Update on the Basic Science Concepts and Applications of Adipose-Derived Stem Cells in Hand and Craniofacial Surgery.

SUMMARY: Adipose-derived stem cell therapy offers plastic surgeons a novel treatment alternative for conditions with few therapeutic options. Adipose-derived stem cells are a promising treatment because of their broad differentiation potential, capacity for self-renewal, and ease of isolation. Over the past decade, plastic surgeons have attempted to harness adipose-derived stem cells' unique cellular characteristics to improve the survival of traditional fat grafting procedures, a process known as cell-assisted lipotransfer. However, the full implications of cell-assisted lipotransfer in clinical practice remain incompletely understood, stressing the urgent need to assess the scientific evidence supporting adipose-derived stem cell-based interventions. Furthermore, with the strict regulatory climate surrounding tissue explantation therapies, reviewing the safety and efficacy of these treatments will clarify their regulatory viability moving forward. In this report, the authors provide a comprehensive, up-to-date appraisal of best evidence-based practices supporting adipose-derived stem cell-derived therapies, highlighting the known mechanisms behind current clinical applications in tissue engineering and regenerative medicine specific to plastic and reconstructive surgery. The authors outline best practices for the harvest and isolation of adipose-derived stem cells and discuss why procedure standardization will elucidate the scientific bases for their broad use. Finally, the authors discuss challenges posed by U.S. Food and Drug Administration oversight of these cell-based therapies and examine the role of adipose-derived stem cell-based applications in the future of plastic surgery.

Three-Dimensional Computed Tomography Scan Analysis of Secondary Labial Deformities in Patients With Unilateral Cleft Lip/Cleft Palate After Revisional Surgery.

BACKGROUND: Surgical reconstruction of secondary labial deformities associated with isolated unilateral cleft lip (UCL) and/or UCL and palate (UCLP) is challenging. There have been few studies in the literature looking at labial soft tissues quantitatively to assess surgical results. OBJECTIVE: To apply a novel computer-aided, 3-dimensional reconstruction technique based on CT scan images to conduct quantitative preoperative and postoperative assessments in patients with UCL/UCLP undergoing surgical revision of secondary labial deformities. METHODS: Preoperative and postoperative spiral computed tomographic (CT) scans of the face were performed in 21 randomly selected UCL or UCLP patients, who underwent secondary lip revision surgery. The data was then imported to the SimPlant 11.04 software system. Fixed point-to-point, linear distance, and angles were measured, statistically analyzed and used to assess the effect of the surgery. RESULTS: Preoperative measurements showed that the thickness of the upper vermilion at the apex of the Cupid's bow on the affected side was greater than that on the unaffected side. The distance from the apex of the Cupid's bow to the ipsilateral subnasal point of the affected side was smaller than that of the unaffected side (P < 0.05). After surgery, the subjects were rescanned at an average of 9 months, and the curative effects were evaluated. The statistically significant preoperative differences between the affected and unaffected sides were not found postoperatively indicating surgical success. CONCLUSIONS: This study demonstrates the utility of a novel method to measure and assess results in the surgical revision of UCL/UCLP patients with secondary lip deformities. This knowledge can aid the surgeon in selection of treatment techniques.

Long-Term Neurocognitive Outcomes in Sagittal Synostosis: The Impact of Reoperation.

INTRODUCTION: Optimal age at surgery in nonsyndromic sagittal craniosynostosis continues to be debated. Previous reports suggest that earlier age at whole vault cranioplasty more frequently requires reoperation. It is unknown, however, whether reoperation affects neurocognitive outcome. This study examined the impact of reoperation on neurocognitive outcome in children with nonsyndromic sagittal craniosynostosis using comprehensive neurocognitive testing. METHODS: Forty-seven school-age children (age 5-16 years) with nonsyndromic sagittal craniosynostosis who underwent whole-vault cranioplasty were included in this analysis. Participants were administered a battery of standardized neuropsychological testing to measure neurocognitive outcomes. RESULTS: Thirteen of the 47 participants underwent reoperation (27.7%); 11 out of the 13 reoperations were minor revisions while 2 reoperations were cranioplasties. Reoperation rate was not statistically different between patients who had earlier surgery (at age ≤6 months) versus later surgery (at age >6 months) (P > 0.05). Nonreoperated patients who had only one later-in-life surgery did not perform statistically better than reoperated patients on any outcome measure of neurocognitive function, including IQ, academic achievement, visuomotor integration, executive function, and behavior. Comparing reoperated earlier surgery patients with nonreoperated later surgery patients, reoperated earlier surgery patients had higher full-scale and verbal IQ (P < 0.05), scored higher on word reading, reading comprehension, spelling, numerical operations, and visuomotor integration (P < 0.05), and had fewer indicators of suspected learning disabilities (P < 0.01) compared to nonreoperated later surgery patients. CONCLUSION: Reoperation rate after whole vault cranioplasty was 27.7%, with few cases of repeat cranioplasty (4.2% of all patients). Reoperation was not associated with worse neurocognitive outcome. Reoperated earlier surgery patients in fact performed better in IQ, academic achievement and visuomotor integration when compared to nonreoperated later surgery patients.

