Tissue Engineering

Introduction
Tissue Train® System
Equipment, Devices, & Consumables

 

Introduction
Formation of tissues in vitro that are structurally and functionally viable requires several basic conditions, such as 1) cells, 2) matrix, 3) media and growth factors, and 4) mechanical stimulation. These conditions are linked to each other and act in conjunction to form a structurally robust tissue that can withstand biomechanical forces. As a tissue develops, the cells fabricate an extracellular matrix in a given geometry according to developmental pathway cues. Several signal transduction pathways may be involved in generating the composition of the extracellular matrix. Some of these pathways are regulated by mechanical deformation of cell matrix and transmitted into the cell via membrane bound proteins such as integrins, focal adhesion complexes (mechanosensory complex), cell adhesion molecules, and ion channels. Cells can also respond to ligands, such as cytokines, hormones, or growth factors that are released as a result of matrix deformation.

In order to maintain the integrity and strength of musculoskeletal tissues, the cells may require maintaining a certain level of intrinsic strain. In the absence of this intrinsic strain, the tissue will lose its strength leading to failures or fractures. It is well accepted that immobilization of limbs, bed rest, or a reduction in the intrinsic strain level in a tissue leads to bone mineral loss, tissue atrophy, weakness and in general, a reduction in anabolic activity and an increase in catabolic activity. Physical activity, on the other hand, results in anabolic effects including an increase in biomechanical strength and an increase in the intrinsic strain in a tissue.

To generate a tissue in vitro that is more or less equivalent to the native tissues, it is of utmost importance to create an environment that would mimic the in vivo conditions. Culturing cells in a mechanically active environment increases cell metabolism and alters cell shape and other properties. Therefore, it is vital to create and maintain a mechanically active environment (i.e., tension, shear stress, or compression) for the cells during the formation of tissues in vitro. In addition to the dynamic environment, culturing cells in a 3D environment more closely simulates the native environment than a static 2D culture method.

The size and shape of the tissue matrix would also directly affect the type, magnitude, direction and distribution of physiological forces within the tissue matrix. The composition of tissue may also depend on the types of forces that the tissue undergoes. Depending on the anatomical location, some tissues may experience both tensile and compressive forces within the tissue leading to multiple compositions. For example, the midsubstance (where tensile forces exist) of an Achilles tendon is comprised of dense fibrous connective tissue, while the area where tendon presses against calcaneus (where compressive forces exist) is comprised of fibrocartilaginous tissue. The shape of the tissue also plays a major role in the location of its failure. Most failures in Achilles tendons occur at the calcaneal junction where it joins the bone and has the least thickness. Therefore, it is clear that the native shape of the tissue needs to be simulated in vitro to facilitate studying the failure mechanism as well as the healing mechanism of tissues. The Flexcell® Tissue Train® Culture System was developed to address these segments of the culture world, providing a 3D matrix, dynamic strain to cells and matrix, and multiple geometries for creating bioartificial tissues of different shapes (i.e., linear, trapezoidal, and circular).
 

Tissue Train® System
Flexcell®'s Tissue Train® Culture System is a stand-alone 3D culture system that allows investigators to create 3D geometries for cell culture in a matrix gel or allow the cells to build a self-assembled matrix that connects to the anchors in a Tissue Train® culture plate. Flexcell® currently has molds and/or plates for creating three different shaped hydrogels: linear, trapezoidal, and circular. The Tissue Train® System can be used to create bioartificial constructs with cells from the cardiac, musculoskeletal, dermal, lung, gastrointestinal, bone marrow, and adipose tissues to name a few. (See our Publication Database to see how researchers are currently using this system).

Figure 1 illustrates how a linear bioartificial tissue (BAT) is created with the Tissue Train® Culture System. In brief, a Tissue Train® culture plate is set atop a Trough Loader™ and a vacuum is applied with the FX-6000™ Tension System pulling the flexible-bottomed rubber membrane of the culture plate downward into the linear trough. A cell and gel matrix suspension is dispensed into the trough between the two anchor stems with a pipette. After polymerization, the vacuum is released and a linear hydrogel, or bioartificial tissue, has been created that is attached to the culture plate via the anchor stems at the east and west poles.


Tissue Train
Figure 1. Bioartificial tissue development with the Tissue Train® Culture System.

The FX-6000™ Tension System provides the investigator with a tool to apply regulated uniaxial or equibiaxial strain to the growing bioartificial tissues. A user can define a frequency, elongation and duration of strain in a regimen that simulates the strain environment of the native tissue in the body (see Applying Mechanical Load to Cells in 3D Culture for further information).

View video Creating a Bioartifical Construct with the Tissue Train® System

Additionally, the cells will remodel their extracellular matrix over time (Figure 2). A measure of this remodeling is gel compaction over time. ScanFlex™ is an automated image collection system that allows users to periodically scan items placed on a scanner bed. The ScanFlex™ software controls a digital scanner and allows users to program the number of times and the time intervals when digital scans are taken. When used in conjunction with the Tissue Train® culture plates, ScanFlex™ can be used to determine the change in area of a bioartificial tissue. Furthermore, the area of a BAT can be measured using the XyFlex™ image analysis software. XyFlex™ software allows the user to automatically measure the BAT area in a large sequence of images.


Pathways
Figure 2. Illustration of gel compaction in a bioartificial tissue.


 

Equipment, Devices, & Consumables

 

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