Name: gradient strain chip for stimulating cellular behaviors in cell-laden hydrogel
Uploaded: 2017-08-08T12:30:00
Description: Watch this Scientific Journal Video about Gradient Strain Chip for Stimulating Cellular Behaviors in Cell-laden Hydrogel at JoVE.com

This project was supported by the Graduate Student Study Abroad Program (NSC-101-2917-I-007-010); the Biomedical Engineering Program (NSC-101-2221-E-007-032-MY3); and the Nanotechnology National Program (NSC-101-2120-M-007-001-); and the Ministry of Science and Technology (MOST-104-2221-E-007-072-MY3), National Science Council of the R.O.C., Taiwan. The authors would like to thank Prof. Ali Khademhosseini, Gulden Camci-Unal, Arghya Paul, and Ronglih Liao at Harvard Medical School for sharing the hydrogel and cell encapsulation technology.

1. GelMA Synthesis
<ol>
	<li>Weigh 10 g of gelatin powder and add it to a glass flask with 100 mL ofDulbecco&#39;s phosphate-buffered saline (DPBS). Put a magnetic stir bar into the flask and place the flask on a stirring hot plate.</li>
	<li>Cover the flask with aluminum foil to avoid water evaporation. Set the hot plate temperature to 50-60 &#176;C and the stirrer at 100 rpm for 1 h to dissolve the gelatin powder well.</li>
	<li>After the gelatin has dissolved, add 8 mL of methacrylic anhydride very slowly (one drop ...

<ol>
	<li>Simmons, C. S., Petzold, B. C., Pruitt, B. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microsystems+for+biomimetic+stimulation+of+cardiac+cells.">Microsystems for biomimetic stimulation of cardiac cells.</a> Lab Chip. 12 (18), 3235-3248 (2012).</li><li>Aubin, H., 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=Directed+3D+cell+alignment+and+elongation+in+microengineered+hydrogels.">Directed 3D cell alignment and elongation in microengineered hydrogels.</a> Biomaterials. 31 (27), 6941-6951 (2010).</li><li>Guan, J., 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+stimulation+of+the+cardiac+differentiation+of+mesenchymal+stem+cells+in+tissue+constructs+that+mimic+myocardium+structure+and+biomechanics.">The stimulation of the cardiac differentiation of mesenchymal stem cells in tissue constructs that mimic myocardium structure and biomechanics.</a> Biomaterials. 32 (24), 5568-5580 (2011).</li><li>Wan, C. R., Chung, S., Kamm, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differentiation+of+embryonic+stem+cells+into+cardiomyocytes+in+a+compliant+microfluidic+system.">Differentiation of embryonic stem cells into cardiomyocytes in a compliant microfluidic system.</a> Ann Biomed Eng. 39 (6), 1840-1847 (2011).</li><li>Huh, 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=Reconstituting+organ-level+lung+functions+on+a+chip.">Reconstituting organ-level lung functions on a chip.</a> Science. 328 (5986), 1662-1668 (2010).</li><li>Li, X., Chu, J. S., Yang, L., Li, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anisotropic+effects+of+mechanical+strain+on+neural+crest+stem+cells.">Anisotropic effects of mechanical strain on neural crest stem cells.</a> Ann. Biomed. Eng. 40 (3), 598-605 (2012).</li><li>Butcher, J. T., Barrett, B. C., Nerem, R. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Equibiaxial+strain+stimulates+fibroblastic+phenotype+shift+in+smooth+muscle+cells+in+an+engineered+tissue+model+of+the+aortic+wall.">Equibiaxial strain stimulates fibroblastic phenotype shift in smooth muscle cells in an engineered tissue model of the aortic wall.</a> Biomaterials. 27 (30), 5252-5258 (2006).</li><li>Ramon-Azcon, J., 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=Gelatin+methacrylate+as+a+promising+hydrogel+for+3D+microscale+organization+and+proliferation+of+dielectrophoretically+patterned+cells.">Gelatin methacrylate as a promising hydrogel for 3D microscale organization and proliferation of dielectrophoretically patterned cells.</a> Lab Chip. 12 (16), 2959-2969 (2012).</li><li>Park, S. H., Sim, W. Y., Min, B. H., Yang, S. S., Khademhosseini, A., Kaplan, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chip-Based+Comparison+of+the+Osteogenesis+of+Human+Bone+Marrow-+and+Adipose+Tissue-Derived+Mesenchymal+Stem+Cells+under+Mechanical+Stimulation.">Chip-Based Comparison of the Osteogenesis of Human Bone Marrow- and Adipose Tissue-Derived Mesenchymal Stem Cells under Mechanical Stimulation.</a> PLoS One. 7 (9), e46689 (2012).</li><li>Gould, R. 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=Cyclic+Strain+Anisotropy+Regulates+Valvular+Interstitial+Cell+Phenotype+and+Tissue+Remodeling+in+3D+Culture.">Cyclic Strain Anisotropy Regulates Valvular Interstitial Cell Phenotype and Tissue Remodeling in 3D Culture.</a> Acta Biomater. 8 (5), 1710-1719 (2012).</li><li>Kurpinski, K., Chu, J., Hashi, C., Li, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proc+Anisotropic+mechanosensing+by+mesenchymal+stemcells.">Proc Anisotropic mechanosensing by mesenchymal stemcells.</a> Natl Acad Sci USA. 103 (44), 16095-16100 (2006).</li><li>Sim, W. Y., Park, S. W., Park, S. H., Min, B. H., Park, S. R., Yang, S. 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+pneumatic+micro+cell+chip+for+the+differentiation+of+human+mesenchymal+stem+cells+under+mechanical+stimulation.">A pneumatic micro cell chip for the differentiation of human mesenchymal stem cells under mechanical stimulation.</a> Lab Chip. 7 (12), 1775-1782 (2007).</li><li>Vader, D., Kabla, A., Weitz, D., Mahadevan, L. <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+Alignment+in+Collagen+Gels.">Strain-Induced Alignment in Collagen Gels.</a> PLoS One. 4 (6), e5902 (2009).</li><li>Aguado, B. A., Mulyasasmita, W., Su, J., Lampe, K. J., 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=Improving+viability+of+stem+cells+during+syringe+needle+flow+through+the+design+of+hydrogel+cell+carriers.">Improving viability of stem cells during syringe needle flow through the design of hydrogel cell carriers.</a> Tissue Eng Part A. 18 (7-8), 806-815 (2012).</li><li>Wan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic-Based+Synthesis+of+Hydrogel+Particles+for+Cell+Microencapsulation+and+Cell-Based+Drug+Delivery.">Microfluidic-Based Synthesis of Hydrogel Particles for Cell Microencapsulation and Cell-Based Drug Delivery.</a> Polymers. 4 (2), 1084-1108 (2012).</li><li>Moraes, C., Wang, G., Sun, Y., Simmons, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+microfabricated+platform+for+high-throughput+unconfined+compression+of+micropatterned+biomaterial+arrays.">A microfabricated platform for high-throughput unconfined compression of micropatterned biomaterial arrays.</a> Biomaterials. 31 (3), 577-584 (2010).</li><li>Keung, A. J., Kumar, S., Schaffer, D. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Presentation+Counts:+Microenvironmental+Regulation+of+Stem+Cells+by+Biophysical+and+Material.+Cues.">Presentation Counts: Microenvironmental Regulation of Stem Cells by Biophysical and Material. Cues.</a> Annu Rev Cell Dev Biol. 26, 533-556 (2010).</li><li>Segers, V. F., Lee, 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=Stem-cell+therapy+for+cardiac+disease.">Stem-cell therapy for cardiac disease.</a> Nature. 451 (7181), 937-942 (2008).</li><li>Hsieh, H. Y., 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=Gradient+static-strain+stimulation+in+a+microfluidic+chip+for+3D+cellular+alignment.">Gradient static-strain stimulation in a microfluidic chip for 3D cellular alignment.</a> Lab Chip. 14 (3), 482-493 (2014).</li></ol>