The Evolution of Breast Implant Infections: Serratia marcescens Is an Emerging Pathogen in Implant-Based Breast Reconstruction.

The flora of implant-based breast infections has changed over the last decade, including at our institution. A 10-year retrospective chart review was performed on 561 implant-based breast reconstructions in 378 patients performed by a single surgeon in an academic university hospital setting. Thirty-two breast infections requiring explantation of the implants occurred during those 10 years. During that time frame, a new pre- and peri-operative protocol was implemented in an effort to diminish infections. We believe that this protocol played a major role in the evolution of changing breast periprosthetic infection flora, from nearly one-third being gram-negative infections to 100% being gram-negative infections. Gram-negative antibiotic coverage should be considered in light of these evolving trends to prevent infections particularly with Serratia marcescens. This may be especially important in patients with BRCA1/2 or ATM mutations. Diabetic patients are more likely to develop Methicillin-resistant Staphylococcus aureus (MRSA) infections and preoperative MRSA decolonization is essential in these patients. Many patients have late-onset breast periprosthetic infection, the majority of which occurred after 30 days. Regular follow-up and patient education is important to provide timely treatment.

La flore des infections mammaires par implant s'est modifiée depuis dix ans, y compris dans l'établissement des auteurs. Dans un hôpital universitaire, un chirurgien a procédé à une analyse rétrospective sur dix ans des dossiers de 561 reconstructions mammaires par implant réalisées chez 378 patientes. Pendant ces dix ans, 32 infections mammaires ont entraîné l'explantation des implants. Un nouveau protocole préopératoire et périopératoire a été adopté au cours de cette période, afin de réduire les infections. Les auteurs sont d'avis que ce protocole a joué un rôle majeur dans l'évolution de la flore des infections mammaires périprothétiques, qui sont passées de près du tiers à 100 % d'infections à Gram négatif. Il faudrait examiner la couverture antibiotique de ces infections à la lumière de ces tendances afin de prévenir les infections, notamment celles à Serratia marcescens. Cette décision serait particulièrement importante pour les patientes présentant des mutations BRCA1/2 ou ATM. Les patientes diabétiques sont plus susceptibles de contracter des infections à Staphylococcus aureus résistantes à la méthicilline (SARM); la décolonisation préopératoire à SARM est essentielle dans ce cas. De nombreuses patientes ont souffert d'une infection mammaire périprothétique tardive, dont la majorité s'est déclarée plus de 30 jours après l'implant. Il est important d'assurer un suivi régulier et d'éduquer les patientes pour pouvoir leur offrir un traitement au moment opportun.

Treatment With Topical Deferoxamine Improves Cutaneous Vascularity and Tissue Pliability in an Irradiated Animal Model of Tissue Expander-Based Breast Reconstruction.

