent stem cells.</a> Methods in Molecular Biology. 1202, 1-9 (2014).</li><li>Lian, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+differentiation+of+human+pluripotent+stem+cells+to+endothelial+progenitors+via+small-molecule+activation+of+WNT+signaling.">Efficient differentiation of human pluripotent stem cells to endothelial progenitors via small-molecule activation of WNT signaling.</a> Stem Cell Reports. 3 (5), 804-816 (2014).</li><li>Kusuma, S., Shen, Y. -. I., Hanjaya-Putra, D., Mali, P., Cheng, L., Gerecht, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-organized+vascular+networks+from+human+pluripotent+stem+cells+in+a+synthetic+matrix.">Self-organized vascular networks from human pluripotent stem cells in a synthetic matrix.</a> Proceedings of the National Academy of Sciences of the United States of America. 110 (31), 12601-12606 (2013).</li><li>Charwat, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Potential+and+limitations+of+microscopy+and+Raman+spectroscopy+for+live-cell+analysis+of+3D+cell+cultures.">Potential and limitations of microscopy and Raman spectroscopy for live-cell analysis of 3D cell cultures.</a> Journal of Biotechnology. 205, 70-81 (2015).</li><li>Holnthoner, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adipose-derived+stem+cells+induce+vascular+tube+formation+of+outgrowth+endothelial+cells+in+a+fibrin+matrix.">Adipose-derived stem cells induce vascular tube formation of outgrowth endothelial cells in a fibrin matrix.</a> Journal of Tissue Engineering and Regenerative Medicine. 9 (2), 127-136 (2012).</li><li>Zudaire, E., Gambardella, L., Kurcz, C., Vermeren, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+computational+tool+for+quantitative+analysis+of+vascular+networks.">A computational tool for quantitative analysis of vascular networks.</a> PLoS ONE. 6 (11), 1-12 (2011).</li><li>Khoo, C. P., Micklem, K., Watt, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+comparison+of+methods+for+quantifying+angiogenesis+in+the+Matrigel+assay+in+vitro.">A comparison of methods for quantifying angiogenesis in the Matrigel assay in vitro.</a> Tissue Engineering Part C: Methods. 17 (9), 895-906 (2011).</li><li>Rytlewski, J. A., Geuss, L. R., Anyaeji, C. I., Lewis, E. W., Suggs, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+image+quantification+as+a+new+morphometry+method+for+tissue+engineering.">Three-dimensional image quantification as a new morphometry method for tissue engineering.</a> Tissue Engineering Part C: Methods. 18 (7), 507-516 (2012).</li><li>Kerschnitzki, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Architecture+of+the+osteocyte+network+correlates+with+bone+material+quality.">Architecture of the osteocyte network correlates with bone material quality.</a> Journal of Bone and Mineral Research. 28 (8), 1837-1845 (2013).</li><li>Bao, X., Lian, X., Palecek, S. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Directed+endothelial+progenitor+differentiation+from+human+pluripotent+stem+cells+via+Wnt+activation+under+defined+conditions.">Directed endothelial progenitor differentiation from human pluripotent stem cells via Wnt activation under defined conditions.</a> Wnt Signaling: Methods and Protocols. 1481 (1), 183-196 (2016).</li><li>Crosby, C. O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantifying+the+vasculogenic+potential+of+iPSC-derived+endothelial+progenitors+in+collagen+hydrogels.">Quantifying the vasculogenic potential of iPSC-derived endothelial progenitors in collagen hydrogels.</a> Tissue Engineering Part A. , (2019).</li><li>Li, C. H., Tam, P. K. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+iterative+algorithm+for+minimum+cross+entropy+thresholding.">An iterative algorithm for minimum cross entropy thresholding.</a> Pattern Recognition Letters. 19 (8), 771-776 (1998).</li><li>Kollmannsberger, P., Kerschnitzki, M., Repp, F., Wagermaier, W., Weinkamer, R., Fratzl, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+small+world+of+osteocytes:+Connectomics+of+the+lacuno-canalicular+network+in+bone.">The small world of osteocytes: Connectomics of the lacuno-canalicular network in bone.</a> New Journal of Physics. 19 (7), (2017).</li><li>Crosby, C. O., Zoldan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mimicking+the+physical+cues+of+the+ECM+in+angiogenic+biomaterials.">Mimicking the physical cues of the ECM in angiogenic biomaterials.</a> Regenerative Biomaterials. , (2019).</li><li>Watanabe, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+ROCK+inhibitor+permits+survival+of+dissociated+human+embryonic+stem+cells.">A ROCK inhibitor permits survival of dissociated human embryonic stem cells.</a> Nature Biotechnology. 25 (6), 681-686 (2007).</li><li>Kniazeva, E., Kachgal, S., Putnam, A. J., Ph, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+extracellular+matrix+density+and+mesenchymal+stem+cells+on+neovascularization+in+vivo.">Effects of extracellular matrix density and mesenchymal stem cells on neovascularization in vivo.</a> Tissue Engineering Part A. 17 (7-8), 905-914 (2011).</li><li>Ghajar, C. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+matrix+density+on+the+regulation+of+3-D+capillary+morphogenesis.">The effect of matrix density on the regulation of 3-D capillary morphogenesis.</a> Biophysical Journal. 94 (5), 1930-1941 (2008).</li><li>Morin, K. T., Smith, A. O., Davis, G. E., Tranquillo, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aligned+human+microvessels+formed+in+3D+fibrin+gel+by+constraint+of+gel+contraction.">Aligned human microvessels formed in 3D fibrin gel by constraint of gel contraction.</a> Microvascular Research. 90, 12-22 (2013).</li><li>Frantz, C., Stewart, K. M., Weaver, V. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+extracellular+matrix+at+a+glance.">The extracellular matrix at a glance.</a> Journal of Cell Science. 123 (24), 4195-4200 (2010).</li></ol>

