This study was performed in the research program of the BioMedical Materials (BMM) institute. BMM is cofunded by the Dutch Ministry of Economic affairs, Agriculture and Innovation. The financial contribution of the Nederlandse Hartstichting is gratefully acknowledged.

1. Culture of Human Vena Saphena Cells
<ol>
	<li>Isolate cells from the vena saphena magna, acquired from a donor in accordance to guidelines for secondary use material, according to the protocol by Schnell et al.9 and store these in liquid nitrogen. From the part of the vena saphena magna from one donor cut pieces of 2 x 2 mm to culture in a six-well plate. Use 2 pieces per well. Generally enough cells can be obtained to fill about 3 vials with 0.25 x 106 cells in liquid nitrogen. HVSCs are characterized as myofibroblasts, by showing expression of vimentin, no expression of desmin and a subpopulation expressing &#945; smooth muscle actin10. Next start the protocol to thaw the cells from the liquid nitrogen to increase the number of cells.</li>
	<li>Place the cells from one&#160;vial into a T75 tissue culture flask and add growth medium (GM), consisting of Advanced DMEM, 50 ml fetal bovine serum (FBS), 5 ml penicillin/streptomycin and 5 ml L-glutamax. Change medium every 2-3 days.</li>
	<li>Cells grow confluent approximately after 14 days. Store 0.5 x 106 cells in one&#160;vial in liquid nitrogen, referred to as passage 1.</li>
</ol>
Note: Freezing of the cells is not necessary when harvesting HVSCs, but is solely used for storage.
<ol start="4">
	<li>Place the HVSCs from one&#160;vial into a T175 tissue culture flask and add GM. Change medium every 2-3 days. Cells grow confluent after approximately 7 days. Store 3 x 106 cells in one&#160;vial in liquid nitrogen, referred to as passage 2.</li>
	<li>Place the cells from one&#160;vial into a rollerbottle and add GM. Change medium every 2-3 days. Cells grow confluent approximately after 8 days. One rollerbottle contains approximately 20-30 x 106 cells. Culture cells up to passage 6. Do not use cells of a passage higher than passage 9, since cell phenotype may change.</li>
</ol>
2. Engineering of Fibrin-based Tissue Constructs
<ol>
	<li>Prepare silicone glue by mixing the elastomer with the curing agent (10:1). Cut 7 x 3 mm rectangular segments of Velcro. Glue the Velcro into a 6 well culture plate, with flexible membranes&#160;to form a cross, and to leave a squared space of 3 mm between the Velcro strips.</li>
</ol>
Notes: Only use the soft side of the Velcro and face this side upwards. When gluing the Velcro, only cover the Velcro with silicone glue, do not spread glue throughout the well. Since the culture plates have silicone membrane bottoms, use something underneath the well plate for reinforcing the flexible membranes, to ensure easy gluing in to the plate.
<ol start="2">
	<li>Dry the silicone glue overnight in an oven at 60 &#176;C to ensure hardening of the glue. Sterilize by adding 70% EtOH to the wells and incubate for 30 min. Rinse 3x with PBS and remove all PBS from the wells and the Velcro. Put under UV for 30 min and keep sterile until use.</li>
	<li>Prepare Tissue Engineering medium (TM), consisting of GM and 130 mg of L-ascorbic acid 2-phosphate.</li>
	<li>Add 1 mg/ml &#949;-Amino Caproic Acid (ACA) to the TM. ACA is used for the first 7 days of culture to prevent the fibrin from degrading11. Alternatively, aprotinin can be used.</li>
	<li>To prevent clump formation, allow fibrinogen to reach room temperature before opening. Without clumps fibrinogen will dissolve more easily. Dissolve fibrinogen to a concentration of 10 mg actual protein/ml TM7 supplemented with ACA. Sterile filter the fibrinogen solution, multiple filters might be needed. Store on ice until use.</li>
</ol>
Note: To dissolve fibrinogen mix gently to prevent too much foam formation.
<ol start="6">
	<li>Dissolve thrombin to a concentration of 10 IU/ml TM7 supplemented with ACA. Store on ice until use.</li>
</ol>
Note: Storage on ice is needed to prevent early gelation of thrombin and fibrinogen.
<ol start="7">
	<li>Trypsinize the cells, resuspend in GM and count.</li>
	<li>Use 15 x 106 cells/ml for seeding the fibrin-based tissue constructs (concentration based on methods for tissue engineering heart valves7). A single gel consists of 100 &#956;l gof el, and thus of 1.5 x 106 cells. Put 1.5 x 106 cells in one centrifuge tube, resulting in as many centrifuge tubes as the number of gels that will be made.</li>