PURPOSE: Postmastectomy radiation therapy is an important component of the multimodality approach to later-stage breast cancers. Unfortunately, despite its proven survival benefits, postmastectomy radiation therapy is deleterious to the skin and soft tissue, causing increased complications and worse aesthetic outcomes after breast reconstruction.There is currently no effective pharmaceutical agent to mitigate the soft tissue fibrosis and hypovascularity associated with soft tissue radiation. We hypothesized that a novel topical formulation of deferoxamine (DFX) will result in improved cutaneous vascularity and soft tissue pliability in an animal model of irradiated tissue expander-based breast reconstruction. METHODS: This study consisted of 16 hairless rats divided into 4 equal groups: a control group (expander only), a tissue expanded and irradiated group, a tissue expanded + DFX group, and a tissue expanded/irradiated/DFX group. A novel topical formulation of DFX consisted of reconstituted drug dissolved in agents designed to enhance dermal penetrance. Vessels per high-power field (vHPF) were quantified histologically; micro-computed tomography angiography was used to assess vessel volume fraction (VVF) and vessel length density. RESULTS: Irradiated skin had less vascularity compared with control (3.81 vHPF vs 8.25 vHPF, P = 0.03; 0.79% VVF vs 1.53% VVF, P = 0.06). Treatment of irradiated skin with topical DFX reversed these effects, resulting in vascular findings similar to the control group histologically (7.94 vHPF vs 8.25 HPF, P = 0.985) and via micro-computed tomography angiography (1.05% VVF vs 1.53% VVF, P = 0.272). Similarly, radiation resulted in less volume expansion compared with controls (0.72 vs 0.8 mL, P = 0.04), whereas treatment with topical DFX reversed this effect, allowing for an expansion volume similar to the control group (0.81 vs 0.80 mL, P = 0.999). CONCLUSIONS: In an animal model of irradiated tissue expander-based breast reconstruction, treatment with topical DFX improved the cutaneous vascularity and tissue pliability, resulting in vascular density and final tissue expansion volumes similar to those found in the nonirradiated control group. Topical DFX may be an effective agent for the treatment of soft tissue radiation injury; future studies are indicated to further characterize this novel drug formulation.

Reducing Infection Rates in Implant-Based Breast Reconstruction: Impact of an Evidence-based Protocol.

BACKGROUND: Infection is the most significant complication in implant-based breast reconstruction, potentially leading to reconstructive failure. We hypothesized that implementation of an evidence-based protocol marked by preoperative decolonization and sterility optimization would result in a decline in postoperative infection rates. METHODS: Informed by a literature review, we developed an evidence-based, perioperative infection prevention protocol implemented in 2015. Surgical outcomes were compared between patients who had undergone implant-based breast reconstruction before and after protocol implementation. A Fisher exact test was used to compare infection rates before and after protocol implementation. A logistic regression analysis was modeled to evaluate the impact of the protocol on infection rate while controlling for nonmodifiable risk factors. RESULTS: Three hundred fifty-eight breasts underwent reconstruction before protocol implementation and 135 afterward. Patients were similar in terms of demographics and surgical characteristics. There was a significantly reduced incidence of clinically relevant infection after protocol implementation (9.5%-2.9%, P = 0.013). Logistic regression analysis confirmed that the protocol was independently associated with a decrease in infection risk (odds ratio, 0.244; P = 0.021). After protocol implementation, no gram-positive bacteria were isolated among cultures obtained from infected periprosthetic fluid. Radiation and drain duration greater than 21 days were independently associated with greater risk for infection. CONCLUSIONS: Our evidence-based protocol was associated with a significant decline in infection rates among implant-based breast reconstruction patients and was particularly effective for gram-positive infections. We will continue to use this protocol in our practice and will consider future directions for addressing gram-negative infections as well.

A Novel Small-Animal Model of Irradiated, Implant-Based Breast Reconstruction.

BACKGROUND: There is currently a need for a clinically relevant small-animal model for irradiated, implant-based breast reconstruction. Present models are inadequate in terms of suboptimal location of expander placement and mode of radiation delivery, correlating poorly with the human clinical scenario. The authors hypothesized that by delivering fractionated radiation and placing an expander under the scalp of the animal, they would achieve soft-tissue changes histologically analogous to those seen in human irradiated, implant-based breast reconstruction. METHODS: This study consisted of 11 immunocompetent, hairless rats divided into three groups as follows: untreated control (n = 3), tissue-expanded scalps (n = 4), and fractionated irradiation plus tissue expansion of the scalp (n = 4). At the completion of the experiment for each group, skin tissue samples were analyzed histologically for vascularity, epidermal and dermal thickness, and collagen fiber alignment or scar formation. RESULTS: Expanded rat epidermis was significantly thicker and dermis was more vascular than nonexpanded skin. The authors observed a greater degree of collagen fiber alignment in the expanded group compared with nonexpanded skin. The combination of irradiation and expansion resulted in significant dermal thinning, vascular depletion, and increased scar formation compared with expanded skin alone. CONCLUSIONS: The authors describe a novel small-animal model for irradiated, implant-based breast reconstruction where histologic analysis shows structural changes in the skin consistent with known effects of radiation therapy and expansion in human skin. This model represents a significant improvement from previous ones and, as such, holds the potential to be used to test new therapeutic agents to improve clinical outcomes.

