The Effect of MHC Antigen Matching Between Donors and Recipients on Skin Tolerance of Vascularized Composite Allografts.

The emergence of skin-containing vascularized composite allografts (VCAs) has provided impetus to understand factors affecting rejection and tolerance of skin. VCA tolerance can be established in miniature swine across haploidentical MHC barriers using mixed chimerism. Because the deceased donor pool for VCAs does not permit MHC antigen matching, clinical VCAs are transplanted across varying MHC disparities. We investigated whether sharing of MHC class I or II antigens between donors and recipients influences VCA skin tolerance. Miniature swine were conditioned nonmyeloablatively and received hematopoietic stem cell transplants and VCAs across MHC class I (n = 3) or class II (n = 3) barriers. In vitro immune responsiveness was assessed, and VCA skin-resident leukocytes were characterized by flow cytometry. Stable mixed chimerism was established in all animals. MHC class II-mismatched chimeras were tolerant of VCAs. MHC class I-mismatched animals, however, rejected VCA skin, characterized by infiltration of recipient-type CD8+ lymphocytes. Systemic donor-specific nonresponsiveness was maintained, including after VCA rejection. This study shows that MHC antigen matching influences VCA skin rejection and suggests that local regulation of immune tolerance is critical in long-term acceptance of all VCA components. These results help elucidate novel mechanisms underlying skin tolerance and identify clinically relevant VCA tolerance strategies.

Rapid and reliable healing of critical size bone defects with genetically modified sheep muscle.

Large segmental defects in bone fail to heal and remain a clinical problem. Muscle is highly osteogenic, and preliminary data suggest that autologous muscle tissue expressing bone morphogenetic protein-2 (BMP-2) efficiently heals critical size defects in rats. Translation into possible human clinical trials requires, inter alia, demonstration of efficacy in a large animal, such as the sheep. Scale-up is fraught with numerous biological, anatomical, mechanical and structural variables, which cannot be addressed systematically because of cost and other practical issues. For this reason, we developed a translational model enabling us to isolate the biological question of whether sheep muscle, transduced with adenovirus expressing BMP-2, could heal critical size defects in vivo. Initial experiments in athymic rats noted strong healing in only about one-third of animals because of unexpected immune responses to sheep antigens. For this reason, subsequent experiments were performed with Fischer rats under transient immunosuppression. Such experiments confirmed remarkably rapid and reliable healing of the defects in all rats, with bridging by 2 weeks and remodelling as early as 3-4 weeks, despite BMP-2 production only in nanogram quantities and persisting for only 1-3 weeks. By 8 weeks the healed defects contained well-organised new bone with advanced neo-cortication and abundant marrow. Bone mineral content and mechanical strength were close to normal values. These data demonstrate the utility of this model when adapting this technology for bone healing in sheep, as a prelude to human clinical trials.

Vascularized composite allograft tolerance across MHC barriers in a large animal model.

Vascularized composite allograft (VCA) transplantation can restore form and function following severe craniofacial injuries, extremity amputations or massive tissue loss. The induction of transplant tolerance would eliminate the need for long-term immunosuppression, realigning the risk-benefit ratio for these life-enhancing procedures. Skin, a critical component of VCA, has consistently presented the most stringent challenge to transplant tolerance. Here, we demonstrate, in a clinically relevant miniature swine model, induction of immunologic tolerance of VCAs across MHC barriers by induction of stable hematopoietic mixed chimerism. Recipient conditioning consisted of T cell depletion with CD3-immunotoxin, and 100 cGy total body irradiation prior to hematopoietic cell transplantation (HCT) and a 45-day course of cyclosporine A. VCA transplantation was performed either simultaneously to induction of mixed chimerism or into established mixed chimeras 85-150 days later. Following withdrawal of immunosuppression both VCAs transplanted into stable chimeras (n=4), and those transplanted at the time of HCT (n=2) accepted all components, including skin, without evidence of rejection to the experimental end point 115-504 days posttransplant. These data demonstrate that tolerance across MHC mismatches can be induced in a clinically relevant VCA model, providing proof of concept for long-term immunosuppression-free survival.

Induction of tolerance to an allogeneic skin flap transplant in a preclinical large animal model.