	<li>Centrifuge the cells at 350 x g for 7 min and discard the supernatant. Resuspend the cells in 50 &#956;l thrombin. Add 4 &#956;l of blue fluorescent polystyrene microspheres to this suspension. Put 50 &#956;l of fibrinogen in a vial. Use a pipette to take up the 50 &#956;l thrombin with cells. Increase the volume of the pipette to 100 &#956;l. Pipette the 50 &#956;l solution of thrombin with cells into the vial with fibrinogen to mix the thrombin and fibrinogen, and take up the 100 &#956;l mixture.</li>
</ol>
Note: The fluorescent polystyrene microspheres are used as internal reference markers for image analysis. When mixing thrombin and fibrinogen, prevent the formation of air bubbles by carefully pipetting the mixture. Air bubbles will result in holes in the fibrin gel.
<ol start="10">
	<li>Pipette the gel mixture into and in between the Velcro strips. The typical gelation time for the fibrin gels, once the components are mixed, is of the order of 20 sec.</li>
</ol>
Notes: Do this as quickly as possible to prevent gelation before the mixture is pipetted in the well plate. Practice before using cells and beads.
<ol start="11">
	<li>Incubate suspensions for 30 min at 37 &#176;C in a humidified 95% air/5%&#160;CO2 incubator to allow gelation before adding culture TM with ACA.</li>
	<li>Replace TM with ACA every 2-3 days for the first 7 days. Gels are stable enough after one week to be cultured without ACA. After the first week replace TM every 2-3 days. Add 6 ml of TM per well. On day 12 cells have produced enough collagen for visualization.</li>
</ol>
3. Applying Strain and Inducing Changes in Strain and Constraints
<ol>
	<li>Static strain is applied by the cells directly, due to cell traction and compaction12. To induce collagen reorganization, cut the tissue construct loose from two Velcro strips in one direction. This results in unidirectional constraints (Figure 1A). Perform this on day 12, since the construct is then stable enough and collagen has been deposited.</li>
	<li>Apply cyclic strain by applying a vacuum to the bottom of the culture plates with flexible membranes, with the use of the Flexcell FX4000T system. Place the plates on a baseplate, on top of loading posts (which are a part of the cyclic loading device). The pump applies a vacuum onto the membrane and thereby pulls the membrane over the rectangular post lying underneath. Due to the cross shaped attachments of the tissue constructs to the membrane (via the Velcro) and the rectangular post the applied cyclic strain is uniaxial (Figure 1B).</li>
	<li>Initially, use static culture for 5 days to achieve initial mechanical integrity, before application of cyclic strain starting on day 6. Program a cyclic stain protocol into the controller for the vacuum pump. For example use a previously established intermittent cyclic strain protocol, with uniaxial direction13. This consists of an intermittent strain of a sine wave with 1 Hz, straining from 0 to 5% strain, for periods of 3 hr, alternated with 3 hr resting periods. Perform this cyclic strain protocol for 7 days, inducing an aligned collagen organization.</li>
	<li>Typically after 12 days HVSCs have produced an aligned collagen organization. After an aligned collagen organization is reached, change the uniaxial cyclic strain direction, to be perpendicular to the original strain direction. Do this by turning the rectangular posts by 90&#176;.</li>
</ol>
4. Visualizing Cells and Collagen
<ol>
	<li>To visualize active, real-time collagen remodeling, label samples with probes that do not interfere with cell viability or collagen formation. Use probes to fluorescently stain the cell cytoplasm and collagen.</li>
	<li>Alternatively second harmonic generation using confocal laser scanning microscopy can be used to visualize collagen structures using autofluorescence14, with excitation at 780 nm and detection between 500-550 nm.</li>
	<li>Remove the culture plates from the setup and the incubator, for the cyclically strained samples during the 3 hr rest period, and transport them to a confocal laser scanning microscope to visualize cells and collagen.</li>
</ol>