Effects of Collagenase Digestion and Stromal Vascular Fraction Supplementation on Volume Retention of Fat Grafts.

OBJECTIVE: The use of autologous fat as a soft tissue filler has increased over the past decade in both reconstructive and aesthetic surgeries. Enhancement of autologous fat grafts with the addition of the stromal vascular fraction (SVF) has been reported to improve long-term volume retention. Stromal vascular fraction is most commonly isolated using enzymatic digestion, but it is unknown what effect the digestion process has on the adipocytes and SVF cells that comprise the graft. Some clinicians have reported use of enzymatically digested fat grafts to alter the physical properties of the tissue in specialized applications. We have previously reported that increasing collagenase digestion duration adversely affects the viability of adipocytes and SVF cells. Here, we aimed to determine if collagenase digestion of adipocytes before grafting is detrimental to long-term graft retention and if SVF supplementation can abrogate these potential deleterious effects. METHODS AND RESULTS: We used a published xenograft model in which human lipoaspirate was implanted into the scalp of immunocompromised mice to study the effects of collagenase digestion on in vivo graft survival after 12 weeks. We used 4 experimental groups: grafts composed of collagenase-digested and nondigested adipocytes (50-minute digestion) and grafts with and without SVF supplementation. We used microcomputed tomography to serially and noninvasively quantify graft volume, in conjunction with hematoxylin-eosin staining of histological cross-sections of implanted and excised grafts to assess overall tissue viability. We found that adipocytes that were collagenase-digested before implantation had significantly lower retention rates at 12 weeks and poorer tissue health, which was assessed by quantifying the number of intact adipocytes, the number of cystic formations, and by scoring the degree of inflammation and fibrosis. Further, we found that SVF supplementation of the digested grafts improved graft survival, but not to the level observed in undigested grafts. CONCLUSIONS: We conclude that collagenase digestion adversely affects the long-term volume retention of fat grafts, but that graft retention is improved by SVF supplementation. These experimental results can serve as an initial framework to further elucidate the reported efficacy and safety of using collagenase-digested fat grafts and SVF in the clinical setting.

A Collective Review on Hazards of Powder on Surgical and Examination Gloves.

This article reviews information on the hazards associated with dusting powders on latex surgical and examination gloves. Dusting powders were first applied to latex gloves to facilitate donning. After 1980, manufacturers devised innovative techniques to manufacture gloves without dusting powders. It has been well documented that the powders on gloves present a health hazard to patients, as well as to operating-room personnel. First, these powders elicit tissue toxicity in every tissue in the body. Second, these powders serve as carriers of latex allergen and may precipitate a life-threatening allergic reaction in sensitized patients. These well-documented hazards of glove powders have caused a growing number of hospitals in the world to abandon the use of examination and surgical gloves coated with powder, and instead to use only powder-free gloves.

Evaluation of Sidestream Darkfield Microscopy for Real-Time Imaging Acellular Dermal Matrix Revascularization.

BACKGROUND: Acellular dermal matrices (ADMs) serve as a regenerative framework for host cell integration and collagen deposition to augment the soft tissue envelope in ADM-assisted breast reconstruction-a process dependent on vascular ingrowth. To date noninvasive intra-operative imaging techniques have been inadequate to evaluate the revascularization of ADM. METHODS: We investigated the safety, feasibility, and efficacy of sidestream darkfield (SDF) microscopy to assess the status of ADM microvascular architecture in 8 patients at the time of tissue expander to permanent implant exchange during 2-stage ADM-assisted breast reconstruction. The SDF microscopy is a handheld device, which can be used intraoperatively for the real-time assessment of ADM blood flow, vessel density, vessel size, and branching pattern. The SDF microscopy was used to assess the microvascular architecture in the center and border zone of the ADM and to compare the native, non-ADM-associated capsule in each patient as a within-subject control. RESULTS: No incidences of periprosthetic infection, explantation, or adverse events were reported after SDF image acquisition. Native capsules demonstrate a complex, layered architecture with an average vessel area density of 14.9 mm/mm and total vessel length density of 12.3 mm/mm. In contrast to native periprosthetic capsules, ADM-associated capsules are not uniformly vascularized structures and demonstrate 2 zones of microvascular architecture. The ADM and native capsule border zone demonstrates palisading peripheral vascular arcades with continuous antegrade flow. The central zone of the ADM demonstrates punctate perforating vascular plexi with intermittent, sluggish flow, and intervening 2- to 3-cm watershed zones. CONCLUSIONS: Sidestream darkfield microscopy allows for real-time intraoperative assessment of ADM revascularization and serves as a potential methodology to compare revascularization parameters among commercially available ADMs. Thr SDF microscopy demonstrates that the periprosthetic capsule in ADM-assisted implant-based breast reconstruction is not a uniformly vascularized structure.

