Does Cryopreservation Affect the Biological Properties of Stem Cells from Dental Tissues? A Systematic Review

Abstract

This systematic review evaluated if different cryopreservation protocols could affect biological properties (Cell survival rate (CSR), proliferation, differentiation, maintenance of stem cell markers) of stem cells obtained from dental tissues (DSC) post-thaw. An electronic search was carried out within PubMed and ISI Web Science by using specific keyword. Two independent reviewers read the titles and abstracts of all reports respecting predetermined inclusion/exclusion criteria. Data were extracted considering the biological properties of previously cryopreserved DSCs and previously cryopreserved dental tissues. DSCs cryopreserved as soon as possible after their isolation presents a CSR quite similar to the non-cryopreserved DSC. Dimethyl sulfoxide (DMSO) [10%] showed good results related to cell recovery post-thaw to cryopreserve cells and tissues for periods of up to 2 years. The cryopreservation of DSC in a mechanical freezer (-80°C) allows the recovery of stem cells post-thaw. The facilities producing magnetic field (MF), demand a lower concentration of cryoprotectant, but their use is not dispensable. It is possible to isolate and cryopreserve dental pulp stem cell (DPSC) from healthy and diseased vital teeth. Cryopreservation of dental tissues for late DSC isolation, combined with MF dispensability, could be valuable to reduce costs and improve the logistics to develop teeth banks.

Key Words:
cryopreservation; mesenchymal stem cells; dental tissues; tooth banking; cell viability

Resumo

Essa revisão sistemática avaliou se diferentes protocolos de criopreservação podem afetar as propriedades biológicas (taxa de sobrevivência celular, proliferação, diferenciação, manutenção dos marcadores de superfície) de células-tronco isoladas de tecidos dentais (DSC) após o descongelamento. Uma busca eletrônica foi realizada no PubMed e no ISI Web of Science utilizando palavras-chave específicas. Dois revisores independentes avaliaram os títulos e resumos de todos os estudos respeitando critérios de inclusão e exclusão previamente determinados. Os dados foram extraídos considerando as propriedades biológicas de DSC, e DSC isoladas de tecidos previamente criopreservados. DSC criopreservadas logo após seu isolamento apresentaram propriedades biológicas muito semelhantes às observadas em DSC não criopreservadas. Dimetil sulfóxido (DMSO) [10%] demonstrou bons resultados relacionados com a recuperação celular após descongelamento de células e tecidos, por períodos de até 2 anos. A criopreservação de DSC em freezer mecânico (-80 °C) permite a recuperação de células-tronco pós-descongelação. A utilização de freezer com campo magnético (MF), proporciona a utilização de uma menor concentração de crioprotector, mas a sua utilização não é dispensável. É possível isolar e criopreservar e criopreservar células-tronco da polpa dental (DPSC) de dentes vitais saudáveis e doentes. Criopreservação de tecidos dentais após o isolamento de DSC, combinados com MF, podem ser valiosas estratégias para reduzir custos e melhorar a logística no desenvolvimento de bancos de dentes.

Introduction

The discovery and isolation of responsive mesenchymal stem cells (MSC) from dental tissues ^1,2,3,4 raised the possibility for developing new regenerative therapies. Such MSC populations possess high proliferative, self-renewal, and multipotency, expressing the ability to differentiate in tissues, such as fat, bone, cartilage, and neural cells. MSCs could proliferate in vitro, maintaining their differentiation capacity, for a limited period, ranging for 15 to 50 population doublings ⁵. Currently it is clear that the number of passaging reduces the differentiation potential and the capacity of proliferation ⁶. Cryopreservation comprises a key method to keep MSC at early passages maintaining their biological properties.

Cryopreservation relies on a complex balance established by a well-controlled cooling rate and the cryoprotectant agent's (CA) concentration ^7,8. Thus, a proper CA should allow water to leave the cell slowly enough to avoid impairment on cells' membranes, but sufficiently fast to avoid ice crystals formation inside the cell. Dimethyl sulfoxide (DMSO) - (CH₃)₂SO - a polar aprotic solvent, is widely applied as CA. Due to its hydrophilic nature, DMSO induces the water output in an ideal speed reducing the heat stress during transition from liquid to solid state ^8,9,10,11.

