Point-of-Care Adipose-Derived Stromal Vascular Fraction Cell Isolation and Expanded Polytetrafluoroethylene Graft Sodding

Adipose-derived stromal vascular fraction (SVF) cell populations are being evaluated for numerous clinical applications. The current study evaluated a point-of-care technology, the Tissue Genesis "TGI 1000" Cell Isolation System™, to perform an automated isolation of adipose-derived SVF cells to be used in the fabrication of a tissue-engineered vascular graft in the operating room. A total of seven patients were enrolled in this study and received femoral to tibial expanded polytetrafluoroethylene bypass grafts to treat peripheral arterial disease. Lipoaspiration of fat was performed on five patients, and the fat sample was processed immediately in the automated system in the operating room. The mean processing time, from the point of fat delivery into the instrument to removal of the SVF-containing syringe, was 70 min. The SVF cell population was evaluated for cell yield, cell viability, endotoxin levels, and microbial contamination. Samples of the SVF preparation were further subjected to microbiologic evaluation both microscopically before implantation of the graft and through a microbiologic screening using aerobic and anaerobic culture conditions. Mean cell yield was 1E5 cells per cc of fat, and endotoxin levels were below the FDA recognized standards. All SVF preparations were released for graft preparation, and the intimal surface of 90-cm-long grafts was pressure sodded with cells at a concentration of 2E5 cells/cm². The sodded grafts (n = 5) and control grafts (n = 2) were immediately implanted and graft patency assessed for 1 year. One year patency was 60% for sodded grafts and 50% for control grafts. Automated preparation of autologous adipose-derived SVF cells for immediate use to create cellular linings on vascular grafts is feasible and safe.

The Neointima Formed in Endothelial Cell Sodded ePTFE Vascular Grafts Results from Both Cellular-Hyperplasia and Extracellular-Hypertrophy

Endothelial cell transplantation onto polymeric vascular grafts results in the formation of a neointima. The formation of this neointima is often suggested to result from a chronic cellular hyperplasia where the terms intimal hyperplasia and intimal thickening are used interchangeably. While the formation of a midgraft neointima in sodded grafts involves a level of cell proliferation, the synthesis and deposition of extracellular matrix proteins is also a ubiquitous observation in these grafts. To assess the composition of midgraft neointima in sodded grafts, a morphometric method was developed to provide a differential quantitation of the cellular-hyperplastic and extracellular-hypertrophic elements of intimal thickening. The formed neointima on microvessel endothelial cell sodded and control (noncell-treated) ePTFE vascular grafts was quantified after 3, 12, and 52 wk of graft implantation in a canine carotid artery model. Midgraft sections of grafts were evaluated for both intimal thickness (IT) and cell density per unit volume and quantified using a PC-based image analysis program. Sodded grafts explanted at 3 wk exhibited an average neointimal cell density (3 × 109 cells/cm3; IT 30 μm) equivalent to cell densities observed in normal arterial media. After 12 wk the mean cell density approached a hyperplastic value (3.7 × 109 cells/cm3; IT 76 μm), while grafts explanted after 52 wk exhibited a mean cell density (2.8 × 109 cells/cm3; IT 30 μm) similar to 3-wk values. Control grafts that received no cells exhibited no midgraft cellular coverage. These results indicate that neointima formation in the midgraft region of sodded grafts occurred via mechanisms involving both a cellular hyperplasia and an extracellular hypertrophy. Differential responses occur presumably due to localized differences in cellular proliferation and cellular biosynthetic activity.

Adipose stromal vascular fraction cells isolated using an automated point of care system improve the patency of expanded polytetrafluoroethylene vascular grafts

We evaluated the use of an automated, point-of-care instrument to derive canine adipose stromal vascular fraction cells, and the subsequent deposition of these cells onto the luminal surface of an expanded polytetrafluoroethylene (ePTFE) vascular graft for use as a bypass graft. The hypothesis evaluated was that an instrument requiring minimal user interface will provide a therapeutic dose of cells to improve the patency of synthetic vascular grafts in an autologous animal model of graft patency. The stromal vascular fraction (SVF) cells were isolated using an automated adipose tissue processing and cell isolation system and cells sodded onto the surface of an ePTFE vascular graft. Control grafts, used off-the-shelf without cell treatment were used as a control to assess patency effects. Each animal received a control, untreated graft implanted in one carotid artery, and the cell-treated graft implanted in the carotid artery on the contralateral side. The grafts were implanted for 6 months utilizing 12 animals. Results indicate a fully automated adipose tissue processing system will consistently produce functional autologous cells for immediate use in the operating room. Cell-sodded polymeric grafts exhibited improved patency compared to control grafts after 6 month implantation in the canine carotid artery model.