In this paper, we report on a simple approach to compare cell alignment behavior after hydrogel shape guidance and tensile stretch. A flexible PDMS membrane creates a dome-shaped curvature for generating different heights of concentric circular hydrogels. After releasing the pressure, the PDMS membrane automatically applies force to the micro-patterned hydrogels to form gradient strain/elongation, with a maximum at the center and a minimum at the outer boundary. As the formation of the gradient strain is designed by the ...

Serving as a block material that mimics a native microenvironment, a hydrogel containing extracellular matrix (ECM) can re-build biomimetic tissue scaffolds to support cell growth. To possess the functions of a tissue, organized cell alignment is an essential requirement. Various 2D (i.e., cells cultured on a surface) and 3D (i.e., cells encapsulated in a hydrogel) cell alignments have been achieved by culturing or encapsulating cells in or on flexible substrates with micro-or nano-patterns1. 3D cell alignment in microarchitecture is more attractive, as the microenvironment is closer to the native tissue construct2,3,4. One common approach for 3D cell alignment is the geometric cue of hydrogel shape2,3. Because of the restricted space for cell proliferation in the short-axis direction, cells aim to align along the long-axis direction in a micro-patterned hydrogel. Another approach is to apply tensile stretch to the hydrogels to achieve cell alignment parallel to the stretch direction4,5.
Biophysical stimulation on ECM hydrogels, such as compressive strain or an electrical field, can regulate cell functions for proper tissue integration, proliferation, and differentiation1,2,3. Much research has been done to investigate cellular behavior by applying one strain condition at a time using multiple mechanical control units4,6,7,8,9. For example, the use of mechanical step motors squeezed or stretched on a 3D cell-encapsulated collagen hydrogel has been a common approach7,10. However, such controlling equipment requires extra space and faces the issue of contamination in the incubator7,9,11,12. In addition, the large instrument cannot give a precise control environment to provide high reproducibility13.
Considering that cell-laden hydrogels are usually employed on the micro-scale for biomedical applications, it is advantageous to combine MEMS techniques to generate a range of strain/stretch stimulation to simultaneously investigate cell behaviors in 3D biomimetic constructs in vitro2,14,15,16,17,18. For example, using gas pressure to deform the PDMS membrane in microfluidic chips can give rise to various strains, driving cell differentiation to different lineages9,16. However, there are many technical challenges, such as complicated chip fabrication processes in a clean room and the software control integration of motors, pumps, valves, and compressed gases.
In this work, we demonstrate a simple approach to obtain a self-sustaining gradient static-strain microfluidic chip by employing a concentric circular hydrogel pattern and a flexible PDMS membrane. Unlike most of the existing approaches, our platform is a portable and disposable miniature device that can be fabricated outside a yellow room and that possesses self-generating gradient strains on concentric cell-encapsulated hydrogels, without external mechanical equipment during the incubation. 3T3 fibroblast cell behaviors influenced by a combination of hydrogel shape and a variety of tensile stretch guidance cues were demonstrated during the observation of cell alignment within 3D ECM-mimetic environments in the gradient strain chip for 3 days.

