Alterations of cytological and karyological profile of human mesenchymal stem cells during in vitro culturing

Uterine Leiomyomas with an Apparently Normal Karyotype Comprise Minor Heteroploid Subpopulations Differently Represented in vivo and in vitro

In the present study, we aimed to check whether uterine leiomyomas (ULs) with an apparently normal karyotype in vitro comprise "hidden" cell subpopulations with numerical chromosome abnormalities (heteroploid cells). A total of 32 ULs obtained from 32 patients were analyzed in the study. Each UL was sampled for in vivo and in vitro cytogenetic studies. Karyotyping was performed on metaphase preparations from the cultured UL samples. A normal karyotype was revealed in 20 out of the 32 ULs, of which 9 were selected for further study based on the good quality of the interphase preparations. Then, using interphase FISH with centromeric DNA probes, we analyzed the copy number of chromosomes 7 and 16 in 1,000 uncultured and 1,000 cultured cells of each selected UL. All of the ULs included both disomic cells representing a predominant subpopulation and heteroploid cells reaching a maximum frequency of 21.6% (mean 9.8%) in vivo and 11.5% (mean 6.1%) in vitro. The spectrum of heteroploid cells was similar in vivo and in vitro and mostly consisted of monosomic and tetrasomic cells. However, their frequencies in the cultured samples differed from those in the uncultured ones: while the monosomic cells decreased in number, the tetrasomic cells became more numerous. The frequency of either monosomic or tetrasomic cells both in vivo and in vitro was not associated with the presence of MED12 exon 2 mutations in the tumors. Our results suggest that ULs with an apparently normal karyotype consist of both karyotypically normal and heteroploid cells, implying that the occurrence of minor cell subpopulations with numerical chromosome abnormalities may be considered a characteristic of UL tumorigenesis. Different frequencies of heteroploid cells in vivo and in vitro suggest their dependence on microenvironmental conditions, thus providing a pathway for regulation of their propagation, which may be important for the UL pathogenesis.

Age and Serum AMH and FSH Levels as Predictors of the Number of Oocytes Retrieved from Chromosomal Translocation Carriers after Controlled Ovarian Hyperstimulation: Applicability and Limitations

We studied the impact of age and the serum anti-Müllerian hormone (AMH)/follicle-stimulating hormone (FSH) levels on the number of cumulus–oocyte complexes (COCs) retrieved from female reciprocal and Robertsonian translocation carriers after controlled ovarian hyperstimulation (COH). The number of COCs retrieved after COH was retrospectively analyzed in female translocation carriers and 46,XX partners of male translocation carriers from 100 couples. The median number of COCs varied from nine to 16 and did not differ among subgroups of women categorized by age, presence and type of a translocation. The number of COCs correlated negatively with the woman’s age in both the reciprocal and the Robertsonian translocation carriers, while in 46,XX women no correlation was detected. The number of COCs did not differ between the reciprocal and the Robertsonian translocation carriers aged either <35 or ≥35 years. In translocation carriers, the number of COCs correlated with the serum AMH level only in the younger-age subgroups; the correlation was strong positive in reciprocal and moderate positive in Robertsonian translocation carriers. The 46,XX women aged both <35 and ≥35 years showed similar moderate positive correlations. Across all subgroups, the number of COCs correlated moderately negatively with the serum FSH level only in Robertsonian translocation carriers aged <35 years. Our results suggest that chromosomal translocations per se do not increase the risk of poor oocyte retrieval outcome after COH. In translocation carriers, oocyte retrieval outcome depends to a large extent on their age. The serum AMH level strongly predicts oocyte retrieval outcomes only in young reciprocal translocation carriers, while the serum FSH level has a moderate predictive value in young Robertsonian translocation carriers.

Comparison of chromosomal instability of human amniocytes in primary and long-term cultures in AmnioMAX II and DMEM media: A cross-sectional study

Background

The genomic stability of stem cells to be used in cell therapy and other clinical applications is absolutely critical. In this regard, the relationship between in vitro expansion and the chromosomal instability (CIN), especially in human amniotic fluid cells (hAFCs) has not yet been completely elucidated.

Objective

To investigate the CIN of hAFCs in primary and long-term cultures and two different culture mediums.