The authors have nothing to disclose.

This protocol involves the long-term culture of cells in three types of cell culture media: E8, LaSR Basal, and EGM-2. Therefore, great care should be taken to appropriately sterilize all materials. Additionally, lab coats and ethanol-cleaned gloves should always be worn when working in the laboratory&#8217;s laminar flow hood. It is recommended to frequently test for mycoplasma contamination; if a large amount of debris is observed during iPSC culture or a sudden drop in differentiation efficiency is noted, mycoplasma c...

Human umbilical vein endothelial cells (HUVECs) and other primary endothelial cell types have been utilized for two decades to model blood vessel sprouting and development in vitro1. Such vascular platforms promise to illuminate molecular and tissue-level mechanisms of cardiovascular disease and may present physiological insight into the development of primitive vascular networks2,3. Though the field of vascular modeling has witnessed significant advances, a &#8220;gold standard&#8221; assay that can quantitatively model and assess physiological vascular development remains elusive. Most published protocols do not adequately recapitulate the vascular niche to encourage the formation of mature, functional blood vessels or do not have a method to quantitatively compare the vasculogenic potential of the assessed cell types in three dimensions (3D).
Many current vascular models are limited in their ability to mimic the physiological vascular niche. One of the most commonly employed in vitro platforms is the gelatinous protein mixture-based tube formation assay. Briefly, HUVECs are seeded as single cells on a thin layer of gel that consists of proteins harvested from murine sarcoma extracellular matrix (ECM); within one to two days, the HUVECs self-assemble into primitive tubes4. However, this process occurs in two-dimensions (2D) and the endothelial cells (ECs) utilized in this assay do not form enclosed, hollow lumens, thereby limiting the physiological significance of these studies. More recently, ECs and supporting cells (e.g., mesenchymal stem cells (MSCs) and pericytes) have been co-cultured in 3D microenvironments that simulate the fibrous architecture of the native ECM, such as collagen or fibrin hydrogels5. To model vascular development in this microenvironment, polymeric beads coated with ECs are typically employed6. The addition of exogenous growth factors and/or growth factors secreted by other cells interstitially embedded in the hydrogel can induce the ECs, coating the polymeric beads, to sprout and form single lumen; the number and diameter of sprouts and vessels can then be computed. However, these sprouts are singular and do not form an enclosed, connected network as is seen in physiological conditions and thus is more reminiscent of a tumor vasculature model. Microfluidic devices have also been utilized to mimic the vascular niche and to promote the formation of vasculature in EC-laden hydrogels7,8. Typically, an angiogenic growth factor-gradient is applied to the circulating cell culture medium to induce EC migration and sprouting. ECs that constitute the lumen of developed vessels are sensitive to the shear stress induced by the application of fluid flow through the microfluidic device; thus, these microfluidic devices capture key physiological parameters that are not accessible in the static models. However, these devices require costly microfabrication abilities.
Most importantly, all three vascular models (2D, 3D, microfluidic) overwhelmingly utilize primary ECs as well as primary supporting cell types. Primary cells cannot be developed into an effective cardiovascular therapy because the cells would engender an immune response upon implantation; furthermore, HUVECs and similar primary cell types are not patient-specific and do not capture vascular abnormalities that occur in patients with a genetic disposition or pre-existing health conditions, e.g., diabetes mellitus. Induced pluripotent stem cells (iPSCs) have emerged in the past decade as a patient-specific, proliferative cell source that can be differentiated into all somatic cells in the human body9. In particular, protocols have been published that outline the generation and isolation of iPSC-derived endothelial progenitors (iPSC-EPs)10,11; iPSC-EPs are bipotent and can, therefore, be further differentiated into endothelial cells and smooth muscle cells/pericytes, the building blocks of mature, functional vasculature. Only one study has convincingly detailed the development of a primary capillary plexus from iPSC-EPs in a 3D microenvironment12; though this study is critical to an understanding of iPSC-EP assembly and differentiation in natural and synthetic hydrogels, it did not quantitatively compare the network topologies of the resulting vasculature. Another recent study has used the polymeric bead model to compare the sprouting of HUVECs and iPSC-derived ECs5. Therefore, there is a clear need to further elucidate the physical and chemical signaling mechanisms that regulate iPSC-EP vasculogenesis in 3D microenvironments and to determine if these cells are suitable for ischemic therapy and cardiovascular disease modeling.
In the past decade, different open-source computational pipelines and skeletonization algorithms have been developed to quantify and compare vascular network length and connectivity. For example, Charwat et al. developed a Photoshop-based pipeline to extract a filtered, binarized image of vascular networks derived from a co-culture of adipose-derived stem cells and outgrowth ECs in a fibrin matrix13,14. Perhaps the most widely used topology comparison tool is AngioTool, a program published online by the National Cancer Institute15; despite the program&#8217;s widespread adoption and well-documented fidelity, the program is limited to analyzing vessel-like structures in two dimensions and other programs, including AngioSys and Wimasis, share the same dimensionality limitation16. Powerful software suites such as Imaris, Lucis, and Metamorph have been developed to analyze the network topology of engineered microvasculature; however, these suites are cost-prohibitive for most academic labs and limit access to the source code, which may hinder the ability of the end-user to customize the algorithm to their specific application. 3D Slicer, an open-source magnetic resonance imaging/computed tomography package, contains a Vascular Modeling Toolkit that can effectively analyze the topology of 3D vascular networks17; however, the analysis is dependent on the user manually placing the end-points of the network, which may become tedious when analyzing a large dataset and can be influenced by the user&#8217;s subconscious biases. In this manuscript, a computational pipeline that can quantify 3D vascular networks is described in detail. To overcome the above-outlined limitations, this open source computational pipeline utilizes ImageJ to pre-process acquired confocal images to load the 3D volume into a skeleton analyzer. The skeleton analyzer uses a parallel medial axis thinning algorithm, and was originally developed by Kerschnitzki et al. to analyze the length and connectivity of osteocyte networks18; this algorithm can be effectively applied to characterize the length and connectivity of engineered microvasculature.
Altogether, this protocol outlines the creation of microvascular networks in 3D microenvironments and provides an open-source and user-bias free computational pipeline to readily compare the vasculogenic potential of iPSC-EPs.