<table border="0" cellpadding="0" cellspacing="0" width="1383">
	<tbody>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">1.5-mL black microcentrifuge tube</td>
			<td style="width:225px;">Argos Technologies&#160;</td>
			<td style="width:156px;">03-391-161</td>
			<td style="width:665px;">This one can be replaced with a neutral color of 1.5-mL tube covered with aluminun foil</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">10X DPBS</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">56064C</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Alexa Fluor 488 phalloidin&#160;</td>
			<td style="width:225px;">Invitrogen</td>
			<td style="width:156px;">A12379&#160;</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">BSA</td>
			<td style="width:225px;">Sigma</td>
			<td style="width:156px;">A1595</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Calcein</td>
			<td style="width:225px;">Molecular Probe</td>
			<td style="width:156px;">C1430</td>
			<td style="width:665px;">For labeling viable cells</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">CCD</td>
			<td style="width:225px;">PCO. Imaging</td>
			<td style="width:156px;">Pixelfly qe</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Cell membrane permeating solution</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">X100</td>
			<td style="width:665px;">0.5% Triton X-100 for permeating cell membrane</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">DAPI</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">D8417</td>
			<td style="width:665px;">Cell nucleus staining</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Dialysis membrane</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">D9527</td>
			<td style="width:665px;">Molecular weight cut-off = 14,000</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">DMEM</td>
			<td style="width:225px;">Gibco</td>
			<td style="width:156px;">11995-065</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Double-side tape</td>
			<td style="width:225px;">3M</td>
			<td style="width:156px;">8003</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">FBS</td>
			<td style="width:225px;">Hyclone</td>
			<td style="width:156px;">SH30071.03</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Gelatin</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">G2500</td>
			<td style="width:665px;">gel strength 300, type A, from porcine skin</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">High frequency electronic corona generator</td>
			<td style="width:225px;">Electro-technic products</td>
			<td style="width:156px;">MODEL BD-20</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Methacrylic Anhydride</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">276685</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Micro syringe</td>
			<td style="width:225px;">Hamilton</td>
			<td style="width:156px;">80501</td>
			<td style="width:665px;">50 &#956;L&#160;</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Microscope</td>
			<td style="width:225px;">Olympus</td>
			<td style="width:156px;">IX71</td>
			<td style="width:665px;">Include two filter sets: LF405/LP-B-000 and LF488/LP-C-000 from Semrock</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Oxygen plasma machine</td>
			<td style="width:225px;">Harrick plasma</td>
			<td style="width:156px;">PDC-001</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Paraformaldehyde</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">P6148</td>
			<td style="width:665px;">For fixing cell</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">PDMS</td>
			<td style="width:225px;">DOW CORNING</td>
			<td style="width:156px;">Sylgard 184</td>
			<td style="width:665px;">Mixture for PDMS chip cast-molding fabrication</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Pen-Strep</td>
			<td style="width:225px;">Gibco</td>
			<td style="width:156px;">10378-016</td>
			<td style="width:665px;">penicillin/streptomycin</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Photoinitiator</td>
			<td style="width:225px;">CIBA</td>
			<td style="width:156px;">Irgacure 2959</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Propidium iodide</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">P4170</td>
			<td style="width:665px;">For labeling dead cells</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Sterile Filtration cup</td>
			<td style="width:225px;">Millipore</td>
			<td style="width:156px;">SCGPT05RE</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">TMSPMA</td>
			<td style="width:225px;">Sigma-Aldrich</td>
			<td style="width:156px;">440159</td>
			<td style="width:665px;">For hydrogel immobilization</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Ultrasonicator</td>
			<td style="width:225px;">Delta</td>
			<td style="width:156px;">D150H</td>
			<td style="width:665px;">150W, 43kHz</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">UV light</td>
			<td style="width:225px;">DAIHAN</td>
			<td style="width:156px;">WUV-L10</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">Freeze Dryer</td>
			<td style="width:225px;">FIRSTEK</td>
			<td style="width:156px;">150311025</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">NIH3T3(fibroblast)</td>
			<td style="width:225px;">Food Industry Research and Development Institute(FIRDI)</td>
			<td style="width:156px;">08C0011</td>
			<td style="width:665px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:337px;">MOXI Z Mini Automated Cell Counter</td>
			<td style="width:225px;">ORFLO</td>
			<td style="width:156px;">MXZ001</td>
			<td style="width:665px;"></td>
		</tr>
	</tbody>
</table>