Clinical composite tissue allotransplantation can adequately reconstruct defects that are not possible by other means. However, immunosuppressant toxicity limits the use of these techniques. Clinical attempts to reduce the amount of immunosuppression required by induction of an immunologically permissive state have so far been unsuccessful. The aim of this study was to induce tolerance in a preclinical large animal model. Donor hematopoietic stem cell (HSC) engraftment was induced by T-cell depletion, irradiation, and a short course of cyclosporine administered to the recipient, along with a hematopoietic cell infusion from a single haplotype major histocompatibility complex (MHC) mismatched donor. Skin was then allotransplanted from the donor. Both primarily vascularized skin flaps and secondarily vascularized conventional skin grafts were allotransplanted to investigate if the mode of transplantation affected outcome. Control animals received the skin allotransplants without conditioning. Tolerance was defined as no evidence of rejection at 90 days following transplantation. Conventional skin grafts only achieved prolonged survival (<65 days) in HSC-engrafted animals (P < .01). In contrast, there was indefinite skin flap survival with the achievement of tolerance in HSC-engrafted animals; this was confirmed on histology with donor-specific unresponsiveness on MLR and CML. Furthermore, a conventional skin donor graft subsequently applied to an animal tolerant to a skin flap was not rejected and did not trigger skin flap rejection. To our knowledge, this is the first time skin tolerance has been achieved across a MHC barrier in a large animal model. This is a significant step toward the goal of clinical skin tolerance induction.

Auricular reconstruction using biofabrication-based tissue engineering strategies.

Auricular malformations, which impose a significant social and psychological burden, are currently treated using ear prostheses, synthetic implants or autologous implants derived from rib cartilage. Advances in the field of regenerative medicine and biofabrication provide the possibility to engineer functional cartilage with intricate architectures and complex shapes using patient-derived or donor cells. However, the development of a successful auricular cartilage implant still faces a number of challenges. These challenges include the generation of a functional biochemical matrix, the fabrication of a customized anatomical shape, and maintenance of that shape. Biofabrication technologies may have the potential to overcome these challenges due to their ability to reproducibly deposit multiple materials in complex geometries in a highly controllable manner. This topical review summarizes this potential of biofabrication technologies for the generation of implants for auricular reconstruction. In particular, it aims to discuss how biofabrication technologies, although still in pre-clinical phase, could overcome the challenges of generating and maintaining the desired auricular shapes. Finally, remaining bottlenecks and future directions are discussed.

The effects of different schedules of total-body irradiation in heterotopic vascularized bone transplantation. An experimental study in the Lewis rat.

To evaluate the effects of irradiation on heterotopically placed vascularized knee isografts, a single dose of 10 Gy of total-body irradiation was given to Lewis donor rats. Irradiation was delivered either 2 or 6 days prior to harvesting or subsequent transplantation, and evaluated at 1, 2, and 4 weeks after grafting. Irradiation caused endothelial depopulation of the graft artery, although vascular pedicle patency was maintained throughout the study. Bone graft viability and mineralization were normal. Dramatic changes in the bone marrow were seen that included an increase of its fat content (P less than 0.001), and a concomitant decrease in bone marrow-derived immunocompetent cells. These changes were more prominent in recipients of grafts from day -6 irradiated donor rats. Total-body irradiation did not prejudice the use of vascularized bone grafts, and exhibited an associated immunosuppresant effect over the vascular endothelium and bone marrow. This may be a further rational conditioning procedure to avoid recipient manipulation in vascularized bone allotransplantation.

Tolerance to musculoskeletal allografts with transient lymphocyte chimerism in miniature swine.

BACKGROUND: Although transplantation of musculoskeletal allografts in humans is technically feasible, the adverse effects of long-term immunosuppression subject the patient to high risks for correcting a non-life-threatening condition. Achieving immunologic tolerance to musculoskeletal allografts, without the need for chronic immunosuppression, could expand the clinical application of limb tissue allografting. Tolerance to musculoskeletal allografts has been accomplished previously in miniature swine in our laboratory. Although stable, mixed chimerism has been suggested as the mechanism underlying long-term tolerance in a rat limb model, the mechanism of this tolerance induction has not been established. This report explores the possible relationship between hematopoietic chimerism and tolerance to musculoskeletal allografts in swine. METHODS: Twelve miniature swine underwent vascularized musculoskeletal allograft transplantation from histocompatibility complex (MHC) matched, minor antigen-mismatched donors. Eight animals received a 12-day coprse of cyclosporine, one of which was excluded due to subtherapeutic levels. Four recipients were not immunosuppressed. Serial biopsies to assess graft viability and flow cytometry to assess chimerism were performed. Donor and third-party skin grafts were placed on recipients with surviving allografts greater than 100 days to validate tolerance. RESULTS: Both groups developed early peripheral chimerism, but this chimerism became undetectable by postoperative day 19 in the cyclosporine group and by day 13 in the control group. Animals receiving cyclosporine developed permanent tolerance to their allografts, whereas those not receiving cyclosporine rejected their allografts in 6-9 weeks. Animals demonstrating tolerance to their bone allografts also demonstrated prolonged donor skin graft survival. CONCLUSIONS: Induction of tolerance to musculoskeletal allografts can be achieved in the MHC matched swine. Although hematopoietic chimerism is present in the immediate postoperative period, persistent, long-term chimerism does not seem to be necessary for maintenance of such tolerance.

