Animal Models in Tissue Engineering. Part I

Translational tissue-engineered vascular grafts: From bench to bedside

Cardiovascular disease is a primary cause of mortality worldwide, and patients often require bypass surgery that utilizes autologous vessels as conduits. However, the limited availability of suitable vessels and the risk of failure and complications have driven the need for alternative solutions. Tissue-engineered vascular grafts (TEVGs) offer a promising solution to these challenges. TEVGs are artificial vascular grafts made of biomaterials and/or vascular cells that can mimic the structure and function of natural blood vessels. The ideal TEVG should possess biocompatibility, biomechanical mechanical properties, and durability for long-term success in vivo. Achieving these characteristics requires a multi-disciplinary approach involving material science, engineering, biology, and clinical translation. Recent advancements in scaffold fabrication have led to the development of TEVGs with improved functional and biomechanical properties. Innovative techniques such as electrospinning, 3D bioprinting, and multi-part microfluidic channel systems have allowed the creation of intricate and customized tubular scaffolds. Nevertheless, multiple obstacles must be overcome to apply these innovations effectively in clinical practice, including the need for standardized preclinical models and cost-effective and scalable manufacturing methods. This review highlights the fundamental approaches required to successfully fabricate functional vascular grafts and the necessary translational methodologies to advance their use in clinical practice.

Animal Models of Rotator Cuff Injury and Repair: A Systematic Review

There are a large number of animal studies on rotator cuff injury and repair, but a lack of detailed research and evaluation on the animal models. This systematic review aims to provide a framework for animal studies and repair patches for rotator cuff injury. Four hundred nine animal studies were included, of which the most common animal model of rotator cuff injury is rat (53.56%), the most common site of rotator cuff injury is the supraspinatus tendon (62.10%), and the most common injury type (degree) is acute tear (full thickness) (48.41%). The most common research purpose is to evaluate the repair effect of the patch (24.94%), followed by the observation of pathophysiological changes after rotator cuff injury (20.87%). Among the five types of repair patch materials including nondegradable and degradable synthetic materials, autologous and allogeneic tissues, and naturally derived biomaterial, the last one is the mostly used (52.74%). For different animal models, the rodent models (rat and mouse) are the most commonly used and probably the most suitable species for preliminary studies of rotator cuff injury; the rabbit, canine, sheep, and goat models are more suitable for biomechanical performance testing, rehabilitation training, and validation of surgical methods; and the nonhuman primate models (monkey and baboon) are the closest to human, but it is more difficult to carry out the animal studies on them because of ethical issues, high feeding cost, and management difficulties.

Towards Clinical Translation of In Situ Cartilage Engineering Strategies: Optimizing the Critical Facets of a Cell-Laden Hydrogel Therapy

BACKGROUND

Articular cartilage repair using implantable photocrosslinkable hydrogels laden with chondrogenic cells, represents a promising in situ cartilage engineering approach for surgical treatment. The development of a surgical procedure requires a minimal viable product optimized for the clinical scenario. In our previous work we demonstrated how gelatin based photocrosslinkable hydrogels in combination with infrapatellar derived stem cells allow the production of neocartilage in vitro. In this study, we aim to optimize the critical facets of the in situ cartilage engineering therapy: the cell source, the cell isolation methodology, the cell expansion protocol, the cell number, and the delivery approach.

METHODS

We evaluated the impact of the critical facets of the cell-laden hydrogel therapy in vitro to define an optimized protocol that was then used in a rabbit model of cartilage repair. We performed cells counting and immunophenotype analyses, chondrogenic potential evaluation via immunostaining and gene expression, extrusion test analysis of the photocrosslinkable hydrogel, and clinical assessment of cartilage repair using macroscopic and microscopic scores.

RESULTS

We identified the adipose derived stem cells as the most chondrogenic cells source within the knee joint. We then devised a minimally manipulated stem cell isolation procedure that allows a chondrogenic population to be obtained in only 85 minutes. We found that cell expansion prior to chondrogenesis can be reduced to 5 days after the isolation procedure. We characterized that at least 5 million of cells/ml is needed in the photocrosslinkable hydrogel to successfully trigger the production of neocartilage. The maximum repairable defect was calculated based on the correlation between the number of cells retrievable with the rapid isolation followed by 5-day non-passaged expansion phase, and the minimum chondrogenic concentration in photocrosslinkable hydrogel. We next optimized the delivery parameters of the cell-laden hydrogel therapy. Finally, using the optimized procedure for in situ tissue engineering, we scored superior cartilage repair when compared to the gold standard microfracture approach.