<ol>
	<li>Beamish, J. A., He, P., Kottke-Marchant, K., Marchant, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+regulation+of+contractile+smooth+muscle+cell+phenotype:+implications+for+vascular+tissue+engineering.">Molecular regulation of contractile smooth muscle cell phenotype: implications for vascular tissue engineering.</a> Tissue Eng. Part B Rev. 16, 467-491 (2010).</li><li>Isenberg, B. C., 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=Long-term+cyclic+distention+enhances+the+mechanical+properties+of+collagen-based+media-equivalents.">Long-term cyclic distention enhances the mechanical properties of collagen-based media-equivalents.</a> Ann. Biomed. Eng. 31, 937-949 (2003).</li><li>Nichol, J. W., Khan, A. R., Birbach, M., Gaynor, J. W., Gooch, K. 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Eng. 36, 1322-1334 (2008).</li><li>Mol, A., 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=Fibrin+as+a+cell+carrier+in+cardiovascular+tissue+engineering+applications.">Fibrin as a cell carrier in cardiovascular tissue engineering applications.</a> Biomaterials. 26, 3113-3121 (2005).</li><li>de Jonge, N., Kanters, F. M., Baaijens, F. P., Bouten, C. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strain-induced+Collagen+Organization+at+the+Micro-level+in+Fibrin-based+Engineered+Tissue+Constructs.">Strain-induced Collagen Organization at the Micro-level in Fibrin-based Engineered Tissue Constructs.</a> Ann. Biomed. Eng. 41 (4), 763-774 (2012).</li><li>Schnell, A. 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=Optimal+cell+source+for+cardiovascular+tissue+engineering:+venous+vs.+aortic+human+myofibroblasts.">Optimal cell source for cardiovascular tissue engineering: venous vs. aortic human myofibroblasts.</a> Thorac. Cardiovasc. Surg. 49, 221-225 (2001).</li><li>Mol, A., 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=Autologous+human+tissue-engineered+heart+valves:+prospects+for+systemic+application.">Autologous human tissue-engineered heart valves: prospects for systemic application.</a> Circulation. , I152-I158 (2006).</li><li>Ahmann, K. A., Weinbaum, J. S., Johnson, S. L., 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=Fibrin+degradation+enhances+vascular+smooth+muscle+cell+proliferation+and+matrix+deposition+in+fibrin-based+tissue+constructs+fabricated+in+vitro.">Fibrin degradation enhances vascular smooth muscle cell proliferation and matrix deposition in fibrin-based tissue constructs fabricated in vitro.</a> Tissue Eng. Part A. 16, 3261-3270 (2010).</li><li>John, J., Quinlan, A. T., Silvestri, C., Billiar, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Boundary+stiffness+regulates+fibroblast+behavior+in+collagen+gels.">Boundary stiffness regulates fibroblast behavior in collagen gels.</a> Ann. Biomed. Eng. 38, 658-673 (2010).</li><li>Rubbens, M. P., 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=Intermittent+straining+accelerates+the+development+of+tissue+properties+in+engineered+heart+valve+tissue.">Intermittent straining accelerates the development of tissue properties in engineered heart valve tissue.</a> Tissue Eng. Part A. 15, 999-1008 (2009).</li><li>Chen, W. L., 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=Multiphoton+imaging+and+quantitative+analysis+of+collagen+production+by+chondrogenic+human+mesenchymal+stem+cells+cultured+in+chitosan+scaffold.">Multiphoton imaging and quantitative analysis of collagen production by chondrogenic human mesenchymal stem cells cultured in chitosan scaffold.</a> Tissue Eng. Part C Methods. 16, 913-920 (2010).</li><li>de Jonge, N., Kanters, F. M., Baaijens, F. P., Bouten, C. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strain-induced+Collagen+Organization+at+the+Micro-level+in+Fibrin-based+Engineered+Tissue+Constructs.">Strain-induced Collagen Organization at the Micro-level in Fibrin-based Engineered Tissue Constructs.</a> Ann. Biomed. Eng. 41, 763-774 (2013).</li><li>Merryman, W. D., 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=Correlation+between+heart+valve+interstitial+cell+stiffness+and+transvalvular+pressure:+implications+for+collagen+synthesis.">Correlation between heart valve interstitial cell stiffness and transvalvular pressure: implications for collagen synthesis.</a> Am. J. Physiol. 290, (2006).</li><li>Ingber, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=From+cellular+mechanotransduction+to+biologically+inspired+engineering:+2009+Pritzker+Award+Lecture,+BMES+Annual+Meeting+October+10,+2009.">From cellular mechanotransduction to biologically inspired engineering: 2009 Pritzker Award Lecture, BMES Annual Meeting October 10, 2009.</a> Ann. Biomed. Eng. 38, 1148-1161 (2009).</li><li>Sander, E. A., Barocas, V. H., 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=Initial+fiber+alignment+pattern+alters+extracellular+matrix+synthesis+in+fibroblast-populated+fibrin+gel+cruciforms+and+correlates+with+predicted+tension.">Initial fiber alignment pattern alters extracellular matrix synthesis in fibroblast-populated fibrin gel cruciforms and correlates with predicted tension.</a> Ann. Biomed. Eng. 39, 714-729 (2010).</li><li>van der Schaft, D. 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=Engineering+Skeletal+Muscle+Tissues+from+Murine+Myoblast+Progenitor+Cells+and+Application+of+Electrical+Stimulation.">Engineering Skeletal Muscle Tissues from Murine Myoblast Progenitor Cells and Application of Electrical Stimulation.</a> J. Vis. Exp. (73), e4267 (2013).</li></ol>

