The effect of serum types on Chondrogenic differentiation of adipose-derived stem cells

Optimizing aseptic and serum milieu for the isolation of human whole umbilical cord tissue-derived mesenchymal stem cells

Background

Mesenchymal stem cells (MSCs) have become an attractive tool for tissue engineering and targets in clinical transplantation due to their regeneration potential and immune-suppressive capacity. The human umbilical cord, which is discarded at birth, can provide an inexhaustible source of stem cells for therapy. They are reported to contain immune privilege cells which may be suitable for allogenic-based therapies. However, the use of MSCs for therapeutic application is based on their subsequent large-scale in vitro expansion. A fast and efficient protocol for the generation of large quantities of MSCs is required to meet the clinical demand and biomedical research needs.

Results

MSCs were isolated from the umbilical cord by explants and enzymatic digestion and cultured in the appropriate growth medium resulted in the propagation of more than 1X 108 cells within 15 days from the single umbilical cord.

Conclusion

The isolation efficiency, cell yield, colony-forming unit fibroblast (CFU-F), growth kinetics, phenotypic characteristics of UCMSCs were determined.

Human gelatin-based composite hydrogels for osteochondral tissue engineering and their adaptation into bioinks for extrusion, inkjet, and digital light processing bioprinting

The investigation of novel hydrogel systems allows for the study of relationships between biomaterials, cells, and other factors within osteochondral tissue engineering. Three-dimensional (3D) printing is a popular research method that can allow for further interrogation of these questions via the fabrication of 3D hydrogel environments that mimic tissue-specific, complex architectures. However, the adaptation of promising hydrogel biomaterial systems into 3D-printable bioinks remains a challenge. Here, we delineated an approach to that process. First, we characterized a novel methacryloylated gelatin composite hydrogel system and assessed how calcium phosphate and glycosaminoglycan additives upregulated bone- and cartilage-like matrix deposition and certain genetic markers of differentiation within human mesenchymal stem cells (hMSCs), such as RUNX2 and SOX9. Then, new assays were developed and utilized to study the effects of xanthan gum and nanofibrillated cellulose, which allowed for cohesive fiber deposition, reliable droplet formation, and non-fracturing digital light processing (DLP)-printed constructs within extrusion, inkjet, and DLP techniques, respectively. Finally, these bioinks were used to 3D print constructs containing viable encapsulated hMSCs over a 7 d period, where DLP printed constructs facilitated the highest observed increase in cell number over 7 d (∼2.4×). The results presented here describe the promotion of osteochondral phenotypes via these novel composite hydrogel formulations, establish their ability to bioprint viable, cell-encapsulating constructs using three different 3D printing methods on multiple bioprinters, and document how a library of modular bioink additives affected those physicochemical properties important to printability.

Synovial fluid niche promoted differentiation of dental follicle mesenchymal stem cells toward chondrogenesis in rheumatoid arthritis

Objectives

In this study, we aimed to investigate the differentiation potential of dental follicle mesenchymal stem cells (MSCs) in the synovial fluid (SF) niche of early-onset or end-stage rheumatoid arthritis (RA).

Patients and methods

Between May 2020 and January 2021, six patients (1 male, 5 females; mean age: 57.5±11.2 years; range, 49 to 65 years) who were diagnosed with RA with the indication of SF aspiration were included in the study. The third passage dental follicle stem cells (DFSCs) were cocultured with fresh SF samples of end-stage or early-onset RA patients in micromass culture system for 21 days. SF samples were analyzed for secreted cytokines. Chondrogenic markers (CD49e, CD49f) were analyzed in DFSCs, gene expression analysis was performed for the expressions of Col I, Col II, Aggrecan and Sox-9, and histochemical analysis was performed by staining three-dimensional pellets with anti-collagen II antibody. The neutralization assay was performed with anti-interleukin (IL)-6, anti-interferon-gamma (IFN-g), and anti-IL-1beta(b).

Results

The high levels of IL-1b and IL-6 were observed in end-stage RA patients’ SF samples compared to the early-onset patients (p<0.05). The CD49e and CD49f expressions in DFSCs were significantly higher in the SF samples of end-stage RA patients (p<0.05). Also, the Col II, Sox-9 and Aggrecan messenger ribonucleic acid (mRNA) expressions increased in the DFSCs, when cultured with end-stage RA patients’ SF samples (p<0.01). Collagen-II expression in histochemical analysis of micromass pellets was higher in the DFSCs cultured with end-stage RA patients’ SF samples. The neutralization of IL-6 significantly decreased the CD49e and CD49f expressions (p<0.05).

