Tissue engineering.

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human health care. A new field, tissue engineering, applies the principles of biology and engineering to the development of functional substitutes for damaged tissue. This article discusses the foundations and challenges of this interdisciplinary field and its attempts to provide solutions to tissue creation and repair.

Bioengineered dental tissues grown in the rat jaw.

Our long-term objective is to develop methods to form, in the jaw, bioengineered replacement teeth that exhibit physical properties and functions similar to those of natural teeth. Our results show that cultured rat tooth bud cells, seeded onto biodegradable scaffolds, implanted into the jaws of adult rat hosts and grown for 12 weeks, formed small, organized, bioengineered tooth crowns, containing dentin, enamel, pulp, and periodontal ligament tissues, similar to identical cell-seeded scaffolds implanted and grown in the omentum. Radiographic, histological, and immunohistochemical analyses showed that bioengineered teeth consisted of organized dentin, enamel, and pulp tissues. This study advances practical applications for dental tissue engineering by demonstrating that bioengineered tooth tissues can be regenerated at the site of previously lost teeth, and supports the use of tissue engineering strategies in humans, to regenerate previously lost and/or missing teeth. The results presented in this report support the feasibility of bioengineered replacement tooth formation in the jaw.

Engineering of implantable cartilaginous structures from bone marrow-derived mesenchymal stem cells.

Fabrication of implantable cartilaginous structures that could be secured in the joint defect could provide an alternative therapeutic approach to prosthetic joint replacement. Herein we explored the possibility of using biodegradable hydrogels in combination with a polyglycolic acid (PGA) scaffold to provide an environment propitious to mesenchymal stem cells (MSCs) chondrogenic differentiation. We examined the influence of type I collagen gel and alginate combined with PGA meshes on the extracellular matrix composition of tissue-engineered transplants. MSCs were isolated from young rabbits, expanded in monolayers, suspended in each hydrogel, and loaded on PGA scaffolds. All constructs (n=48) were cultured in serum-free medium containing transforming growth factor beta-1, under dynamic conditions in specially designed bioreactors for 3-6 weeks. All cell-polymer constructs had a white, shiny aspect, and retained their initial size and shape over the culture period. Their thickness increased substantially over time, and no shrinkage was observed. All specimens developed a hyalin-like extracellular matrix containing glycosaminoglycans (GAGs) and type II collagen, but significant differences were observed among the three different groups. In PGA/MSCs and collagen-PGA/MSCs constructs, the cell growth phase and the chondrogenic differentiation phase of MSCs occurred during the first 3 weeks. In alginate-PGA/MSCs constructs, cells remained round in the hydrogel and cartilage extracellular matrix deposition was delayed. However, at 6 weeks, alginate-PGA/MSCs constructs exhibited higher contents of GAGs and lower contents of type I collagen. These results suggest that the implied time for the transplantation of in vitro engineered constructs depends, among other factors, on the nature of the scaffold envisioned. In this study, we demonstrated that the use of a composite hydrogel-PGA scaffold supported the in vitro growth of implantable cartilaginous structures cultured in a bioreactor system.

Integrin binding and cell spreading on extracellular matrix act at different points in the cell cycle to promote hepatocyte growth.

This study was undertaken to determine the importance of integrin binding and cell shape changes in the control of cell-cycle progression by extracellular matrix (ECM). Primary rat hepatocytes were cultured on ECM-coated dishes in serum-free medium with saturating amounts of growth factors (epidermal growth factor and insulin). Integrin binding and cell spreading were promoted in parallel by plating cells on dishes coated with fibronectin (FN). Integrin binding was separated from cell shape changes by culturing cells on dishes coated with a synthetic arg-gly-asp (RGD)-peptide that acts as an integrin ligand but does not support hepatocyte extension. Expression of early (junB) and late (ras) growth response genes and DNA synthesis were measured to determine whether these substrata induce G0-synchronized hepatocytes to reenter the growth cycle. Cells plated on FN exhibited transient increases in junB and ras gene expression (within 2 and 8 h after plating, respectively) and synchronous entry into S phase. Induction of junB and ras was observed over a similar time course in cells on RGD-coated dishes, however, these round cells did not enter S phase. The possibility that round cells on RGD were blocked in mid to late G1 was confirmed by the finding that when trypsinized and replated onto FN-coated dishes after 30 h of culture, they required a similar time (12-15 h) to reenter S phase as cells that had been spread and allowed to progress through G1 on FN. We have previously shown that hepatocytes remain viable and maintain high levels of liver-specific functions when cultured on these RGD-coated dishes. Thus, these results suggest that ECM acts at two different points in the cell cycle to regulate hepatocyte growth: first, by activating the G0/G1 transition via integrin binding and second, by promoting the G1/S phase transition and switching off the default differentiation program through mechanisms related to cell spreading.