The authors declare that they have no competing financial interests.

The described model system of cell-populated fibrin constructs has great potential for the study of cell and collagen (re)organization (de Jonge et al.15), e.g. to be used for tissue engineering purposes. By using fibrin as the initial cell carrier, after fibrin degradation, a tissue is created with cells and endogenous matrix only. In this way, cells are stimulated to react to strain, either static or cyclic in nature, by applying contractile forces16,17, sensing boundary stiffnes...

Cardiovascular tissues have a prominent load-bearing function. In particular content and organization of collagen fibers in the extracellular matrix contribute to the load-bearing properties and dominate overall tissue strength1. In tissue engineering mechanical conditioning of the construct is used - typically consisting of (cyclic) straining regimens - to enhance tissue organization and mechanical properties2,3. Full understanding of strain-induced collagen organization in complex tissue geometries to create tissues with predefined collagen architecture has not yet been achieved. This is mainly due to our limited knowledge of collagen remodeling in developing tissues. Existing models mainly give information about the final net outcome of collagen remodeling with use of static strain4-6. Here we provide a highly tunable model system that allows the study of collagen (re)organization in a real-time fashion, in 3D, under influence of static or cyclic strain. The tissue constructs are fibrin-based, ensuring that all collagen in the construct is endogenous. Cell and collagen organization in the constructs is visualized, and an internal reference system allows us to relocate cells and collagen structures for time-lapse analysis. In this protocol we will describe the use of the model system for Human Vena Saphena Cells (HVSCs), since these cells are known for their enhanced extra cellular matrix production and ability to remodel the matrix and our established use in engineered cardiovascular tissues7, based on the work of de Jonge et al.8

<table border="1">
	<tbody>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Culture plastic</td>
			<td>Greiner</td>
			<td></td>
			<td>Includes culture flasks and pipettes</td>
		</tr>
		<tr>
			<td>Advanced DMEM</td>
			<td>Gibco</td>
			<td>12491</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal bovine serum</td>
			<td>Greiner</td>
			<td>758075</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin/streptomycin</td>
			<td>Gibco</td>
			<td>10378016</td>
			<td></td>
		</tr>
		<tr>
			<td>GlutaMax</td>
			<td>Gibco</td>
			<td>35050-079</td>
			<td></td>
		</tr>
		<tr>
			<td>Elastomer and curing agent</td>
			<td>Dow Corning Corporation</td>
			<td>3097358-1004</td>
			<td>Silastic MDX 4-4210#</td>
		</tr>
		<tr>
			<td>Velcro</td>
			<td>Regular store</td>
			<td></td>
			<td>You can buy this at a regular store, only use the soft side</td>
		</tr>
		<tr>
			<td>Bioflex culture plates</td>
			<td>Flexcell Int</td>
			<td>BF-3001U</td>
			<td>Untreated</td>
		</tr>
		<tr>
			<td>L-Ascorbic Acid 2-phosphatase</td>
			<td>Sigma</td>
			<td>A8960</td>
			<td></td>
		</tr>
		<tr>
			<td>&#949;-Amino Caproic Acid</td>
			<td>Sigma-Aldrich</td>
			<td>D7754</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine thrombin</td>
			<td>Sigma</td>
			<td>T4648</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine fibrinogen</td>
			<td>Sigma</td>
			<td>F8630</td>
			<td></td>
		</tr>
		<tr>
			<td>0.45 syringe filter</td>
			<td>Whatmann (Schleicher and Scheul)</td>
			<td>10462100</td>
			<td></td>
		</tr>
		<tr>
			<td>Polystyrene microspheres</td>
			<td>Invitrogen</td>
			<td>F-8829</td>
			<td>Blue fluorescent, 10 &#956;m diameter</td>
		</tr>
		<tr>
			<td>Flexcell FX-4000T</td>
			<td>Flexcell Int</td>
			<td></td>
			<td>Includes rectangular loading posts</td>
		</tr>
		<tr>
			<td>Cell Tracker Orange</td>
			<td>Invitrogen Molecular Probes</td>
			<td>C2927</td>
			<td></td>
		</tr>
		<tr>
			<td>CNA35-OG488</td>
			<td></td>
			<td></td>
			<td>Cordially provided by the Laboratory for Macromolecular and Organic Chemistry, Department of Biomedical Engineering, Eindhoven University of Technology</td>
		</tr>
		<tr>
			<td>Confocal laser scanning microscope</td>
			<td>Carl Zeiss</td>
			<td></td>
			<td>LSM 510 Meta laser scanning microscope and Two-Photon-LSM mode</td>
		</tr>
		<tr>
			<td>Amphotericin</td>
			<td>Gibco</td>
			<td>15290-018</td>
			<td>Needed for cell isolation</td>
		</tr>
	</tbody>
</table>

engineering fibrin-based tissue constructs from myofibroblasts and application of constraints and strain to induce cell and collagen reorganization

Collagen content and organization in developing collagenous tissues can be influenced by local tissue strains and tissue constraint. Tissue engineers aim to use these principles to create tissues with predefined collagen architectures. A full understanding of the exact underlying processes of collagen remodeling to control the final tissue architecture, however, is lacking. In particular, little is known about the (re)orientation of collagen fibers in response to changes in tissue mechanical loading conditions. We developed an in&#160;vitro model system, consisting of biaxially-constrained myofibroblast-seeded fibrin constructs, to further elucidate collagen (re)orientation in response to i) reverting biaxial to uniaxial static loading conditions and ii) cyclic uniaxial loading of the biaxially-constrained constructs before and after a change in loading direction, with use of the Flexcell FX4000T loading device. Time-lapse confocal imaging is used to visualize collagen (re)orientation in a nondestructive manner.
Cell and collagen organization in the constructs can be visualized in real-time, and an internal reference system allows us to relocate cells and collagen structures for time-lapse analysis. Various aspects of the model system can be adjusted, like cell source or use of healthy and diseased cells. Additives can be used to further elucidate mechanisms underlying collagen remodeling, by for example adding MMPs or blocking integrins. Shape and size of the construct can be easily adapted to specific needs, resulting in a highly tunable model system to study cell and collagen (re)organization.