gradient strain chip for stimulating cellular behaviors in cell-laden hydrogel

Artificial guidance for cellular alignment is a hot topic in the field of tissue engineering. Most of the previous research has investigated single strain-induced cellular alignment on a cell-laden hydrogel by using complex experimental processes and mass controlling systems, which are usually associated with contamination issues. Thus, in this article, we propose a simple approach to building a gradient static strain using a fluidic chip with a plastic PDMS cover and a UV transparent glass substrate for the stimulation of cellular behavior in a 3D hydrogel. Overloading photo-patternable cell prepolymer in the fluidic chamber can generate a convex curved PDMS membrane on the cover. After UV crosslinking, through a concentric circular micropattern under the curved PDMS membrane, and buffer washing, a microenvironment for investigating cell behaviors under a variety of gradient strains is self-established in a single fluidic chip, without external instruments. NIH3T3 cells were demonstrated after observing the change in the cellular alignment trend under geometry guidance, in cooperation with strain stimulation, which varied from 15 - 65% on hydrogels. After a 3-day incubation, the hydrogel geometry dominated the cell alignment under low compressive strain, where cells aligned along the hydrogel elongation direction under high compressive strain. Between these, the cells showed random alignment due to the dissipation of the radical guidance of hydrogel elongation and the geometry guidance of the patterned hydrogel.

To compare the mechanical variations between each circular hydrogel in the completed gradient strain stimulation chip, we measured the line widths of each circular hydrogel in two of the same chips, with injection volumes of 0 &#181;L (Figure 4a) and 40 &#181;L (Figure 4b), respectively. The percent elongations at each circle were calculated by dividing the elongations in the 40 &#181;L-injected chip by the line widths of the cor...

Watch this Scientific Journal Video about Gradient Strain Chip for Stimulating Cellular Behaviors in Cell-laden Hydrogel at JoVE.com

Gradient Strain Chip for Stimulating Cellular Behaviors in Cell-laden Hydrogel

This article introduces a simple approach to providing non-continuous gradient static strains on a concentric cell-laden hydrogel to regulate cell alignment for tissue engineering.

Artificial guidance for cellular alignment is a hot topic in the field of tissue engineering. Most of the previous research has investigated single ...

