0031-3998/08/6305-0472 Vol. 63, No. 5, 2008 PEDIATRIC RESEARCH Printed in U.S.A. Copyright © 2008 International Pediatric Research Foundation, Inc. Stem Cells and Tissue Engineering Applications of the Genitourinary Tract JONATHAN L. YAMZON, PAUL KOKOROWSKI, AND CHESTER J. KOH Department of Urology [J.Y., P.K.], University of Southern California, Keck School of Medicine, Los Angeles, California 90033; Childrens Hospital Los Angeles, Division of Pediatric Urology [C.J.K.], University of Southern California, Keck School of Medicine, Los Angeles, California 90027 ABSTRACT: The field of regenerative medicine continues to make native tissue rehabilitation, or by development of functional substantial advancements in therapeutic strategies addressing uro- reproducible tissue substitutes with minimal immunogenicity logic diseases. Tissue engineering borrows principles from the fields and which resemble native tissues in biologic and mechanical of cell biology, materials science, transplantation and engineering in properties. an effort to repair or replace damaged tissues. This review is intended When autologous tissue is lacking, other possible sources of to provide a current overview of the use of stem cells and tissue tissue include homologous tissues from cadavers or donors, engineering technologies specifically in the treatment of genitouri- nary diseases. Current themes in the field include the use of adult heterologous tissues from animals (bovine), and synthetic stem cells seeded onto biocompatible resorbable matrices for implan- materials (silicone, polyurethane, Teflon, poly(lactic acid), tation as tissue substitutes, which is conducive to host tissue in- poly(glycolic acid), and poly(lactic-co-glycolic acid)), which growth. Injection therapy of adult stem cells for organ rehabilitation are often referred to as alloplastic materials. The use of is also making strong headway toward the restoration of organ allogeneic tissues, such as with organ transplantation, can be structure and function. With new data describing the molecular limited by the need for tissue matching, donor organ avail- mechanisms for differentiation, work has begun on targeting tissues ability, and lifelong immunosuppression. On the other hand, for regeneration by genetic modification methods. Promising labora- biocompatible and structurally similar alloplastic materials tory discoveries portend the emergence of a new class of clinical can be used for prosthetics and other clinical devices. How- therapies for regenerative medicine applications in the genitourinary tract. (Pediatr Res 63: 472–477, 2008) ever, functional recovery to the caliber of the original tissue is seldom achieved. TE offers the potential to circumvent many of these difficulties. he field of tissue engineering (TE) has evolved substan- TE approaches can be classified into two categories: acel- Ttially over the past four decades into an international area lular and cellular techniques. Acellular techniques entail the of science that is being investigated in virtually every country use of acellular matrices as a scaffold for organ regeneration, in the world. Early advances in the field were the result of requiring the host organ to incorporate new tissue onto the groundbreaking discoveries of the pioneers in the regenerative scaffold with proper layering and orientation. Collagen-rich medicine field including Joseph and Charles Vacanti, Robert matrices can be completely synthesized in the laboratory for Langer, and Eugene Bell. In fact, it was Joseph and Charles subsequent cellular in-growth. These matrices slowly degrade Vacanti who first used the term “Tissue Engineering” (1). They and eventually are replaced by host extracellular matrix in- eloquently described the interplay required by cells, scaffolds, vested with in-growing cells. Scaffolds can also be harvested and added growth factors in the microenvironment of mechano- from other autologous, allogeneic, or xenogenic tissues, and transducing bioreactors to develop cellular constructs that could then processed by chemical and mechanical means to remove ultimately serve as functional tissues suitable for transplantation. cellular components for eventual implantation (8–10). The field of TE has exponentially grown in size such that it now Common cellular techniques employ the use of donor cells, claims its own international academic society, Tissue Engineer- which are processed before implantation. These cells can be ing Regenerative Medicine International Society, which the ex- directly injected into the host, or expanded and processed in panding community of scientists and physicians in the field have culture, seeded onto a support matrix or scaffold, and then steadfastly supported (1–4). As of today, the principles of TE are