CONCLUSION

This study demonstrates the possibility to repair a critical size articular cartilage defect by means of a surgical streamlined procedure with optimized conditions.

Effects of recipient age, heparin release and allogeneic bone marrow-derived stromal cells on vascular graft remodeling

Small-caliber vascular grafts are used in a wide range of clinical conditions. However, there remains a substantial unfulfilled need for readily-available, synthetic vascular grafts with high long-term patency rate. To fulfill the translational goal for bioengineered vascular grafts, important considerations for the pre-clinical evaluation include the graft design, cell incorporation and selection of an animal model. To assess the three factors, we used vascular grafts consisting of core/shell-structured microfibers of polycaprolactone/gelatin with a thin polycaprolactone overlay. The respective influences of the heparin release mode, animal age, and allogeneic bone marrow-derived stromal cells (MSCs) seeded in the lumen on the graft remodeling were assessed after four-and-half-month implantation on an interposition graft of abdominal aorta model. Except two rats dying from graft-unrelated issues, all other rats (18 out of 20) showed good graft patency upon explantation. The cell phenotype, matrix content and structure in the neotissues around the graft, as well as the flow perfusion through the graft were examined. More grafts in the aged rats showed local narrowing and flow incongruence than the other grafts in young adult rats. Compared to acellular grafts, cellular grafts showed efficient recruitment of vascular cells to form more organized structures with elastin in the vascular wall. Endothelialization and α-smooth muscle actin-positive cells were shown in all four types of vascular grafts. This study revealed the significant effects of MSC and recipient age but not heparin release pattern on graft remodeling. STATEMENT OF SIGNIFICANCE: The vascular graft is a mainstream of surgical intervention to treat vascular diseases. Currently, vascular grafts, particularly small-diameter ones, still show high failure rates. This study has evaluated the respective impacts of heparin release pattern, allogeneic bone marrow-derived stromal cell seeding, and recipient age on the long-term remodeling of vascular grafts. There is a dearth of literature which considers the recipient age as an influencing factor for vascular grafting. However, adults particularly elderly constitute the majority of vascular graft recipients in the "real" clinical environment. While juvenile animals were widely used for graft evaluations, this study involved adult animals. The study outcomes provided important implications regarding graft designs and evaluation approaches.

Donor Cell Fate in Particulated Juvenile Allograft Cartilage for the Repair of Articular Cartilage Defects

BACKGROUND

Particulated juvenile allograft cartilage (PJAC) has demonstrated good clinical efficacy in repairing articular cartilage defects, but the related repair mechanism after transplant and the biological characteristics of the transplanted cells are still unclear.

PURPOSE

To study the efficacy of PJAC in repairing full-thickness cartilage defects and the specific fate of donor cells to provide experimental evidence for its clinical application.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twenty female Guizhou minipigs were randomly divided into an experimental group and a control group. An 8-mm cylindrical full-thickness cartilage defect was created in the femoral trochlea of 1 knee in all minipigs. The experimental group received transplant of PJAC from 5 male juvenile Guizhou minipigs (PJAC group; n = 10) and the control group received autologous cartilage chips (ACC group; n = 10). Follow-up assessments were conducted at 1 month and 3 months to track the transplanted cells by the male-specific sex-determining region Y-linked (SRY) gene; tissue sections were hybridized in situ, and O'Driscoll histological scoring was performed according to hematoxylin and eosin staining, safranin O and fast green staining, and toluidine blue O staining, as well as immunohistochemical evaluation of aggrecan and Sry-type HMG-box 9 (SOX9).

RESULTS

All 20 Guizhou minipigs were followed; no infection or incision healing disorder occurred after the operation. By SRY in situ hybridization, the SRY signal of the transplanted cells was positive in the repaired tissue of the defect, and the SRY positive signal could still be detected in repaired tissue at 3 months postoperatively. The average number of positive cells was 68.6 ± 11.91 at 1 month and 32.6 ± 3.03 at 3 months (confocal microscope: ×400), and the difference was statistically significant. The O'Driscoll histological scores were 14 ± 0.71 in the ACC group and 9.8 ± 0.84 in the PJAC group at 1 month, and 18 ± 1.20 in the ACC group and 17.4 ± 1.14 in the PJAC group at 3 months. The scores were statistically significant between the ACC group and PJAC group at 1 month. The positive rates of SOX9 in the PJAC and ACC groups at 1 month were 67.6% ± 3.78% and 63.4% ± 5.30%, respectively, and the difference was not statistically significant (P > .05). The positive rates of SOX9 in the PJAC and ACC groups at 3 months were 68.8% ± 2.69% and 17.1% ± 1.26%, respectively, and the difference was statistically significant (P < .05). The positive rates of aggrecan in the PJAC and ACC groups at 1 month were 40.5% ± 2.78% and 42.4% ± 0.54% respectively, and the difference was not statistically significant (P > .05). The positive rates of aggrecan in the PJAC and ACC groups at 3 months were 40.8% ± 1.50% and 30.1% ± 2.44%, respectively, and the difference was not statistically significant (P > .05).