However, DMSO has been shown to be cytotoxic, since DMSO can decrease the MSC's ability to proliferate and differentiate post-thaw ¹¹. Thus, researchers have been testing alternative substances, such as glycerol and ethyleneglycol and sugars such as sucrose and trehalose, due to their reduced cytotoxicity as CA ¹². Such CAs are widely used in vitrification techniques, which are based on eliminating the phase-glazing glass transition through a fast-freezing process. After that, the solution is converted into an amorphous solid, which should be free from ice crystal. In addition, new specialized facilities, such as magnetic field freezers ^13,14,15,16 and programed freezers ¹², have been applied to improve cell biological properties and minimize the toxic effects of CA.

There is a lack of information regarding the behavior of well-characterized cryopreserved MSC from dental tissues (dental stem cells - DSC). MSCs come from tissues with specific-related characteristics containing a heterogeneous cells population ⁵, thus making the definition of the best CA or the best facilities to cryopreserve cells and tissues a hard task. Davies et al. ¹⁷ have shown that the effects of in vitro expansion, after 10% DMSO cryopreservation, on the viability and differentiation capacity is cell-dependent. The objective of this study has been to systematically review the literature in order to identify the influence of different cryopreservation protocols over the biological properties of dental stem cells post-thaw.

Material and Methods

This systematic review was carried out according to PRISMA statement ¹⁸. To structure the research question (Which is the influence of cryopreservation protocol over biological properties of stem cells obtained from dental tissues?) the PICO model has been applied. PICO stands for: Population: Stem cells from dental tissues; Intervention: Cryopreservation; Comparison: Fresh cells or tissues; Outcome: DSC biological properties (Cell survival rate (CSR), proliferation, differentiation, maintenance of stem cell markers).

As for eligibility criteria, the search was limited to: a) Experimental research articles; b) Studies evaluating the biological properties of previously cryopreserved DSC or cells isolated from previously cryopreserved dental tissues; c) Studies meeting the minimal criteria for MSC characterization as described by Dominici et al. ⁵.

As for exclusion criteria, the presented systematic review did not include studies evaluating the viability of cryopreserved MSC from tissues other than dental pulp, periodontal ligaments, apical papilla, and tooth germ.

Regarding the information sources, studies were identified by searching electronic databases, including PubMed (PM) and the ISI Web of Science(r) (IWS), up to April 2015. The keywords selected included relevant entry terms associated with each MeSH Term database and the search was developed considering the relevant entry terms associated with each MeSH Term (Table 1).

Table 1
Database syntax applied to investigate the selected MeSH terms and its respective entry terms. TS= (At the ISI Web of Knowledge it field searches into abstracts, keywords, and article titles)

With respect to selection of studies and data collection, pre-selected keywords were combined to retrieve the studies (Table 2). The records retrieved have been uploaded into the ENDNOTE(r) basic (www.myendnoteweb.com) to delete the duplicated ones. Two independent reviewers read the titles and abstracts of all found reports considering the inclusion criteria to perform later the full report evaluation. For data collection, the full version of all included studies were obtained and the data were extracted (Table 3). If any disagreement was found in relation to the inclusion of some study, the reviewers discussed the matter to obtain consensus.

Table 2
Keywords combination and records recovered in each database.

Table 3
Studies matching the inclusion criteria selected for data extraction full text selected for data extraction

Table 3
Continuation

• °C - Degree Celsius; °C/min - Degree Celsius per minute; CA - Cryoprotectant agent; DFSC - Dental Foicle Stem Cells; DMSO - Dimethyl sulfoxide; DPSC - Dental Pulp Stem Cells; EG - Ethylene Glycol; FBS - Fetal Bovine serum; G - Glycerol; h - Hours; M - Molar; MF - Magnetic Freezing; min - Minute; N2 - Liquid Nitrogen; NaB - Boran Nitrate; PDLSC - Periodontal Ligament Stem Cells; PSA - Penicillin-streptomicyn-anpicillin; RT - Room Temperature; SCAP - Stem Cells from Apical Papilla; sDPSC - Suine Dental Pulp Stem Cells; SHED - Stem Cells from Human Exfoliated deciduous teeth; T - Tesla; TGSC - Tooth Germ Stem Cells; PBS - Phosphate Buffered Saline; NR - Not reported

