In vivo visualization of the origination of skin graft vasculature in a wild-type/GFP crossover model

Postoperative Time and Anatomic Location Influence Skin Graft Reperfusion Assessed With Laser Speckle Contrast Imaging

OBJECTIVES

Under optimal conditions, afferent and efferent human skin graft microcirculation can be restored 8-12 days postgrafting. Still, the evidence about the reperfusion dynamics beyond this period in a dermato-oncologic setting is scant. We aimed to characterise the reperfusion of human skin grafts over 4 weeks according to the necrosis extension (less than 20%, or 20%-50%) and anatomic location using laser speckle contrast imaging (LSCI).

METHODS

Over 16 months, all eligible adults undergoing skin grafts following skin cancer removal on the scalp, face and lower limb were enroled. Perfusion was assessed with LSCI on the wound margin (control skin) on day 0 and on the graft surface on days 7, 14, 21 and 28. Graft necrosis extension was determined on day 28.

RESULTS

Forty-seven grafts of 47 participants were analysed. Regardless of necrosis extension, graft perfusion equalled the control skin by day 7, surpassed it by day 21, and stabilised onwards. Grafts with less than 20% necrosis on the scalp and lower limb shared this reperfusion pattern and had a consistently better-perfused centre than the periphery for the first 21 days. On the face, the graft perfusion did not differ from the control skin from day 7 onwards, and there were no differences in reperfusion within the graft during the study.

CONCLUSION

Skin graft reperfusion is a protracted process that evolves differently in the graft centre and periphery, influenced by postoperative time and anatomic location. A better knowledge of this process can potentially enhance the development of strategies to induce vessel ingrowth into tissue-engineered skin substitutes.

A Histological and Clinical Study of MatriDerm® Use in Burn Reconstruction

Dermal substitutes are well established in the reconstructive ladder. MatriDerm® (Dr. Otto Suwelack Skin & Health Care AG, Billerbeck, Germany) is a single-layer dermal substitute composed of a bovine collagen (type I, III, and V) and elastin hydrolysate, that allows for immediate split-thickness skin grafting (SSG). The aim of this study was to histologically characterize the integration of MatriDerm® when used during burns surgery reconstruction. Eight subjects with nine burn scars and one acute burn wound underwent reconstruction with MatriDerm® and an immediate SSG. MatriDerm® integration and skin graft take were assessed with serial biopsies performed at weeks 1, 2, 3, and 4 and months 2, 3, 6, 9, and 12. Biopsies were assessed with standard special stains and immunohistochemistry, and representative slides were imaged with a transmission electron microscope. Patient satisfaction and clinical scar outcome were assessed with the Vancouver Scar Scale and a patient questionnaire. Histological analysis showed similar stages of wound healing as shown in other dermal templates but on a different timescale. There is early evidence of vascularization and an inflammatory infiltrate in the first 2 weeks. MatriDerm® is resorbed earlier than other dermal substitutes, with evidence of resorption at week 3, to be completely replaced by a neodermis at 2 months. The use of MatriDerm® in reconstruction with immediate skin grafting is supported histologically with early evidence of vascularization to support an epidermal autograft. Future histological studies may help further characterize the ideal dermal substitute.

Minimal invasiveness in gingival augmentation and root coverage procedures

Minimally invasive surgical procedures aim at optimal wound healing, a reduction of postoperative morbidity and, thus, at increased patient satisfaction. The present article reviews the concept of minimal invasiveness in gingival augmentation and root coverage procedures, and critically discusses the influencing factors, technical and nontechnical ones, and relates them to the underlying biological mechanisms. Furthermore, the corresponding outcomes of the respective procedures are assessed and evaluated in relation to a possible impact of a minimized surgical invasiveness on the clinical, aesthetic, and patient-related results.

