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Decellularization

Decellularization (also spelt decellularisation in British English) is the process used in biomedical engineering to isolate the extracellular matrix (ECM) of a tissue from its inhabiting cells, leaving an ECM scaffold of the original tissue, which can be used in artificial organ and tissue regeneration. Organ and tissue transplantation treat a variety of medical problems, ranging from end organ failure to cosmetic surgery. One of the greatest limitations to organ transplantation derives from organ rejection caused by antibodies of the transplant recipient reacting to donor antigens on cell surfaces within the donor organ. Because of unfavorable immune responses, transplant patients suffer a lifetime taking immunosuppressing medication. Stephen F. Badylak pioneered the process of decellularization at the McGowan Institute for Regenerative Medicine at the University of Pittsburgh. This process creates a natural biomaterial to act as a scaffold for cell growth, differentiation and tissue development. By recellularizing an ECM scaffold with a patient’s own cells, the adverse immune response is eliminated. Nowadays, commercially available ECM scaffolds are available for a wide variety of tissue engineering. Using peracetic acid to decellularize ECM scaffolds have been found to be false and only disinfects the tissue. Decellularization (also spelt decellularisation in British English) is the process used in biomedical engineering to isolate the extracellular matrix (ECM) of a tissue from its inhabiting cells, leaving an ECM scaffold of the original tissue, which can be used in artificial organ and tissue regeneration. Organ and tissue transplantation treat a variety of medical problems, ranging from end organ failure to cosmetic surgery. One of the greatest limitations to organ transplantation derives from organ rejection caused by antibodies of the transplant recipient reacting to donor antigens on cell surfaces within the donor organ. Because of unfavorable immune responses, transplant patients suffer a lifetime taking immunosuppressing medication. Stephen F. Badylak pioneered the process of decellularization at the McGowan Institute for Regenerative Medicine at the University of Pittsburgh. This process creates a natural biomaterial to act as a scaffold for cell growth, differentiation and tissue development. By recellularizing an ECM scaffold with a patient’s own cells, the adverse immune response is eliminated. Nowadays, commercially available ECM scaffolds are available for a wide variety of tissue engineering. Using peracetic acid to decellularize ECM scaffolds have been found to be false and only disinfects the tissue. With a wide variety of decellularization-inducing treatments available, combinations of physical, chemical, and enzymatic treatments are carefully monitored to ensure that the ECM scaffold maintains the structural and chemical integrity of the original tissue. Scientists can use the acquired ECM scaffold to reproduce a functional organ by introducing progenitor cells, or adult stem cells (ASCs), and allowing them to differentiate within the scaffold to develop into the desired tissue. The produced organ or tissue can be transplanted into a patient. In contrast to cell surface antibodies, the biochemical components of the ECM are conserved between hosts, so the risk of a hostile immune response is minimized. Proper conservation of ECM fibers, growth factors, and other proteins is imperative to the progenitor cells differentiating into the proper adult cells. The success of decellularization varies based on the components and density of the applied tissue and its origin. The applications to the decellularizing method of producing a biomaterial scaffold for tissue regeneration are present in cardiac, dermal, pulmonary, renal, and other types of tissues. Complete organ reconstruction is still in the early levels of development. Researchers are able to take the tissue from a donor or cadaver, lyse and kill the cells within the tissue without damaging the extracellular components, and finish with a product that is the natural ECM scaffold that has the same physical and biochemical functions of the natural tissue. After acquiring the ECM scaffold, scientists can recellularize the tissue with potent stem or progenitor cells that will differentiate into the original type of tissue. By removing the cells from a donor tissue, the immunogenic antibodies from the donor will be removed. The progenitor cells can be taken from the host, therefore they will not have an adverse response to the tissue. This process of decellularizing tissues and organs is still being developed, but the exact process of taking a tissue from a donor and removing all the cellular components is considered to be the decellularization process. The steps to go from a decellularized ECM scaffold to a functional organ is under the umbrella of recellularization. Because of the diverse applications of tissue in the human body, decellularization techniques have to be tailored to the specific tissue