<table>
	<tbody>
		<tr>
			<td>&#181;-Slide Angiogenesis</td>
			<td>Ibidi</td>
			<td>N/A</td>
			<td>A flat, glass bottom tissue-culture plate with side walls enables facile confocal imaging</td>
		</tr>
		<tr>
			<td>96 well, round bottom, ultra low attachment microplate, sterile</td>
			<td>Corning</td>
			<td>7007</td>
			<td>Prevents the binding of cell-laden collagen hydrogels to the cell culture dish</td>
		</tr>
		<tr>
			<td>Accutase</td>
			<td>STEMCELL Technologies</td>
			<td>7920</td>
			<td>Gentle cell detachment solution; does not degrade extracellular epitopes vital for FACS</td>
		</tr>
		<tr>
			<td>Advanced DMEM/F12</td>
			<td>Thermo Scientific</td>
			<td>12634010</td>
			<td>The base media for iPSC-EP differentiation.&#160;</td>
		</tr>
		<tr>
			<td>Barnstead GenPure xCAD Plus&#160;</td>
			<td>Thermo Fisher Scientific</td>
			<td>50136165</td>
			<td>Water purification system; others can be readily substituted</td>
		</tr>
		<tr>
			<td>Bovine Serum Albumin solution,7.5% in DPBS, sterile-filtered, BioXtra, suitable for cell culture</td>
			<td>Fisher Scientific</td>
			<td>A8412</td>
			<td>To preserve cell viability when FACs sorting</td>
		</tr>
		<tr>
			<td>CD34-PE, human (clone: AC136)</td>
			<td>Miltenyi Biotec</td>
			<td>130-098-140</td>
			<td>Antibody used for FACs isolation of iPSC-EPs</td>
		</tr>
		<tr>
			<td>CHIR99021</td>
			<td>LC Laboratories</td>
			<td>C-6556</td>
			<td>Induces the formation of mesoderm from pluripotent stem cells</td>
		</tr>
		<tr>
			<td>Collagen I Rat Tail High Protein 100 mg</td>
			<td>VWR</td>
			<td>354249</td>
			<td>Main component of the 3D microenvironment</td>
		</tr>
		<tr>
			<td>Conical centrifuge tubes (15/50 mL)</td>
			<td>Fisher Scientific</td>
			<td>14-959-49D/A</td>
			<td>Used to store and mix relatively large volumes of reagents and cell culture media</td>
		</tr>
		<tr>
			<td>DAPI (4&#39;,6-Diamidino-2-Phenylindole, Dihydrochloride)</td>
			<td>Thermo Fisher Scientific</td>
			<td>D1306</td>
			<td>To counterstain and visualize cell nuclei</td>
		</tr>
		<tr>
			<td>DMEM/F12</td>
			<td>Thermo Fisher Scientific</td>
			<td>11320-082</td>
			<td>For dilution of Matrigel and thawing of pluripotent stem cells</td>
		</tr>
		<tr>
			<td>Dulbecco&#39;s phosphate-buffered saline (DPBS)</td>
			<td>ThermoFisher</td>
			<td>14190-250</td>
			<td>To wash monolayer cultures</td>
		</tr>
		<tr>
			<td>EDTA</td>
			<td>Sigma-Aldrich</td>
			<td>E8008</td>
			<td>For passaging of pluripotent stem cell colonies and to prevent cell aggregation when FACs sorting</td>
		</tr>
		<tr>
			<td>Endothelial Cell Growth Medium 2</td>
			<td>PromoCell</td>
			<td>C-22011</td>
			<td>Promotes endothelial cell viability and proliferation</td>
		</tr>
		<tr>
			<td>Essential 8 Medium</td>
			<td>Thermo Fisher Scientific</td>