This model system allows for culturing myofibroblast-seeded fibrin gels. Figure 1A shows a tissue cultured first under static biaxial constraints. Tissue constraints are released by cutting the fibrin gel from two constraints, to create uniaxial static constraints, and tissue compacts and remodels afterwards (Figure 1A). For cyclic strain, the tissue is cultured under static biaxial constraints as well. After 5 days cyclic uniaxial strain can be applied (Figure 1B). To i...

Watch this Scientific Journal Video about Engineering Fibrin-based Tissue Constructs from Myofibroblasts and Application of Constraints and Strain to Induce Cell and Collagen Reorganization at JoVE.co

Engineering Fibrin-based Tissue Constructs from Myofibroblasts and Application of Constraints and Strain to Induce Cell and Collagen Reorganization

This model system starts from a myofibroblast-populated fibrin gel that can be used to study endogenous collagen (re)organization real-time in a nondestructive manner. The model system is very tunable, as it can be used with different cell sources, medium additives, and can be adapted easily to specific needs.

Collagen content and organization in developing collagenous tissues can be influenced by local tissue strains and tissue constraint. Tissue engineers aim ...

engineering-fibrin-based-tissue-constructs-from-myofibroblasts

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JoVE Journal

Bioengineering

10.7K Views.  Eindhoven University of Technology. This model system starts from a myofibroblast-populated fibrin gel that can be used to study endogenous collagen (re)organization real-time in a nondestructive manner. The model system is very tunable, as it can be used with different cell sources, medium additives, and can be adapted easily to specific needs.

Video: Engineering Fibrin-based Tissue Constructs from Myofibroblasts and Application of Constraints and Strain to Induce Cell and Collagen Reorganization

Directed Cellular Self-Assembly to Fabricate Cell-Derived Tissue Rings for Biomechanical Analysis and Tissue Engineering

Each year, hundreds of thousands of patients undergo coronary artery bypass surgery in the United States.1 Approximately one third of these patients do not have suitable autologous donor vessels due to disease progression or previous harvest. The aim of vascular tissue engineering is to develop a suitable alternative source for these bypass grafts. In addition, engineered vascular tissue may prove valuable as living vascular models to study cardiovascular diseases. Several promising approaches to engineering blood vessels have been explored, with many recent studies focusing on development and analysis of cell-based methods.2-5 Herein, we present a method to rapidly self-assemble cells into 3D tissue rings that can be used in vitro to model vascular tissues.
To do this, suspensions of smooth muscle cells are seeded into round-bottomed annular agarose wells. The non-adhesive properties of the agarose allow the cells to settle, aggregate and contract around a post at the center of the well to form a cohesive tissue ring.6,7 These rings can be cultured for several days prior to harvesting for mechanical, physiological, biochemical, or histological analysis. We have shown that these cell-derived tissue rings yield at 100-500 kPa ultimate tensile strength8 which exceeds the value reported for other tissue engineered vascular constructs cultured for similar durations (&lt;30 kPa).9,10 Our results demonstrate that robust cell-derived vascular tissue ring generation can be achieved within a short time period, and offers the opportunity for direct and quantitative assessment of the contributions of cells and cell-derived matrix (CDM) to vascular tissue structure and function.

This article outlines a versatile method to create cell-derived tissue rings by cellular self-assembly. Smooth muscle cells seeded into ring-shaped agarose wells aggregate and contract to form robust three-dimensional (3D) tissues within 7 days. Millimeter-scale tissue rings are conducive to mechanical testing and serve as building blocks for tissue assembly.

Each year, hundreds of thousands of patients undergo coronary artery bypass surgery in the United States.1 Approximately one third of these patients do ...

Non-contact, Label-free Monitoring of Cells and Extracellular Matrix using Raman Spectroscopy

Non-destructive, non-contact and label-free technologies to monitor cell and tissue cultures are needed in the field of biomedical research.1-5 However, currently available routine methods require processing steps and alter sample integrity. Raman spectroscopy is a fast method that enables the measurement of biological samples without the need for further processing steps. This laser-based technology detects the inelastic scattering of monochromatic light.6 As every chemical vibration is assigned to a specific Raman band (wavenumber in cm-1), each biological sample features a typical spectral pattern due to their inherent biochemical composition.7-9 Within Raman spectra, the peak intensities correlate with the amount of the present molecular bonds.1 Similarities and differences of the spectral data sets can be detected by employing a multivariate analysis (e.g. principal component analysis (PCA)).10
Here, we perform Raman spectroscopy of living cells and native tissues. Cells are either seeded on glass bottom dishes or kept in suspension under normal cell culture conditions (37 &deg;C, 5% CO2) before measurement. Native tissues are dissected and stored in phosphate buffered saline (PBS) at 4 &deg;C prior measurements. Depending on our experimental set up, we then either focused on the cell nucleus or extracellular matrix (ECM) proteins such as elastin and collagen. For all studies, a minimum of 30 cells or 30 random points of interest within the ECM are measured. Data processing steps included background subtraction and normalization.