Accelerated neovascularization and endothelialization of vascular grafts promoted by covalently bound laminin type 1

Development of a small diameter (<6 mm) synthetic vascular graft with clinically acceptable patency must overcome the inherent thrombogenicity of polymers and the development of neointimal thickening. Establishment of an endothelial cell lining on the lumenal surface has been hypothesized as a mechanism to improve the function of vascular grafts. The major aim of this study is to evaluate the use of laminin type 1, covalently bound to all surfaces of expanded polytetrafluoroethylene (ePTFE) grafts, on neovascularization of the interstices and lumenal surface endothelialization. One millimeter i.d. vascular grafts were surface modified through covalent attachment of laminin type 1. Grafts were subsequently implanted as interpositional aortic grafts in rats. Following 5-weeks implantation, the grafts were explanted and morphologically evaluated using scanning electron microscopy and light microscopy. Scanning electron microscopy identified an extensive coverage of antithrombogenic cells on the lumenal flow surface of laminin type 1 modified grafts. Histological evaluation confirmed the presence of endothelial cells on the midgraft lumenal surface of laminin 1 modified grafts. Extensive neovascularization of the interstices of the laminin-modified grafts occurred as compared with control grafts. We conclude that surface modification using laminin type 1 accelerates both the neovascularization and endothelialization of porous ePTFE vascular grafts.

Prophylactic gatifloxacin therapy in prevention of bacterial keratitis in a rabbit laser in situ keratomileusis model

PURPOSE

Use the ID(50) (infectious dose to 50% of experimental animals) to quantify the most effective prophylactic dosing regimen to use with gatifloxacin 0.3% (Zymar) for the prevention of keratitis in a rabbit laser in situ keratomileusis model of Staphylococcus epidermidis infection.

SETTING

University Laboratory, University of Arizona, Tucson, Arizona, USA.

METHODS

Two groups of rabbits were compared in each of 2 experiments that were separated by 12 months. In the first experiment, rabbits receiving no postoperative antibiotic therapy (Group 1) were compared with rabbits receiving postoperative antibiotic therapy (Group 2). In the second experiment, postoperative antibiotic therapy (Group 3) was compared with preoperative and postoperative antibiotic therapy (Group 4). All antibiotic regimens used gatifloxacin 0.3%. Before antibiotic therapy began, corneal pockets were created in the right eye of each rabbit and all rabbits received balanced salt solution (BSS) only or BSS and S epidermidis inoculations in the corneal pocket. Rabbits were monitored for corneal infiltrates after surgery.

RESULTS

The ID(50) of the first, second, third, and fourth groups of rabbits was 10(2), 10(4), 10(5), and 10(7) organisms, respectively. The data showed a statistically significant difference between rabbits receiving BSS only and most rabbits receiving BSS plus inoculate at each postoperative measurement (P<.05).

CONCLUSIONS

The findings suggest that the use of both preoperative and postoperative antibiotic therapy may be most effective in preventing infection. Postoperative antibiotic therapy increased the number of S epidermidis necessary to cause infection by at least 100-fold over no antibiotic intervention. Preoperative plus postoperative antibiotic therapy increased the number of bacteria necessary to cause infection by at least 100-fold over postoperative therapy alone and by more than 10000-fold over no antibiotic intervention.

Covalent modification of porous implants using extracellular matrix proteins to accelerate neovascularization