implanted into the recipient. Tissue or cell sources can vary being applied widely to create new tissue constructs in virtually from autologous, allogenic (same species, different individ- every organ system. ual), or heterologous (such as bovine), with the most preferred In general, the field of TE combines the principles of cell source being autologous so as to eliminate the risk of rejection biology, materials science, and engineering to devise thera- and associated complications of immunosuppression. For au- peutic strategies in various acquired and congenital diseases tologous sources, a tissue sample can be obtained from the (5–7). From a clinical perspective, the goal of regenerative patient by biopsy (10–15). Improvements in culture tech- medicine efforts is to restore end organ function, either by Abbreviations: hESC, human embryonic stem cells; NIH, National Insti- Received October 31, 2007; accepted January 4, 2008. Correspondence: Chester J. Koh, M.D., Childrens Hospital Los Angeles, 4650 Sunset tutes of Health; SIS, small intestine submucosa; SUI, stress urinary inconti- Boulevard, Mailstop 114, Los Angeles, CA 90027; e-mail: [email protected] nence; TE, tissue engineering 472 STEM CELLS AND ENGINEERING IN UROLOGY 473 niques have enabled the isolation of individual cell types from these cells is challenging due to their propensity to form these tissue biopsies, which are then selectively expanded to teratomas in vivo (24,25). amounts sufficient for implantation (6,9,16,17). Similar to The harvest of hESC requires the destruction of human acellular techniques, the scaffold material must be biocompat- embryos and has raised significant ethical and political con- ible, bioresorbable, and illicit minimal immunogenicity while cerns. In August 2001, the United States federal government expanded cellular components integrate into the local envi- ordered that only previously generated human embryonic stem ronment. These scaffold structures may be seeded or impreg- cell lines could be used in research supported by federal nated with growth factors and other cell signaling peptides to funding. Although over 70 existing cell lines met this criteria, regulate cell activity and mimic the microenvironment provided the National Institutes of Health reported that only 11 were by the extracellular matrix. Ideal scaffolds should also provide the available, most of which were grown on mouse feeder cells appropriate three-dimensional lattice where cell-adhesion may and were at one point in time potentially exposed to murine occur while performing the mechanical functions of the damaged viruses or proteins (26). These barriers to the development of tissues. Thus, the final steps of the regenerative process occur in hESC technologies have prompted the search for alternative vivo (18–20). stem cell sources including fetal tissues, parthenogenesis, Over the past two decades, research in TE techniques and amniotic fluid-derived stem cells, somatic cell nuclear trans- stem cell tissue sources has led to potentially viable replace- fer, and adult multipotent stem cells. These are briefly men- ments for a variety of genitourinary tissues including ureter, tioned below, and extensive discussion on these topics can be bladder, prostate, urethra, external urinary sphinctor, and pe- found elsewhere (27–35). nile structures. Multipotent stem cells are harvested from adult organs or developing tissues, thus avoiding any controversy surrounding STEM CELLS hESC. They can be extracted from many different tissues including bone marrow, striated muscle, fat, skin, testicle, and Many current strategies for TE rely on the presence of autol- synovial membrane. Adult-derived stem cells are gaining ogous tissue samples from which specific cells types can be popularity as researchers are finding a more extensive differ- isolated, expanded, and seeded onto a matrix for subsequent entiation potential than previously thought and were fre- reimplantation. However, in instances of severe end organ failure quently used in the studies below (29–31). or neoplasia, retrieval of normal cells is often problematic. The In addition, several multipotent or pluripotent stem cell ability of stem cells to expand and differentiate into desired tissue populations derived from fetal tissues have been shown to types makes them an attractive alternative cell source for regen- produce a number of lineages including bone marrow, hepatic, erative medicine applications. Novel therapeutic strategies are and neural tissues. Fetal mesenchymal stem cells do not emerging and utilize stem cells as the primary cellular component express human leukocyte antigen class II antigens and are of various TE constructs. thought to exist in a preimmune state. Both differentiated and Stem cells are