Materials and Methods

After completing prenatal genetic diagnoses (PND) using karyotype technique and chromosomal analysis, a total of 15 samples of hAFCs from 650 samples were randomly selected and cultured in two different mediums as AmnioMAX II and DMEM. Then, proliferative cells were fixed on the slide to be used in standard chromosome G-banding analysis. Also, the senescent cells were screened for aneuploidy considering 8 chromosomes by FISH technique using two probe sets including PID I (X-13-18-21) & PID II (Y-15-16-22).

Results

Karyotype and interphase fluorescence in situ hybridization (iFISH) results from 650 patients who were referred for prenatal genetic diagnosis showed that only 6 out of them had culture- derived CIN as polyploidy, including mosaic diploid-triploid and diploid-tetraploid. Moreover, the investigation of aneuploidies in senesced hAFCs demonstrated the rate of total chromosomal abnormalities as 4.3% and 9.9% in AmnioMAX- and DMEM-cultured hAFCs, respectively.

Conclusion

hAFCs showed a low rate of CIN in two AmnioMAX II and DMEM mediums and also in the proliferative and senescent phases. Therefore, they could be considered as an attractive stem cell source with therapeutic potential in regenerative medicine.

Reproductive History of a Woman With 8p and 18p Genetic Imbalance and Minor Phenotypic Abnormalities

We report on the phenotype and the reproductive history of an adult female patient with an unbalanced karyotype: 8p23 and 18p11.3 terminal deletions and 8p22 duplication. The indication for karyotyping of the 28-year-old patient was a structural rearrangement in her miscarriage specimen: 45,ХХ,der(8;18)t(8;18)(p23;p11.3). Unexpectedly, the patient had the same karyotype with only one normal chromosome 8, one normal chromosome 18, and a derivative chromosome, which was a product of chromosomes 8 and 18 fusion with loss of their short arm terminal regions. Fluorescence in situ hybridization revealed that derivative chromosome was a pseudodicentric with an active centromere of chromosome 8. Array comparative genomic hybridization confirmed 8p and 18p terminal deletions and additionally revealed 8p22 duplication with a total of 43 OMIM annotated genes being affected by the rearrangement. The patient had minor facial and cranial dysmorphia and no pronounced physical or mental abnormalities. She was socially normal, had higher education and had been married since the age of 26 years. Considering genetic counseling, the patient had decided to conceive the next pregnancy through in vitro fertilization (IVF) with preimplantation genetic testing for structural chromosomal aberrations (PGT-SR). She underwent four IVF/PGT-SR cycles with a total of 25 oocytes obtained and a total of 10 embryos analyzed. Only one embryo was balanced regarding chromosomes 8 and 18, while the others were unbalanced and demonstrated different combinations of the normal chromosomes 8 and 18 and the derivative chromosome. The balanced embryo was transferred, but the pregnancy was not registered. After four unsuccessful IVF/PGT-SR cycles, the patient conceived naturally. Non-invasive prenatal testing showed additional chromosome 18. The prenatal cytogenetic analysis of chorionic villi revealed an abnormal karyotype: 46,ХХ,der(8;18)t(8;18)(p23;p11.3)mat,+18. The pregnancy was terminated for medical reasons. The patient has a strong intention to conceive a karyotypically normal fetus. However, genetic counseling regarding this issue is highly challenging. Taking into account a very low chance of balanced gametes, emotional stress caused by numerous unsuccessful attempts to conceive a balanced embryo and increasing age of the patient, an IVF cycle with a donor oocyte should probably be considered.

Principal Criteria for Evaluating the Quality, Safety and Efficacy of hMSC-Based Products in Clinical Practice: Current Approaches and Challenges

Human Mesenchymal Stem Cells (hMSCs) play an important role as new therapeutic alternatives in advanced therapies and regenerative medicine thanks to their regenerative and immunomodulatory properties, and ability to migrate to the exact area of injury. These properties have made hMSCs one of the more promising cellular active substances at present, particularly in terms of the development of new and innovative hMSC-based products. Currently, numerous clinical trials are being conducted to evaluate the therapeutic activity of hMSC-based products on specific targets. Given the rapidly growing number of hMSC clinical trials in recent years and the complexity of these products due to their cellular component characteristics and medicinal product status, there is a greater need to define more stringent, specific, and harmonized requirements to characterize the quality of the hMSCs and enhance the analysis of their safety and efficacy in final products to be administered to patients. These requirements should be implemented throughout the manufacturing process to guarantee the function and integrity of hMSCs and to ensure that the hMSC-based final product consistently meets its specifications across batches. This paper describes the principal phases involved in the design of the manufacturing process and updates the specific technical requirements needed to address the appropriate clinical use of hMSC-based products. The challenges and limitations to evaluating the safety, efficacy, and quality of hMSCs have been also reviewed and discussed.