Extracellular matrix controls tubulin monomer levels in hepatocytes by regulating protein turnover.

Cells have evolved an autoregulatory mechanism to dampen variations in the concentration of tubulin monomer that is available to polymerize into microtubules (MTs), a process that is known as tubulin autoregulation. However, thermodynamic analysis of MT polymerization predicts that the concentration of free tubulin monomer must vary if MTs are to remain stable under different mechanical loads that result from changes in cell adhesion to the extracellular matrix (ECM). To determine how these seemingly contradictory regulatory mechanisms coexist in cells, we measured changes in the masses of tubulin monomer and polymer that resulted from altering cell-ECM contacts. Primary rat hepatocytes were cultured in chemically defined medium on bacteriological petri dishes that were precoated with different densities of laminin (LM). Increasing the LM density from low to high (1-1000 ng/cm2), promoted cell spreading (average projected cell area increased from 1200 to 6000 microns2) and resulted in formation of a greatly extended MT network. Nevertheless, the steady-state mass of tubulin polymer was similar at 48 h, regardless of cell shape or ECM density. In contrast, round hepatocytes on low LM contained a threefold higher mass of tubulin monomer when compared with spread cells on high LM. Furthermore, similar results were obtained whether LM, fibronectin, or type I collagen were used for cell attachment. Tubulin autoregulation appeared to function normally in these cells because tubulin mRNA levels and protein synthetic rates were greatly depressed in round cells that contained the highest level of free tubulin monomer. However, the rate of tubulin protein degradation slowed, causing the tubulin half-life to increase from approximately 24 to 55 h as the LM density was lowered from high to low and cell rounding was promoted. These results indicate that the set-point for the tubulin monomer mass in hepatocytes can be regulated by altering the density of ECM contacts and changing cell shape. This finding is consistent with a mechanism of MT regulation in which the ECM stabilizes MTs by both accepting transfer of mechanical loads and altering tubulin degradation in cells that continue to autoregulate tubulin synthesis.

Early in vivo experience with tissue-engineered trileaflet heart valves.

BACKGROUND: Tissue engineering is a new approach in which techniques are being developed to transplant autologous cells onto biodegradable scaffolds to ultimately form new functional autologous tissue. Workers at our laboratory have focused on tissue engineering of heart valves. The present study was designed to evaluate the implantation of a whole trileaflet tissue-engineered heart valve in the pulmonary position in a lamb model. METHODS AND RESULTS: We constructed a biodegradable and biocompatible trileaflet heart valve scaffold that was fabricated from a porous polyhydroxyalkanoate (pore size 180 to 240 microm; Tepha Inc). Vascular cells were harvested from ovine carotid arteries, expanded in vitro, and seeded onto our heart valve scaffold. With the use of cardiopulmonary bypass, the native pulmonary leaflets were resected, and 2-cm segments of pulmonary artery were replaced by autologous cell-seeded heart valve constructs (n=4). One animal received an acellular valved conduit. No animal received any anticoagulation therapy. Animals were killed at 1, 5, 13, and 17 weeks. Explanted valves were examined histologically with scanning electron microscopy, biochemically, and biomechanically. All animals survived the procedure. The valves showed minimal regurgitation, and valve gradients were <20 mm Hg on echocardiography. The maximum gradient was 10 mm Hg with direct pressures. Macroscopically, the tissue-engineered constructs were covered with tissue, and there was no thrombus formation on any of the specimens. Scanning electron microscopy showed smooth flow surfaces during the follow-up period. Histological examination demonstrated laminated fibrous tissue with predominant glycosaminoglycans as extracellular matrix. 4-Hydroxyproline assays demonstrated an increase in collagen content as a percentage of native pulmonary artery (1 week 45.8%, 17 weeks 116%). DNA assays showed a comparable number of cells in all explanted samples. There was no tissue formation in the acellular control. CONCLUSIONS: Tissue-engineered heart valve scaffolds fabricated from polyhydroxyalkanoates can be used for implantation in the pulmonary position with an appropriate function for 120 days in lambs.