			<td>A1517001&#160;&#160;</td>
			<td>For maintenance of pluripotent stem cells</td>
		</tr>
		<tr>
			<td>Glycine,BioUltra, for molecular biology, &#62;=99.0% (NT)</td>
			<td>Sigma-Aldrich</td>
			<td>50046</td>
			<td>Neutralizes remaining detergent</td>
		</tr>
		<tr>
			<td>L-Ascorbic acid 2-phosphate sesquimagnesium salt hydrate,&#62;=95%</td>
			<td>Sigma-Aldrich</td>
			<td>A8960</td>
			<td>Component of iPSC-EP differentiation medium</td>
		</tr>
		<tr>
			<td>MATLAB</td>
			<td>MathWorks</td>
			<td>1.8.0_152</td>
			<td>Multi-paradigm numerical computing environment (free available at most academic institutions)</td>
		</tr>
		<tr>
			<td>Matrigel Matrix GFR PhenolRF Mouse 10 mL (gelatinous protein mixture)</td>
			<td>Fisher Scientific</td>
			<td>356231</td>
			<td>Diluted in DMEM/F12 to coat plates for iPSC-EP differentiation</td>
		</tr>
		<tr>
			<td>Medium-199 10X</td>
			<td>Thermo Fisher Scientific</td>
			<td>1825015</td>
			<td>Used to balance final hydrogel osmolarity and pH</td>
		</tr>
		<tr>
			<td>Microcentrifuge tubes (1.7 mL)</td>
			<td>VWR</td>
			<td>87003-294</td>
			<td>Stores small volumes of reagents</td>
		</tr>
		<tr>
			<td>Phosphate-buffered saline (PBS)</td>
			<td>Sigma-Aldrich</td>
			<td>P3813</td>
			<td>The main ingredient of the immunostaining solutions</td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin (10,000 U/mL)</td>
			<td>Thermo Fisher Scientific</td>
			<td>15140122</td>
			<td>Antibiotic used after sorting to remove possible contamination from FACS instrument</td>
		</tr>
		<tr>
			<td>Recombinant Human VEGF 165 Protein</td>
			<td>R&#38;D Systems</td>
			<td>293-VE</td>
			<td>Mitogen that stimulates endothelial cell proliferation and tubulogenesis</td>
		</tr>
		<tr>
			<td>Rhodamine phalloidin</td>
			<td>Themo Fisher Scientific</td>
			<td>R415</td>
			<td>To identify F-actin deposition and therfore outline the borders of the vascular networks</td>
		</tr>
		<tr>
			<td>Triton X-100 (nonionic surfactant)</td>
			<td>Sigma-Aldrich</td>
			<td>X-100</td>
			<td>Detergent used to gently permeabilize cells</td>
		</tr>
		<tr>
			<td>Tween-20 (emulsifying reagent)</td>
			<td>Fisher Scientific</td>
			<td>BP337</td>
			<td>Increases the binding specificity of the added antibodies</td>
		</tr>
		<tr>
			<td>VE-Cadherin (F-8)</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-9989&#160;</td>
			<td>To identify 3D endothelial lumen in collagen hydrogels</td>
		</tr>
		<tr>
			<td>Vitronectin</td>
			<td>ThermoFisher</td>
			<td>A14700</td>
			<td>For maintenance of pluripotent stem cells</td>
		</tr>
		<tr>
			<td>Y-27632</td>
			<td>Selleck Chemicals</td>
			<td>S1049</td>
			<td>Preserves pluripotent stem cell and iPSC-EP viability when dissociated and re-seeded</td>
		</tr>
	</tbody>
</table>