Healing associated with many polymeric biomedical implants commonly involves the formation of an avascular fibrous capsule. The lack of either formation or persistence of blood vessels in formed fibrous capsules, as well as a lack of new blood vessels within porous polymeric implants, often results in poor performance of the implant. The current study evaluated the use of extracellular matrix protein modification of a commonly used biomedical implant material, expanded polytetrafluoroethylene (ePTFE), as a mechanism to increase the neovascularization both within these porous implants and in tissue that forms in the peri-implant area. Discs of ePTFE were covalently modified with different extracellular matrix proteins including collagen type IV, fibronectin, and laminin type I. Discs were implanted into the adipose tissue of adult rats, and following a 5-week implant phase, histologic analysis of peri-implant tissue angiogenesis and implant neovascularization was performed. Striking differences were observed in angiogenic and neovascularization responses to matrix-modified ePTFE when compared with control, untreated ePTFE. Fibronectin treatment resulted in an extensive inflammatory response but, relative to the degree of inflammation, limited evidence of tissue angiogenesis or polymer neovascularization. Collagen type IV treatment groups exhibited a significant increase in angiogenesis in the peri-implant tissue with minimal evidence of implant neovascularization. In contrast to all other implant modifications, laminin type 1-treated ePTFE samples stimulated an extensive peri-implant tissue angiogenic response and a coordinate neovascularization of the porous interstices of the biomaterial.

Dual Porosity Expanded Polytetrafluoroethylene for Soft-Tissue Augmentation

BACKGROUND

A variety of nondegradable polymers have been evaluated for use as soft-tissue augmentation devices. This study compared a novel dual porosity expanded polytetrafluoroethylene with current, clinically used devices.

METHODS

Studies were performed in a porcine model of soft-tissue healing with both histologic evaluations and determination of biomechanical strength of tissue incorporation. Five different samples of expanded polytetrafluoroethylene were used in this study. Control devices were clinically available soft-tissue augmentation devices manufactured by W. L. Gore and Associates (Newark, Del.). Atrium Medical Corporation (Hudson, NH) manufactured three test devices with modified porosities. A total of 12 animals were used with implant evaluations performed after 1, 3, 6, and 12 months.

RESULTS

Significant differences in tissue incorporation were observed morphologically with the dual porosity material, including reduced inflammation and increased cellular and extracellular matrix incorporation of the material. Significant increases in both angiogenesis (new vessel formation in the peri-implant tissue) and neovascularization (blood vessel penetration into the interstices of the implants) were observed with the dual porosity expanded polytetrafluoroethylene material.

CONCLUSIONS

This novel dual porosity expanded polytetrafluoroethylene is associated with reduced inflammation and more extensive tissue incorporation as compared with the currently available form. These results suggest a dual porosity expanded polytetrafluoroethylene may provide a superior material for soft-tissue augmentation.

Characterization of angiogenesis and inflammation surrounding ePTFE implanted on the epicardium

The response of epicardial tissue to the implantation of expanded polytetrafluoroethylene (ePTFE) was evaluated and compared with identical material implanted within subcutaneous and adipose tissues. These two tissue environments were selected for comparison with epicardial implants because they represent tissue often involved in device implantation. Discs of ePTFE (6 mm) were implanted into three different tissue sites in Sprague-Dawley rats. At 5 weeks, polymers and surrounding tissues were harvested and processed for light microscopy. General histology and histochemistry data indicated all polymers to be well incorporated with new tissue. Subcutaneous implants were covered by a dense fibrous capsule (55-70 microm). Epicardial and adipose implants had no fibrous capsule and a significantly greater number of microvessels (arterioles, capillaries, and venules) within the surrounding tissues compared with subcutaneous implants. An increased level of inflammation was also observed around epicardial implants compared with the other implants. Additionally, the new vasculature surrounding epicardially implanted ePTFE revealed an altered microvessel density and vessel type distribution compared with normal (control) epicardium. These results suggest that epicardial tissue responds to implanted ePTFE with a robust inflammatory response that may support the formation of a new microvasculature that is uniquely different from the native epicardial microvasculature.

Cellular proliferation and macrophage populations associated with implanted expanded polytetrafluoroethylene and polyethyleneterephthalate