Contributions of the Musculus Uvulae to Velopharyngeal Closure Quantified With a 3-Dimensional Multimuscle Computational Model.

The convexity of the dorsal surface of the velum is critical for normal velopharyngeal (VP) function and is largely attributed to the levator veli palatini (LVP) and musculus uvulae (MU). Studies have correlated a concave or flat nasal velar surface to symptoms of VP dysfunction including hypernasality and nasal air emission. In the context of surgical repair of cleft palates, the MU has been given relatively little attention in the literature compared with the larger LVP. A greater understanding of the mechanics of the MU will provide insight into understanding the influence of a dysmorphic MU, as seen in cleft palate, as it relates to VP function. The purpose of this study was to quantify the contributions of the MU to VP closure in a computational model. We created a novel 3-dimensional (3D) finite element model of the VP mechanism from magnetic resonance imaging data collected from an individual with healthy noncleft VP anatomy. The model components included the velum, posterior pharyngeal wall (PPW), LVP, and MU. Simulations were based on the muscle and soft tissue mechanical properties from the literature. We found that, similar to previous hypotheses, the MU acts as (i) a space-occupying structure and (ii) a velar extensor. As a space-occupying structure, the MU helps to nearly triple the midline VP contact length. As a velar extensor, the MU acting alone without the LVP decreases the VP distance 62%. Furthermore, activation of the MU decreases the LVP activation required for closure almost 3-fold, from 20% (without MU) to 8% (with MU). Our study suggests that any possible salvaging and anatomical reconstruction of viable MU tissue in a cleft patient may improve VP closure due to its mechanical function. In the absence or dysfunction of MU tissue, implantation of autologous or engineered tissues at the velar midline, as a possible substitute for the MU, may produce a geometric convexity more favorable to VP closure. In the future, more complex models will provide further insight into optimal surgical reconstruction of the VP musculature in normal and cleft palate populations.

Use of tranexamic acid in craniosynostosis surgery.

BACKGROUND: Intraoperative tranexamic acid (TXA) administration has been used to abate blood loss in a variety of surgical procedures. Several recent studies have supported its efficacy in reducing transfusion requirements in pediatric cranial vault reconstruction (CVR). OBJECTIVE: To conduct a retrospective chart review to determine whether a significant reduction in packed red blood cell (PRBC) and fresh frozen plasma (FFP) transfusions exists when TXA is used. METHODS: A retrospective cohort study of 28 patients who underwent CVR for sagittal craniosynostosis was performed. Transfusion requirements for 14 patients who did not receive TXA were compared with 14 patients who did. Predictors of increased blood product transfusion were also studied. RESULTS: Total volume of PRBC transfusion was reduced by 50% with the use of TXA (P=0.004) with a 34% reduction in intraoperative PRBC transfusion (P=0.017) and a 67% reduction in postoperative PRBC transfusion (P<0.001). Total volume of FFP transfusion was reduced by 46% (P=0.002) and postoperative FFP transfusion was reduced by 100% (P=0.001). The use of TXA was associated with a lower total volume of PRBC (P=0.003) and FFP (P=0.003) transfusions. Older patient age was associated with lower total volume of PRBC transfused (P=0.046 and P=0.002), but not with FFP (P=0.183 and P=0.099) transfusion volumes. Increasing patient weight was associated with lower PRBC (P=0.010 and P=0.020) and FFP (P=0.045 and P=0.016) transfusion volumes. CONCLUSION: TXA decreased blood product transfusion requirements in patients undergoing CVR for sagittal craniosynostosis, and should be a routine part of the strategy to reduce blood loss in these procedures.