Functional living trileaflet heart valves grown in vitro.

BACKGROUND: Previous tissue engineering approaches to create heart valves have been limited by the structural immaturity and mechanical properties of the valve constructs. This study used an in vitro pulse duplicator system to provide a biomimetic environment during tissue formation to yield more mature implantable heart valves derived from autologous tissue. METHODS AND RESULTS: Trileaflet heart valves were fabricated from novel bioabsorbable polymers and sequentially seeded with autologous ovine myofibroblasts and endothelial cells. The constructs were grown for 14 days in a pulse duplicator in vitro system under gradually increasing flow and pressure conditions. By use of cardiopulmonary bypass, the native pulmonary leaflets were resected, and the valve constructs were implanted into 6 lambs (weight 19+/-2.8 kg). All animals had uneventful postoperative courses, and the valves were explanted at 1 day and at 4, 6, 8, 16, and 20 weeks. Echocardiography demonstrated mobile functioning leaflets without stenosis, thrombus, or aneurysm up to 20 weeks. Histology (16 and 20 weeks) showed uniform layered cuspal tissue with endothelium. Environmental scanning electron microscopy revealed a confluent smooth valvular surface. Mechanical properties were comparable to those of native tissue at 20 weeks. Complete degradation of the polymers occurred by 8 weeks. Extracellular matrix content (collagen, glycosaminoglycans, and elastin) and DNA content increased to levels of native tissue and higher at 20 weeks. CONCLUSIONS: This study demonstrates in vitro generation of implantable complete living heart valves based on a biomimetic flow culture system. These autologous tissue-engineered valves functioned up to 5 months and resembled normal heart valves in microstructure, mechanical properties, and extracellular matrix formation.

Developmental analysis and computer modelling of bioengineered teeth.

Here we present the developmental progression of bioengineered pig teeth from 1 to 25 weeks of development. We demonstrate that 2-25 week implants contained embryonic tooth bud- and cap-stage tooth structures consisting of dental epithelium expressing the sonic hedgehog gene and condensed dental mesenchyme. Implants harvested at 18-25 weeks also contained tooth bud-like structures, as well as mature tooth structures containing enamel, dentin and pulp tissues. Immunohistochemical analyses confirmed the expression of dentin- and enamel-specific proteins in differentiated bioengineered tooth tissues. Three-dimensional computer modelling further demonstrated a spatial organization of enamel, dentin and pulp tissues resembling that of natural teeth. We conclude that bioengineered teeth commonly exhibit morphological stages characteristic of naturally forming teeth. Furthermore, the presence of immature tooth buds at all times assayed and increased numbers of bioengineered tooth structures over time suggests that porcine dental progenitor cells maintain the ability to form teeth for at least 25 weeks.

Delivery of whole liver-equivalent hepatocyte mass using polymer devices and hepatotrophic stimulation.

Using hepatocytes injected into prevascularized polymer sponge devices, we studied hepatocyte survival and function after delivery of a whole liver-equivalent of cells into rats. LEW rats and enzyme-deficient Gunn rats served as recipients, respectively. Totally, 28.5 cm2 (0.5-cm thick) of polyvinyl alcohol sponges were implanted per animal. Hepatotrophic stimulation was induced by portacaval shunt and partial (70% or 30%) hepatectomy. Recipient rats received 5 x 10(8) hepatocytes (equivalent to whole rat liver) which were harvested from LEW and Wistar donors, respectively. After engraftment, histologic examination revealed hepatocyte remodeling in the device with capillaries lining plates of hepatocytes, and also tubular structures resembling early biliary radicles. BrdU staining revealed DNA synthesis in hepatocytes, providing evidence of regeneration within the grafts. Quantification of viable hepatocyte area at various time points was performed using computer-assisted morphometry. We then estimated a range of cell numbers from the quantitated cell area. The number of hepatocytes viable at day 7 was estimated at 27.5-46.0% and 6.6-11.0% in the mesentery and subcutaneous site, respectively. Thus the average number was estimated between 10.8% and 18.0% of initially injected hepatocytes. In the Gunn rat experiment, experimental rats that received normal Wistar hepatocytes showed a significantly greater decrease in total serum bilirubin compared with the concurrent control Gunn rats (P < 0.01). At week 1, serum bilirubin in experimental rats decreased to 74.7% (6.80 +/- 0.46 mg/dl) of pretransplantation level (9.10 +/- 0.47 mg/dl) and this was 71.4% of the control rats' bilirubin level (9.53 +/- 0.37 mg/dl). In conclusion, a hepatocyte mass equivalent to a whole rat liver can be delivered into prevascularized polymer sponge devices. At day 7 between 10.8% and 18.0% of these hepatocytes were estimated to be engrafted and functioning. Further optimization of this technique is necessary before clinical application is considered.