Inter-Cell and Inter-Chromosome Variability of 5-Hydroxymethylcytosine Patterns in Noncultured Human Embryonic and Extraembryonic Cells

5-hydroxymethylcytosine (5hmC) is an oxidative derivative of 5-methylcytosine (5mC). Recent studies have revealed a sharp difference in the levels of 5hmC in 2 opposite DNA strands of a given chromosome and a chromosome-wide cell-to-cell variability in mammalian cells. This asymmetric 5hmC distribution was found in cultured cells, which may not fully mimic in vivo epigenetic processes. We have checked whether inter-chromosome and inter-cell variability of 5hmC patterns is typical for noncultured human cells. Using indirect immunofluorescence, we analyzed the localization of 5hmC and its co-distribution with 5mC on direct preparations of mitotically active cells from human embryonic lung and chorionic cytotrophoblast samples. We demonstrated 3 types of chromosomes according to the 5hmC accumulation pattern: hydroxymethylated (5hmC in both sister chromatids), hemihydroxymethylated (5hmC in only 1 sister chromatid), and nonhydroxymethylated ones. Each accumulation type was not specific to any particular chromosome, resulting in different 5hmC patterns between homologous chromosomes, among chromosomes within each metaphase plate, among metaphases in one tissue, and between the tissues. The 5mC distribution was stable: chromosomes were methylated in R-bands and, especially in embryonic lung cells, in the heterochromatic regions 1q12, 9q12, and 16q11.2. Our results provide the first evidence of inter-cell and inter-chromosome variability of 5hmC patterns in human noncultured embryonic and extraembryonic cells.

Historical Perspectives and Advances in Mesenchymal Stem Cell Research for the Treatment of Liver Diseases

Liver transplantation is the only effective therapy for patients with decompensated cirrhosis and fulminant liver failure. However, due to a shortage of donor livers and complications associated with immune suppression, there is an urgent need for new therapeutic strategies for patients with end-stage liver diseases. Given their unique function in self-renewal and differentiation potential, stem cells might be used to regenerate damaged liver tissue. Recent studies have shown that stem cell-based therapies can improve liver function in a mouse model of hepatic failure. Moreover, acellular liver scaffolds seeded with hepatocytes produced functional bioengineered livers for organ transplantation in preclinical studies. The therapeutic potential of stem cells or their differentiated progenies will depend on their capacity to differentiate into mature and functional cell types after transplantation. It will also be important to devise methods to overcome their genomic instability, immune reactivity, and tumorigenic potential. We review directions and advances in the use of mesenchymal stem cells and their derived hepatocytes for liver regeneration. We also discuss the potential applications of hepatocytes derived from human pluripotent stem cells and challenges to using these cells in treating end-stage liver disease.

The future of mesenchymal stem cell-based therapeutic approaches for cancer - From cells to ghosts

Mesenchymal stem cells (MSCs) are multipotent stromal cells which can differentiate into a variety of cell types including osteoblasts, adipocytes and chondrocytes. They are normally resident in adipose tissue, bone marrow and the umbilical cord, but can also be found in other tissues and are known to be recruited to sites of wound healing as well as growing tumours. The therapeutic potential of MSCs has been explored in a number of phase I/II and III clinical trials, of which several were targeted against graft-versus-host disease and to support engraftment of haematopoietic stem cells (HSCs), but currently only very few in the oncology field. There are now three clinical trials either ongoing or recruiting patients that use MSCs to treat tumour disease. In these, MSCs target gastrointestinal, lung and ovarian cancer, respectively. The first study uses MSCs loaded with a HSV-TK expression construct under the control of the CCL5 promoter, and has recently reported successful completion of Phase I/II. While no adverse side effects were seen during this study, no outcomes with respect to therapeutic benefits have been published. The other clinical trials targeting lung and ovarian cancer will be using MSCs expressing cytokines as therapeutic payload. Despite these encouraging early steps towards their clinical use, many questions are still unanswered regarding the biology of MSCs in normal and pathophysiological settings. In this review, in addition to summarising the current state of MSC-based therapeutic approaches for cancer, we will describe the remaining questions, obstacles and risks, as well as novel developments such as MSC-derived nanoghosts.