Engineering transplantable human lymphatic and blood capillary networks in a porous scaffold

Due to a relative paucity of studies on human lymphatic assembly in vitro and subsequent in vivo transplantation, capillary formation and survival of primary human lymphatic (hLEC) and blood endothelial cells (hBEC) ± primary human vascular smooth muscle cells (hvSMC) were evaluated and compared in vitro and in vivo. hLEC ± hvSMC or hBEC ± hvSMC were seeded in a 3D porous scaffold in vitro, and capillary percent vascular volume (PVV) and vascular density (VD)/mm² assessed. Scaffolds were also transplanted into a sub-cutaneous rat wound with morphology/morphometry assessment. Initially hBEC formed a larger vessel network in vitro than hLEC, with interconnected capillaries evident at 2 days. Interconnected lymphatic capillaries were slower (3 days) to assemble. hLEC capillaries demonstrated a significant overall increase in PVV (p = 0.0083) and VD (p = 0.0039) in vitro when co-cultured with hvSMC. A similar increase did not occur for hBEC + hvSMC in vitro, but hBEC + hvSMC in vivo significantly increased PVV (p = 0.0035) and VD (p = 0.0087). Morphology/morphometry established that hLEC vessels maintained distinct cell markers, and demonstrated significantly increased individual vessel and network size, and longer survival than hBEC capillaries in vivo, and established inosculation with rat lymphatics, with evidence of lymphatic function. The porous polyurethane scaffold provided advantages to capillary network formation due to its large (300-600 μm diameter) interconnected pores, and sufficient stability to ensure successful surgical transplantation in vivo. Given their successful survival and function in vivo within the porous scaffold, in vitro assembled hLEC networks using this method are potentially applicable to clinical scenarios requiring replacement of dysfunctional or absent lymphatic networks.

A New Model for Specific Visualization of Skin Graft Neoangiogenesis Using Flt1-tdsRed BAC Transgenic Mice

BACKGROUND

Neovascularization plays a critical role in skin graft survival. Up to date, the lack of specificity to solely track the newly sprouting blood vessels has remained a limiting factor in skin graft transplantation models. Therefore, the authors developed a new model by using Flt1-tdsRed BAC transgenic mice. Flt1 is a vascular endothelial growth factor receptor expressed by sprouting endothelial cells mediating neoangiogenesis. The authors determined whether this model reliably visualizes neovascularization by quantifying tdsRed fluorescence in the graft over 14 days.

METHODS

Cross-transplantation of two full-thickness 1 × 1-cm dorsal skin grafts was performed between 6- to 8-week-old male Flt1 mice and KSN/Slc nude mice (n = 5). The percentage of graft area occupied by tdsRed fluorescence in the central and lateral areas of the graft on days 3, 5, 9, and 14 was determined using confocal-laser scanning microscopy.

RESULTS

Flt1+ endothelial cells migrating from the transgenic wound bed into the nude graft were first visible in the reticular dermis of the graft center on day 3 (0.5 ± 0.1; p < 0.05). Peak neovascularization was observed on day 9 in the lateral and central parts, increasing by 2- to 4-fold (4.6 ± 0.8 and 4.2 ± 0.9; p < 0.001). Notably, some limited neoangiogenesis was displayed within the Flt grafts on nude mice, particularly in the center. No neovascularization was observed from the wound margins.

CONCLUSION

The ability of the Flt1-tdsRed transgenic mouse model to efficiently identify the origin of the skin-graft vasculature and visualize graft neovascularization over time suggests its potential utility for developing techniques that promote graft neovascularization.

Analysis of biomonitoring data after full-thickness skin grafting

Skin graft vascularization is investigated mainly by histological evaluation. Immunohistochemical analysis has been conducted only in mice. Transcutaneous oxygen tension (TcPO₂ ), which is an index of blood flow, has not been evaluated in skin grafts and only a few studies have reported biologic monitoring data using color tone evaluation and surface temperature. In humans, these tests can be performed non-invasively. To evaluate human skin graft vascularization, we analyzed biomonitoring data after skin grafting. We evaluated 14 patients who underwent skin grafting surgery at Nagoya City University Hospital. The TcPO₂ , color tone, surface temperature, and dermoscopic observations at recipient sites were measured at postoperative day (POD) 4, 6, and 11. Mean TcPO₂ levels at POD4, 6, and 11 were 12.7, 15.2, and 33.5 mmHg, respectively, and significantly higher at POD11 than at POD4 (p = 0.003, Steel-Dwass test). Dermoscopic observation revealed gradually increasing redness and yellowness. Color tone evaluation measured by spectrophotometry supported the appearance. The a*(redness) value at POD4, 6, and 11 was 6.19, 9.20, and 11.27, respectively, and significantly higher at POD11 than at POD4 (p < 0.001, Steel-Dwass test). The b*(yellowness) value at POD4, 6, and 11 was 8.83, 9.24, and 13.02, respectively, and significantly higher at POD11 than at POD4 (p = 0.020, Steel-Dwass test). The surface temperature did not significantly differ between graft and control sites. These findings suggest that skin graft vascularization started by POD6 and stabilized by POD11. Because TcPO₂ increases after POD4, skin grafts should remain undisturbed until at least POD11.