Outcome of subcutaneous islet transplantation improved by polymer device.

Syngeneic transplantation of rat islets into subcutaneous tissue failed to cure streptozocin diabetes. The reason is unknown, but poor vascularization may play a role. We hypothesize that if a well-vascularized subcutaneous site could be created, islet grafts would do well. Four hundred freshly isolated mouse islets were transplanted syngeneically under the renal capsule or into the intraperitoneal cavity and compared with 800 islets in subcutaneous tissue of streptozocin-diabetic mice. Four weeks after transplantation, 14 of 14 under the renal capsule, 4 of 8 in the intraperitoneal site, and 0 of 7 in the subcutaneous tissue site achieved normoglycemia. To create vascularized organoids, we transplanted 800 mouse islets into polyvinyl alcohol (PVA) or polyglycolic acid (PGA) polymers in subcutaneous tissue of streptozocin-diabetic mice either immediately (four in PVA and six in PGA) or 7 days (four in PVA and four in PGA) after implantation. Four weeks after transplantation, the mean blood glucose level and body weight had no change with PVA. However, the mean body weight increased significantly with PGA and 3/10 became normoglycemic. When transplanting 400 islets with PGA polymers intraperitoneally, all animals (n=5) remained hyperglycemic 3 months later. In contrast, four of five recipients transplanted with 800 islets with PGA polymers subcutaneously became normoglycemic. The grafts from successful animals contained numerous revascularized islets containing a substantial amount of insulin. These preliminary results indicate that subcutaneous islet transplantation using PGA polymers can improve the metabolic status and, in some cases, even cure diabetes in streptozocin-diabetic mice.

Cell transplantation of genetically altered cells on biodegradable polymer scaffolds in syngeneic rats.

Many severe metabolic deficiencies in children are caused by a single gene defect with a resultant single gene product deficiency. These diseases may be amenable to permanent cure using new techniques of gene transfer and cell transplantation. In many in vivo models of retroviral mediated gene therapy, a significant limiting factor is the ability to transplant a sufficient number of modified cells. To potentially circumvent this problem, we have developed a biodegradable polymer implant system capable of supporting large numbers of genetically modified cells. In this study, we inserted a reporter gene into syngeneic cultured normal fibroblasts and then transplanted these genetically modified cells into animals using synthetic biodegradable polymer fibers as temporary cell delivery scaffolds. To begin to develop a system capable of delivering desirable proteins secreted by genetically modified cells, Fischer 344 adult rat fibroblasts were transduced in tissue culture with a retrovirus containing the reporter gene Lac Z. These genetically modified cells (1.1 x 10(7) cells/graft) were then attached to the biodegradable polymer fibers and the polymer-cell graft was transplanted subdermally into syngeneic recipients (n = 9). There was persistence of the modified cells with expression of the reporter gene for at least 30 days. The estimated number of genetically modified cells per implanted graft decreased from a pretransplant value of 1.1 +/- 0.6 x 10(7) to 3.2 +/- 0.7 x 10(6) by 15 days after transplantation (P < 0.01). Thereafter, the cell number did not vary significantly to the conclusion of the study at day 30 (3.6 +/- 1.0 x 10(6) cells/graft). Evidence of ingrowth and incorporation of other stromal elements was present in the graft by 1 week post-transplantation, as judged by counterstained hematoxylin and eosin micrograph sections. Migration of modified cells to areas outside of the polymer-cell graft was not detected. Over the course of the study, there was little degradation of the polymer implant, although by day 30, evidence of early dissolution was evident. The number of polymer fibers per high power field increased slightly from 62.5 +/- 5.8 on day 1 to 77.3 +/- 26.6 on day 30 (P > 0.2). These data suggest that the use of biodegradable polymer fibers may permit the transplantation of genetically modified cells in sufficient numbers to deliver a therapeutically useful product. Polymer matrices allow for the attachment and site-specific transplantation of genetically modified cells.