an in vitro 3d model and computational pipeline to quantify the vasculogenic potential of ipsc-derived endothelial progenitors

Induced pluripotent stem cells (iPSCs) are a patient-specific, proliferative cell source that can differentiate into any somatic cell type. Bipotent endothelial progenitors (EPs), which can differentiate into the cell types necessary to assemble mature, functional vasculature, have been derived from both embryonic and induced pluripotent stem cells. However, these cells have not been rigorously evaluated in three-dimensional environments, and a quantitative measure of their vasculogenic potential remains elusive. Here, the generation and isolation of iPSC-EPs via fluorescent-activated cell sorting are first outlined, followed by a description of the encapsulation and culture of iPSC-EPs in collagen hydrogels. This extracellular matrix (ECM)-mimicking microenvironment encourages a robust vasculogenic response; vascular networks form after a week of culture. The creation of a computational pipeline that utilizes open-source software to quantify this vasculogenic response is delineated. This pipeline is specifically designed to preserve the 3D architecture of the capillary plexus to robustly identify the number of branches, branching points, and the total network length with minimal user input.

After differentiation (Figure 1), FACS&#160;and encapsulation of iPSC-EPs in collagen hydrogels, the cells will typically remain rounded for 24 h before beginning to migrate and form initial lumens. After about 6 days of culture, a primitive capillary plexus will be visible in the hydrogel when viewed with brightfield microscopy (Figure 2). After imaging the fixed, stained cell-laden hydrogels on a confocal microscope (Mo...

Watch this Scientific Journal Video about An In Vitro 3D Model and Computational Pipeline to Quantify the Vasculogenic Potential of iPSC-Derived Endothelial Progenitors at JoVE.com

An In Vitro 3D Model and Computational Pipeline to Quantify the Vasculogenic Potential of iPSC-Derived Endothelial Progenitors

Endothelial progenitors derived from induced pluripotent stem cells (iPSC-EPs) have the potential to revolutionize cardiovascular disease treatments and to enable the creation of more faithful cardiovascular disease models. Herein, the encapsulation of iPSC-EPs in three-dimensional (3D) collagen microenvironments and a quantitative analysis of these cells&#8217; vasculogenic potential are described.

Induced pluripotent stem cells (iPSCs) are a patient-specific, proliferative cell source that can differentiate into any somatic cell type. Bipotent ...

an-vitro-3d-model-computational-pipeline-to-quantify-vasculogenic

Research

JoVE Journal

Developmental Biology

6.0K Views.  University of Texas at Austin. Endothelial progenitors derived from induced pluripotent stem cells (iPSC-EPs) have the potential to revolutionize cardiovascular disease treatments and to enable the creation of more faithful cardiovascular disease models. Herein, the encapsulation of iPSC-EPs in three-dimensional (3D) collagen microenvironments and a quantitative analysis of these cells’ vasculogenic potential are described.

Video: An In Vitro 3D Model and Computational Pipeline to Quantify the Vasculogenic Potential of iPSC-Derived Endothelial Progenitors

Characterization of Thymic Settling Progenitors in the Mouse Embryo Using In Vivo and In Vitro Assays

Characterizing thymic settling progenitors is important to understand the pre-thymic stages of T cell development, essential to devise strategies for T cell replacement in lymphopenic patients. We studied thymic settling progenitors from murine embryonic day 13 and 18 thymi by two complementary in vitro and in vivo techniques, both based on the &#8220;hanging drop&#8221; method. This method allowed colonizing irradiated fetal thymic lobes with E13 and/or E18 thymic progenitors distinguished by CD45 allotypic markers and thus following their progeny. Colonization with mixed populations allows analyzing cell autonomous differences in biologic properties of the progenitors while colonization with either population removes possible competitive selective pressures. The colonized thymic lobes can also be grafted in immunodeficient male recipient mice allowing the analysis of the mature T cell progeny in vivo, such as population dynamics of the peripheral immune system and colonization of different tissues and organs. Fetal thymic organ cultures revealed that E13 progenitors developed rapidly into all mature CD3+ cells and gave rise to the canonical &#947;&#948; T cell subset, known as dendritic epithelial T cells. In comparison, E18 progenitors have a delayed differentiation and were unable to generate dendritic epithelial T cells. The monitoring of peripheral blood of thymus-grafted CD3-/- mice further showed that E18 thymic settling progenitors generate, with time, larger numbers of mature T cells than their E13 counterparts, a feature that could not be appreciated in the short term fetal thymic organ cultures.

Characterization of Thymic Settling Progenitors in the Mouse Embryo Using In Vivo and In Vitro Assays

This article describes in vivo and in vitro methodology to characterize the thymic settling progenitors by the analysis of the kinetics of generation, phenotype and numbers of their T cell progeny.

Characterizing thymic settling progenitors is important to understand the pre-thymic stages of T cell development, essential to devise strategies for T ...

In Vitro and In Vivo Models to Study Corneal Endothelial-mesenchymal Transition

Corneal endothelial cells (CECs) play a crucial role in maintaining corneal clarity through active pumping. A reduced CEC count may lead to corneal edema and diminished visual acuity. However, human CECs are prone to compromised proliferative potential. Furthermore, stimulation of cell growth is often complicated by gradual endothelial-mesenchymal transition (EnMT). Therefore, understanding the mechanism of EnMT is necessary for facilitating the regeneration of CECs with competent function. In this study, we prepared a primary culture of bovine CECs by peeling the CECs with Descemet's membrane from the corneal button and demonstrated that bovine CECs exhibited the EnMT process, including phenotypic change, nuclear translocation of &#946;-catenin, and EMT regulators snail and slug, in the in vitro culture. Furthermore, we used a rat corneal endothelium cryoinjury model to demonstrate the EnMT process in vivo. Collectively, the in vitro primary culture of bovine CECs and in vivo rat corneal endothelium cryoinjury models offers useful platforms for investigating the mechanism of EnMT.