Cell-based tissue-engineered allogeneic implant for cartilage repair.

The potential for using of allogeneic cartilage chips, transplanted in a biologic polymer with articular chondrocytes, as a tool for articular cartilage repair was studied. Small lyophilized articular cartilage chips were mixed with a cell/fibrinogen solution and thrombin to obtain implantable constructs made of fibrin glue, chondrocytes, and cartilage chips. Specimens were implanted in the subcutaneous tissue on the backs of nude mice (experimental group A). Three groups of controls (groups B, C, and D) were also prepared. Group B consisted of fibrin glue and cartilage chips without chondrocytes. Group C consisted of fibrin glue and chondrocytes without cartilage chips, and group D was composed solely of fibrin glue. All samples were carefully weighed before implantation in the mice. The constructs were harvested from the animals at 6, 9, and 12 weeks, examined grossly, and weighed. The samples were then processed and stained with hematoxylin and eosin for histological examination. Gross evaluation and weight analysis of the constructs at the time of retrieval showed retention of the original mass in the samples made of fibrin glue, chondrocytes, and cartilage chips (group A) and demonstrated a cartilaginous consistency upon probing. Specimens from constructs of fibrin glue and cartilage chips without chondrocytes (control group B) retained most of their volume, but were statistically lighter than specimens from group A and were much softer and more pliable than those in group A. Samples of specimens from constructs of fibrin glue and chondrocytes (groups C) and fibrin glue alone (group D) both showed a substantial reduction of their original masses over the experimental time periods when compared to the samples in groups A and B, although specimens from group C demonstrated new cartilage matrix formation. Histological analysis of specimens in experimental group A demonstrated the presence of cartilage chips surrounded by newly formed cartilaginous matrix, while specimens of control group B showed only fibrotic tissue surrounding the devitalized cartilage pieces. Cartilaginous matrix was also observed in control group C, in which cartilage chips were absent, whereas only fibrin glue debris was observed in control group D. This study demonstrated that a composite of fibrin glue and devitalized cartilage can serve as a scaffold for chondrocyte transplantation, preserve the original phenotype of the chondrocytes, and maintain the original mass of the implant. This may represent a valid option for addressing the problem of articular cartilage repair.

Biomechanical analysis of a chondrocyte-based repair model of articular cartilage.

The objective of this study was to evaluate the biomechanical properties of newly formed cartilaginous tissue synthesized from isolated chondrocytes. Cartilage from articular joints of lambs was either digested in collagenase to isolated chondrocytes or cut into discs that were devitalized by multiple freeze-thaw cycles. Isolated cells were incubated in suspension culture in the presence of devitalized cartilage matrix for 3 weeks. Multiple chondrocyte/matrix constructs were assembled with fibrin glue and implanted subcutaneously in nude mice for up to 6 weeks. Testing methods were devised to quantify integration of cartilage pieces and mechanical properties of constructs. These studies showed monotonic increase with time in tensile strength, fracture strain, fracture energy, and tensile modulus to values 5-10% of normal articular cartilage by 6 weeks in vivo. Histological analysis indicated that chondrocytes grown on dead cartilage matrix produced new matrix that integrated individual cartilage pieces with mechanically functional tissue.

Cultured chondrocytes produce injectable tissue-engineered cartilage in hydrogel polymer.