Long-term follow-up of tissue-engineered intestine after anastomosis to native small bowel.

BACKGROUND: Our laboratory has investigated the fabrication of a tissue-engineered intestine using biodegradable polymer scaffolds. Previously we reported that isolated intestinal epithelial organoid units on biodegradable polymer scaffolds formed cysts and the neointestine was successfully anastomosed to the native small bowel. The purpose of this study was to observe the development of tissue-engineered intestine after anastomosis and to demonstrate the effect of the anastomosis over a 9-month period. METHODS: Microporous biodegradable polymer tubes were created from polyglycolic acid. Intestinal epithelial organoid units were harvested from neonatal Lewis rats and seeded onto the polymers, which were implanted into the abdominal cavity of adult male Lewis rats followed by 75% small bowel resection (n=24). Three weeks after implantation, the unit/polymer constructs were anastomosed to the native jejunum in a side-to-side fashion. The anastomosed tissue-engineered intestine was measured by laparotomy 10, 24, and 36 weeks after the implantation (n= 14). During the laparotomy, all rats with an obstruction in their anastomosis were killed and excluded from the statistical analysis. Another five rats were also killed at 10 and 36 weeks for histological and morphometric studies. RESULTS: All analyzed rats survived this study and significantly increased their body weight by 36 weeks. Obstruction of the anastomosis was observed in one rat at 24 weeks and in two rats at 36 weeks; however, the anastomosis was patent in the other 11 rats by 36 weeks. The tissue-engineered intestine of these 11 rats increased in length and diameter at 10, 24, and 36 weeks after anastomosis; there were statistically significant differences between each time point except between the length of 10 and 24 weeks (P<0.016 by Wilcoxon signed rank test). Histologically the inner surface of the tissue-engineered intestine was lined with well-developed neomucosa at 10 and 36 weeks; however, there were small bare areas lacking neomucosa in the tissue-engineered intestine at 36 weeks. Morphometric analysis demonstrated no significant differences in villus number, villus height, and surface length of the neomucosa at 10 and 36 weeks. CONCLUSIONS: Anastomosis between tissue-engineered intestine and native small bowel resulted in no complications after operation and maintained a high patency rate for up to 36 weeks. The tissue-engineered intestine increased in size and was lined with well-developed neomucosa for the duration of the study.

Regenerative signals for intestinal epithelial organoid units transplanted on biodegradable polymer scaffolds for tissue engineering of small intestine.

BACKGROUND: Our laboratory is investigating the tissue engineering of small intestine using intestinal epithelial organoid units seeded onto highly porous biodegradable polymer tubes. This study investigated methods of stimulation for optimizing neointestinal regeneration. METHODS: Intestinal epithelial organoid units harvested from neonatal Lewis rats were seeded onto porous biodegradable polymer tubes and implanted into the omentum of adult Lewis rats in the following groups: (1) the control group (group C), implantation alone (n=9); (2) the small bowel resection (SBr) group, after 75% SBr (n=9); (3) the portacaval shunt (PCS) group, after PCS (n=8); and (4) the partial hepatectomy (PH) group, after 75% PH (n=8). Neointestinal cyst size was recorded using ultrasonography. Constructs were harvested at 10 weeks and were examined using histology. Morphometric analysis of the neomucosa was obtained using a computer image analysis program (NIH Image, version 1.59). RESULTS: Cyst development was noted in all animals. Cyst lengths and diameters were significantly larger in the SBr group at 7 and 10 weeks compared with the other three groups (P<0.05; analysis of variance [ANOVA], Fisher's protected least significant difference). Histology revealed a well-vascularized tissue with a neomucosa lining the lumen with invaginations resembling crypt-villus structures. Morphometric analysis demonstrated a significantly greater villus number, height, area, and mucosal surface in the SBr group compared with the other three groups and a significantly greater crypt number and area in the PCS group compared with group C (P<0.05; ANOVA, Fisher's protected least significant difference). CONCLUSIONS: Intestinal epithelial organoid units transplanted on porous biodegradable polymer tubes can successfully vascularize, survive, and regenerate into complex tissue resembling small intestine. SBr and, to a lesser extent, PCS provide significant regenerative stimuli for the morphogenesis and differentiation of tissue-engineered small intestine.

Successful anastomosis between tissue-engineered intestine and native small bowel.