Results

The initial search (Fig. 1) resulted in 1773 initial records corresponding to 482 individual, from which 32 were selected by evaluating their title and abstract for complete reading. After that, 21 papers were selected for the data extraction (Table 3). Dental pulp stem cells (DPSC) were the most cryopreserved DSC (52%). DMSO has been applied as CA in 100% of the selected studies, at concentrations between 3% and 20% ^16,19,20. Only 19% of the included studies performed the cryopreservation for periods higher than one year ^20,21,22,23. Specialized facilities (magnetic or programed freezers) were applied in 19% of the studies ^12,13,14,16.

Figure 1
Flowchart from selected studies

Biological Properties of Previously Cryopreserved Dental Stem Cells: Selected studies (Table 3) showed that the applied cryopreservation protocols, in periods ranging from 24h up to 2 years, have been allowed to recover DSC presenting suitable biological properties ⁵. DMSO in a concentration of 10% has been the most applied CA for DPSC (^{14,20,21,22,24}. Besides, it was possible to cryopreserve DPSC by the fast-freezing technique (-80 °C) for up to 1 year, thus eliminating the pre-freezing steps ²⁰. Lower DMSO concentration 3% (DPSC) and 5% (Human Tooth Germ Stem Cells - HTGSC) were effective to maintain cell biological properties when cryopreserved into a magnetic field freezer ¹⁴ or by adding NaB (20 µg) to CA ¹⁹. DSC cryopreserved with 10% glycerol and 10% ethylene glycol ^12,25 presented similar cell biological properties when compared to those cryopreserved with 10% DMSO. When Pluronic 188 (F68) was incorporated into 10% DMSO, a slight improvement in HTGSC viability was reported ²⁶.

Biological Properties of Stem Cells Isolated from Previously Cryopreserved Dental Tissues: Selected studies depicted to be possible isolate viable DSC from previously cryopreserved dental tissues. DPSC isolated from cryopreserved dental pulps , for up to 2 years in N₂^{11,13,23,27,28}, has been able to retain their ability to differentiate and express mesenchymal stem cell surface markers. In addition, pulp tissues' cryopreservation, for up to 6 months at -85°C mechanical freezer, has been shown to be a viable option to store DPSC ²⁷. 10% or 15% DMSO has not been deleterious for DSC biological properties, being better than vitrification agents such as glycerol and ethylene glycol ²⁷. However, a modified cryoprotectant (0.05 m glucose, 0.05 m sucrose and 1.5 m ethylene glycol) allowed > 70% of the survival rate of human follicle stem cells after 3 months of tissue storage ¹². The cells from cryopreserved human dental follicles (HDFSC) expressed mesenchymal stem cell markers in a manner similar to fresh cells ¹². Moreover, it has been possible to isolate viable DPSCs from diseased 6 month-cryopreserved pulp tissue ⁸; DPSCs has been able to maintain their biological properties and differentiate toward hepatic cells ²⁸.

DPSC obtained from whole-tooth cryopreserved dental pulps has been shown a small recuperation rate (20%) ^8,27. However, DPSC has been successfully recovered when the whole tooth has been submitted to a previous treatment with a Nd:YAG laser beam to generate micro-channels across the enamel and dentin layers, improving the cryoprotectant diffusion until the pulp tissue ²⁹.

Discussion

The presented systematic review has shown If DSC are cryopreserved after their isolation, then the biological properties such as CSR, adhesion ability, high proliferation rate, and differentiation ability (multipotency), remains quite similar to those observed in non-cryopreserved DSC. All of included studies applied DMSO in different concentration and 10% DMSO was the most-applied CA, providing post-thaw CSR above 90%. Interestingly, we could observe that DMSO concentrations under 10% were suitable for DPSC cryopreservation, thus allowing DPSC survival (-85°C or N₂ -196°C) ²⁷ and normal karyotype maintenance ³⁰.