The purpose of this study was to determine if chondrocytes cultured through several subcultures at very low plating density would produce new cartilage matrix after being reimplanted in vivo with or without a hydrogel polymer scaffold. Chondrocytes were initially plated in low-density monolayer culture, grown to confluence, and passaged four times. After each passage cells were suspended in purified porcine fibrinogen and injected into the subcutaneous space of nude mice while simultaneously polymerizing with thrombin to reach a final concentration of 40 million cells/cc. Controls were made by injecting fresh, uncultured cells with fibrin polymer and by injecting the cultured cells in saline (without polymer). All samples were harvested at 6 weeks. When injected in polymer, both fresh cells and cells that had undergone only one passage in culture produced cartilaginous nodules. Cultured cells did not produce cartilage, regardless of length of time spent in culture, when injected without polymer. Cartilage was also not recovered from samples with cells kept in culture for longer than one passage, even when provided with a polymer matrix. All samples harvested were subjected to histological analysis and assayed for total DNA, glycosaminoglycan (GAG), and type II collagen. There was histological evidence of cartilage in the groups that used fresh cells and cultured cells suspended in fibrin polymer that only underwent one passage. No other group contained areas that would be consistent with cartilage histologically. All experimental samples had a higher percent of DNA than native swine cartilage, and there was no statistical difference between the DNA content of the groups containing cultured or fresh cells in fibrin polymer. Whereas the GAG content of native cartilage was 8.39% of dry weight and fresh cells in fibrin polymer was 12.85%, cultured cells in fibrin polymer never exceded the 2.48% noted from first passage cells. In conclusion, this study demonstrates that porcine chondrocytes that have been cultured in monolayer for one passage will produce cartilage in vivo when suspended in fibrin polymer.

Quantitative histologic study of the influence of spinal instrumentation on lumbar fusions: a canine model.

UNLABELLED: Histomorphometric and microradiographic studies were performed on 28 beagle hounds 1 year of age, followed up for 6 months after an L5-L6 anterior and posterior spinal destabilization procedure--Group I (n = 7), destabilized surgical controls; Group II (n = 7), posterolateral bone grafting after destabilization; Group III (n = 7), Harrington rod instrumentation and posterolateral bone grafting after destabilization; and Group IV (n = 7), Luque instrumentation, and bone grafting after destabilization. Six months postoperatively, device-related osteoporosis occurred in spines treated with spinal instrumentation. Within the L5 vertebral body the mean trabecular width was less for the two groups with instrumentation (Groups III and IV) compared with the two groups without instrumentation (Groups I and II) (p less than 0.001). The bone formation rate [mm3/(mm3 x year)] x 10(3), which is based on the mean distance between sequenced fluorochrome labels, for Group I (destabilized, nonfused, noninstrumented dogs) was more than twice that of the other three groups, which were all equivalent (p less than 0.05). CLINICAL RELEVANCE: Stress shielding, or more correctly, device-related osteoporosis, probably can occur within vertebrae in response to rigid spinal instrumentation. However, the overall mechanical properties of vertebrae underlying spinal instrumentation are probably not at increased risk of fracture because the increase in cross-sectional area of the vertebra and incorporated fusion mass more than compensate for the loss of volumetric bone density.

Allogenic transplants of bone revascularized by microvascular anastomoses: a preliminary study.

An area of experimental bone grafting that needs further study is the use of free vascularized allografts of bone. In 35 outbred mongrel dogs, the viability of vascularized bone allografts with and without azathioprine immunosuppression was compared to vascularized autogenous bone grafts. Viability was assessed by histologic techniques, fluorochrome bone labeling, and electron microscopy. Autogenous vascularized bone grafts remained viable, and it was concluded that microvascular technique was not the limiting factor in attaining survival of the grafts. The behavior of autogenous vascularized bone grafts with and without the influence of azathioprine was similar. Allogenic vascularized bone transplants uniformly failed at a period between 2 and 3 weeks. Immunosuppression with azathioprine did not appreciably affect survival of the osteocytes. However, the host response to the foreign tissue was slightly modified. The clinical ramifications of bone transplantations in humans are not analogous to the clinical situation of transplantation of other organs. If vascularized bone transplants are performed in humans, a relatively safe form of immunosuppression is necessary. This study suggests that azathioprine alone does not offer sufficient immunosuppression to insure viability of the vascularized transplant.

Bonding of cartilage matrices with cultured chondrocytes: an experimental model.

