Stem cells in tissue engineering

insight
Nature 414, 118-121 (1 November 2001) | doi:10.1038/35102181

Stem cells in tissue engineering
Paolo Bianco1 and Pamela Gehron Robey2

In this “Stem Cells in Tissue Engineering” paper, the authors describe the diverse range of tissue that can now be engineered via stem cells, including epithelial surfaces (skin, cornea, and mucosal membranes) to skeletal tissues. The two most important factors when reconstructing tissues using stem cells include their inherently different rates of self-renewal as well as physical structures. This paper examines two specific applications of stem cells to tissue engineering—the regeneration of skin involving the structural formation of two-dimensional sheets as well as a more complex formation of bone, which involves the reconstruction of three-dimensional shapes and scaffolding.
In engineering the skin and other surfaces, permanent restoration of tissues that are characterized by high and continuous self-renewal require stem cells that are self-renewing. In most recent studies, a small but pure population of holoclone-generating cells is what is needed to generate epidermal grafts; the identification of a keratinocyte stem cell markers may provide a new tool for this approach with respect to epidermis. Success for these procedures have varied—usually with graft failure after a promising initial engraftment. In clinical trials, skin autografts are produced by culturing keratinocytes to generate an epidermal sheet, and then transplanting this sheet along with a suitable skin-like substrate. The long-term success of a skin graft depends on appropriate replenishment of stem cells in the graft. To engineer the skeleton, skeletal stem cells are needed, and can be found in bone-marrow stromal stem cells, which are able to undergo extensive replication in culture. The stem cells obtained in culture must be then combined with appropriate carriers before transplantation, which provide a three-dimensional scaffold in which a vascular bed can be established so that transplanted progenitor cells can differentiate and form a bone/marrow organ. Suitable materials that have been used include synthetic hydroxyapatite/tricalcium phosphates and polyglycolic and polylactic acids. Studies have yet to show their compatibility with long-term maintenance of stem cell properties, or the fate of bone-biomaterial composites being generated at the site of transplantation. One procedure for bone reconstruction includes loading the skeletal stem cells into appropriate carriers and transplanting it into a non skeletal site surrounding an artery and vein. It is here that they generate a vascularized segment of bone, the size and shape of which are dictated by the carrier geometry.
This paper is particularly important because it details the potential of tissue-engineering—both in its approach as well as with its usage. Specifically, researchers hope to utilize stem cell’s ability of restoration to develop continuously self-renewing tissues such as skin. The list of tissues with the potential to be engineered is growing steadily—the most important contributing factor being that there has been recent progress in stem cell biology and recognition of the unique biological properties of stem cells. Tissue engineering is not simple reconstruction of tissue, but engineering of tissue function.