Basic Principles Of Tissue Engineering

The source of donor tissue can be heterologous, allogenic, or autologous; autologous is preferred as this method avoids tissue rejection by the immune system and immunosuppressive drugs can be avoided. When autologous cells are used, a biopsy is obtained from the host and the cells are dissociated and expanded, and later returned to the same host as new tissue that is not immunogenic.

Tissue engineering "entails" the use of donor tissue that is dissociated into individual cells. The cells are either implanted directly into the host, or expanded in culture, attached to a biodegradable support matrix, and reimplanted after expansion (1-3).

In its early phases, cell-based tissue engineering was limited by the inability to grow specific cell types in sufficient quantities for implantation. Many cell types, including urothelium, seemed to have a natural senescence that prevented their expansion in vitro. However, several protocols have been developed over the past two decades that have improved urothelial growth and expansion in the laboratory (4,5). Improved understanding of the privileged sites for precursor cells in specific organs and the conditions that promote differentiation has led to techniques that have overcome the difficulties of in vitro cell expansion. Cilento et al. demonstrated that

Reported studies demonstrated that it would be possible to harvest autologous bladder cells from human patients, grow and expand them in vitro, attach them to a support matrix, and use them in the same patient for reconstructive purposes.

a urothelial strain from a single specimen, initially covering a surface area of 1 cm2, could be expanded to cover an area of 4202 m2 (the equivalent area of one football field) in eight weeks.

Reported studies demonstrated that it would be possible to harvest autologous bladder cells from human patients, grow and expand them in vitro, attach them to a support matrix, and use them in the same patient for reconstructive purposes.

Bladder, ureter, and renal pelvis cells can equally be harvested, cultured, and expanded in a similar fashion. The next step is clinical application of these techniques in humans, and major advances have been achieved in the past decade, which will make the use of autologous cells in patients for expansion a possibility.

Biomaterials should provide a three-dimensional space for cells to form into new tissues; they should allow for delivery of the desired cells for tissue replacement and appropriate bioactive factors (cell adhesion peptides, growth factors) to desired sites in the body, and guide the development of new tissues with appropriate function.

The ideal biomaterial should also be biocompatible. The biomaterial should persist for an appropriate amount of time to allow for adequate replacement of normal tissue, but it should be absorbed by the host without inflammation.

Three basic classes of biomaterials have been utilized in the engineering of genitourinary organs: naturally-derived materials (e.g., collagen and alginate), acellular tissue matrices (e.g., bladder submucosa and small intestinal submucosa), and synthetic polymers [e.g., polyglycolic acid, polylactic acid, and poly(lactic-co-glycolic acid)].

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