Developing a xenograft model of human vasculature in the mouse ear pinna

Humanised xenograft models allow for the analysis of human tissue within a physiological environment in vivo. However, current models often rely on the angiogenesis and ingrowth of recipient vasculature to perfuse tissues, preventing analysis of biological processes and diseases involving human blood vessels. This limits the effectiveness of xenografts in replicating human physiology and may lead to issues with translating findings into human research. We have designed a xenograft model of human vasculature to address this issue. Human subcutaneous fat was cultured in vitro to promote blood vessel outgrowth prior to implantation into immunocompromised mice. We demonstrate that implants survived, retained human vasculature and anastomosed with the circulatory system of the recipient mouse. Significantly, by performing transplants into the ear pinna, this system enabled intravital observation of xenografts by multiphoton microscopy, allowing us to visualise the steps leading to vascular cytoadherence of erythrocytes infected with the human parasite Plasmodium falciparum. This model represents a useful tool for imaging the interactions that occur within human tissues in vivo and permits visualization of blood flow and cellular recruitment in a system which is amenable to intervention for various studies in basic biology together with drug evaluation and mechanism of action studies.

Bio-engineering a tissue flap utilizing a porous scaffold incorporating a human induced pluripotent stem cell-derived endothelial cell capillary network connected to a vascular pedicle

Tissue flaps are used to cover large/poorly healing wounds, but involve complex surgery and donor site morbidity. In this study a tissue flap is assembled using the mammalian body as a bioreactor to functionally connect an artery and vein to a human capillary network assembled from induced pluripotent stem cell-derived endothelial cells (hiPSC ECs). In vitro: Porous NovoSorb™ scaffolds (3 mm × 1.35 mm) were seeded with 200,000 hiPSC ECs ± 100,000 human vascular smooth muscle cells (hvSMC), and cultured for 1-3 days, with capillaries formed by 24 h which were CD31⁺, VE-Cadherin⁺, EphB4⁺, VEGFR2⁺ and Ki67⁺, whilst hvSMCs (calponin⁺) attached abluminally. In vivo: In SCID mice, bi-lateral epigastric vascular pedicles were isolated in a silicone chamber for a 3 week 'delay period' for pedicle capillary sprouting, then reopened, and two hiPSC EC ± hvSMCs seeded scaffolds transplanted over the pedicle. The chamber was either resealed (Group 1), or removed and surrounding tissue secured around the pedicle + scaffolds (Group 2), for 1 or 2 weeks. Human capillaries survived in vivo and were CD31⁺, VE-Cadherin⁺ and VEGFR2⁺. Human vSMCs remained attached, and host mesenchymal cells also attached abluminally. Systemically injected FITC-dextran present in human capillary lumens indicated inosculation to host capillaries. Human iPSC EC capillary morphometric parameters at one week in vivo were equal to or higher than the same parameters measured in human abdominal skin. This 'proof of concept' study has demonstrated that bio-engineering an autologous human tissue flap based on hiPSC EC could minimize the use of donor flaps and has potential applications for complex wound coverage. STATEMENT OF SIGNIFICANCE: Tissue flaps, used for surgical reconstruction of wounds, require complex surgery, often associated with morbidity. Bio-engineering a simpler alternative, we assembled a human induced pluripotent stem cell derived endothelial cell (hiPSC ECs) capillary network in a porous scaffold in vitro, which when transplanted over a mouse vascular pedicle in vivo formed a functional tissue flap with mouse blood flow in the human capillaries. Therefore it is feasible to form an autologous tissue flap derived from a hiPSC EC capillary network assembled in vitro, and functionally connect to a vascular pedicle in vivo that could be utilized in complex wound repair for chronic or acute wounds.

