Routine assessment of viability in split-thickness skin

The viability change of pigskin in vitro

BACKGROUND

It is widely recognised that take of grafts is strongly influenced by tissue viability. Although porcine skin is currently the most widely used xenograft, the viability change of pigskin in vitro has not been extensively studied. The purpose of this study was to assess the change of the viability of Bama miniature pigskin after harvest and cryopreservation, and to set up a guideline for pigskin preservation and storage that would allow the skin to retain the highest viability after treatment and still be used in the clinical applications.

METHODS

Harvested pigskin grafts were divided into five groups: normal saline medium/4 degrees C (group 1), Dulbecco's minimum essential medium (DMEM)/4 degrees C (group 2), normal saline medium/25 degrees C (group 3), DMEM/25 degrees C (group 4) and cryopreserved (group 5). In our experiment, the viability was investigated by 3-(4,5)-dimethylthiazol-2,5-diphenyl tetrasolium bromide (MTT) salt assay. We also evaluated the transplantation performance of preserved skin in different conditions by using a rat recipient model, in which primary take was evaluated by gross observation and predetermined histological criteria after 7 days.

RESULTS

Skin stored at 4 degrees C showed a very slow viability decrease with time. The sample showed a viability decrease of about 70% after 3 days in normal saline and 4 days in DMEM medium. Nevertheless, skin stored in DMEM at 25 degrees C underwent a viability increase during the first 4h and then decreased gradually to about 70% after 20 h, while the viability declined very quickly for skin grafts stored in normal saline medium at 25 degrees C, and maintained the same viability only within 6h of preservation. On the other hand, cryopreserved skin has been shown to maintain a level of skin metabolism equal to 77% of the fresh sample when measured immediately after thawing, and the viability remained about 70% after 6h at 25 degrees C and 2 days at 4 degrees C in DMEM. The graft performance of skin specimens with 70% viability of fresh skin stored in different conditions has not shown statistical significance compared with fresh pigskin.

CONCLUSIONS

Based on these results, we suggest that the conservation period of fresh pigskin should not exceed 72 or 96 h when stored in normal saline or DMEM at 4 degrees C, and should not exceed 6 or 18 h when stored in normal saline or DMEM at 25 degrees C. Cryopreserved pigskin should be stored in DMEM for a maximum period of 48 h at 4 degrees C and 6h at 25 degrees C after thawing.

Islet assessment for transplantation

PURPOSE OF REVIEW

There is a critical need for meaningful viability and potency assays that characterize islet preparations for release prior to clinical islet cell transplantation. Development, testing, and validation of such assays have been the subject of intense investigation for the last decade. These efforts are reviewed, highlighting the most recent results while focusing on the most promising assays.

RECENT FINDINGS

Assays based on membrane integrity do not reflect true viability when applied to either intact islets or dispersed islet cells. Assays requiring disaggregation of intact islets into individual cells for assessment introduce additional problems of cell damage and loss. Assays evaluating mitochondrial function, specifically mitochondrial membrane potential, bioenergetic status, and cellular oxygen consumption rate, especially when conducted with intact islets, appear most promising in evaluating their quality prior to islet cell transplantation. Prospective, quantitative assays based on measurements of oxygen consumption rate with intact islets have been developed, validated, and their results correlated with transplant outcomes in the diabetic nude mouse bioassay.

CONCLUSION

More sensitive and reliable islet viability and potency tests have been recently developed and tested. Those evaluating mitochondrial function are most promising, correlate with transplant outcomes in mice, and are currently being evaluated in the clinical setting.

Quality control for clinical islet transplantation: organ procurement and preservation, the islet processing facility, isolation, and potency tests

Pancreatic islet transplantation has become one of the ideal treatments for patients with type 1 diabetes mellitus due to improvements in isolation techniques and immunosuppression regimens. In order to ensure the safety and rights of patients, isolated islets need to meet the criteria for regulation as both a biological product and a drug product. For the constant success of transplantation, therefore, all investigators involved in clinical islet transplantation must strive to ensure the safety, purity, and potency of islets in all the phases of clinical islet isolation and transplantation. In this review, we summarize the quality control for clinical islet isolation and transplantation, and the latest topics of pre-transplant islet assessment.

A stirred microchamber for oxygen consumption rate measurements with pancreatic islets

Improvements in pancreatic islet transplantation for treatment of diabetes are hindered by the absence of meaningful islet quality assessment methods. Oxygen consumption rate (OCR) has previously been used to assess the quality of organs and primary tissue for transplantation. In this study, we describe and characterize a stirred microchamber for measuring OCR with small quantities of islets. The device has a titanium body with a chamber volume of about 200 microL and is magnetically stirred and water jacketed for temperature control. Oxygen partial pressure (pO(2)) is measured by fluorescence quenching with a fiber optic probe, and OCR is determined from the linear decrease of pO(2) with time. We demonstrate that measurements can be made rapidly and with high precision. Measurements with betaTC3 cells and islets show that OCR is directly proportional to the number of viable cells in mixtures of live and dead cells and correlate linearly with membrane integrity measurements made with cells that have been cultured for 24 h under various stressful conditions.