The capacity of isolated chondrocytes to join separate masses of cartilage matrix was investigated with composites implanted in subcutaneous pouches in nude mice. Slices of articular cartilage were harvested from lambs and were devitalized by cyclic freezing and thawing. The slices were then either co-cultured with viable allogeneic lamb chondrocytes (experimental) or cultured without such chondrocytes (control). Composites of three slices were constructed with use of fibrin glue and were implanted in nude mice for periods ranging from 7 to 42 days. Bonding of the experimental matrices with viable chondrocytes was achieved at 28 and 42 days, as assessed by direct examination, histology, thymidine uptake, and fluorescence. No bonding occurred in the control composites without viable chondrocytes. We conclude that devitalized cartilage matrix is a scaffold to which isolated chondrocytes can attach and begin to repopulate.

Meniscal repair using engineered tissue.

In this study, devitalized meniscal tissue pre-seeded with viable cultured chondrocytes was used to repair a bucket-handle incision in meniscal tissue transplanted to nude mice. Lamb knee menisci were devitalized by cyclic freezing and thawing. Chips measuring four by two by one-half millimeters were cut from this devitalized tissue to serve as scaffolds. These chips were then cultured either with or without viable allogeneic lamb chondrocytes. From the inner third of the devitalized meniscal tissue, rectangles were also cut approximately 8 x 6 mm. A 4 mm bucket-handle type incision was made in these blocks. The previously prepared chips either with (experimental group) or without viable chondrocytes (control group) were positioned into the incisions and secured with suture. Further control groups included blocks of devitalized menisci with incisions into which no chips were positioned and either closed with suture or left open with no suture. Specimens were transplanted to subcutaneous pouches of nude mice for 14 weeks. After 14 weeks, seven of eight experimental specimens (chips with viable chondrocytes) demonstrated bridging of the incision assessed by gross inspection and manual distraction. All the control groups were markedly different from the experimental group in that the incision remained grossly visible. Histological analysis was consistent with the differences apparent at the gross level. Only the experimental specimens (chips with viable chondrocytes) with gross bridging demonstrated obliteration of the interface between incision and scaffold. None of the control specimens revealed any cells or tissue filling the incision. Tissue engineering using scaffolds and viable cells may have an application in meniscal repair in vivo.

Free microvascular epiphyseal-plate transplantation. An experimental study in dogs.

To evaluate the feasibility of transplanting vascularized epiphyseal plates while maintaining normal growth in the recipient site, twenty-two puppies from known, large breeds were divided into one control and three experimental groups of four animals each and one long-term group of six animals. The control group underwent insertion of a radiopaque marker in the fibular metaphysis bilaterally, and, in addition, a fibular osteotomy was performed on one side. In the experimental groups, a fibular switch was carried out, selecting one fibula as a vascularized graft and the other as a non-vascularized graft. Both the controls and the experimental groups were evaluated using serial roentgenograms, histological examination, fluorescent bone-labeling, and microangiography. One week, six weeks, three months, and seven months postoperatively, animals from each group were killed. Continuous growth was observed in the vascularized epiphyseal transplants and in the controls, with no statistical difference noted, whereas the non-vascularized transplants exhibited considerably less or no growth. Vascularized transplants demonstrated an average 21.2-millimeter increase in length while non-vascularized transplants showed a 6.6-millimeter increase. Histological examination, fluorochrome bone-labeling, and microangiography confirmed the continued viability of the vascularized epiphyseal transplants in contrast to the non-vascularized transplants.

Histological characteristics of acute rejection in vascularized allografts of bone.