[The applications of periodontal gingival surgery. Ⅱ: alternative materials]

The main purposes of periodontal graft surgery include achieving root coverage, improving the clinical attachment level and keratinized tissue, and advancing the procedure of periodontal plastic surgery. Autogenous graft, such as subepithelial connective tissue graft-based procedure, provide the best outcomes for mean and complete root coverage, as well as increase in keratinized tissue. However, a disadvantage of the procedure is in the location of the operation itself: the additional surgical site (palate). Therefore, clinicians are always looking for graft substitutes. This article will discuss the evidence supporting the use of 1) acellular dermal matrix (ADM); 2) xenogeneic collagen matrix (XCM); 3) recombinant human platelet-derived growth factor (rhPDGF); 4) enamel matrix derivative (EMD); 5) guided tissue regeneration (GTR); 6) living cellular construct (LCC), all of which are used in conjunction with coronally advanced flaps as alternatives to autogenous donor tissue. The decision tree for treatments of Miller recession-type defects are also discussed.

Endothelial Rictor is crucial for midgestational development and sustained and extensive FGF2-induced neovascularization in the adult

To explore the general requirement of endothelial mTORC2 during embryonic and adolescent development, we knocked out the essential mTORC2 component Rictor in the mouse endothelium in the embryo, during adolescence and in endothelial cells in vitro. During embryonic development, Rictor knockout resulted in growth retardation and lethality around embryonic day 12. We detected reduced peripheral vascularization and delayed ossification of developing fingers, toes and vertebrae during this confined midgestational period. Rictor knockout did not affect viability, weight gain, and vascular development during further adolescence. However during this period, Rictor knockout prevented skin capillaries to gain larger and heterogeneously sized diameters and remodeling into tortuous vessels in response to FGF2. Rictor knockout strongly reduced extensive FGF2-induced neovascularization and prevented hemorrhage in FGF2-loaded matrigel plugs. Rictor knockout also disabled the formation of capillary-like networks by FGF2-stimulated mouse aortic endothelial cells in vitro. Low RICTOR expression was detected in quiescent, confluent mouse aortic endothelial cells, whereas high doses of FGF2 induced high RICTOR expression that was associated with strong mTORC2-specific protein kinase Cα and AKT phosphorylation. We demonstrate that the endothelial FGF-RICTOR axis is not required during endothelial quiescence, but crucial for midgestational development and sustained and extensive neovascularization in the adult.

Barriers in contribution of human mesenchymal stem cells to murine muscle regeneration

AIM: To study regeneration of damaged human and murine muscle implants and the contribution of added xenogeneic mesenchymal stem cells (MSCs).

METHODS: Minced human or mouse skeletal muscle tissues were implanted together with human or mouse MSCs subcutaneously on the back of non-obese diabetic/severe combined immunodeficient mice. The muscle tissues (both human and murine) were minced with scalpels into small pieces (< 1 mm³) and aliquoted in portions of 200 mm³. These portions were either cryopreserved in 10% dimethylsulfoxide or freshly implanted. Syngeneic or xenogeneic MSCs were added to the minced muscles directly before implantation. Implants were collected at 7, 14, 30 or 45 d after transplantation and processed for (immuno)histological analysis. The progression of muscle regeneration was assessed using a standard histological staining (hematoxylin-phloxin-saffron). Antibodies recognizing Pax7 and von Willebrand factor were used to detect the presence of satellite cells and blood vessels, respectively. To enable detection of the bone marrow-derived MSCs or their derivatives we used MSCs previously transduced with lentiviral vectors expressing a cytoplasmic LacZ gene. X-gal staining of the fixed tissues was used to detect β-galactosidase-positive cells and myofibers.

RESULTS: Myoregeneration in implants of fresh murine muscle was evident as early as day 7, and progressed with time to occupy 50% to 70% of the implants. Regeneration of fresh human muscle was slower. These observations of fresh muscle implants were in contrast to the regeneration of cryopreserved murine muscle that proceeded similarly to that of fresh tissue except for day 45 (P < 0.05). Cryopreserved human muscle showed minimal regeneration, suggesting that the freezing procedure was detrimental to human satellite cells. In fresh and cryopreserved mouse muscle supplemented with LacZ-tagged mouse MSCs, β-galactosidase-positive myofibers were identified early after grafting at the well-vascularized periphery of the implants. The contribution of human MSCs to murine myofiber formation was, however, restricted to the cryopreserved mouse muscle implants. This suggests that fresh murine muscle tissue provides a suboptimal environment for maintenance of human MSCs. A detailed analysis of the histological sections of the various muscle implants revealed the presence of cellular structures with a deviating morphology. Additional stainings with alizarin red and alcian blue showed myofiber calcification in 50 of 66 human muscle implants, and encapsulated cartilage in 10 of 81 of murine muscle implants, respectively.