Raman spectroscopy is a suitable technique for the non-contact, label-free analysis of living cells, tissue-engineered constructs and native tissues. Source-specific spectral fingerprints can be generated and analyzed using multivariate analysis.

Non-destructive, non-contact and label-free technologies to monitor cell and tissue cultures are needed in the field of biomedical research.1-5 However, ...

Engineering Skeletal Muscle Tissues from Murine Myoblast Progenitor Cells and Application of Electrical Stimulation

Engineered muscle tissues can be used for several different purposes, which include the production of tissues for use as a disease model in vitro, e.g. to study pressure ulcers, for regenerative medicine and as a meat alternative 1. The first reported 3D muscle constructs have been made many years ago and pioneers in the field are Vandenburgh and colleagues 2,3. Advances made in muscle tissue engineering are not only the result from the vast gain in knowledge of biochemical factors, stem cells and progenitor cells, but are in particular based on insights gained by researchers that physical factors play essential roles in the control of cell behavior and tissue development. State-of-the-art engineered muscle constructs currently consist of cell-populated hydrogel constructs. In our lab these generally consist of murine myoblast progenitor cells, isolated from murine hind limb muscles or a murine myoblast cell line C2C12, mixed with a mixture of collagen/Matrigel and plated between two anchoring points, mimicking the muscle ligaments. Other cells may be considered as well, e.g. alternative cell lines such as L6 rat myoblasts 4, neonatal muscle derived progenitor cells 5, cells derived from adult muscle tissues from other species such as human 6 or even induced pluripotent stem cells (iPS cells) 7. Cell contractility causes alignment of the cells along the long axis of the construct 8,9 and differentiation of the muscle progenitor cells after approximately one week of culture. Moreover, the application of electrical stimulation can enhance the process of differentiation to some extent 8. Because of its limited size (8 x 2 x 0.5 mm) the complete tissue can be analyzed using confocal microscopy to monitor e.g. viability, differentiation and cell alignment. Depending on the specific application the requirements for the engineered muscle tissue will vary; e.g. use for regenerative medicine requires the up scaling of tissue size and vascularization, while to serve as a meat alternative translation to other species is necessary.

Engineered muscle tissue has great potential in regenerative medicine, as disease model and also as an alternative source for meat. Here we describe the engineering of a muscle construct, in this case from mouse myoblast progenitor cells, and the stimulation by electrical pulses.

Engineered muscle tissues can be used for several different purposes, which include the production of tissues for use as a disease model in vitro, e.g. to ...

Tissue Engineering of Tumor Stromal Microenvironment with Application to Cancer Cell Invasion

3D organotypic cultures of epithelial cells on a matrix embedded with mesenchymal cells are widely used to study epithelial cell differentiation and invasion. Rat tail type I collagen and/or matrix derived from Engelbreth-Holm-Swarm mouse sarcoma cells have been traditionally employed as the substrates to model the matrix or stromal microenvironment into which mesenchymal cells (usually fibroblasts) are populated. Although experiments using such matrices are very informative, it can be argued that due to an overriding presence of a single protein (such as in type I Collagen) or a high content of basement membrane components and growth factors (such as in matrix derived from mouse sarcoma cells), these substrates do not best reflect the contribution to matrix composition made by the stromal cells themselves. To study native matrices produced by primary dermal fibroblasts isolated from patients with a tumor prone, genetic blistering disorder (recessive dystrophic epidermolysis bullosa), we have adapted an existing native matrix protocol to study tumor cell invasion. Fibroblasts are induced to produce their own matrix over a prolonged period in culture. This native matrix is then detached from the culture dish and epithelial cells are seeded onto it before the entire coculture is raised to the air-liquid interface. Cellular differentiation and/or invasion can then be assessed over time. This technique provides the ability to assess epithelial-mesenchymal cell interactions in a 3D setting without the need for a synthetic or foreign matrix with the only disadvantage being the prolonged period of time required to produce the native matrix. Here we describe the application of this technique to assess the ability of a single molecule expressed by fibroblasts, type VII collagen, to inhibit tumor cell invasion.

Tissue engineered fibroblast-derived native matrix is an emerging tool to generate a stromal substrate which supports epithelial cell proliferation and differentiation. Here a protocol applying this methodology to assess the impact of different stromal cell types on tumor cell biology is presented.