The drawback related to DMSO consists on its inherent cytotoxicity, which would be detrimental for cell viability ³¹; thus it has been emphasized the need to reduce DMSO concentrations to ensure maximum cell yield post-thaw. However, Kumar et al. ²⁰ after evaluating eight different DMSO-based cryopreservation for DPSC (Table 3), could recover, in every tested protocols, DPSC able to differentiate in adipose-, osteoblast- and neural-like cells. They could conclude that uncontrolled freezing (fast freezing) at -80°C has been as effective as controlled freezing (slow freezing), independently of DMSO concentration ²⁰. Furthermore, 15% DMSO allowed the survival of DPSC from previously cryopreserved dental pulp tissues [-80°C mechanical freezer or N₂ -196°C for 6 months] ²⁷. In both studies ^20,27, every evaluated cell population has been retained their stemness surface markers (CD-90, CD-105, CD-73, CD-34, CD-45, CD-11b, CD-19, and HLA-DR) post-thaw depicting multipotency ability. DMSO concentrations up to 5% were effective (CSR ≥ 90%) when associated to chemical reagents or specific facilities producing magnetic fields ^14,19. Demirci et al. ¹⁹ showed that 5% DMSO supplemented with NaB provided a CSR similar to 10% DMSO, when HTGSC has been cryopreserved.

Ice crystal formation starts in temperatures between 0 and 4°C ³¹ generating a weak electric current which is able to disrupt cell membranes affecting CSR post-thaw. Magnetic fields could exert a positive influence during MSC cryopreservation since prevent ice crystal formation by allowing the water molecules to be instantly frozen. DSC's CSR in a DMSO-free environment has been increased 2 or 2.5 fold, depending on the configuration of the magnetic field ¹⁶. The addition of trehalose as CA's additive has not been improved DMSO's performance during cryopreservation of dental pulp or intact teeth under magnetic field ¹⁵. Thus, considering the limitations related to the selected studies, is possible to hypothesize that lower DMSO concentration and shorter preequilibration time are needed when cryopreservation is performed in magnetic-emitting field freezer. In fact, Lee Huang et al. ¹⁴ has been shown that DPSC's biological properties post-thaw, such as viability, adhesion, proliferation, MSC surface markers expression and multipotency, were quite similar after cryopreservation with 10% DMSO in a regular freezer or 3% DMSO, FBS-free medium, in a magnetic field; both DSC population presented lower rates of apoptotic cells, suggesting a freezing process with less cell damage under magnetic fields.

Another important factor to be considered during cryopreservation is the fetal bovine serum (FBS) concentration applied. FBS possess a central role in maintaining biological properties reducing the risk of cell damage during freezing and thawing cycles ³². FBS comprises a complex mixture of growth factors, proteins, carbohydrates, cytokines, and indispensable nutrients for cell development in vitro³³. In fact, high FBS concentration (90%) has induced less cell damage when compared to concentration under 90% concentration in animal cells ³². However, there is a concern regarding FBS due to risk related to pathogens transmissions and internalization of animal's proteins, which can unleash antigenic responses in the patient post-DSC implantation ³⁴. FBS content protein remain present in human cells after consecutive cell washings ³⁴ and may change the cell surface markers even as induce unforeseen modification of cell biology ³⁵. Due to such characteristics, FBS is not recommended in studies aiming to perform Stem Cell-Based Therapy (SC-BT) for clinical transition. A free-FBS cell-freezing (DMEM, 10% hetastarch, human albumin, DMSO) allowed for cryopreservation of swine DSCs for one year. Cryopreserved swine DSCs were positive for CD90, CD105, and CD146 ²². Besides, a commercially available FBS-medium (Cryostor-CS-10), worked as well as DMEM containing 10% FBS ²⁷.

When 10% DMSO was compared to another CAs, 10% glycerol and 10% ethylene glycol, a similar effect was observed for SCAP cell viability, colony-forming efficiency ²⁵. In addition, DPSC has been cryopreserved in different concentrations (0.5 M-1.5 M) of ethylene glycol, propylene glycol, or DMSO (1 month - LN2). The DPSC viability cryopreserved in 1.0 M DMSO (90.6%) and 1.5 M DMSO (91.0%) were significantly better than the other CAs at corresponding concentrations. A modified CA (0.05 M glucose, 0.05 M sucrose and 1.5 M ethylene glycol in PBS) allowed for previously cryopreserved DFCS present a CSR of 70% ¹⁶. Furthermore, chemical-like pluronic 188-F68 was slight better than DMSO 10% in preserving HSTG viability after 6 months of cryopreservation ²⁶.