Conclusion

The high levels of IL-6 in SF niche of end-stage RA patients were found to differentiate DFSCs toward chondrogenesis. Based on these findings, DFSCs can be used as a new cell-based treatment in RA patients for the cartilage damage.

'Fat chance': a review of adipose tissue engineering and its role in plastic and reconstructive surgery

Soft tissue reconstruction remains a continuing challenge for plastic and reconstructive surgeons. Standard methods of reconstruction such as local tissue transfer and free autologous tissue transfer are successful in addressing soft tissue cover, yet they do not come without the additional morbidity of donor sites. Autologous fat transfer has been used in reconstruction of soft tissue defects in different branches of plastic surgery, specifically breast and facial defect reconstruction, while further maintaining a role in body contouring procedures. Current autologous fat transfer techniques come with the drawbacks of donor-site morbidity and, more significantly, resorption of large amounts of fat. Advancement in tissue engineering has led to the use of engineered adipose tissue structures based on adipose-derived stem cells. This enables a mechanically similar reconstruct that is abundantly available. Cosmetic and mechanical similarity with native tissue is the main clinical goal for engineered adipose tissue. Development of novel techniques in the availability of natural tissue is an exciting prospect; however, it is important to investigate the potential of cell sources and culture strategies for clinical applications. We review these techniques and their applications in plastic surgery.

Differentiation of canine adipose mesenchymal stem cells into insulin-producing cells: comparison of different culture medium compositions

The aim of this study was to differentiate canine adipose-derived mesenchymal stem cells (ADMSCs) into insulin-producing cells by using culture media with different compositions to determine the most efficient media. Stem cells isolated from the fat tissues close to the bitch uterus were distributed into 6 groups: (1) Dulbecco's modified Eagle medium (DMEM)-high glucose (HG), β-mercaptoethanol, and nicotinamide; (2) DMEM-HG, β-mercaptoethanol, nicotinamide, and exendin-4; (3) DMEM-HG, β-mercaptoethanol, nicotinamide, exendin-4, B27, nonessential amino acids, and l-glutamine; (4) DMEM-HG, β-mercaptoethanol, and nicotinamide (for the initial 8-d period), and DMEM-HG, β-mercaptoethanol, nicotinamide, exendin-4, B27, nonessential amino acids, l-glutamine, and basic fibroblast growth factor (for the remaining 8-d period); (5) DMEM-HG and fetal bovine serum; and (6) DMEM-low glucose and fetal bovine serum (standard control group). Adipose-derived mesenchymal stem cells from groups 1 to 5 gradually became round in shape and gathered in clusters. These changes differed between the groups. In group 3, the cell clusters were apparently more in numbers and gathered as bigger aggregates. Dithizone staining showed that groups 3 and 4 were similar in terms of the mean area of each aggregate stained for insulin. However, only in group 4, the number of insulin aggregates and the total area of aggregates stained were significantly bigger than in the other groups. The mRNA expression of PDX1, BETA2, MafA, and Insulin were also confirmed in all the groups. We conclude that by manipulating the composition of the culture medium it is possible to induce canine ADMSCs into insulin-producing cells, and the 2-staged protocol that was used promoted the best differentiation.

Bioconversion Products of Whey by Lactic Acid Bacteria Exert Anti-Adipogenic Effect

Microbial bioconversion using lactic acid bacteria (LAB) provides several human health benefits. Although whey and whey-derived bioactive compounds can contribute to an improvement in human health, the potential anti-obesity effect of whey bioconversion by LAB has not been well studied. This study aimed to investigate whether bioconversion of whey by Pediococcus pentosaceus KI31 and Lactobacillus sakei KI36 (KI31-W and KI36-W, respectively) inhibits 3T3-L1 preadipocyte differentiation. Both KI31-W and KI36-W reduced intracellular lipid accumulation significantly, without decreasing 3T3-L1 preadipocyte proliferation. In addition, obesity-related transcription factor (peroxisome proliferator-activated receptor γ) and genes (adipocyte fatty acid-binding protein and lipoprotein lipase) were down-regulated significantly in 3T3-L1 cells in the presence of KI31-W and KI36-W. Collectively, these results suggest that bioconversion of whey by LAB exhibits anti-adipogenic activity and may be applied as a therapeutic agent for obesity.