The overall goal of this gradient strain chip is to provide a simple approach to generate non continuous gradient static strains on a 3D hydrogel for the investigation of cell behaviors under a series of strain conditions. This method can help answer key questions in the bioengineering field such as the stimulations of a stem cell differentiation and the regulation of cellular alignment for artificial tissue organ development. The main advantage of this technique is that various strains can be applied on the cell layer and hydrogels in the same micro environment for comparison of cell behavior.Visual demonstration of this method is critical as the chip preparation steps are difficult to learn since the light should be complete in three to four hours for base of experiment to ensure high cell variability. Weigh 10 grams of gelatin powder and add it to a glass flask with 100 milliliters of Dulbecco's phosphate buffered saline, or DPBS. Put a magnetic stir bar into the flask and place the flask on a stirring hot plate.Cover the flask with aluminum foil to avoid water evaporation. Set the hot plate temperature to 50 to 60 degrees and the stirrer at 100 RPM for one hour to dissolve the gelatin powder. After the gelatin has dissolved, add eight milliliters of methacrylic anhydride very slowly using a pipette.Let it react at 60 degrees Celsius for three hours. Then add prewarmed DPBS into the flask to a final volume of 500 milliliters. Allow the solution to mix well for 15 minutes to stop the reaction.Meanwhile cut a dialysis membrane into several 25 millimeter long tubes. Immerse them in deionized water for 15 minutes. And make a knot to close one end of the dialysis tubes.Load the appropriate amount of the polymer solution into the dialysis tubes and close the other end. Then place the filled tubes in a five liter plastic beaker with deionized water for a week. Renew the deionized water twice a day and maintain the solution at 40 to 50 degrees Celsius during the dialysis process.Following dialysis collect the solution from the tubes into a 500 milliliter glass bottle. Pour approximately 450 to 500 milliliters of solution into a 500 milliliter filter cup. And apply a vacuum to force the solution to pass through the filter membrane for sterilization.Then transfer the sterilized polymer into several 50 milliliter sterilized tubes. Transfer the sterilized polymer into several 50 milliliter sterilized tubes and store them in a minus 80 degrees Celsius freezer for three to five days. Freeze dry the minus 80 degrees Celsius polymer for one week using a freeze drier to form GelMA which can then be stored in a minus 80 degree Celsius freezer.For chip fabrication first prepare the polymethacrylate or PMMA, played and molded as described in the text protocol. Then prepare the PTMS cover and PTMS plug by properly mixing 30 grams of PTMS elastomer and three grams of PTMS curing agent. Degas the mixture under a vacuum chamber for one hour.Cast 1.8 to two grams of the mixture into the PMMA mold for the PTMS cover. Also use the appropriate amount to fill each cavity for the PMMA mold for the PTMS plug. In addition cast 10 grams of uncured PTMS mixture in a blank 10 centimeter plastic plate.Put these molds in a vacuum chamber to degas for 30 minutes. Cure the PTMS for two hours at 80 degrees Celsius. After cooling down the cured PTMS on the mold, detach the PTMS covers and the PTMS plugs from the PMMA molds.Next punch two holes with diameters of about three millimeters at the ends of the flow channels of the PTMS covers. Cut the PTMS sheet molded from the 10 centimeter plastic plate into many one centimeter by one centimeter cubes. And punch a three millimeter hole in each PTMS cube.Glue two uncured one centimeter by one centimeter PTMS cubes onto the openings of the PTMS cover to serve as medium reservoirs and to aid in the curing process of the gradient circular hydrogel patterns. Then cure for one hour at 80 degrees Celsius. Next bond the PTMS covers with the two PTMS reservoirs onto a TMSPMA coated slide.To do so, pretreat the bonding side of the PTMS cover and the TMSPMA coated slide under an oxygen plasma machine for 90 seconds of oxygen plasma treatment. Contact the plasma treated surface of the PTMS covers and the TMSPMA slide and press them closely for permanent bonding through the formation of the silicon oxygen silicon bond. Print a photomask by printing the established layout on a transparent film.Then cut the printed photomask to 25 millimeters by 37.5 millimeters in size. Weigh 25 milligrams of freeze dried GelMA into 0.3 milliliters of prewarmed cell culture medium in a 1.5 milliliter black microcentrifuge tube. Put the microcentrifuge tube on a laboratory stirrer hot plate until the GelMA dissolves in the medium.Add 50 milligrams of photoinitiator to one milliliters of DPBS in a microcentrifuge tube. And place it in an 80 degree Celsius oven for 15 minutes or until the photoinitiator has dissolved. Add 25 microliters of the 10%photoinitiator to the microcentrifuge tube.Pipette several times to mix well. Next count three million NIH3T3 cells using an automated cell counter. After centrifuging the suspension at 200 Gs for five minutes discard the supernatant and resuspend the cells in 175 microliters of cell culture medium.Add the cell solution to the microcentrifuge tube to get a pre polymer cell solution of 5%GelMA, 0.5%photoinitiator and about six million 3T3 cells per milliliter. After mixing load 100 microliters of cell prepolymer into a 100 microliter microsyringe. Manually align a piece of the photomask onto the bottom slide of the sterilized gradient strain chip.And simply fix the position using a small drop of deionized water in between. Connect the 100 microliter microsyringe, loaded with prepolymer cell solution to the inlet of the chip. Then place 50 microliters of prepolymer cell solution in the flow channel using the microsyringe and plug the outlet using a PTMS plug.Inject an extra 40 microliters of solution to create a convex bulge in the circular PTMS membrane. Move the chip with the photomask, microsyringe, and PTMS plug to under a UV lamp. And expose it for 30 to 45 seconds to cross link the concentric circular hydrogel in the fluidic chamber.After cross linking remove the PTMS plug and the microsyringe to release the liquid pressure. Use a one milliliter syringe loaded with prewarmed DPBS to wash out uncrosslinked resins three times by flushing from the inlet to the outlet. Next fill the flow channel with about 100 microliters of cell culture medium.Place the chip in a sterilized culture dish and culture in a 5%carbon dioxide atmosphere at 37 degrees Celsius for a week. Refresh the medium every day. Take images of three chips on day zero after four hours of incubation as the control group.And three chips on day three as the experimental set using a microscope with a 20 times objective. Measure the line width of each hydrogel from line one to line 12 using software to calculate the compress stains. Shown here are representative results of hydrogel patterns with zero microliter and 40 microliters over injection, after UV cross linking and four hour incubation.The line width of the 3D hydrogel in the non over injection control looks uniform as expected. However the hydrogel patterns with 40 microliters over injection gives a gradient line width of hydrogels decreasing from line one to line 12. After three days of incubation, the cells in the hydrogel extend to different directions.The cells in line one align along the radial direction due to the stimulation of hydrogel elongation. On the contrary the cells in line 12 align along the hydrogel shape where the geometry cue dominates the cellular alignment. There is no specific cell alignment in line 7 where the two cellular alignment cues of line one and line 12 balance each other out.Once measuring the micro environment of gradient strains on 3D hydrogel can be done by the gradient strain chip in two hours if it's performed properly. While attempting this procedure, it is important to remember to sterilize the gradient strain chip before loading the cell prepolymer solution. Also thoroughly wipe the surface of the chip with 75%ethanol before placing the chips into the cell culture incubator.After watching this video you should have a very good understanding of how to generate a gradient strain chip to investigate cell behaviors under different strains imperial.

gradient-strain-chip-for-stimulating-cellular-behaviors-cell-laden

Research

JoVE Journal

Bioengineering

Encapsulation of Cardiomyocytes in a Fibrin Hydrogel for Cardiac Tissue Engineering