Using a genetically defined rat model for the heterotopic transplantation of a vascularized knee in the rat, histological and histochemical studies of acute rejection in vascularized allografts of bone were carried out. The graft consisted of the knee joint with the distal end of the femur, the proximal part of the tibia, the cartilaginous growth plates, the articular cartilage, and a minimum cuff of muscle, which was transferred to a location under the abdominal skin. A total of 160 transplants, including vascularized and non-vascularized isografts, vascularized and non-vascularized allografts that were transplanted across a strong histocompatibility barrier, and vascularized allografts of bone that were transplanted across a weak histocompatibility barrier, were studied by light microscopy at intervals for as long as twelve weeks after transplantation. Vascularized allografts of bone that were transplanted across a strong histocompatibility barrier showed evidence of rapid rejection, similar to that after transplantation of allografts of visceral organs. This was manifested at one week by necrosis of osteocytes, cessation of microcirculatory flow, massive extravasation of red cells, and deposition of fibrin in the marrow. The large vessels demonstrated changes that were characteristic of vascular rejection. Allografts that were transplanted across a weak histocompatibility barrier showed a more gradual, less intense process of rejection that allowed observation of the evolution of the process. In these grafts, the osteoblasts and marrow in the primary spongiosa of the metaphysis were early targets of rejection, as indicated by necrosis of osteoblasts, extravasation of red blood cells, and deposition of fibrin in the marrow spaces. Loss of osteoblasts from the surfaces of osteoid as well as from bone on spicules of calcified cartilage resulted in the cessation of new-bone formation. Calcification of the longitudinal septa between the lowermost hypertrophic chondrocytes was decreased. However, the proliferation and maturation of chondrocytes in the zone of proliferating chondrocytes and in the upper hypertrophic zone continued and resulted in the formation of a thickened growth plate. The loss of osteocytes in other areas of the graft occurred later and only in the areas where the microcirculation had been lost. These data suggest that ischemic damage, which is probably secondary to an immune-related vascular compromise, is a significant factor in the failure of grafts. In the grafts that were transplanted across a weak histocompatibility barrier, the growth of new bone and revascularization by the host occurred by twelve weeks.

Prolonged general anesthesia for experimental craniofacial surgery in fetal swine.

A protocol utilizing high preoperative doses of altrenogest (Regu-Mate) and a "balanced" general anesthesia regimen consisting of isoflurane at subanesthetic doses supplemented with intravenous doses of sodium thiopental was developed to prevent preterm labor, minimize intracranial fetal cerebral edema, and decrease postpartum mortality of fetal swine after undergoing complex in utero craniofacial procedures. A total of 20 fetal piglets at 75% gestation were exposed to prolonged (> 3 h) anesthesia conditions of which 7 piglets were randomly selected to undergo experimental craniofacial procedures consisting of periosteal stripping of frontal and parietal bone segments with/without extensive coronal suture fusion procedures. Neither sows nor piglets were lost to anesthetic complications during the initial laparotomy or subsequent cesarean delivery. None of the sows experienced uterine sepsis or underwent preterm labor. The overall survival rate for all piglets exposed to prolonged anesthesia conditions was 95% at 4 weeks and 45% at 11 weeks after surgery. The experimental group's survival was 85.7% at 4 weeks and 28.5% at 11 weeks after surgery.

Prolonged survival in fetal rabbit surgery.

Timing and outcome of antenatal surgical intervention is being explored using fetal animal models. Models that are currently used range from larger animals with fewer offspring and higher cost to smaller animals with larger litters and lower cost. The rabbit is an ideal "small" animal model for experimentation in the third trimester, with a large litter, short gestation and a relatively large fetus. This paper reports methods by which prolonged survival (greater than 110 days) may be achieved in as many as 60% of operated fetuses following complex fetal surgery in the rabbit.

Use of swine model in transplantation of vascularized skeletal tissue allografts.

Permanent tolerance to vascularized skeletal tissue allografts can be induced in miniature swine with minor antigen differences using a 12-day course of CsA. Demonstration of skeletal tissue allograft survival in a large animal model without long-term immunosuppression represents an important step toward transplantation of skeletal tissue allografts in humans.

Heterotopic microvascular growth plate transplantation of the proximal fibula: an experimental canine model.

The reconstructive potential of microvascular transplantation of skeletal growth plates was investigated through heterotopic transfers. The distal radius was resected in two series of puppies of a known large breed and substituted with a microsurgically revascularized transplant from the proximal fibula. Evaluation was conducted through serial roentgenograms, goniometric registration of joint mobility, volume measurements, histology, and fluorescent bone labeling. In the first series, development of neuropathic-like destruction of the weight-bearing graft ensued in the majority of the animals. In the second series, prolonged protection from weight bearing inhibited this destruction and resulted in hypertrophy of the revascularized epiphyseal end of the transplant but clearly reduced longitudinal growth, with only one transplant exhibiting longitudinal growth that exceeded 50 percent of the value for the control. This experiment demonstrates that skeletal growth plates possess a capacity for hypertrophy under the influence of increased loads. Whether this adaptability is sufficient to allow microvascular transplantation of growth plates to become a clinically useful procedure in children remains unclear. Further laboratory investigations are mandatory prior to clinical application of microvascular transfers of epiphyseal growth plates.