Genome-wide 5-hydroxymethylcytosine patterns in human spermatogenesis are associated with semen quality

We performed immunofluorescent analysis of DNA hydroxymethylation and methylation in human testicular spermatogenic cells from azoospermic patients and ejaculated spermatozoa from sperm donors and patients from infertile couples. In contrast to methylation which was present throughout spermatogenesis, hydroxymethylation was either high or almost undetectable in both spermatogenic cells and ejaculated spermatozoa. On testicular cytogenetic preparations, 5-hydroxymethylcytosine was undetectable in mitotic and meiotic chromosomes, and was present exclusively in interphase spermatogonia Ad and in a minor spermatid population. The proportions of hydroxymethylated and non-hydroxymethylated diploid and haploid nuclei were similar among samples, suggesting that the observed alterations of 5-hydroxymethylcytosine patterns in differentiating spermatogenic cells are programmed. In ejaculates, a few spermatozoa had high 5-hydroxymethylcytosine level, while in the other ones hydroxymethylation was almost undetectable. The percentage of highly hydroxymethylated (5-hydroxymethylcytosine-positive) spermatozoa varied strongly among individuals. In patients from infertile couples, it was higher than in sperm donors (P<0.0001) and varied in a wider range: 0.12-21.24% versus 0.02-0.46%. The percentage of highly hydroxymethylated spermatozoa correlated strongly negatively with the indicators of good semen quality – normal morphology (r=-0.567, P<0.0001) and normal head morphology (r=-0.609, P<0.0001) – and strongly positively with the indicator of poor semen quality: sperm DNA fragmentation (r=0.46, P=0.001). Thus, the immunocytochemically detected increase of 5hmC in individual spermatozoa is associated with infertility in a couple and with deterioration of sperm parameters. We hypothesize that this increase is not programmed, but represents an induced abnormality and, therefore, it can be potentially used as a novel indicator of semen quality.

Exosomes from mesenchymal stem cells induce the conversion of hepatocytes into progenitor oval cells

Background

We previously reported that mesenchymal stem cells (MSCs) possess therapeutic effects in a murine model of carbon tetrachloride-induced acute liver failure. In the study, we observed that the majority of repopulated hepatocytes were of recipient origin and were adjacent to transplanted MSCs; only a low percentage of repopulated hepatocytes were from transplanted MSCs. The findings indicate that MSCs guided the formation of new hepatocytes. Exosomes are important messengers for paracrine signaling delivery. The aim of this study is to investigate the paracrine effects, in particular, the effects of exosomes from MSCs, on hepatocytes.

Methods

Mature hepatocytes were isolated from murine liver by a two-step perfusion method with collagenase digestion. MSCs were obtained from murine bone marrow, and conditioned medium (CM) from MSC culture was then collected. Time-lapse imaging was used for observation of cell morphological change induced by CM on hepatocytes. In addition, expression of markers for hepatic progenitors including oval cells, intrahepatic stem cells, and hepatoblasts were analyzed.

Results

Treatment with the CM promoted the formation of small oval cells from hepatocytes; time-lapse imaging demonstrated the change from epithelial to oval cell morphology at the single hepatocyte level. Additionally, expression of EpCAM and OC2, markers of hepatic oval cells, was upregulated. Also, the number of EpCAM^high cells was increased after CM treatment. The EpCAM^high small oval cells possessed colony-formation ability; they also expressed cytokeratin 18 and were able to store glycogen upon induction of hepatic differentiation. Furthermore, exosomes from MSC-CM could induce the conversion of mature hepatocytes to EpCAM^high small oval cells.

Conclusions

In summary, paracrine signaling through exosomes from MSCs induce the conversion of hepatocytes into hepatic oval cells, a mechanism of action which has not been reported regarding the therapeutic potentials of MSCs in liver regeneration. Exosomes from MSCs may therefore be used to treat liver diseases. Further studies are required for proof of concept of this approach.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0560-z) contains supplementary material, which is available to authorized users.