An integrated microfluidic device for stem cell differentiation based on cell-imprinted substrate designed for cartilage regeneration in a rabbit model

Separating cells from the body and cultivating them in vitro will alter the function of cells. Therefore, for optimal cell culture in the laboratory, conditions similar to those of their natural growth should be provided. In previous studies, it has been shown that the use of cellular shape at the culture surface can regulate cellular function. In this work, the efficiency of the imprinting method increased by using microfluidic chip design and fabrication. In this method, first, a cell-imprinted substrate of chondrocytes was made using a microfluidic chip. Afterwards, stem cells were cultured on a cell-imprinted substrate using a second microfluidic chip aligned with the substrate. Therefore, stem cells were precisely placed on the chondrocyte patterns on the substrate and their fibroblast-like morphology was changed to chondrocyte's spherical morphology after 14-days culture in the chip without using any chemical growth factor. After chondrogenic differentiation and in vitro assessments (real-time PCR and immunocytotoxicity), differentiated stem cells were transferred on a collagen-hyaluronic acid scaffold and transplanted in articular cartilage defect of the rabbit. After 6 months, the post-transplantation analysis showed that the articular cartilage defect had been successfully regenerated in differentiated stem cell groups in comparison with the controls. In conclusion, this study showed the potency of the imprinting method for inducing chondrogenicity in stem cells, which can be used in clinical trials due to the safety of the procedure.

Extracellular matrix-based biomaterials as adipose-derived stem cell delivery vehicles in wound healing: a comparative study between a collagen scaffold and two xenografts

BACKGROUND

Stem cell therapies represent a promising tool in regenerative medicine. Considering the drawbacks of direct stem cell injections (e.g. poor cell localisation), extracellular matrix-based biomaterials (e.g. scaffolds and tissue grafts), due to their compositional biofunctionality and cytocompatibility, are under investigation as potential stem cell carriers.

METHODS

The present study assessed the potential of three commercially available extracellular matrix-based biomaterials [a collagen/glycosaminoglycan scaffold (Integra™ Matrix Wound Dressing), a decellularised porcine peritoneum (XenoMEM™) and a porcine urinary bladder (MatriStem™)] as human adipose-derived stem cell delivery vehicles.

RESULTS

Both tissue grafts induced significantly (p < 0.01) higher human adipose-derived stem cell proliferation in vitro over the collagen scaffold, especially when the cells were seeded on the basement membrane side. Human adipose-derived stem cell phenotype and trilineage differentiation potential was preserved in all biomaterials. In a splinted wound healing nude mouse model, in comparison to sham, biomaterials alone and cells alone groups, all biomaterials seeded with human adipose-derived stem cells showed a moderate improvement of wound closure, a significantly (p < 0.05) lower wound gap and scar index and a significantly (p < 0.05) higher proportion of mature collagen deposition and angiogenesis (the highest, p < 0.01, was observed for the cell loaded at the basement membrane XenoMEM™ group). All cell-loaded biomaterial groups retained more cells at the implantation side than the direct injection group, even though they were loaded with half of the cells than the cell injection group.

CONCLUSIONS

This study further advocates the use of extracellular matrix-based biomaterials (in particular porcine peritoneum) as human adipose-derived stem cell delivery vehicles. Comparative analysis of a collagen scaffold (Integra™ Matrix Wound Dressing) and two tissue grafts [decellularised porcine peritoneum (XenoMEM™) and porcine urinary bladder (MatriStem™)] as human adipose-derived stem cells carriers.

A comparison of the effects of fetal bovine serum and newborn calf serum on cell growth and maintenance of cryopreserved mouse spermatogonial stem cells