In Vitro and In Vivo Models to Study Corneal Endothelial-mesenchymal Transition

A primary culture of bovine corneal endothelial cells was used to investigate the mechanism of corneal endothelial-mesenchymal transition. Furthermore, a rat corneal endothelium cryoinjury model was used to demonstrate corneal endothelial-mesenchymal transition in vivo.

Corneal endothelial cells (CECs) play a crucial role in maintaining corneal clarity through active pumping. A reduced CEC count may lead to corneal edema ...

Generation of iPSC-derived Human Brain Organoids to Model Early Neurodevelopmental Disorders

The restricted availability of suitable in vitro models that can reliably represent complex human brain development is a significant bottleneck that limits the translation of basic brain research into clinical application. While induced pluripotent stem cells (iPSCs) have replaced the ethically questionable human embryonic stem cells, iPSC-based neuronal differentiation studies remain descriptive at the cellular level but fail to adequately provide the details that could be derived from a complex, 3D human brain tissue. 
This gap is now filled through the application of iPSC-derived, 3D brain organoids, "Brains in a dish," that model many features of complex human brain development. Here, a method for generating iPSC-derived, 3D brain organoids is described. The organoids can help with modeling autosomal recessive primary microcephaly (MCPH), a rare human neurodevelopmental disorder. A widely accepted explanation for the brain malformation in MCPH is a depletion of the neural stem cell pool during the early stages of human brain development, a developmental defect that is difficult to recreate or prove in vitro. 
To study MCPH, we generated iPSCs from patient-derived fibroblasts carrying a mutation in the centrosomal protein CPAP. By analyzing the ventricular zone of microcephaly 3D brain organoids, we showed the premature differentiation of neural progenitors. These 3D brain organoids are a powerful in vitro system that will be instrumental in modeling congenital brain disorders induced by neurotoxic chemicals, neurotrophic viral infections, or inherited genetic mutations.

Modeling human brain development has been hindered due to the unprecedented complexity of neural epithelial tissue. Here, a method for the robust generation of brain organoids to delineate early events of human brain development and to model microcephaly in vitro is described.

The restricted availability of suitable in vitro models that can reliably represent complex human brain development is a significant bottleneck that ...

The Aortic Ring Co-culture Assay: A Convenient Tool to Assess the Angiogenic Potential of Mesenchymal Stromal Cells In Vitro

Angiogenesis is a complex, highly regulated process responsible for providing and maintaining adequate tissue perfusion. Insufficient vasculature maintenance and pathological malformations can result in severe ischemic diseases, while overly abundant vascular development is associated with cancer and inflammatory disorders. A promising form of pro-angiogenic therapy is the use of angiogenic cell sources, which can provide regulatory factors as well as physical support for newly developing vasculature.
Mesenchymal Stromal Cells (MSCs) are extensively investigated candidates for vascular regeneration due to their paracrine effects and their ability to detect and home to ischemic or inflamed tissues. In particular, first trimester human umbilical cord perivascular cells (FTM HUCPVCs) are a highly promising candidate due to their pericyte-like properties, high proliferative and multilineage potential, immune-privileged properties, and robust paracrine profile. To effectively evaluate potentially angiogenic regenerative cells, it is a requisite to test them in reliable and &#34;translatable&#34; pre-clinical assays. The aortic ring assay is an ex vivo angiogenesis model that allows for easy quantification of tubular endothelial structures, provides accessory supportive cells and extracellular matrix (ECM) from the host, excludes inflammatory components, and is fast and inexpensive to set up. This is advantageous when compared to in vivo models (e.g., corneal assay, Matrigel plug assay); the aortic ring assay can track the administered cells and observe intercellular interactions while avoiding xeno-immune rejection.
We present a protocol for a novel application of the aortic ring assay, which includes human MSCs in co-cultures with developing rat aortic endothelial networks. This assay allows for the analysis of the MSC contribution to tube formation and development through physical pericyte-like interactions and of their potency for actively migrating to sites of angiogenesis, and for evaluating their ability to perform and mediate ECM processing. This protocol provides further information on changes in MSC phenotype and gene expression following co-culture.

The Aortic Ring Co-culture Assay: A Convenient Tool to Assess the Angiogenic Potential of Mesenchymal Stromal Cells In Vitro

Here, we present a novel application of the aortic ring assay where prelabelled mesenchymal cells are co-cultured with rat aorta-derived endothelial networks. This novel method allows visualization of Mesenchymal Stromal Cells (MSCs) homing and integration with endothelial networks, quantification of network properties, and evaluation of MSC immunophenotypes and gene expression.

Angiogenesis is a complex, highly regulated process responsible for providing and maintaining adequate tissue perfusion. Insufficient vasculature ...