Effects of human umbilical cord blood mononuclear cells on respiratory system mechanics in a murine model of neonatal lung injury

BACKGROUND

Mononuclear cells (MNCs) have well-documented beneficial effects in a wide range of adult pulmonary diseases. The effects of human umbilical cord blood-derived MNCs on neonatal lung injury, highly relevant for potential autologous application in preterm newborns at risk for bronchopulmonary dysplasia (BPD), remain incompletely established. The aim of this study was to determine the long-term morphologic and functional effects of systemically delivered MNCs in a murine model of neonatal lung injury.

MATERIALS AND METHODS

MNCs from cryopreserved cord blood (1 × 10⁶ cells per pup) were given intravenously to newborn mice exposed to 90% O₂ from birth; controls received cord blood total nucleated cells (TNCs) or granular cells, or equal volume vehicle buffer (sham controls). In order to avoid immune rejection, we used SCID mice as recipients. Lung mechanics (flexiVent™), engraftment, growth, and alveolarization were evaluated eight weeks postinfusion.

RESULTS

Systemic MNC administration to hyperoxia-exposed newborn mice resulted in significant attenuation of methacholine-induced airway hyperreactivity, leading to reduction of central airway resistance to normoxic levels. These bronchial effects were associated with mild improvement of alveolarization, lung compliance, and elastance. TNCs had no effects on alveolar remodeling and were associated with worsened methacholine-induced bronchial hyperreactivity. Granular cell administration resulted in a marked morphologic and functional emphysematous phenotype, associated with high mortality. Pulmonary donor cell engraftment was sporadic in all groups.

CONCLUSIONS

These results suggest that cord blood MNCs may have a cell type-specific role in therapy of pulmonary conditions characterized by increased airway resistance, such as BPD and asthma. Future studies need to determine the active MNC subtype(s), their mechanisms of action, and optimal purification methods to minimize granular cell contamination.

Chromosomal Abnormalities in Embryonic and Somatic Stem Cells

The potential use of stem cells (SCs) for tissue engineering, regenerative medicine, disease modeling, toxicological studies, drug delivery, and as in vitro model for the study of basic developmental processes implies large-scale in vitro culture. Here, after a brief description of the main techniques used for karyotype analysis, we will give a detailed overview of the chromosome abnormalities described in pluripotent (embryonic and induced pluripotent SCs) and somatic SCs, and the possible causes of their origin during culture.

Stem Cells and Their Mediators - Next Generation Therapy for Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) remains a major complication of premature birth. Despite great achievements in perinatal medicine over the past decades, there is no treatment for BPD. Recent insights into the biology of stem/progenitor cells have ignited the hope of regenerating damaged organs. Animal experiments revealed promising lung protection/regeneration with stem/progenitor cells in experimental models of BPD and led to first clinical studies in infants. However, these therapies are still experimental and knowledge on the exact mechanisms of action of these cells is limited. Furthermore, heterogeneity of the therapeutic cell populations and missing potency assays currently limit our ability to predict a cell product’s efficacy. Here, we review the therapeutic potential of mesenchymal stromal, endothelial progenitor, and amniotic epithelial cells for BPD. Current knowledge on the mechanisms behind the beneficial effects of stem cells is briefly summarized. Finally, we discuss the obstacles constraining their transition from bench-to-bedside and present potential approaches to overcome them.

Chromosome hydroxymethylation patterns in human zygotes and cleavage-stage embryos

We report the sequential changes in 5-hydroxymethylcytosine (5hmC) patterns in the genome of human preimplantation embryos during DNA methylation reprogramming. We have studied chromosome hydroxymethylation and methylation patterns in triploid zygotes and blastomeres of cleavage-stage embryos. Using indirect immunofluorescence, we have analyzed the localization of 5hmC and its co-distribution with 5-methylcytosine (5mC) on the QFH-banded metaphase chromosomes. In zygotes, 5hmC accumulates in both parental chromosome sets, but hydroxymethylation is more intensive in the poorly methylated paternal set. In the maternal set, chromosomes are highly methylated, but contain little 5hmC. Hydroxymethylation is highly region specific in both parental chromosome sets: hydroxymethylated loci correspond to R-bands, but not G-bands, and have well-defined borders, which coincide with the R/G-band boundaries. The centromeric regions and heterochromatin at 1q12, 9q12, 16q11.2, and Yq12 contain little 5mC and no 5hmC. We hypothesize that 5hmC may mark structural/functional genome 'units' corresponding to chromosome bands in the newly formed zygotic genome. In addition, we suggest that the hydroxymethylation of R-bands in zygotes can be treated as a new characteristic distinguishing them from G-bands. At cleavages, chromosomes with asymmetrical hydroxymethylation of sister chromatids appear. They decrease in number during cleavages, whereas totally non-hydroxymethylated chromosomes become numerous. Taken together, our findings suggest that, in the zygotic genome, 5hmC is distributed selectively and its pattern is determined by both parental origin of chromosomes and type of chromosome bands - R, G, or C. At cleavages, chromosome hydroxymethylation pattern is dynamically changed due to passive and non-selective overall loss of 5hmC, which coincides with that of 5mC.