Culturing cells in a three dimensional hydrogel environment is an important technique for developing constructs for tissue engineering as well as studying cellular responses under various culture conditions in vitro. The three dimensional environment more closely mimics what the cells observe in vivo due to the application of mechanical and chemical stimuli in all dimensions 1. Three-dimensional hydrogels can either be made from synthetic polymers such as PEG-DA 2 and PLGA 3 or a number of naturally occurring proteins such as collagen 4, hyaluronic acid 5 or fibrin 6,7. Hydrogels created from fibrin, a naturally occurring blood clotting protein, can polymerize to form a mesh that is part of the body's natural wound healing processes 8. Fibrin is cell-degradable and potentially autologous 9, making it an ideal temporary scaffold for tissue engineering.
Here we describe in detail the isolation of neonatal cardiomyocytes from three day old rat pups and the preparation of the cells for encapsulation in fibrin hydrogel constructs for tissue engineering. Neonatal myocytes are a common cell source used for in vitro studies in cardiac tissue formation and engineering 4. Fibrin gel is created by mixing fibrinogen with the enzyme thrombin. Thrombin cleaves fibrinopeptides FpA and FpB from fibrinogen, revealing binding sites that interact with other monomers 10. These interactions cause the monomers to self-assemble into fibers that form the hydrogel mesh. Because the timing of this enzymatic reaction can be adjusted by altering the ratio of thrombin to fibrinogen, or the ratio of calcium to thrombin, one can injection mold constructs with a number of different geometries 11,12. Further we can generate alignment of the resulting tissue by how we constrain the gel during culture 13.
After culturing the engineered cardiac tissue constructs for two weeks under static conditions, the cardiac cells have begun to remodel the construct and can generate a contraction force under electrical pacing conditions 6. As part of this protocol, we also describe methods for analyzing the tissue engineered myocardium after the culture period including functional analysis of the active force generated by the cardiac muscle construct upon electrical stimulation, as well as methods for determining final cell viability (Live-Dead assay) and immunohistological staining to examine the expression and morphology of typical proteins important for contraction (Myosin Heavy Chain or MHC) and cellular coupling (Connexin 43 or Cx43) between myocytes.

We describe the isolation of neonatal cardiomyocytes and the preparation of the cells for encapsulation in fibrin hydrogel constructs for tissue engineering. We describe methods for analyzing the tissue engineered myocardium after the culture period including active force generated upon electrical stimulation and cell viability and immunohistological staining.

Culturing cells in a three dimensional hydrogel environment is an important technique for developing constructs for tissue engineering as well as studying ...

Cellular Lipid Extraction for Targeted Stable Isotope Dilution Liquid Chromatography-Mass Spectrometry Analysis

The metabolism of fatty acids, such as arachidonic acid (AA) and linoleic acid (LA), results in the formation of oxidized bioactive lipids, including numerous stereoisomers1,2. These metabolites can be formed from free or esterified fatty acids. Many of these oxidized metabolites have biological activity and have been implicated in various diseases including cardiovascular and neurodegenerative diseases, asthma, and cancer3-7. Oxidized bioactive lipids can be formed enzymatically or by reactive oxygen species (ROS). Enzymes that metabolize fatty acids include cyclooxygenase (COX), lipoxygenase (LO), and cytochromes P450 (CYPs)1,8. Enzymatic metabolism results in enantioselective formation whereas ROS oxidation results in the racemic formation of products.
While this protocol focuses primarily on the analysis of AA- and some LA-derived bioactive metabolites; it could be easily applied to metabolites of other fatty acids. Bioactive lipids are extracted from cell lysate or media using liquid-liquid (l-l) extraction. At the beginning of the l-l extraction process, stable isotope internal standards are added to account for errors during sample preparation. Stable isotope dilution (SID) also accounts for any differences, such as ion suppression, that metabolites may experience during the mass spectrometry (MS) analysis9. After the extraction, derivatization with an electron capture (EC) reagent, pentafluorylbenzyl bromide (PFB) is employed to increase detection sensitivity10,11. Multiple reaction monitoring (MRM) is used to increase the selectivity of the MS analysis. Before MS analysis, lipids are separated using chiral normal phase high performance liquid chromatography (HPLC). The HPLC conditions are optimized to separate the enantiomers and various stereoisomers of the monitored lipids12. This specific LC-MS method monitors prostaglandins (PGs), isoprostanes (isoPs), hydroxyeicosatetraenoic acids (HETEs), hydroxyoctadecadienoic acids (HODEs), oxoeicosatetraenoic acids (oxoETEs) and oxooctadecadienoic acids (oxoODEs); however, the HPLC and MS parameters can be optimized to include any fatty acid metabolites13.
Most of the currently available bioanalytical methods do not take into account the separate quantification of enantiomers. This is extremely important when trying to deduce whether or not the metabolites were formed enzymatically or by ROS. Additionally, the ratios of the enantiomers may provide evidence for a specific enzymatic pathway of formation. The use of SID allows for accurate quantification of metabolites and accounts for any sample loss during preparation as well as the differences experienced during ionization. Using the PFB electron capture reagent increases the sensitivity of detection by two orders of magnitude over conventional APCI methods. Overall, this method, SID-LC-EC-atmospheric pressure chemical ionization APCI-MRM/MS, is one of the most sensitive, selective, and accurate methods of quantification for bioactive lipids.

This protocol will demonstrate the extraction and analysis of free and esterified bioactive fatty acids from cells. Fatty acids are accurately quantified using stable isotope dilution, chiral liquid chromatography, electron capture atmospheric chemical ionization multiple reaction monitoring mass spectrometry (SID-LC-ECAPCI-MRM/MS).

The metabolism of fatty acids, such as arachidonic acid (AA) and linoleic acid (LA), results in the formation of oxidized bioactive lipids, including ...