The cryopreservation of previous isolated DSCs seems to be a process that maintains the proliferation and differentiation ability of these cells. It was interesting to note that is possible to store the teeth, after extraction, for up to 120 h in PBS (-4°C) to obtain high proliferative DPSCs ³⁶. It was also possible to recover 70% of DPSCs from a cryopreserved whole tooth after one month in liquid nitrogen ³⁶. Conversely, other studies showed that only small rates (around 20%) of viable DPSCs could be retrieved from cryopreserved healthy ²⁷ or diseased teeth ⁸. To be effective, the CA must penetrate by passive diffusion into the whole tissue ^27,36, which is affected by the surface area available for CA contact. A tooth is constituted by highly mineralized tissues surrounding the dental pulp, which presents just the apical foramen for CA access the pulp. To obtain viable cells from the pulp tissue, the apical foramen from human teeth should have a minimum dimension of 9.42 mm²³⁷ to allow preservation agents to be effective. Cells isolated from the apical (root) portion of rat teeth showed a high CSR when compared with cells from the other two sections-the middle and the coronal ¹⁵. Another alternative to improve the CA diffusion is to build up artificial openings to improve the CA diffusion through dental tissues. Gioventu et al. ²⁹ showed that cells from cryopreserved teeth, submitted to previous laser-piercing (the coronal portion), showed mesenchymal stem cells' morphology, immunophenotype, viability, and a proliferation rate similar to those cells from non-cryopreserved teeth.

This systematic review depicted that DSC could be cryopreserved, mainly with DMSO [10% - 20%], for periods up to 2 years maintaining their high proliferation rate, multipotency, karyotype and stem cells surface markers. The whole tooth cryopreservation (to isolate DSC) seemed to be a not reliable method and future investigation are needed in this filed. On the other hand, the cryopreservation of the intact pulp tissue seemed to constitute an attractive and reliable source to isolate DPSC. It could be valuable since avoids the immediate stem isolation before cryopreservation. However, evaluated cryopreservation times evaluated in selected studies has been too short; just 19% of the included studies have been evaluated the cryopreservation for periods longer than one year. Thus, the behavior of DSC in long times storage cannot be securely predicted. The conclusions presented in this systematic review should be interpreted with caution.