HISTORIQUE: L'administration d'acide tranexamique (ATX) intraopératoire vise à limiter la perte de sang dans le cadre de diverses interventions chirurgicales. Plusieurs études récentes en appuient l'efficacité pour réduire les transfusions lors des reconstructions de la voûte crânienne (RVC) en pédiatrie. OBJECTIF: Effectuer une analyse rétrospective des dossiers pour déterminer si l'utilisation d'ATX s'associe à une diminution importante des transfusions de culots globulaires (CG) et de plasma frais congelé (PFC). MÉTHODOLOGIE: Les chercheurs ont réalisé une étude rétrospective de cohorte auprès de 28 patients qui ont subi une RVC pour corriger une craniosystose sagittale. Ils ont comparé les transfusions chez 14 patients qui n'avaient pas reçu d'ATX à celles des 14 patients qui en avaient reçu. Ils ont également étudié les prédicteurs d'une transfusion accrue de produits sanguins. RÉSULTATS: Le volume total de transfusion de CG diminuait de 50 % grâce à l'utilisation d'ATX (P=0,004), y compris une réduction de 34 % des transfusions de CG intraopératoire (P=0,017) et de 67 % des transfusions de CG postopératoire (P<0,001). Le volume total de transfusion de PFC intraopératoire diminuait de 46 % (P=0,002) et celui de transfusion postopératoire, de 100 % (P=0,001). L'utilisation d'ATX s'associait à une diminution du volume total de transfusions de CG (P=0,003) et de PFC (P=0,003). Un âge plus avancé était lié à un volume total de CG transfusé plus faible (P=0,046 et P=0,002), mais pas à celui de PFC (P=0,183 et P=0,099). Un poids plus élevé s'associait à un volume CG (P=0,010 et P=0,020) et de PFC (P=0,045 et P=0,016) plus faible. CONCLUSION: L'ATX réduisait le volume de transfusion de produits sanguins chez les patients subissant une RVC pour corriger une craniosynostose sagittale. Son utilisation devrait être systématique pour réduire la perte de sang lors de ces interventions.

Calvarial Regeneration With Use of Acellular Dermal Matrix in Aplasia Cutis Congenita.

Aplasia cutis congenital (ACC) is a rare congenital anomaly, most commonly affecting the scalp, with a variable penetrance ranging from a small (<2 cm) area of missing skin to large defects characterized by absent skin, subcutaneous tissue, calvarium, and dura. Calvarial reconstruction in ACC can be challenging. Due to exposed neurologic structures, in large defects, ACC has a high mortality rate. A stable reconstruction is optimally achieved shortly after birth to minimize complications. Herein the authors present a case of a neonate with an extensive (4.5 × 7 cm) cutis aplasia defect associated with absent skin, subcutaneous tissue, calvarium, dura, and with exposed cortical surface and sagittal sinus. This defect was successfully reconstructed in a single stage with the use of an acellular dermal matrix/skin graft construct. The acelluar dermal matrix served as a scaffold for tissue ingrowth, promoting regeneration of the bony calvarium as well as soft tissue. At 18-month follow-up, the patient exhibits a 50% smaller calvarial defect as well as stable skin coverage.

Differential Effects of Processing Time and Duration of Collagenase Digestion on Human and Murine Fat Grafts.

BACKGROUND: Autologous fat graft retention is unpredictable, and mechanisms of optimization are poorly understood. Attempts at improving retention use collagenase experimentally and clinically to isolate the stromal vascular fraction to "enhance" fat grafts. However, no standardized duration for collagenase digestion or time following fat graft harvest has been established. This study investigates the effect of (1) time after fat graft harvest and (2) collagenase digestion time on interstitial cell and adipocyte viability in murine fat and human lipoaspirate. METHODS: Murine fat and human lipoaspirate were incubated ex vivo after harvest at room temperature for 120 minutes. Additional groups were incubated with collagenase for increasing 5-minute intervals from 30 to 60 minutes. Samples from each group were stained with BODIPY to quantify intact adipocytes and the LIVE/DEAD kit to quantify interstitial cell viability. RESULTS: With increased time after harvest, the number of intact adipocytes in murine fat and human lipoaspirate remained unchanged. Human interstitial cells were resistant to the effect of increased time ex vivo, whereas murine interstitial cells decreased in viability. In both populations, increased collagenase digestion time significantly decreased the number of viable adipocytes (murine, p ≤ 0.001; human, p ≤ 0.001) and interstitial cells (murine, p ≤ 0.001; human, p ≤ 0.001). CONCLUSIONS: Human and murine adipocytes and human interstitial cells appear resistant to deleterious effects of increasing time following harvest. However, murine interstitial cells are sensitive to increased time and prolonged collagenase digestion. These studies highlight the complex cellular components of fat grafts and how they respond differentially to time and collagenase digestion.