Electrotaxis Studies of Lung Cancer Cells using a Multichannel Dual-electric-field Microfluidic Chip

The behavior of directional cell migration under a direct current electric-field (dcEF) is referred to as electrotaxis. The significant role of physiological dcEF in guiding cell movement during embryo development, cell differentiation, and wound healing has been demonstrated in many studies. By applying microfluidic chips to an electrotaxis assay, the investigation process is shortened and experimental errors are minimized. In recent years, microfluidic devices made of polymeric substances (e.g., polymethylmethacrylate, PMMA, or acrylic) or polydimethylsiloxane (PDMS) have been widely used in studying the responses of cells to electrical stimulation. However, unlike the numerous steps required to fabricate a PDMS device, the simple and rapid construction of the acrylic micro&#64258;uidic chip makes it suitable for both device prototyping and production. Yet none of the reported devices facilitate the efficient study of the simultaneous chemical and dcEF effects on cells. In this report, we describe our design and fabrication of an acrylic-based multichannel dual-electric-field (MDF) chip to investigate the concurrent effect of chemical and electrical stimulation on lung cancer cells. The MDF chip provides eight combinations of electrical/chemical stimulations in a single test. The chip not only greatly shortens the required experimental time but also increases accuracy in electrotaxis studies.

Many microfluidic devices have been developed for use in the study of electrotaxis. Yet, none of these chips allows the efficient study of the simultaneous chemical and electric-field (EF) effects on cells. We developed a polymethylmethacrylate-based device that offers better-controlled coexisting EF and chemical stimulation for use in electrotaxis research.

The behavior of directional cell migration under a direct current electric-field (dcEF) is referred to as electrotaxis. The significant role of ...

Construction of Modular Hydrogel Sheets for Micropatterned Macro-scaled 3D Cellular Architecture

Hydrogels can be patterned at the micro-scale using microfluidic or micropatterning technologies to provide an in vivo-like three-dimensional (3D) tissue geometry. The resulting 3D hydrogel-based cellular constructs have been introduced as an alternative to animal experiments for advanced biological studies, pharmacological assays and organ transplant applications. Although hydrogel-based particles and fibers can be easily fabricated, it is difficult to manipulate them for tissue reconstruction. In this video, we describe a fabrication method for micropatterned alginate hydrogel sheets, together with their assembly to form a macro-scale 3D cell culture system with a controlled cellular microenvironment. Using a mist form of the calcium gelling agent, thin hydrogel sheets are easily generated with a thickness in the range of 100 - 200 &#181;m, and with precise micropatterns. Cells can then be cultured with the geometric guidance of the hydrogel sheets in freestanding conditions. Furthermore, the hydrogel sheets can be readily manipulated using a micropipette with an end-cut tip, and can be assembled into multi-layered structures by stacking them using a patterned polydimethylsiloxane (PDMS) frame. These modular hydrogel sheets, which can be fabricated using a facile process, have potential applications of in vitro drug assays and biological studies, including functional studies of micro- and macrostructure and tissue reconstruction.

We describe the fabrication of micropatterned hydrogel sheets using a simple process, which can be assembled and manipulated in a freestanding form. Using these modular hydrogel sheets, a simple macro-scaled 3D cell culture system can be generated with a controlled cellular microenvironment.

Hydrogels can be patterned at the micro-scale using microfluidic or micropatterning technologies to provide an in vivo-like three-dimensional (3D) tissue ...

Improved 3D Hydrogel Cultures of Primary Glial Cells for In Vitro Modelling of Neuroinflammation

In the central nervous system, numerous acute injuries and neurodegenerative disorders, as well as implanted devices or biomaterials engineered to enhance function result in the same outcome: excess inflammation leads to gliosis, cytotoxicity, and/or formation of a glial scar that collectively exacerbate injury or prevent healthy recovery. With the intent of creating a system to model glial scar formation and study inflammatory processes, we have generated a 3D cell scaffold capable of housing primary cultured glial cells: microglia that regulate the foreign body response and initiate the inflammatory event, astrocytes that respond to form a fibrous scar, and oligodendrocytes that are typically vulnerable to inflammatory injury. The present work provides a detailed step-by-step method for the fabrication, culture, and microscopic characterization of a hyaluronic acid-based 3D hydrogel scaffold with encapsulated rat brain-derived glial cells. Further, protocols for characterization of cell encapsulation and the hydrogel scaffold by confocal immunofluorescence and scanning electron microscopy are demonstrated, as well as the capacity to modify the scaffold with bioactive substrates, with incorporation of a commercial basal lamina mixture to improved cell integration.

Improved 3D Hydrogel Cultures of Primary Glial Cells for In Vitro Modelling of Neuroinflammation

Herein, we present a protocol for the 3D culture of rat brain-derived glia cells, including astrocytes, microglia, and oligodendrocytes. We demonstrate primary cell culture, methacrylated hyaluronic acid (HAMA) hydrogel synthesis, HAMAphoto-polymerization and cell encapsulation, and sample processing for confocal and scanning electron microscopic imaging.

In the central nervous system, numerous acute injuries and neurodegenerative disorders, as well as implanted devices or biomaterials engineered to enhance ...