Assessment of organ culture for the conservation of human skin allografts

OBJECTIVE

Human skin allografts are used in the treatment of severe burns and their preservation is therefore critical for optimal clinical benefit. Current preservation methods, such as 4 degrees C storage or cryopreservation, cannot prevent the decrease of tissue viability. The aim of this study was to assess viability and function of skin allografts in a new skin organ culture model, allowing conservation parameters as close as possible to physiological conditions: 32 degrees C, air-liquid interface and physiological skin tension.

DESIGN

Twelve skin samples, harvested from 6 living surgical donors, were conserved 35 days in two conditions: conservation at 4 degrees C and organ culture. Viability and function of skin samples were investigated at Day 0, 7, 14, 21, 28 and 35 using cell culture methods (trypan blue exclusion, Colony Forming Efficiency and Growth Rate), histopathological and histoenzymological studies (Ki67 immunostaining).

RESULTS

In the two conditions, fibroblast and keratinocyte viability was progressively affected by storage, with a significant decrease observed after 35 days. No statistical difference could be observed between the two conditions. The two methods were also comparable regarding alterations of fibroblast and keratinocyte culture parameters, which were respectively significantly reduced at Day 7 and 21, compared to fresh skin. By contrast, histopathological and histoenzymological studies revealed a better preservation of skin architecture and proliferative potential at 4 degrees C, as compared to organ culture.

CONCLUSION

These results indicate that skin organ culture does not provide significant advantages for skin allograft preservation. However, its potential use as an experimental model to study skin physiology and wound healing should be further evaluated.

Human islet oxygen consumption rate and DNA measurements predict diabetes reversal in nude mice

There is a need for simple, quantitative and prospective assays for islet quality assessment that are predictive of islet transplantation outcome. The current state-of-the-art athymic nude mouse bioassay is costly, technically challenging and retrospective. In this study, we report on the ability of 2 parameters characterizing human islet quality: (1) oxygen consumption rate (OCR), a measure of viable volume; and (2) OCR/DNA, a measure of fractional viability, to predict diabetes reversal in nude mice. Results demonstrate that the probability for diabetes reversal increases as the graft's OCR/DNA and total OCR increase. For a given transplanted OCR dose, diabetes reversal is strongly dependent on OCR/DNA. The OCR and OCR/DNA (the 'OCR test') data exhibit 89% sensitivity and 77% specificity in predicting diabetes reversal in nude mice (n = 86). We conclude that the prospective OCR test can effectively replace the retrospective athymic nude mouse bioassay in assessing human islet quality prior to islet transplantation.

Dimethyl sulfoxide toxicity kinetics in intact articular cartilage

Osteochondral defects can degenerate into osteoarthritis and currently there are no good treatment alternatives available to most Orthopaedic surgeons. Osteochondral allografting can restore damaged joint surfaces but its clinical use is limited by poor access to high quality tissue. Vitrification of osteochondral tissue would allow the banking of this tissue but requires high concentrations of cryoprotective agents. This study was designed to ascertain dimethyl sulfoxide (DMSO) toxicity kinetics to chondrocytes in situ after exposure to DMSO at different temperatures recorded as a function of time. Porcine osteochondral dowels were exposed to 1, 3, 5, and 6M DMSO at 4, 22, and 37 degrees C for 0.5 min to 120 min. Chondrocyte recovery was determined by membrane integrity (Syto 13 and ethidium bromide) and mitochondrial (WST-1) assays. Results demonstrated that cell recovery was concentration, temperature and time dependent. At higher concentrations and temperatures, significant cell loss occurred within minutes. A rate constant calculated for chondrocyte death was dependent on temperature. 1 M DMSO appeared relatively non-toxic. This experiment established a method to examine systematically toxicity parameters for chondrocytes in situ and this data can be used to tailor vitrification protocols by limiting exposure temperature and time or lowering DMSO concentrations below toxic levels recorded.

Performance and safety of skin allografts

Skin preservation for transplantation began almost 200 years ago with the pioneering work of Baronio (cited by in Ref. ). Since that time, hundreds of papers have been published on the preservation of skin for later application in wound treatment. This interest stems from the excellent clinical results obtained with skin as a permanent autograft or temporary allograft for wound cover, coupled with the relative ease of preservation and storage methods. The general recognition of the need for human skin allografts has stimulated the establishment of banking facilities and research to improve the methods for harvesting, processing, storage, and subsequent evaluation of transplantation performance.