The chronic inflammatory response associated with the abluminal surface of polymeric vascular grafts has been suggested to affect adversely graft neovascularization, the cellular response at the luminal surface of vascular grafts, and overall graft patency. To better understand the source for this chronic inflammation, this study examined two types of macrophages and the amount of cellular proliferation around two widely used graft materials, expanded polytetrafluoroethylene (ePTFE) and polyethyleneterephthalate (PET or Dacron) implanted in the rat for 3 and 5 weeks. Serial sections of explants were analyzed for recruited macrophages (ED1), resident macrophages (ED2), and proliferating cells (PCNA). Results show that Dacron is more inflammatory than ePTFE and that there is a segregated macrophage response; the first 54 micrometer of perigraft tissue were composed predominantly of recruited macrophages (ED1+) while the more distal tissue consisted of resident macrophages (ED2+). Proliferating cells were located predominantly in this same 54 micrometer perigraft region. In subcutaneous tissue they accounted for 23% of all cells present around Dacron after 3 weeks of implantation and 8% after 5 weeks. Conversely, cellular proliferation around ePTFE increased from 4% at 3 weeks to 21% at 5 weeks. In adipose tissue, proliferation levels around the implanted polymers were lower and more similar after 3 and 5 weeks. Serial sections revealed the coordinate expression of PCNA and ED1 antigens by the same individual cells, suggesting that proliferation is a mechanism used to perpetuate the chronic inflammatory response. These results suggest a new target for designing treatments to alter inflammation and improve the healing associated with these biomaterials.

Anastomotic tissue response associated with expanded polytetrafluoroethylene access grafts constructed by using nonpenetrating clips

PURPOSE

The gross, light microscopic, and scanning microscopic appearance of arterial and venous anastomoses in expanded polytetrafluoroethylene (ePTFE) access grafts constructed with nonpenetrating clips were compared with that of those constructed with polypropylene suture. We hypothesized that clip-constructed anastomoses would provide controlled approximation of native vessel intimal and medial components with the ePTFE grafts. We further hypothesized that anastomotic healing with clips would involve primarily an intimal cellular response, as compared with suture-constructed anastomoses in which cells within the media and adventitia walls participate.

METHODS

Femoral artery to femoral vein arteriovenous (AV) grafts were constructed in five dogs using 4-mm internal diameter ePTFE graft material. Each animal received one AV graft with anastomoses constructed by using polypropylene sutures in one leg and one AV graft with anastomoses constructed with Vascular Closure System clips in the contralateral leg. Animals were given aspirin for the duration of the study, and grafts were explanted at 5 weeks. At the time of explantation, graft segments were grossly evaluated and then underwent light and scanning electron microscopic analysis.

RESULTS

At the time of explantation, all access grafts were patent. Joining the ePTFE grafts to the native vessels with clips resulted in minimal vessel wall damage. The lumenal contours of the discontinuous approximation were smooth and without gross endothelial disruption. These observations are in contrast to the lumenal compromise and endothelial disturbance associated with the sutured anastomoses. Furthermore, hemostasis was achieved immediately in the clipped grafts, decreasing the incidence of perianastomic hematoma. Finally, cellular reconstitution occurred at the anastomotic cleft in both the sutured and the clipped junctions. The neointima exhibited an endothelial cell lining on the lumenal surface and the presence of alpha-smooth muscle cell actin positive cells within the subendothelial layer.

CONCLUSION

Vascular Closure System clips are a viable alternative to suture for the approximation of ePTFE AV access grafts to native blood vessels. The use of the clips resulted in a more streamlined anastomosis, with decreased vessel wall damage, immediate hemostasis, and a trend toward shorter procedure times.

Inflammation and neovascularization associated with clinically used vascular prosthetic materials

This study was designed to evaluate and compare healing characteristics, specifically neovascularization and inflammation, of polymeric vascular graft materials commonly used in clinical applications. Our hypotheses were (i) polymeric materials used in vascular graft manufacture stimulate chronic inflammation and (ii) inflammation and neovascularization of polymeric materials are related. Impra and Gore-Tex ePTFE, Meadox weavenit and woven Dacron, Hemashield microvel and woven Dacron, and Golaski microknit Dacron were implanted as 6-mm diameter disks within rat subcutaneous and adipose tissue. Following 5 weeks of implantation samples were evaluated by histological and immunocytochemical analysis. Sections were stained using hematoxylin and eosin or reacted with ED1 antibody and GS1 lectin to quantify inflammation and neovascularization. respectively. The extent of inflammation and neovascularization were influenced by both tissue site of implantation and polymer characteristics. For subcutaneous implants, inflammation was graded as follows: Meadox weavenit > Hemashield woven > Meadox woven > Gore-Tex ePTFE > Hemashield microvel > ImpraePTFE > Golaski microknit, while only the Golaski microknit neovascularized. Inflammation was graded as follows for adipose implants: Hemashield woven > Hemashield microvel > Meadox weavenit > Meadox woven > Gore-Tex ePTFE > Golaski microknit > Imnpra ePTFE, while the following order of neovascularization was observed: Impra ePTFE > Gore-Tex ePTFE > Golaski microknit. The degree of inflammation following biomnaterial implantation has a profound effect on implant neovascularization. These data suggest an inverse relationship exists between inflammation and neovascularization.