Activation of human mesenchymal stem cells impacts their therapeutic abilities in lung injury by increasing interleukin (IL)-10 and IL-1RN levels

Acute respiratory distress syndrome (ARDS) is an important cause of morbidity and mortality, with no currently effective therapies. Several preclinical studies have shown that human mesenchymal stem cells (hMSCs) have therapeutic potential for patients with ARDS because of their immunomodulatory properties. The clinical use of hMSCs has some limitations, such as the extensive manipulation required to isolate the cells from bone marrow aspirates and the heterogeneity in their anti-inflammatory effect in animal models and clinical trials. The objective of this study was to improve the protective anti-inflammatory capacity of hMSCs by evaluating the consequences of preactivating hMSCs before use in a murine model of ARDS. We injected endotoxemic mice with minimally manipulated hMSCs isolated from the bone marrow of vertebral bodies with or without prior activation with serum from ARDS patients. Minimally manipulated hMSCs were more efficient at reducing lung inflammation compared with isolated and in vitro expanded hMSCs obtained from bone marrow aspirates. Where the most important effect was observed was with the activated hMSCs, independent of their source, which resulted in increased expression of interleukin (IL)-10 and IL-1 receptor antagonist (RN), which was associated with enhancement of their protective capacity by reduction of the lung injury score, development of pulmonary edema, and accumulation of bronchoalveolar lavage inflammatory cells and cytokines compared with nonactivated cells. This study demonstrates that a low manipulation during hMSC isolation and expansion increases, together with preactivation prior to the therapeutic use of hMSCs, would ensure an appropriate immunomodulatory phenotype of the hMSCs, reducing the heterogeneity in their anti-inflammatory effect.

Human mesenchymal stem cells: a bank perspective on the isolation, characterization and potential of alternative sources for the regeneration of musculoskeletal tissues

The continuous discovery of human mesenchymal stem cells (hMSCs) in different tissues is stirring up a tremendous interest as a cell source for regenerative medicine therapies. Historically, hMSCs have been always considered a sub-population of mononuclear cells present in the bone marrow (BM). Although BM-hMSCs are still nowadays considered as the most promising mesenchymal stem cell population to reach the clinics due to their capacity to differentiate into multiple tissues, hMSCs derived from other adult and fetal tissues have also demonstrated to possess similar differentiation capacities. Furthermore, different reports have highlighted a higher recurrence of hMSCs in some of these tissues as compared to BM. This offer a fascinating panorama for cell banking, since the creation of a stem cell factory could be envisioned where hMSCs are stocked and used for ad hoc clinical applications. In this review, we summarize the main findings and state of the art in hMSCs isolation, characterization, and differentiation from alternative tissue sources and we attempt to compare their potency for musculoskeletal regeneration.

Risk of tumorigenicity in mesenchymal stromal cell-based therapies--bridging scientific observations and regulatory viewpoints

In the past decade, the therapeutic value of mesenchymal stromal cells (MSCs) has been studied in various indications, thereby taking advantage of their immunosuppressive properties. Easy procurement from bone marrow, adipose tissue or other sources and conventional in vitro expansion culture have made their clinical use attractive. Bridging the gap between current scientific knowledge and regulatory prospects on the transformation potential and possible tumorigenicity of MSCs, the Cell Products Working Party and the Committee for Advanced Therapies organized a meeting with leading European experts in the field of MSCs. This meeting elucidated the risk of potential tumorigenicity related to MSC-based therapies from two angles: the scientific perspective and the regulatory point of view. The conclusions of this meeting, including the current regulatory thinking on quality, nonclinical and clinical aspects for MSCs, are presented in this review, leading to a clearer way forward for the development of such products.