CONCLUSION

An animal model was established with Guizhou minipigs, and the cartilage defect was repaired with PJAC from male minipigs. The SRY gene positive signal could be detected from the repaired tissue by in situ hybridization, indicating that the transplanted cells survived at least 3 months. The key genes of cartilage formation, SOX9 and aggrecan, were expressed at 1 month and 3 months, and SOX9 expression was stronger in the PJAC group than the ACC group at 3 months.

CLINICAL RELEVANCE

This study suggests that it is feasible to study the biological characteristics of transplanted cells in the cartilage region by the sex-determining gene.

Protocols for Culturing and Imaging a Human Ex Vivo Osteochondral Model for Cartilage Biomanufacturing Applications

Cartilage defects and diseases remain major clinical issues in orthopaedics. Biomanufacturing is now a tangible option for the delivery of bioscaffolds capable of regenerating the deficient cartilage tissue. However, several limitations of in vitro and experimental animal models pose serious challenges to the translation of preclinical findings into clinical practice. Ex vivo models are of great value for translating in vitro tissue engineered approaches into clinically relevant conditions. Our aim is to obtain a viable human osteochondral (OC) model to test hydrogel-based materials for cartilage repair. Here we describe a detailed step-by-step framework for the generation of human OC plugs, their culture in a perfusion device and the processing procedures for histological and advanced microscopy imaging. Our ex vivo OC model fulfils the following requirements: the model is metabolically stable for a relevant culture period of 4 weeks in a perfusion bioreactor, the processing procedures allowed for the analysis of 3 different tissues or materials (cartilage, bone and hydrogel) without compromising their integrity. We determined a protocol and the settings for a non-linear microscopy technique on label free sections. Furthermore, we established a clearing protocol to perform light sheet-based observations on the cartilage layer without the need for tedious and destructive histological procedures. Finally, we showed that our OC system is a clinically relevant in terms of cartilage regeneration potential. In conclusion, this OC model represents a valuable preclinical ex vivo tool for studying cartilage therapies, such as hydrogel-based bioscaffolds, and we envision it will reduce the number of animals needed for in vivo testing.

Performance of marrow stromal cell-seeded small-caliber multilayered vascular graft in a senescent sheep model

Failure of small-caliber grafts, used as bypass or reconstructive grafts in cardiovascular treatments, is often caused by thrombosis and stenosis. We have developed a multilayered, compliant graft with an electrospun heparin-encapsulated core and collagen-chitosan shell. Herein, the performances of acellular and cell-seeded grafts were evaluated in adult sheep for preclinical assessment. Allogeneic ovine marrow stroma cells (MSCs) were uniformly attached to the lumen of cell-seeded grafts. Interposition grafts were used for carotid arteries. Four grafts were tested for each type. Upon implantation, all grafts successfully restored perfusion and rhythmically deformed under pulsatile arterial flow. Weekly ultrasonography and Doppler revealed that all grafts remained patent for perfusion during the course of one-month study. No formation of blood clots or other complications were found. The diameter of graft lumen did not vary significantly over the time or with the graft type, while narrowing at anastomosis and significant thickening of graft wall were found in both types of grafts. More significant neotissue formation was found at anastomotic sections of acellular controls compared to cell-seeded grafts. Results from histological and immunofluorescent analyses revealed moderate intimal hyperplasia (IH) at anastomosis. When compared to cell-seeded grafts, acellular controls presented thicker IH composed of α-smooth muscle actin positive cells and ground substances, which correlated with reduced and more disturbing flow. IH was thickest at anastomosis and tapered off to a minimum in the mid-section. Few PECAM-positive cells appeared on cell-seeded grafts but not acellular controls. Additionally, lesser graft thickening was found in cell-seed grafts, which might be associated with the function of stromal cells in altering the fibrotic process during tissue repair. Results suggest that MSCs held the potential to reduce hyperplasia and improve healing in an aged, large animal model for vascular grafting.