Evaluation of donor skin viability: fresh and cryopreserved skin using tetrazolioum salt assay

Cell viability assessment in allograft skin is an essential step to ensure a supply of good quality allograft skin for clinical repair of wounds. It is widely recognised that 'take' of allografts is strongly influenced grafted by tissue viability. The aim of this study was to set-up storage protocols that maintain high viability of the allograft after harvest, treatment and storage. In this study, the viability of post-mortem allografts (n=350) harvested from 35 different donors, was investigated using the MTT salt assay. The conditions of preparation and storage of the allograft included: 1. Fresh skin samples (about 12, 30, and 60h after harvesting). 2. The same specimens (stored at 4 and 37 degrees C) tested for at least 1 month. 3. Samples after cryopreservation and thawing. 4. Thawed specimens tested daily for at least 6 days. Parallel histomorphological analysis performed, under each of these conditions, showed a correlation between changes in structure and changes in viability as measured by the MTT quantitative assay. The viability index (VI) of skin is expressed as the ratio between the optical density (O.D.) produced in the MTT assay by the skin sample and its weight in grams. The percentage viability index is the ratio of the VI of the fresh sample (considered as 100% viability) and the value of specimens from the same harvest batch after storage or cryopreservation. The results indicated that samples tested within 12-30h from harvesting have an average viability index of about 75 with little variation. Samples tested within 60h have an average viability index of 40, showing a viability decrease of about 50%. A protocol to treat skin within a maximum of 30h was, therefore, set-up. The data suggested that skin stored at 37 degrees C, undergoes a viability increase during the first 2 days after harvesting. However, the viability under these conditions then decreased very quickly. After 6 days of preservation at this temperature the samples were no longer viable (PVI = 0). The tissue structure started to become damaged after 3 days. On the other hand, skin stored at 4 degrees C, showed a very slow viability decrease. After 15 days, viability was still almost 25% of the fresh sample. The tissue architecture showed no signs of damage under these conditions until day 7 from harvesting. MTT analysis was performed on the specimens cryopreserved with DMSO at 10%. These measurements were compared to viability assessment of the same fresh skin samples (considered as 100%) that were analysed within 30h from harvesting. The average PVI of thawed skin was 54% of the fresh sample. This result demonstrates that the viability of cryopreserved skin is comparable to the viability of fresh skin stored at 4 degrees C for 4 days. The PVI of thawed skin samples decreased dramatically within 24h, and had reached 0% within 6 days.

The influence of various methods of cold storage of skin on the permeation of melatonin and nimesulide

The purpose of this study was to investigate the influence of different methods of skin preservation on the percutaneous permeation of two drugs with varied physicochemical properties (melatonin and nimesulide). Hairless rat skin was freshly excised and immediately stored at three different storage conditions: (a). 4 degrees C in minimal essential medium eagle (MEM), (b). frozen at -22 degrees C and (c) frozen at -22 degrees C in 10% glycerol (as a cryoprotective agent). The permeation of melatonin and nimesulide from saturated solutions was studied using the skin stored at various cold storage conditions for 6 months. At 4 degrees C, the flux of melatonin was similar to fresh skin for up to 7 days (P>0.05) and increased 2.4-fold at 14 days (P<0.001). The flux of melatonin was similar to fresh skin for 14 days with skin at -22 degrees C (P>0.05) and then steadily increased from 30 days onwards and at 180 days, the flux was 5-fold greater than fresh skin (P<0.001). Freezing of skin at -22 degrees C with 10% glycerol showed similar flux values for melatonin up to 60 days and at 180 days the flux increased 2.2-fold as compared to fresh skin (P<0.001). In the case of nimesulide, the skin stored for 2 days at 4 degrees C showed similar flux as compared to fresh skin (P>0.05) and increased steadily from 4 days onwards and at 14 days the flux was 3.5-fold higher than fresh skin (P<0.001). The skin frozen at -22 degrees C with or without 10% glycerol showed no difference in flux up to 4 days which however increased from 7 days onwards. At 180 days, the skin at -22 degrees C showed 2.5-fold increase in the flux (P<0.001) whereas the skin frozen with 10% glycerol showed 2-fold increase in the flux (P<0.001) as compared to fresh skin. The results of the present study demonstrate that the permeability of the drugs across the stored skin was dependent on the storage condition, the length of storage and the physicochemical properties of the drug under study. Overall, the freezing of skin at -22 degrees C with 10% glycerol was found to be very helpful for the long-term storage of skin for percutaneous permeation studies.

Vitrification and rapid freezing of rabbit fetal tissues and skin samples from rabbits and pigs

Vitrification (3.58 M EG and 2.82 M DMSO in PBS with 20% FCS) and rapid-freezing (0.25 M sucrose, 2.25 M EG, and 2.25 M DMSO in PBS with 20% FCS) procedures were assayed to cryopreserve rabbit tissue samples from 12-day fetuses, and skin samples from live born pups and adult rabbits. These methods were also assayed to cryopreserve pig skin samples obtained from abattoir animals. The ability of rabbit tissue samples to attach and colonize the substratum by cell proliferation was not affected by the assayed cryopreservation procedures, regardless of specimen age. In porcines, sample attachment and cell proliferation capability of primary cultures were not affected by applied cryopreservation procedures. Almost all primary cultures from cryopreserved skin samples reached confluency (from 92 to 100%). Results reported here allow us to establish in both species, rabbit and pig, a cryobank of skin samples from adult specimens classified as outliers for longevity (in rabbits) and prolificacy (in pigs).