References

• ¹
  Gronthos, S; Mankani, M; Brahim, J; Robey, PG; Shi, S. Postnatal human dental pulp stem cells (DPSCs) in vitro and in vivo. Proc Natl Acad Sci U S A 2000;97:13625-13630.
• ²
  Miura, M; Gronthos, S; Zhao, M; Lu, B; Fisher, LW; Robey, PG; et al. SHED: stem cells from human exfoliated deciduous teeth. Proc Natl Acad Sci U S A 2003;100:5807-5812.
• ³
  Seo, BM; Miura, M; Gronthos, S; Bartold, PM; Batouli, S; Brahim, J; et al. Investigation of multipotent postnatal stem cells from human periodontal ligament. Lancet2004;364:149-155.
• ⁴
  Sonoyama, W; Liu, Y; Yamaza, T; Tuan, RS; Wang, S; Shi, S; et al. Characterization of the apical papilla and its residing stem cells from human immature permanent teeth: a pilot study. J Endod 2008;34:166-171.
• ⁵
  Dominici, M; Le Blanc, K; Mueller, I; Slaper-Cortenbach, I; Marini, F; Krause, D; et al. Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy2006;8:315-317.
• ⁶
  Park, YB; Kim, YY; Oh, SK; Chung, SG; Ku, SY; Kim, SH; et al. Alterations of proliferative and differentiation potentials of human embryonic stem cells during long-term culture. Exp Mol Med 2008;40:98-108.
• ⁷
  Dissanayake, SC; Che, ZM; Choi, SH; Lee, SJ; Kim, J;. Evaluation of vitrification for cryopreservation of teeth. J Periodontal Implant Sci 2010;40:111-118.
• ⁸
  Chen, YK; Huang, AH; Chan, AW; Shieh, TY; Lin, LM. Human dental pulp stem cells derived from different cryopreservation methods of human dental pulp tissues of diseased teeth. J Oral Pathol Med 2011;40:793-800.
• ⁹
  Oh, YH; Che, ZM; Hong, JC; Lee, EJ; Lee, SJ; Kim, J. Cryopreservation of human teeth for future organization of a tooth bank--a preliminary study. Cryobiology2005;51:322-329.
• ¹⁰
  Ahn, SJ; Jang, JH; Seo, JS; Cho, KM; Jung, SH; Lee, HW; et al. Influence of 2 cryopreservation methods to induce CCL-13 from dental pulp cells. J Endod 2013;39:1562-1566.
• ¹¹
  Lindemann, D; Werle, SB; Steffens, D; Garcia-Godoy, F; Pranke, P; Casagrande, L. Effects of cryopreservation on the characteristics of dental pulp stem cells of intact deciduous teeth. Arch Oral Biol 2014;59:970-976.
• ¹²
  Park, BW, Jang, SJ, Byun, JH, Kang, YH, Choi, MJ, Park, WU, et al. Cryopreservation of human dental follicle tissue for use as a resource of autologous mesenchymal stem cells. J Tissue Eng Regen Med2014.
• ¹³
  Lee, SY, Chiang, PC, Tsai, YH, Tsai, SY, Jeng, JH, Kawata, T, et al. Effects of cryopreservation of intact teeth on the isolated dental pulp stem cells. J Endod2010;36:1336-1340.
• ¹⁴
  Lee, SY; Huang, GW; Shiung, JN; Huang, YH; Jeng, JH; Kuo, TF; et al. Magnetic cryopreservation for dental pulp stem cells. Cells Tissues Organs 2012;196:23-33.
• ¹⁵
  Lee, SY; Sun, CH; Kuo, TF; Huang, YH; Jeng, JH; Yang, JC; et al. Determination of cryoprotectant for magnetic cryopreservation of dental pulp tissue. Tissue Eng Part C Methods 2012;18:397-407.
• ¹⁶
  Lin, SL; Chang, WJ; Lin, CY; Hsieh, SC; Lee, SY; Fan, KH; et al. Static magnetic field increases survival rate of dental pulp stem cells during DMSO-free cryopreservation. Electromagn Biol Med 2015;34:302-308.
• ¹⁷
  Davies, OG; Smith, AJ; Cooper, PR; Shelton, RM; Scheven, BA. The effects of cryopreservation on cells isolated from adipose, bone marrow and dental pulp tissues. Cryobiology2014;69:342-347.
• ¹⁸
  Moher, D; Liberati, A; Tetzlaf, J; Altman, DG. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Med 2009;6:e1000097.
• ¹⁹
  Demirci, S; Dogan, A; Sisli, B; Sahin, F;. Boron increases the cell viability of mesenchymal stem cells after long-term cryopreservation. Cryobiology2014;68:139-146.
• ²⁰
  Kumar, A; Bhattacharyya, S; Rattan, V. Effect of uncontrolled freezing on biological characteristics of human dental pulp stem cells. Cell Tissue Bank 2015;16:513-522.
• ²¹