Differential healing and neovascularization of ePTFE implants in subcutaneous versus adipose tissue

The preclinical evaluation of polymer biocompatibility is often performed using animal subcutaneous implant models. The choice of subcutaneous tissue as the implant site is due to a number of factors including simplicity of the surgery involved. Results from subcutaneous implants cannot necessarily be extrapolated to other tissues due to the differences in cellular composition of tissues. We have evaluated and compared the healing characteristics of expanded polytetrafluoroethylene (ePTFE) discs implanted in either subcutaneous tissue or epididymal fat pad tissue in rats. Following 3 and 5 weeks of implantation, the healing characteristics of discs were evaluated histologically with particular emphasis on tissue and polymer neovascularization. Implants placed in subcutaneous tissue exhibited limited formation of new microvascular elements within and directly in contact with the polymer, and the formation of an extensive fibrous capsule. In contrast, ePTFE implanted in the epididymal fat pads of rats exhibited extensive neovascularization of tissue surrounding the polymer, penetration of these microvascular cells into the graft interstices for distances < or = 100 microns and no morphological evidence of a fibrous capsule. The rat epididymal fat pad provides an alternative tissue for polymer healing evaluations. Due to the extensive presence of fat in subcutaneous tissue in humans, we suggest the fat pad model provides a more relevant preclinical evaluation of the healing characteristics of polymers used clinically in anatomic positions which contain significant amounts of fat.

The effects of porosity on endothelialization of ePTFE implanted in subcutaneous and adipose tissue

Healing of biomaterial implants varies depending on the type and structure of material and the tissue surrounding the implant. In this study we examined structural differences of 30 microm, 60 microm, and 100 microm expanded polytetrafluoroethylene (ePTFE) using scanning electron microscopy, and we also investigated differences in healing for these three different porosity ePTFE grafts implanted within subcutaneous tissue and adipose tissue. Scanning electron microscopic examination of 30 microm, 60 microm, and 100 microm ePTFE revealed structural differences and differences in fiber density within the internodal space. Circular patches (6 mm in diameter) of 30 microm ePTFE were implanted within subcutaneous tissue and epididymal fat pads of male Sprague-Dawley rats. After 5 weeks, the implants were removed and analyzed for fibrous capsule formation, endothelialization, and for activated monocytes and macrophages in association with the material. Histological evaluation revealed dense fibrous capsule formation surrounding only the 30 microm ePTFE subcutaneous implants. From immunohistochemistry data obtained, we generated an Endothelialization Index (measure of neovascularization) and a Monocyte/Macrophage Index (measure of inflammatory response) for each sample. Consistently, 60 microm ePTFE had the greatest Endothelialization Index at both implant sites while 100 microm ePTFE generally had the largest values for the Monocyte/Macrophage Index. These data indicate that both the structure of the material and the site of implant influence the healing characteristics of ePTFE and suggest that activated monocytes and/or macrophages associated with the implant may inhibit endothelialization of ePTFE.

The neointima formed in endothelial cell sodded ePTFE vascular grafts results from both cellular-hyperplasia and extracellular-hypertrophy

Endothelial cell transplantation onto polymeric vascular grafts results in the formation of a neointima. The formation of this neointima is often suggested to result from a chronic cellular hyperplasia where the terms intimal hyperplasia and intimal thickening are used interchangeably. While the formation of a midgraft neointima in sodded grafts involves a level of cell proliferation, the synthesis and deposition of extracellular matrix proteins is also a ubiquitous observation in these grafts. To assess the composition of midgraft neointima in sodded grafts, a morphometric method was developed to provide a differential quantitation of the cellular-hyperplastic and extracellular-hypertrophic elements of intimal thickening. The formed neointima on microvessel endothelial cell sodded and control (noncell-treated) ePTFE vascular grafts was quantified after 3, 12, and 52 wk of graft implantation in a canine carotid artery model. Midgraft sections of grafts were evaluated for both intimal thickness (IT) and cell density per unit volume and quantified using a PC-based image analysis program. Sodded grafts explanted at 3 wk exhibited an average neointimal cell density (3 x 10(9) cells/cm3; IT 30 microns) equivalent to cell densities observed in normal arterial media. After 12 wk the mean cell density approached a hyperplastic value (3.7 x 10(9) cells/cm3; IT 76 microns), while grafts explanted after 52 wk exhibited a mean cell density (2.8 x 10(9) cells/cm3; IT 30 microns) similar to 3-wk values. Control grafts that received no cells exhibited no midgraft cellular coverage. These results indicate that neointima formation in the midgraft region of sodded grafts occurred via mechanisms involving both a cellular hyperplasia and an extracellular hypertrophy. Differential responses occur presumably due to localized differences in cellular proliferation and cellular biosynthetic activity.