3D organotypic cultures of epithelial cells on a matrix embedded with mesenchymal cells are widely used to study epithelial cell differentiation and ...

Engineering 3D Cellularized Collagen Gels for Vascular Tissue Regeneration

Synthetic materials are known to initiate clinical complications such as inflammation, stenosis, and infections when implanted as vascular substitutes. Collagen has been extensively used for a wide range of biomedical applications and is considered a valid alternative to synthetic materials due to its inherent biocompatibility (i.e., low antigenicity, inflammation, and cytotoxic responses). However, the limited mechanical properties and the related low hand-ability of collagen gels have hampered their use as scaffold materials for vascular tissue engineering. Therefore, the rationale behind this work was first to engineer cellularized collagen gels into a tubular-shaped geometry and second to enhance smooth muscle cells driven reorganization of collagen matrix to obtain tissues stiff enough to be handled.
The strategy described here is based on the direct assembling of collagen and smooth muscle cells (construct) in a 3D cylindrical geometry with the use of a molding technique. This process requires a maturation period, during which the constructs are cultured in a bioreactor under static conditions (without applied external dynamic mechanical constraints) for 1 or 2 weeks. The &#8220;static bioreactor&#8221; provides a monitored and controlled sterile environment (pH, temperature, gas exchange, nutrient supply and waste removal) to the constructs. During culture period, thickness measurements were performed to evaluate the cells-driven remodeling of the collagen matrix, and glucose consumption and lactate production rates were measured to monitor the cells metabolic activity. Finally, mechanical and viscoelastic properties were assessed for the resulting tubular constructs. To this end, specific protocols and a focused know-how (manipulation, gripping, working in hydrated environment, and so on) were developed to characterize the engineered tissues.

In this work, we present a technique for the rapid fabrication of living vascular tissues by direct culturing of collagen, smooth muscle cells and endothelial cells. In addition, a new protocol for the mechanical characterization of engineered vascular tissues is described.

Synthetic materials are known to initiate clinical complications such as inflammation, stenosis, and infections when implanted as vascular substitutes. ...

Sandwich-like Microenvironments to Harness Cell/Material Interactions

Cell culture has been traditionally carried out on bi-dimensional (2D) substrates where cells adhere using ventral receptors to the biomaterial surface. However in vivo, most of the cells are completely surrounded by the extracellular matrix (ECM), resulting in a three-dimensional (3D) distribution of receptors. This may trigger differences in the outside-in signaling pathways and thus in cell behavior.
This article shows that stimulating the dorsal receptors of cells already adhered to a 2D substrate by overlaying a film of a new material (a sandwich-like culture) triggers important changes with respect to standard 2D cultures. Furthermore, the simultaneous excitation of ventral and dorsal receptors shifts cell behavior closer to that found in 3D environments. Additionally, due to the nature of the system, a sandwich-like culture is a versatile tool that allows the study of different parameters in cell/material interactions, e.g., topography, stiffness and different protein coatings at both the ventral and dorsal sides. Finally, since sandwich-like cultures are based on 2D substrates, several analysis procedures already developed for standard 2D cultures can be used normally, overcoming more complex procedures needed for 3D systems.

The following protocol describes the procedure to assemble sandwich-like cultures to be used as an intermediate stage between bi-dimensional (2D) and three-dimensional (3D) cellular environments. The engineered systems can have applications in microscopy, biomechanics, biochemistry and cell biology assays.

Cell culture has been traditionally carried out on bi-dimensional (2D) substrates where cells adhere using ventral receptors to the biomaterial surface. ...

Treatment of Ligament Constructs with Exercise-conditioned Serum: A Translational Tissue Engineering Model

In vitro experiments are essential to understand biological mechanisms; however, the gap between monolayer tissue culture and human physiology is large, and translation of findings is often poor. Thus, there is ample opportunity for alternative experimental approaches. Here we present an approach in which human cells are isolated from human anterior cruciate ligament tissue remnants, expanded in culture, and used to form engineered ligaments. Exercise alters the biochemical milieu in the blood such that the function of many tissues, organs and bodily processes are improved. In this experiment, ligament construct culture media was supplemented with experimental human serum that has been &#39;conditioned&#39; by exercise. Thus the intervention is more biologically relevant since an experimental tissue is exposed to the full endogenous biochemical milieu, including binding proteins and adjunct compounds that may be altered in tandem with the activity of an unknown agent of interest. After treatment, engineered ligaments can be analyzed for mechanical function, collagen content, morphology, and cellular biochemistry. Overall, there are four major advantages versus traditional monolayer culture and animal models, of the physiological model of ligament tissue that is presented here. First, ligament constructs are three-dimensional, allowing for mechanical properties (i.e., function) such as ultimate tensile stress, maximal tensile load, and modulus, to be quantified. Second, the enthesis, the interface between boney and sinew elements, can be examined in detail and within functional context. Third, preparing media with post-exercise serum allows for the effects of the exercise-induced biochemical milieu, which is responsible for the wide range of health benefits of exercise, to be investigated in an unbiased manner. Finally, this experimental model advances scientific research in a humane and ethical manner by replacing the use of animals, a core mandate of the National Institutes of Health, the Center for Disease Control, and the Food and Drug Administration.