being exercised on. The researched methods of decellularization include physical, chemical, and enzymatic treatments. Though some methods are more commonly used, the exact combination of treatments is variable based on the tissue’s origin and what it is needed for. As far as introducing the different liquidized chemicals and enzymes to an organ or tissue, perfusion and immersion decellularization techniques have been used. Perfusion decellularization is applicable when an extensive vasculature system is present in the organ or tissue. It is crucial for the ECM scaffold to be decellularized at all levels, and evenly throughout the structure. Because of this requirement, vascularized tissues can have chemicals and enzymes perfused through the present arteries, veins, and capillaries. Under this mechanism and proper physiological conditions, treatments can diffuse equally to all of the cells within the organ. The treatments can be removed through the veins at the end of the process. Cardiac and pulmonary decellularization often uses this process of decellularization to introduce the treatments because of their heavily vascularized networks. Immersion decellularization is accomplished through the submersion of a tissue in chemical and enzymatic treatments. This process is more easily accomplished than perfusion, but is limited to thin tissues with a limited vascular system. The most common physical methods used to lyse, kill, and remove cells from the matrix of a tissue through the use of temperature, force and pressure, and electrical disruption. Temperature methods are often used in a rapid freeze-thaw mechanism. By quickly freezing a tissue, microscopic ice crystals form around the plasma membrane and the cell is lysed. After lysing the cells, the tissue can be further exposed to liquidized chemicals that degrade and wash out the undesirable components. Temperature methods conserve the physical structure of the ECM scaffold, but are best handled by thick, strong tissues. Direct force of pressure to a tissue will guarantee disruption of the ECM structure, so pressure is commonly used. Pressure decellularization involves the controlled use of hydrostatic pressure applied to a tissue or organ. This is done best at high temperatures to avoid unmonitored ice crystal formation that could damage the scaffold. Electrical disruption of the plasma membrane is another option to lyse the cells housed in a tissue or organ. By exposing a tissue to electrical pulses, micropores are formed at the plasma membrane. The cells eventually turn to death after their homeostatic electrical balance is ruined through the applied stimulus. This electrical process is documented as Non-thermal irreversible electroporation (NTIRE) and is limited to small tissues and the limited possibilities of inducing an electric current in vivo. The proper combination of chemicals is selected for decellularization depending on the thickness, extracellular matrix composition, and intended use of the tissue or organ. For example, enzymes would not be used on a collagenous tissue because they disrupt the connective tissue fibers. However, when collagen is not present in a high concentration or needed in the tissue, enzymes can be a viable option for decellularization. The chemicals used to kill and remove the cells include acids, alkaline treatments, ionic detergents, non-ionic detergents, and zwitterionic detergents. The ionic detergent, sodium dodecyl sulfate (SDS), is commonly used because of its high efficacy for lysing cells without significant damage to the ECM. Detergents act effectively to lyse the cell membrane and expose the contents to further degradation. After SDS lyses the cell membrane, endonucleases and exonucleases degrade the genetic contents, while other components of the cell is solubilized and washed out of the matrix. SDS is commonly used even though it has a tendency to slightly disrupt the ECM structure. Akaline and acid treatments can be effective companions with an SDS treatment due to their ability to degrade nucleic acids and solubilize cytoplasmic inclusions. The most well known non-ionic detergent is Triton X-100, which is popular because of its ability to disrupt the interactions between lipids and between lipids and proteins. Triton X-100 does not disrupt protein-protein interactions, which is beneficial to keeping the ECM intact. EDTA is a chelating agent that binds calcium, which is a necessary component for proteins to interact with one another. By making calcium unavailable, EDTA prevents the integral proteins between cells from binding to one another. EDTA is often used with trypsin, an enzyme that acts as a protease to cleave the already existing bonds between integral proteins of neighboring cells within a tissue. Together, the EDTA-Trypsin combination make a good team for decellularizing tissues.

[ "Tissue engineering", "Extracellular matrix", "Scaffold", "acellular scaffold", "decellularized matrix", "extracellular matrix scaffold", "decellularized scaffold", "Tissue Decellularization" ]
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