BACKGROUND: Previous work from this laboratory has shown that isolated intestinal epithelial organoid units on porous biodegradable polymer scaffolds formed vascularized cysts lined by a neomucosa. The purpose of this study was to demonstrate anastomosis between tissue-engineered intestine and the native small bowel and to observe the effect of this anastomosis on cyst growth. METHODS: Intestinal epithelial organoid units from neonatal Lewis rats were seeded onto porous biodegradable polymer tubes made of polyglycolic acid, and they were implanted into the omentum of adult male Lewis rats. Three weeks after implantation, the unit-polymer constructs were anastomosed in a side-to-side fashion to the native jejunum in 20 rats (group 1). The other 18 rats were closed without anastomosis (group 2). All 38 tissue-engineered constructs were harvested 10 weeks after implantation. Four rats underwent upper gastrointestinal (GI) study before they were killed. RESULTS: The rats in group 1 increased their body weights equal to those in group 2, and there was no statistically significant difference between the two groups. Upper GI examinations revealed no evidence of either bowel stenosis or obstruction at the anastomotic site. Grossly, the patency of the anastomosis was 90% and the lumen of the cyst was visualized by the upper GI study. At the second operation, there was no significant difference in the size of the cysts in either group: however, at the time the rats were killed, the length of the cysts in group 1 was significantly longer than that in group 2 (P<0.05 using Mann-Whitney U test). Histological examination showed that cysts after anastomosis were lined by a neomucosa in continuity to native small bowel across the anastomotic site and also demonstrated crypt-villus structures. Morphometric study demonstrated that cysts in group 1 had significantly greater villus number, height, and surface length than did those in group 2. CONCLUSIONS: Anastomosis between tissue-engineered intestine and native small bowel resulted in no complications after the operation, kept a high patency rate, and maintained mucosal continuity between the tissue-engineered intestine and native small bowel. Furthermore, anastomosis had a positive effect on cyst size and development of the mucosa in the tissue-engineered intestine.

Hepatocyte transplantation using biodegradable matrices in ascorbic acid-deficient rats: comparison with heterotopically transplanted liver grafts.

BACKGROUND: Hepatocyte transplantation using polymeric matrices is under investigation as an alternative therapy for metabolic liver diseases. Long-term engraftment of hepatocytes in polymers has been demonstrated. However, the metabolic activity of hepatocytes in such devices has never been assessed in direct comparison with liver grafts. METHODS: Hepatocyte and partial liver transplantation were evaluated in the scurvy-prone osteogenic disorder Shionogi rat model. Biodegradable poly glycolic acid matrices seeded with hepatocytes equivalent to 20% of the recipient's liver mass, or 20% liver grafts were heterotopically transplanted into ascorbic acid- (AsA) deficient recipients. Recipients of cell-free matrices or AsA-deficient liver grafts served as controls. Recipients were set on AsA-free diet after transplantation. Plasma AsA levels, AsA concentrations in liver and adrenal gland tissue, and body weight ratios were assessed and H&E histology was performed. RESULTS: Recipients from the control groups showed symptoms of scurvy at 1 month after cessation of AsA supply. Hepatocyte transplantation and auxiliary liver transplantation prevented symptoms of scurvy and increased plasma and tissue AsA levels and body weight ratios. AsA levels in recipients of 20% liver grafts were comparable to normal control animals. CONCLUSIONS: Hepatocytes transplanted in polymeric matrices are able to compensate for liver-based metabolic deficiencies. Hepatocyte transplantation improves plasma AsA levels in AsA-deficient recipients. However, auxiliary liver grafts are superior to hepatocyte grafts in improving metabolic parameters. Further research work is needed to increase the efficiency of liver cell transplantation with regard to a clinical application.

A regional experience with emergency liver transplantation.