Endothelial cell transplantation onto polymeric arteriovenous grafts evaluated using a canine model

Prosthetic arteriovenous grafts (AVG) placed for hemodialysis access fail in humans due to the thrombogenicity of the flow surface and development of cellular intimal hyperplasia, particularly at the venous anastomosis. The poor patency rates of prosthetic AVG result in significant morbidity and mortality in dialysis patients. Consequently, investigators have been evaluating methods to improve the patency of prosthetic grafts by examining endothelial cell transplantation as a means of creating an antithrombogenic lining on artificial polymers. A canine model was developed to study the effects of cell transplantation of autologous, fat-derived microvessel endothelial cells (MVEC) onto the luminal surface of expanded polytetrafluoroethylene (ePTFE) grafts. Microvessel endothelial cells were isolated from falciform ligament fat, with each dog receiving its own endothelial cells. Isolated cells were subsequently placed into the lumen of the graft (4 mm by 20 cm ePTFE). The graft lumen was pressurized to 5 pounds per square inch (psi) resulting in the partial denucleation of the graft, due to the flow of buffer into the interstices of the graft, and the forced deposition of cells onto the luminal surface. Animals were maintained on aspirin and persantine during the implant phase. During the implant phase, grafts were evaluated by both duplex ultrasound and magnetic resonance angiography (MRA). At explant, gross observation of the sodded grafts revealed a glistening white flow surface with no evidence of thrombosis. Morphologic and scanning electron microscopic evaluations revealed the presence of a cellular lining on the luminal flow surface that exhibited characteristics of antithrombogenic endothelial cells. Midgraft samples were evaluated by immunocytochemistry and indicated that cells on the luminal surface react positively with antibodies to von Willebrand factor. Results from this study demonstrate that the canine model provides an excellent method of studying the effects of MVEC sodding on the thrombogenicity and hyperplastic response of prosthetic arteriovenous graft.

Origin of endothelial cells that line expanded polytetrafluorethylene vascular grafts sodded with cells from microvascularized fat

PURPOSE

Cell transplantation onto prosthetic vascular grafts remains an attractive technique to reduce the thrombogenicity of polymeric materials. In this study we evaluated whether autologous cells isolated from falciform ligament fat and transplanted onto the lumenal surface of 4 mm expanded polytetrafluorethylene grafts were the same cells present on the surface of these grafts when they were explanted from canine carotid arteries 3 weeks after their implantation.

METHODS

The fluorescent dye PKH-26 was used to label transplanted cells to evaluate their fate after implantation of grafts as carotid artery replacements. This fluorescent dye homogeneously labeled all cells in the primary cell isolate.

RESULTS

In vitro studies indicated that dye labeling was nontoxic, as evidenced by the normal growth characteristics of fluorescently labeled cells compared with nonlabeled cells. Immunocytochemical analysis of microvascularized fat before cell isolation determined that approximately 90% of the cells stained positive for von Willebrand factor2. At the time of explant, seeded grafts exhibited a nonthrombogenic lumenal cell lining as evidenced by the lack of adherent platelets or fibrin. Cells on the lumenal surface of grafts exhibited PKH-26 fluorescence emission. In addition, these cells expressed von Willebrand factor and actively sequestered DiI-acetylated low-density lipoprotein.

CONCLUSIONS

We conclude that sodding of prosthetic grafts with autologous microvascularized fat-derived cells results in the formation of an endothelial cell lining on the lumenal flow surface. These endothelial cells are the same cells placed on the lumenal surface of the graft at the time of initial cell transplantation. Finally, a confluent monolayer forms after high-density cell sodding by the process of cell adherence and spreading, without the need for cell proliferation.