CONCLUSION: In mouse models the engagement of human MSCs in myoregeneration might be underestimated. Furthermore, our model permits the dissection of species-specific factors in the microenvironment.

Simultaneous dermal matrix and autologous split-thickness skin graft transplantation in a porcine wound model: a three-dimensional histological analysis of revascularization

Despite the popularity of a simultaneous application of dermal matrices and split-thickness skin grafts, scarce evidence exists about the process of revascularization involved. In this study, we aimed at analyzing the progression of revascularization by high-resolution episcopic microscopy (HREM) in a porcine excisional wound model. Following the surgical procedure creating 5 × 5 cm(2) full-thickness defects on the back, one area was covered with an autologous split-thickness skin graft alone (control group), the other with a collagen-elastin dermal matrix plus split-thickness skin graft (dermal matrix group). Two skin biopsies per each group and location were performed on day 5, 10, 15, and 28 postoperatively and separately processed for H&E as well as HREM. The dermal layer was thicker in the dermal matrix group vs. control on day 5 and 28. No differences were found for revascularization by conventional histology. In HREM, the dermal matrix did not appear to decelerate the revascularization process. The presence of the dermal matrix could be distinguished until day 15. By day 28, the structure of the dermal matrix could no longer be delineated and was replaced by autologous tissue. As assessed by conventional histology and confirmed by HREM, the revascularization process was comparable in both groups, notably with regard to the vertical ingrowth of sprouting vessels. The presented technique of HREM is a valuable addition for analyzing small vessel sprouting in dermal matrices in the future.

Lessons and limits of mouse models

Seminal studies in rabbits and rodent transplantation models by Peter Medawar revealed that cellular processes, rather than humoral antibodies, are central to the acute rejection of transplanted organs, and much of basic transplantation research continues to be focused on the biology and control of these cells, which were subsequently shown to be T cells. However, the success of current immunosuppression at controlling T-cell-mediated rejection has resulted in an increasing awareness of antibody-mediated rejection in the clinic. This, in turn, has fueled an emerging interest in the biology of allospecific antibodies, the B cells that produce these antibodies, and the development of mouse models that allow their investigation. Here we summarize some of the more widely used mouse models that have been developed to study the immunobiology of alloreactivity, transplantation rejection and tolerance, and used to identify therapeutic strategies that modulate these events.

A simple alternative technique for harvesting split thickness skin grafts

AIM: To assess the use of a simple split skin graft harvesting technique, requiring only a scalpel and a swab.

METHODS: During the last 8 mo, we operated on a consecutive series of 52 patients (30 males, 22 females) with a mean age of 60 years (33-80). We used the technique we present in order to cover small skin defects. All procedures were performed under local anesthesia. Thirty-seven patients underwent bedside surgery, 8 patients were operated on in the outpatient department and the remaining 7 had their graft harvested in the operating room. After antiseptic preparation of the donor site, the margins of the graft were drawn by the use of a surgical marker. A No 15 scalpel was used for the graft elevation, under constant traction with a moist swab.

RESULTS: All procedures were completed successfully without immediate complications. The patients tolerated the procedure well. The mean operative time was 15 min. Twenty-four donor sites were left to heal by secondary intention, whereas 28 were sutured with interrupted 3/0 silk sutures in order to heal by primary intention. All 24 sites that were left to heal by secondary intention healed completely in approximately 14 d. For the sites that were sutured, the sutures were removed on the 10^th postoperative day. Out of the 52 operated cases, 6 patients (11%) developed complications. In 4 patients, the split thickness skin grafts were partially lost, whereas in 2 patients the grafts were completely lost. Wound dehiscence was observed in 2 patients, which were treated with local antiseptic and antibiotic therapy.

CONCLUSION: The skin graft technique described is simple, costless and effective and can be performed even on an outpatient basis, without the need for special equipment.

Randomized, controlled trial of conjunctival autografting combined with subconjunctival bevacizumab for primary pterygium treatment: 1-year follow-up

BACKGROUND

To investigate the efficacy and safety of subconjunctival bevacizumab application as an adjuvant therapy for primary pterygium.