Corneal Tissue Engineering: An In Vitro Model of the Stromal-nerve Interactions of the Human Cornea

Tissue engineering has gained substantial recognition due to the high demand for human cornea replacements with an estimated 10 million people worldwide suffering from corneal vision loss1. To address the demand for viable human corneas, significant progress in three-dimensional (3D) tissue engineering has been made2,3,4. These cornea models range from simple monolayer systems to multilayered models, leading to 3D full-thickness corneal equivalents2.
However, the use of a 3D tissue-engineered cornea in the context of in vitro disease models studied to date lacks resemblance to the multilayered 3D corneal tissue structure, function, and the networking of different cell types (i.e., nerve, epithelium, stroma, and endothelium)2,3. In addition, the demand for in vitro cornea tissue models has increased in an attempt to reduce animal testing for pharmaceutical products. Thus, more sophisticated models are required to better match systems to human physiological requirements, and the development of a model that is more relevant to the patient population is absolutely necessary. Given that multiple cell types in the cornea are affected by diseases and dystrophies, such as Keratoconus, Diabetic Keratopathy, and Fuchs, this model includes a 3D co-culture model of primary human corneal fibroblasts (HCFs) from healthy donors and neurons from the SH-SY5Y cell line. This allows us for the first time to investigate the interactions between the two cell types within the human corneal tissue. We believe that this model could potentially dissect the underlying mechanisms associated with the stromal-nerve interactions of corneal diseases that exhibit nerve damages. This 3D model mirrors the basic anatomical and physiological nature of the corneal tissue in vivo and can be used in the future as a tool for investigating corneal defects as well as screening the efficacy of various agents before animal testing.

Corneal Tissue Engineering: An In Vitro Model of the Stromal-nerve Interactions of the Human Cornea

This protocol describes a novel three-dimensional in vitro model, where corneal stromal cells and differentiated neuronal cells are cultured together to assist in the examination and understanding of the interactions of the two cell types.

Tissue engineering has gained substantial recognition due to the high demand for human cornea replacements with an estimated 10 million people worldwide ...

Incorporating Pericytes into an Endothelial Cell Bead Sprouting Assay

Angiogenesis is the growth of new vessels from pre-existing vasculature and is an important component of many biological processes, including embryogenesis and development, wound healing, tumor growth and metastasis, and ocular and cardiovascular diseases. Effective in vitro models that recapitulate the biology of angiogenesis are needed to appropriately study this process and identify mechanisms of regulation that can be ultimately targeted for novel therapeutic strategies. The bead angiogenesis assay has been previously demonstrated to recapitulate the multiple stages of endothelial sprouting in vitro. However, a limitation of this assay is a lack of endothelial &#8211; mural cell interactions, which are key to the molecular and phenotypic regulation of endothelial cell function in vivo. The protocol given here presents a methodology for the incorporation of mural cells into the bead angiogenesis assay and demonstrates a tight association of endothelial cells and pericytes during sprouting in vitro. The protocol also details a methodology for effective silencing of target genes using siRNA in endothelial cells for mechanistic studies. Altogether, this protocol provides an in vitro assay that more appropriately models the diverse cell types involved in sprouting angiogenesis, and provides a more physiologically-relevant platform for therapeutic assessment and novel discovery of mechanisms of angiogenesis regulation.

This protocol presents a novel in vitro bead assay that more appropriately models the process of in vivo sprouting angiogenesis by incorporating pericytes. This modification enables the bead assay to more faithfully recapitulate the heterotypic cellular interactions between endothelial cells and mural cells that are critical for angiogenesis.

Angiogenesis is the growth of new vessels from pre-existing vasculature and is an important component of many biological processes, including ...

Generation of Scaffold-free, Three-dimensional Insulin Expressing Pancreatoids from Mouse Pancreatic Progenitors In Vitro

The pancreas is a complex organ composed of many different cell types that work together to regulate blood glucose homeostasis and digestion. These cell types include enzyme-secreting acinar cells, an arborized ductal system responsible for the transportation of enzymes to the gut, and hormone-producing endocrine cells. 
Endocrine beta-cells are the sole cell type in the body that produce insulin to lower blood glucose levels. Diabetes, a disease characterized by a loss or the dysfunction of beta-cells, is reaching epidemic proportions. Thus, it is essential to establish protocols to investigate beta-cell development that can be used for screening purposes to derive the drug and cell-based therapeutics. While the experimental investigation of mouse development is essential, in vivo studies are laborious and time-consuming. Cultured cells provide a more convenient platform for screening; however, they are unable to maintain the cellular diversity, architectural organization, and cellular interactions found in vivo. Thus, it is essential to develop new tools to investigate pancreatic organogenesis and physiology.
Pancreatic epithelial cells develop in the close association with mesenchyme from the onset of organogenesis as cells organize and differentiate into the complex, physiologically competent adult organ. The pancreatic mesenchyme provides important signals for the endocrine development, many of which are not well understood yet, thus difficult to recapitulate during the in vitro culture. Here, we describe a protocol to culture three-dimensional, cellular complex mouse organoids that retain mesenchyme, termed pancreatoids. The e10.5 murine pancreatic bud is dissected, dissociated, and cultured in a scaffold-free environment. These floating cells self-assemble with mesenchyme enveloping the developing pancreatoid and a robust number of endocrine beta-cells developing along with the acinar and the duct cells. This system can be used to study the cell fate determination, structural organization, and morphogenesis, cell-cell interactions during organogenesis, or for the drug, small molecule, or genetic screening.

