Principles of living organ reconstruction by tissue engineering

Tissue engineering is a novel sector arising from the biomaterial field, which is developing rapidly as a result of the dramatic cIinicalneed for organ replacement,since there is unfortunately an ever-growing lack of organs for transplantation. Various approaches are presently being developed in different laboratories and companies based on the utilization of biomaterials, extracellular matrix components, and cellsto produce substitutesto aJlowthe replacement of wounded or diseased tissues[1-4]. Theorgan reconstructionby tissue engineering presented in this chapter are of living tissues. This concept entails that the various cells incorporated in our constructs or tissues are not only readily dividing, but also metabolically active. Thus, mesenchymal cells (fibroblasts, smooth muscle cells) incorporated into the stromal component of these substitutes are also significantly involved in the reorganization of the extracellular matrix. Furthermore, the interactions between the mesenchymal cells and the epithelial cells improve the very nature, structure, and function of the resulting organ. Lastly, the presence of living cells, within the in vitro engineered tissues, adds the benefit of tissue remodeling and healing after transplantation in vivo. The source of cells that can be used for tissue reconstruction is dictated by the foreseen application. Autologous cells will be necessary for the production of living tissue substitutes when striving for permanent replacement of organs in order to prevent any histocompatibility mismatch and the ensuing predictable rejection (e.g., skin grafting for full-thickness burns). However, the rejection process has been shown to vary with the type of cells involved, and it may be possible to graft allogeneic engineered tissue under some appropriate conditions. But in such cases as keratinocytes, dentritic cells and endothelial cells that are privileged targets for rejection, autologous ceIls are necessary to permanently replace tissues encompassing these cells. ln sharp contrast, when the living tissue substitute is destined to improve wound healing, such as in the case of uIcers,allogeneic cells are sufficientsince they act as a temporary coverage, enhancing the natural healing process, and will be replaced over time by cells from the receiver. The firststep in reconstructing a living organ by tissue engineering in vitro is the isolation and culture of each cell type. The most stringent conditions must be met during this step since it has a direct impact on the quality of the desired tissue engineered product. The ideal cellsource for tissue reconstructionshouldprovide celIswith extensive proliferation potential (self-renewal capacity) and appropriate differentiation abilities (able to give rise to a differentiated progeny). Each cell culture method must be characterized in such a manner to ensure that the isolation method and culture conditions (e.g., culturé medium and growth factors) during the growth as weil as during the maturation period are the most appropriate to conserve cell purity and phenotype. This chapter wilI focus on the various approachesdevelopedover the years by the Laboratoire d'Organogénèse Expérimental (LOEX) (Hôpital du Saint-Sacrement, Chauq, Quebec) to obtain three-dimensional tissues such as reconstructed epidermis, skin, blood vessel, comea, bronchi, and ligament [2,5-13]