We present a model of ligament tissue in which three-dimensional constructs are treated with the human exercise-conditioned serum and analyzed for collagen content, function, and cellular biochemistry.

In vitro experiments are essential to understand biological mechanisms; however, the gap between monolayer tissue culture and human physiology is large, ...

Recombinant Collagen I Peptide Microcarriers for Cell Expansion and Their Potential Use As Cell Delivery System in a Bioreactor Model

Tissue engineering is a promising field, focused on developing solutions for the increasing demand on tissues and organs regarding transplantation purposes. The process to generate such tissues is complex, and includes an appropriate combination of specific cell types, scaffolds, and physical or biochemical stimuli to guide cell growth and differentiation. Microcarriers represent an appealing tool to expand cells in a three-dimensional (3D) microenvironment, since they provide higher surface-to volume ratios and mimic more closely the in vivo situation compared to traditional two-dimensional methods. The vascular system, supplying oxygen and nutrients to the cells and ensuring waste removal, constitutes an important building block when generating engineered tissues. In fact, most constructs fail after being implanted due to lacking vascular support. In this study, we present a protocol for endothelial cell expansion on recombinant collagen-based microcarriers under dynamic conditions in spinner flask and bioreactors, and we explain how to determine in this setting cell viability and functionality. In addition, we propose a method for cell delivery for vascularization purposes without additional detachment steps necessary. Furthermore, we provide a strategy to evaluate the cell vascularization potential in a perfusion bioreactor on a decellularized&#160;biological matrix. We believe that the use of the presented methods could lead to the development of new cell-based therapies for a large range of tissue engineering applications in the clinical practice.

We propose a cell expansion protocol on macroporous microcarriers and their use as delivery system in a perfusion bioreactor to seed a decellularized tissue matrix. We also include different techniques to determine cell proliferation and viability of cells cultured on microcarriers. Furthermore, we demonstrate functionality of cells after bioreactor cultures.

Tissue engineering is a promising field, focused on developing solutions for the increasing demand on tissues and organs regarding transplantation ...

A Novel Tenorrhaphy Suture Technique with Tissue Engineered Collagen Graft to Repair Large Tendon Defects

Surgical management of large tendon defects with tendon grafts is challenging, as there are a finite number of sites where donors can be readily identified and used. Currently, this gap is filled with tendon auto-, allo-, xeno-, or artificial grafts, but clinical methods to secure them are not necessarily translatable to animals because of the scale. In order to evaluate new biomaterials or study a tendon graft made up of collagen type 1, we have developed a modified suture technique to help maintain the engineered tendon in alignment with the tendon ends. Mechanical properties of these grafts are inferior to the native tendon. To incorporate engineered tendon into clinically relevant models of loaded repair, a strategy was adopted to offload the tissue engineered tendon graft and allow for the maturation and integration of the engineered tendon in vivo until a mechanically sound neo-tendon was formed. We describe this technique using incorporation of the collagen type 1 tissue engineered tendon construct.

In this paper, we present an in vitro and in situ protocol to repair a tendon gap of up to 1.5 cm by filling it with engineered collagen graft. This was performed by developing a modified suture technique to take the mechanical load until the graft matures into the host tissue.

Surgical management of large tendon defects with tendon grafts is challenging, as there are a finite number of sites where donors can be readily ...

Pancreatic Tissue-Derived Extracellular Matrix Bioink for Printing 3D Cell-Laden Pancreatic Tissue Constructs

The transplantation of pancreatic islets is a promising treatment for patients who suffer from type 1 diabetes accompanied by hypoglycemia and secondary complications. However, islet transplantation still has several limitations such as the low viability of transplanted islets due to poor islet engraftment and hostile environments. In addition, the insulin-producing cells differentiated from human pluripotent stem cells have limited ability to secrete sufficient hormones that can regulate the blood glucose level; therefore, improving the maturation by culturing cells with proper microenvironmental cues is strongly required. In this article, we elucidate protocols for preparing a pancreatic tissue-derived decellularized extracellular matrix (pdECM) bioink to provide a beneficial microenvironment that can increase glucose sensitivity of pancreatic islets, followed by describing the processes for generating 3D pancreatic tissue constructs using a microextrusion-based bioprinting technique.

Decellularized extracellular matrix (dECM) can provide suitable microenvironmental cues to recapitulate the inherent functions of target tissues in an engineered construct. This article elucidates the protocols for the decellularization of pancreatic tissue, evaluation of pancreatic tissue-derived dECM bioink, and generation of 3D pancreatic tissue constructs using a bioprinting technique.

The transplantation of pancreatic islets is a promising treatment for patients who suffer from type 1 diabetes accompanied by hypoglycemia and secondary ...