Liver transplantation for patients requiring life-support results in the lowest survival and highest costs. A ten year (1983-1993) regional experience with liver transplantation for critically ill patients was undertaken to ascertain the fate of several subgroups of patients. Of the 828 liver transplants performed at six transplant centers within the region over this period, 168 (20%) were done in patients who met today's criteria for a United Network of Organ Sharing (UNOS) status 1 (emergency) liver transplant candidate. Recipients were classified according to chronicity of disease and transplant number (primary-acute, primary-chronic, reTx-acute, reTx-chronic). Overall one-year survival was 50% for all status 1 recipients. The primary-acute subgroup (n = 63) experienced a 57% one-year survival compared with 50% for the primary-chronic (n = 51) subgroup (P = 0.07). Of the reTx-acute recipients (n = 43), 44% were alive at one year in comparison with 20% for the reTx-chronic (n = 11) group (P = 0.18). There was no significant difference in survival for the following: transplant center, blood group compatibility with donors, age, preservation solution, or graft size. For patients retransplanted for acute reasons (primary graft nonfunction (PGNF) or hepatic artery thrombosis [HAT]), survival was significantly better if a second donor was found within 3 days of relisting (52% vs. 20%; P = 0.012). Over the study period progressively fewer donor organs came from outside the region. No strong survival-based argument can be made for separating, in allocation priority, acute and chronic disease patients facing the first transplant as a status 1 recipient. Clearly patients suffering from PGNF or HAT do far better if retransplanted within 3 days. Establishing an even higher status for recipients with PGNF, perhaps drawing from a supraregional donor pool, would allow surgeons to accept more marginal donors, thus potentially expanding the pool, without significantly compromising patient survival. Retransplantation of the recipient with a chronically failing graft who deteriorates to the point of needing life-support is nearly futile, and in today's health care climate, not an optimal use of scarce donor livers.

Hepatic retransplantation in New England--a regional experience and survival model.

Hepatic retransplantation (reTx) offers the only alternative to death for patients who have failed primary hepatic transplantation (PTx). Assuming a finite number of donor organs, reTx also denies the chance of survival for some patients awaiting PTx. The impact of reTx on overall survival (i.e., the survival of all candidates for transplantation) must therefore be clarified. Between 1983 and 1991, 651 patients from the New England Organ Bank underwent liver transplantation, and 73 reTx were performed in 71 patients (11% reTx rate). The 1-year actuarial survival for reTx (48%) was significantly less than for PTx (70%, P < 0.05). This survival varied, dependent on the interval of time following PTx in which the reTx was performed (0-3 days, 57% survival; 4-30 days, 24%; 30-365 days, 54%; and > 365 days, 83%). Patients on the regional waiting list had an 18% mortality rate while awaiting transplantation. These results were incorporated into a mathematical model describing survival as a function of reTx rate, assuming a limited supply of donor livers. ReTx improves the 1-year survival rate for patients undergoing PTx but decreases overall survival (survival of all candidates) for liver transplantation. In the current era of persistently insufficient donor numbers, strategies based on minimizing the use of reTx, especially in the case of patients in whom chances of success are minimal, will result in the best overall rate of patient survival.

Extracorporeal membrane oxygenation and conventional medical therapy in neonates with persistent pulmonary hypertension of the newborn: a prospective randomized study.

Thirty-nine newborn infants with severe persistent pulmonary hypertension and respiratory failure who met criteria for 85% likelihood of dying were enrolled in a randomized trial in which extracorporeal membrane oxygenation (ECMO) therapy was compared with conventional medical therapy (CMT). In phase I, 4 of 10 babies in the CMT group died and 9 of 9 babies in the ECMO group survived. Randomization was halted after the fourth CMT death, as planned before initiating the study, and the next 20 babies were treated with ECMO (phase II). Of the 20, 19 survived. All three treatment groups (CMT and ECMO in phase I and ECMO, phase II) were comparable in severity of illness and mechanical ventilator support. The overall survival of ECMO-treated infants was 97% (28 of 29) compared with 60% (6 of 10) in the CMT group (P less than .05).

Role of extracorporeal membrane oxygenation in selected pediatric respiratory problems.

Between 1984 and 1988, 89 infants and children with severe respiratory failure were supported by extracorporeal membrane oxygenation. Major clinical diagnoses included congenital diaphragmatic hernias (34), meconium aspiration syndrome (26), and sepsis (8). Extracorporeal membrane oxygenation was used for patients with a predicted mortality rate of at least 80% based on an oxygenation index greater than 0.4. Venoarterial bypass was accomplished by way of right cervical cannulation of the common carotid artery and internal jugular vein. Overall survival was 71% but varied widely by diagnosis and progressively improved over time. The average extracorporeal membrane oxygenation run was 5.7 days. Intracranial hemorrhage was the most serious complication occurring in 16% of patients. Mechanical circuit complications were seen in 22% but rarely related to significant morbidity. Extracorporeal membrane oxygenation appears to provide effective cardiopulmonary support for selected pediatric respiratory problems. It affords those with potentially reversible pathophysiology the temporal opportunity for successful medical or surgical therapies.