Generation of Scaffold-free, Three-dimensional Insulin Expressing Pancreatoids from Mouse Pancreatic Progenitors In Vitro

Here, we present a protocol to generate insulin expressing 3D murine pancreatoids from free-floating e10.5 dissociated pancreatic progenitors and the associated mesenchyme.

The pancreas is a complex organ composed of many different cell types that work together to regulate blood glucose homeostasis and digestion. These cell ...

Computational Analysis of the Caenorhabditis elegans Germline to Study the Distribution of Nuclei, Proteins, and the Cytoskeleton

The Caenorhabditis elegans (C. elegans) germline is used to study several biologically important processes including stem cell development, apoptosis, and chromosome dynamics. While the germline is an excellent model, the analysis is often two dimensional due to the time and labor required for three-dimensional analysis. Major readouts in such studies are the number/position of nuclei and protein distribution within the germline. Here, we present a method to perform automated analysis of the germline using confocal microscopy and computational approaches to determine the number and position of nuclei in each region of the germline. Our method also analyzes germline protein distribution that enables the three-dimensional examination of protein expression in different genetic backgrounds. Further, our study shows variations in cytoskeletal architecture in distinct regions of the germline that may accommodate specific spatial developmental requirements. Finally, our method enables automated counting of the sperm in the spermatheca of each germline. Taken together, our method enables rapid and reproducible phenotypic analysis of the C. elegans germline.

Computational Analysis of the Caenorhabditis elegans Germline to Study the Distribution of Nuclei, Proteins, and the Cytoskeleton

We present an automated method for three-dimensional reconstruction of the Caenorhabditis elegans germline. Our method determines the number and position of each nucleus within the germline and analyses germline protein distribution and cytoskeletal structure.

The Caenorhabditis elegans (C. elegans) germline is used to study several biologically important processes including stem cell development, apoptosis, and ...

An In Vitro Model of a Parallel-Plate Perfusion System to Study Bacterial Adherence to Graft Tissues

Various valved conduits and stent-mounted valves are used for right ventricular outflow tract (RVOT) valve replacement in patients with congenital heart disease. When using prosthetic materials however, these grafts are susceptible to bacterial infections and various host responses.
Identification of bacterial and host factors that play a vital role in endovascular adherence of microorganisms is of importance to better understand the pathophysiology of the onset of infections such as infective endocarditis (IE) and to develop preventive strategies. Therefore, the development of competent models to investigate bacterial adhesion under physiological shear conditions is necessary. Here, we describe the use of a newly designed in vitro perfusion chamber based on parallel plates that allows the study of bacterial adherence to different components of graft tissues such as exposed extracellular matrix, endothelial cells and inert areas. This method combined with colony-forming unit (CFU) counting is adequate to evaluate the propensity of graft materials towards bacterial adhesion under flow. Further on, the flow chamber system might be used to investigate the role of blood components in bacterial adhesion under shear conditions. We demonstrated that the source of tissue, their surface morphology and bacterial species specificity are not the major determining factors in bacterial adherence to graft tissues by using our in-house designed in vitro perfusion model.

An In Vitro Model of a Parallel-Plate Perfusion System to Study Bacterial Adherence to Graft Tissues

We describe an in-house designed in vitro flow chamber model, which allows the investigation of bacterial adherence to graft tissues.

Various valved conduits and stent-mounted valves are used for right ventricular outflow tract (RVOT) valve replacement in patients with congenital heart ...

CRISPR/Cas9 Technology in Restoring Dystrophin Expression in iPSC-Derived Muscle Progenitors

Duchenne muscular dystrophy (DMD) is a severe progressive muscle disease caused by mutations in the dystrophin gene, which ultimately leads to the exhaustion of muscle progenitor cells. Clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) gene editing has the potential to restore the expression of the dystrophin gene. Autologous induced pluripotent stem cells (iPSCs)-derived muscle progenitor cells (MPC) can replenish the stem/progenitor cell pool, repair damage, and prevent further complications in DMD without causing an immune response. In this study, we introduce a combination of CRISPR/Cas9 and non-integrated iPSC technologies to obtain muscle progenitors with recovered dystrophin protein expression. Briefly, we use a non-integrating Sendai vector to establish an iPSC line from dermal fibroblasts of Dmdmdx mice. We then use the CRISPR/Cas9 deletion strategy to restore dystrophin expression through a non-homologous end joining of the reframed dystrophin gene. After PCR validation of exon23 depletion in three colonies from 94 picked iPSC colonies, we differentiate iPSC into MPC by doxycycline (Dox)-induced expression of MyoD, a key transcription factor playing a significant role in regulating muscle differentiation. Our results show the feasibility of using CRISPR/Cas9 deletion strategy to restore dystrophin expression in iPSC-derived MPC, which has significant potential for developing future therapies for the treatment of DMD.

Here, we present a Cas9-based exon23 deletion protocol to restore dystrophin expression in iPSC from Dmdmdx mouse-derived skin fibroblasts and directly differentiate iPSCs into myogenic progenitor cells (MPC) using the Tet-on MyoD activation system.