Repressing Gene Transcription by Redirecting Cellular Machinery with Chemical Epigenetic Modifiers

Regulation of chromatin compaction is an important process that governs gene expression in higher eukaryotes. Although chromatin compaction and gene expression regulation are commonly disrupted in many diseases, a locus-specific, endogenous, and reversible method to study and control these mechanisms of action has been lacking. To address this issue, we have developed and characterized novel gene-regulating bifunctional molecules. One component of the bifunctional molecule binds to a DNA-protein anchor so that it will be recruited to an allele-specific locus. The other component engages endogenous cellular chromatin-modifying machinery, recruiting these proteins to a gene of interest. These small molecules, called chemical epigenetic modifiers (CEMs), are capable of controlling gene expression and the chromatin environment in a dose-dependent and reversible manner. Here, we detail a CEM approach and its application to decrease gene expression and histone tail acetylation at a Green Fluorescent Protein (GFP) reporter located at the Oct4 locus in mouse embryonic stem cells (mESCs). We characterize the lead CEM (CEM23) using fluorescent microscopy, flow cytometry, and chromatin immunoprecipitation (ChIP), followed by a quantitative polymerase chain reaction (qPCR). While the power of this system is demonstrated at the Oct4 locus, conceptually, the CEM technology is modular and can be applied in other cell types and at other genomic loci.

Regulation of the chromatin environment is an essential process required for proper gene expression. Here, we describe a method for controlling gene expression through the recruitment of chromatin-modifying machinery in a gene-specific and reversible manner.

Regulation of chromatin compaction is an important process that governs gene expression in higher eukaryotes. Although chromatin compaction and gene ...

A Microfluidic Platform for Stimulating Chondrocytes with Dynamic Compression

Mechanical stimuli are known to modulate biological functions of cells and tissues. Recent studies have suggested that compressive stress alters growth plate cartilage architecture and results in growth modulation of long bones of children. To determine the role of compressive stress in bone growth, we created a microfluidic device actuated by pneumatic pressure, to dynamically (or statically) compress growth plate chondrocytes embedded in alginate hydrogel cylinders. In this article, we describe detailed methods for fabricating and characterizing this device. The advantages of our protocol are: 1) Five different magnitudes of compressive stress can be generated on five technical replicates in a single platform, 2) It is easy to visualize cell morphology via a conventional light microscope, 3) Cells can be rapidly isolated from the device after compression to facilitate downstream assays, and 4) The platform can be applied to study mechanobiology of any cell type that can grow in hydrogels.

This article provides detailed methods for fabricating and characterizing a pneumatically actuating microfluidic device for chondrocyte compression.

Mechanical stimuli are known to modulate biological functions of cells and tissues. Recent studies have suggested that compressive stress alters growth ...

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 ...

High Throughput Yeast Strain Phenotyping with Droplet-Based RNA Sequencing

The powerful tools available to edit yeast genomes have made this microbe a valuable platform for engineering. While it is now possible to construct libraries of millions of genetically distinct strains, screening for a desired phenotype remains a significant obstacle. With existing screening techniques, there is a tradeoff between information output and throughput, with high-throughput screening typically being performed on one product of interest. Therefore, we present an approach to accelerate strain screening by adapting single cell RNA sequencing to isogenic picoliter colonies of genetically engineered yeast strains. To address the unique challenges of performing RNA sequencing on yeast cells, we culture isogenic yeast colonies within hydrogels and spheroplast prior to performing RNA sequencing. The RNA sequencing data can be used to infer yeast phenotypes and sort out engineered pathways. The scalability of our method addresses a critical obstruction in microbial engineering.

A bottleneck in the &#8216;design-build-test&#8217; cycle of microbial engineering is the speed at which we can perform functional screens of strains. We describe a high-throughput method for strain screening applied to hundreds to thousands of yeast cells per experiment that utilizes droplet-based RNA sequencing.

The powerful tools available to edit yeast genomes have made this microbe a valuable platform for engineering. While it is now possible to construct ...

A Lab-On-A-Chip Platform for Stimulating Osteocyte Mechanotransduction and Analyzing Functional Outcomes of Bone Remodeling

Bone remodeling is a tightly regulated process that is required for skeletal growth and repair as well as adapting to changes in the mechanical environment. During this process, mechanosensitive osteocytes regulate the opposing responses between the catabolic osteoclasts and anabolic osteoblasts. To better understand the highly intricate signaling pathways that regulate this process, our lab has developed a foundationary lab-on-a-chip (LOC) platform for analyzing functional outcomes (formation and resorption) of bone remodeling within a small scale system. As bone remodeling is a lengthy process that occurs on the order of weeks to months, we developed long-term cell culturing protocols within the system. Osteoblasts and osteoclasts were grown on functional activity substrates within the LOC and maintained for up to seven weeks. Afterward, chips were disassembled to allow for the quantification of bone formation and resorption. Additionally, we have designed a 3D printed mechanical loading device that pairs with the LOC platform and can be used to induce osteocyte mechanotransduction by deforming the cellular matrix. We have optimized cell culturing protocols for osteocytes, osteoblasts, and osteoclasts within the LOC platform and have addressed concerns of sterility and cytotoxicity. Here, we present the protocols for fabricating and sterilizing the LOC, seeding cells on functional substrates, inducing mechanical load, and disassembling the LOC to quantify endpoint results. We believe that these techniques lay the groundwork for developing a true organ-on-a-chip for bone remodeling.

Here, we present protocols for analyzing bone remodeling within a lab-on-a-chip platform. A 3D printed mechanical loading device can be paired with the platform to induce osteocyte mechanostransduction by deforming the cellular matrix. The platform can also be used to quantify bone remodeling functional outcomes from osteoclasts and osteoblasts (resorption/formation).