In vitro systems for tissue engineering.

Tissue engineering, by necessity, encompasses a wide array of experimental directions and scientific disciplines. In vitro tissue engineering involves the manipulation of cells in vitro, prior to implantation into the in vivo environment. In contrast, in vivo tissue engineering relies on the body's natural ability to regenerate over non-cell-seeded biomaterials. Cells, biomaterials, and controlled incubation conditions all play important roles in the construction and use of modern in vitro systems for tissue engineering. Gene delivery is also an important factor for controlling or supporting the function of engineered cells both in vitro and post implantation, where appropriate. In this review, systems involved in the context of in vitro tissue engineering are addressed, including bioreactors, cell-seeded constructs, cell encapsulation, and gene delivery. Emphasis is placed upon investigations that are more directly linked to the treatment of clinical conditions.