  Papaccio, G; Graziano, A; d'Aquino, R; Graziano, MF; Pirozzi, G; Menditti D; et al. Long-term cryopreservation of dental pulp stem cells (SBP-DPSCs) and their differentiated osteoblasts: a cell source for tissue repair. J Cell Physiol 2006;208:319-325.
• ²²
  Kodonas, K; Gogos, C; Papadimitriou, S; Kouzi-Koliakou, K; Tziafas, D. Experimental formation of dentin-like structure in the root canal implant model using cryopreserved swine dental pulp progenitor cells. J Endod 2012;38:913-919.
• ²³
  Ma, L; Makino, Y; Yamaza, H; Akiyama, K; Hoshino, Y; Song, G; et al. Cryopreserved dental pulp tissues of exfoliated deciduous teeth is a feasible stem cell resource for regenerative medicine. PLoS One 2012;7:e51777.
• ²⁴
  Vasconcelos, RG; Ribeiro, RA; Vasconcelos, MG; Lima, KC; Barboza, CA. In vitro comparative analysis of cryopreservation of undifferentiated mesenchymal cells derived from human periodontal ligament. Cell Tissue Bank 2012;13:461-469.
• ²⁵
  Ding, G; Wang, W; Liu, Y; An, Y; Zhang, C; Shi, S; et al. Effect of cryopreservation on biological and immunological properties of stem cells from apical papilla. J Cell Physiol 2010;223:415-422.
• ²⁶
  Dogan, A; Yalvac, ME; Yilmaz, A; Rizvanov, A; Sahin, F. Effect of F68 on cryopreservation of mesenchymal stem cells derived from human tooth germ. Appl Biochem Biotechnol 2013;171:1819-1831.
• ²⁷
  Woods, EJ; Perry, BC; Hockema, JJ; Larson, L; Zhou, D; Goebel, WS. Optimized cryopreservation method for human dental pulp-derived stem cells and their tissues of origin for banking and clinical use. Cryobiology2009;59:150-157.
• ²⁸
  Chen, YK; Huang, AH; Chan, AW; Lin, LM. Human dental pulp stem cells derived from cryopreserved dental pulp tissues of vital extracted teeth with disease demonstrate hepatic-like differentiation. J Tissue Eng Regen Med 2013.
• ²⁹
  Gioventu, S; Andriolo, G; Bonino, F; Frasca, S; Lazzari, L; Montelatici, E; et al. A novel method for banking dental pulp stem cells. Transfus Apher Sci 2012;47:199-206.
• ³⁰
  Yalvac, ME; Ramazanoglu, M; Tekguc, M; Bayrak, OF; Shafigullina, AK; Salafutdinov, II; et al. Human tooth germ stem cells preserve neuro-protective effects after long-term cryo-preservation. Curr Neurovasc Res 2010;7:49-58.
• ³¹
  Kaku, M, Kamada, H, Kawata, T, Koseki, H, Abedini, S, Kojima, S, et al. Cryopreservation of periodontal ligament cells with magnetic field for tooth banking. Cryobiology2010;61:73-78.
• ³²
  Renzi, S; Lombardo, T; Dotti, S; Dessi, SS; De Blasio, P; Ferrari, M. Mesenchymal stromal cell cryopreservation. Biopreserv Biobank 2012;10:276-281.
• ³³
  Fekete, N; Rojewski, MT; Furst, D; Kreja, L; Ignatius, A; Dausend, J; et al. GMP-compliant isolation and large-scale expansion of bone marrow-derived MSC. PLoS One 2012;7:e43255.
• ³⁴
  Haque, N; Kasim, NH; Rahman, MT. Optimization of pre-transplantation conditions to enhance the efficacy of mesenchymal stem cells. Int J Biol Sci 2015;11:324-334.
• ³⁵
  Mannello, F; Tonti, GA. Concise review: no breakthroughs for human mesenchymal and embryonic stem cell culture: conditioned medium, feeder layer, or feeder-free; medium with fetal calf serum, human serum, or enriched plasma; serum-free, serum replacement nonconditioned medium, or ad hoc formula? All that glitters is not gold! Stem Cells 2007;25:1603-1609.
• ³⁶
  Perry, BC; Zhou, D; Wu, X; Yang, FC; Byers, MA; Chu, TM; et al. Collection, cryopreservation, and characterization of human dental pulp-derived mesenchymal stem cells for banking and clinical use. Tissue Eng Part C Methods 2008;14:149-156.
• ³⁷
  Temmerman, L; Beele, H; Dermaut, LR; Van Maele, G; De Pauw, GA. Influence of cryopreservation on the pulpal tissue of immature third molars in vitro. Cell Tissue Bank 2010;11:281-289.

Publication Dates

• Publication in this collection
  Oct-Dec 2016

History

• Received
  15 Apr 2016
• Accepted
  12 Aug 2016