Delivery of Molecular Agents in Tissue Engineering

The previous chapter provided some examples of tissue engineering, in which cells that were isolated and engineered outside of the body are introduced into a patient by direct injection of a cell suspension, typically into the circulatory system. But the field of tissue engineering also points to treatments that are conceptually different from variations on cell transfusion technology; tissue engineering promises the regrowth of adult tissue structure through application of engineered cells and synthetic materials. In support of this broad claim, the field of tissue engineering can point to some initial successes. For example, synthetic materials are now available that accelerate healing of burns and skin ulcers. In addition, in vitro cell culture methods now allow the amplification of a patient’s own cells for cartilage repair or bone marrow transplantation. But major obstacles to the widespread application of tissue engineering remain. Tissue engineers have not yet learned how to reproduce complex tissue architectures, such as vascular networks, which are essential for the normal function of many tissues. In fact, the tissue engineering concepts that have been demonstrated in the laboratory to date involve arrangements of cells and materials into precursor tissues (or neotissues) that develop according to natural processes that are already present within the cells or the materials at the time of implantation. These methods may be suitable for production of some tissues in which either the structure is relatively homogeneous (such as cartilage, in which a tissue structure can reform after the implantation of chondrocytes into a tissue defect) or the structure develops naturally (such as in some tissue-engineered skin, in which the stratified epithelium develops naturally by culturing at an air–liquid interface). The engineering of many tissue structures—such as the branching architectures found in many tissues or the intricate network architecture of the nervous system—will probably require methods for introducing and changing molecular signals during the process of neo-tissue development. For example, it is well known that chemical gradients of factors known as morphogens induce the formation of structures during development; some of the attributes of morphogens were introduced in Chapter 3.