A decade ago researchers assumed that cells are smart: if we put the correct cell types in proximity to one another, they would “figure out” what to do to form their native tissues. To some degree, this approach is effective, but we now have a greater appreciation of the intricacy of signals exchanged among cells and their surroundings during organ and tissue development as well as during normal functioning, and we know the importance of providing a tailored environment in our constructs.
Further, every tissue in the body performs specific tasks that engineered replacements must be able to perform, and we are learning that replicating the underlying biology of the tissue in question as closely as possible is critical to generating tissues that can carry out their intended functions. In more complex organs, multiple cell types work in concert—in the liver, for instance, the cells’ jobs include detoxification and nutrient breakdown. Thus, the microarchitecture of tissues and the positioning of cells relative to one another must be re-created in tissue-engineered constructs to reproduce the desired functionality. Early tissue-engineering work used scaffolds made from assorted materials to try to replicate the 3-D shape of the tissue as well as crudely approximate this spatial cell organization. A number of advances in the past few years have enhanced the level of complexity of engineered tissues and reproduced the tissue environment more closely. For example, scaffolds have been made by removing all the cells from natural tissues, leaving only connective fibers. These shells can be used to grow engineered tissues that recreate a significant amount of the function of the original tissue. In one particularly impressive study, decellularized rodent heart scaffolds that were seeded with cardiac and endothelial cells produced cardiac muscle fibers and vascular structures that grew into a beating heart.
Assorted “printing” technologies can also be used to arrange cells precisely. By modifying standard ink-jet printers, engineers can dispense cells themselves or scaffold materials to generate tissues or frameworks onto which cells can be seeded. Mimicking the tissue’s natural topography also helps to guide the cells, and another technology borrowed from the engineering world, electrospinning, can produce scaffolds that resemble the texture of natural tissue matrix. Very thin polymer fibers are spun to form weblike scaffolds, which provide cells with a more natural 3-D environment, and the chemical and mechanical features of the polymer materials can be finely manipulated. David Kaplan of Tufts University has fashioned similar scaffolds from silk materials that resemble spider webs to generate ligaments and bone tissues.
Because the biological, chemical and mechanical properties of hydrogels can be readily manipulated, the gels are proving useful for supporting and encasing cells while enhancing the function of the resulting tissues. Hydrogels containing live cells can be “printed” or otherwise arranged and layered to delineate correct tissue structure. One of us (Khademhosseini) has shown, for example, that hydrogel-encased cell aggregates can be molded into any number of complementary shapes [see box on next page], then pooled together to self-organize into a larger complex pattern. This technique could be used to replicate the natural organization of cells in a tissue such as the liver, which is made up of hexagonal structures that each contain toxin-filtering cells surrounding a central blood vessel.
Some gels are designed so that their polymers link together in response to ultraviolet light, making it possible to sculpt the desired construct shape and then solidify it by exposing all or parts of the construct to light. Kristi Anseth of the University of Colorado at Boulder and Jennifer Elisseeff of Johns Hopkins University have generated cartilage and bone tissue using such photocrosslinkable hydrogels. Gels can also be imbued with a number of signaling molecules to promote tissue growth or differentiation. Samuel Stupp of Northwestern University has shown that neural stem cells differentiate into neurons within a hydrogel that incorporates small proteins that act as environmental signals directing the cells’ behavior. Helen M. Blau of Stanford University has also used hydrogels containing extracellular matrix components to control and study the properties of individual stem cells.
Finally, nanotechnology has been enlisted to generate engineered sheets of cells suitable for transplantation. Teruo Okano of Tokyo Women’s Medical University has produced surfaces coated with a temperature-responsive polymer that swells as the temperature is lowered from 37 to 20 degrees Celsius. Cells are first induced to form a single layer on these nanoengineered surfaces, then the temperature is lowered to swell the underlying substrate and detach the intact cell sheet. These cell sheets, which contain appropriate cellsecreted matrix molecules can then be stacked or rolled to build larger tissue constructs.
Although these advances have made a significant improvement in the range and diversity of scaffolds that can be generated, challenges persist in this area as well. One difficulty I a lack of knowledge of the concentrations and combinations of growth factors and extracellular molecules that are present at specific stages of development and wound healing in various tissues. A better understanding of these design parameters is needed to engineer tissues that mimic the body’s own healing and development. Thus, tissue engineers are looking to other fields for insights, including studies of gene and protein interactions in developing tissues and regenerating wounds. Incorporating these findings with advanced culture systems is helping us to better control the responses of cells outside the body, but more progress is needed.
Source of Information : Scientific American(2009-05)