DESIGN

This study was a clinical randomized trial performed at the Institute of Ophthalmology 'Conde de Valenciana'.

PARTICIPANTS

Forty-nine patients with primary pterygium were enrolled in the study.

METHODS

Each primary pterygium patient was randomized after pterygium excision and a conjunctival autograft to receive either a single 2.5 mg/0.1 mL dose of subconjunctival bevacizumab immediately after surgery (group 1); a double 2.5 mg/0.1 mL injection of subconjunctival bevacizumab, one immediately after surgery and the second 15 days after surgery (group 2); or no injection (group 3).

MAIN OUTCOME MEASURES

Autoconjunctival graft presence or absence of ischaemia, necrosis, infection or detachment; surgical bed appearance; and pterygium recurrence at 1-year follow-up period were determined.

RESULTS

Forty-nine eyes of 49 patients were included. Sixteen patients were assigned to group 1, 17 to group 2 and 16 to group 3. Patients from groups 1 and 2 showed conjunctival autograft ischaemia at 24 h postoperative (37.5% and 58.8%, respectively, P > 0.05), which disappeared by the first postoperative month. No significant difference in the main outcome measures was found among single versus double-dose of subconjunctival bevacizumab injection patients. At the end of the study, pterygium recurrences were observed only in group 3 (P < 0.04).

CONCLUSIONS

A single 2.5 mg/mL subconjunctival bevacizumab injection in conjunction with primary pterygium surgery accomplishing a conjunctival autograft procedure is safe and well tolerated, and is capable of preventing pterygium recurrences when compared with a control group.

The mouse dorsal skin fold chamber as a means for the analysis of tissue engineered skin

The therapy of extensive and deep burn wounds is still a challenging task for reconstructive plastic surgery. The outcome is generally not satisfactory, neither from the functional nor from the aesthetic aspect. Several available skin substitutes are used but there is need for optimization of new skin substitutes which have to be tested in vitro as well as in vivo. Here, we show that the dorsal skin fold chamber preparation of mice is well suited for the testing of skin substitutes in vivo. Dermal skin constructs consisting of matriderm(®) covered with a collagen type I gel were inserted into full thickness skin wounds in the skin fold chambers. The skin substitutes integrated well into the adjacent skin and got epithelialized from the wound edges within 11 days. The epithelialization by keratinocytes is the prerequisite that also cell-free dermal substitutes might be used in the case of the lack of sufficient areas to gain split thickness skin grafts. Further advantage of the chambers is the lack of wound contraction, which is common but undesired in rodent wound healing. Furthermore, this model allows a sophisticated histological as well as immunohistochemical analysis. As such, we conclude that this model is well suited for the analysis of tissue engineered skin constructs. Besides epithelialization the mode and extend of neovascularization and contraction of artificial grafts may be studied under standardized conditions.

Metalloproteinases facilitate connection of wound bed vessels to pre-existing skin graft vasculature

BACKGROUND

Despite advances in tissue engineering of human skin, the exact revascularization processes remain unclear. Therefore it was the aim of this study to investigate the vascular transformations during engraftment and to identify associated proteolytic factors.

METHODS

The modified dorsal skinfold chamber with autologous skin grafting was prepared in C57BL/6J mice, and intravital microscopy was performed. The expression of proteases and vascular factors was quantified by immunohistochemistry.

RESULTS

Reperfusion of the skin graft after 72hours was followed by a temporary angiogenic response of the graft vessels. Wound bed bud formation appeared after 24 to 48hours representing starting points for capillary sprouting. In the reperfused skin graft larger buds developed over several days without transformation into angiogenic sprouts; instead pruning took place. MT1-MMP was detected at sprout tips of in-growing vessels. MMP-2 expression was located at the wound bed/graft connection sites. Pericytes were found to withdraw from the angiogenic vessel in order to facilitate sprouting.

CONCLUSIONS

Skin graft vasculature responded with temporary angiogenesis to reperfusion, which was pruned after several days and exhibited a different morphology than regular sprouting angiogenesis present within the wound bed. Furthermore we identified MT1-MMP as sprout-tip located protease indicating its potential role as sprout growth facilitator as well as potentially in lysing the existing graft capillaries in order to connect to them. The differences between the wound bed and skin graft angiogenesis may represent a relevant insight into the processes of vascular pruning and may help in the engineering of skin substitutes.