Serum is a common supplement that is widely used to protect various cells and tissues from cryopreservation because it provides the necessary active components for cell growth and maintenance. In this study, we compared the effects of newborn calf serum (NCS) and fetal bovine serum (FBS) on the cryopreservation of mouse spermatogonial stem cells (SSCs). The isolated SSCs were cryopreserved in two groups: freezing medium that contained 10% DMSO (dimethyl sulfoxide) and 10% FBS in DMEM (Dulbecco's Modified Eagle's Medium) (group 1) and freezing medium that contained 10% DMSO and 10% NCS in DMEM (group 2). Real-time PCR was performed for stemness gene expression. The SSCs' viability was performed by trypan blue. We observed that the SSCs had increased viability in the NCS-freeze/thaw group (87.82%) compared to the FBS-freeze/thaw group (79.83%), but this increase was not statistically significant (P < 0.105). Promyelocytic leukemia zinc finger (Plzf) and Lin28 gene expression levels in the NCS-frozen/thawed SSCs were not significantly different compared to the FBS-frozen/thawed SSCs; however, Nanog gene expression increased considerably, and Dazl gene expression decreased significantly. The results in this study demonstrated that the presence of NCS in a solution of cryopreserved SSCs increased their viability after freeze/thawing and might promote the proliferation of cultivated SSCs in vitro by increasing the relative expression of Nanog.

Dual delivery of stem cells and insulin-like growth factor-1 in coacervate-embedded composite hydrogels for enhanced cartilage regeneration in osteochondral defects

Exogenous dual delivery of progenitor cell population and therapeutic growth factors (GFs) is one of alternative tissue engineering strategies for osteochondral tissue regeneration. In the present study, an implantable dual delivery platform was developed using coacervates (Coa) (i.e., a tertiary complex of poly(ethylene argininylaspartate diglyceride) (PEAD) polycation, heparin, and cargo insulin-like growth factor-1 (IGF-1), in thiolated gelatin (gelatin-SH)/ poly(ethylene glycol) diacrylate (PEGDA) interpenetrating network (IPN) hydrogels. Since Coa is able to protect cargo GF and maintain its long-term bioactivity, it is speculated that Coa-mediated delivery of chondrogenic factor IGF-1 with the aid of adipose-derived stem cells (ADSCs) would synergistically facilitate osteochondral tissue repair during physiological regeneration process. Our results indicate that gelatin-SH/PEGDA IPN hydrogels demonstrated biocompatibility and mechanical properties for a possible long-term transplantation, and PEAD-base Coa exhibited a sustained release of bioactive IGF-1 over 3 weeks. Subsequently, released IGF-1 from Coa could effectively induce chondrogenic differentiation of embedded ADSCs in the hydrogel, by showing enhanced glycosaminoglycan deposition and expression of chondrogenesis-associated genes. More importantly, at 12 weeks post-implantation in a rabbit full thickness osteochondral defect model, the quality of regenerative tissues in both chondral and subchondral layers was significantly improved in dual delivery of ADSC and IGF-1 in Coa encapsulated in gelatin-SH/PEGDA IPN hydrogels, as compared with a single delivery of ADSC only and a dual delivery without Coa. Therefore, we conclude that our Coa-embedded composite hydrogel platform could effectively augment osteochondral tissue regeneration holds promise for a feasible osteoarthritis therapeutic application.

Supplementation of culture medium with knockout serum replacement improves the survival of bovine secondary follicles when compared with other protein sources during in vitro culture

The present study evaluated the effect of knockout serum replacement (KSR), fetal bovine serum (FBS) and bovine serum albumin (BSA) on the viability and growth of bovine secondary follicles cultured in vitro for 12 days. To this end, secondary follicles were isolated (185-202 μm) and cultured in vitro in TCM-199+ medium supplemented with KSR (5% and 10%), FBS (5% and 10%) or BSA (3 mg/ml) at 38.5°C with 5% CO2 in air. Follicular diameters were evaluated on days 0, 4, 8 and 12. After 12 days of culture, follicular survival analysis was performing by using calcein-AM and ethidium homodimer. Before and after culture, follicles were fixed in paraformaldehyde for histological evaluation. Follicular diameter at different days of culture were compared using the Kruskal-Wallis test, while the percentages of viable follicles were analyzed by chi-squared test (P < 0.05). Results showed that follicles cultured in the presence of KSR at both concentrations presented higher follicular survival rates than those cultured in control medium alone or supplemented with FBS or BSA. Conversely, the presence of KSR, BSA or FBS did not increase follicular diameter after 12 days of culture. Histology analysis showed that, among the tested treatments, follicles cultured in the presence of KSR had preserved rounded oocytes, juxtaposed granulosa cells and intact basal membrane. In conclusion, supplementation of culture medium with KSR increases the follicular survival of bovine secondary follicles cultured in vitro.

Chondrogenic Differentiation of Pluripotent Stem Cells under Controllable Serum-Free Conditions

The repair of damaged articular cartilage using currently available implantation techniques is not sufficient for the full recovery of patients. Pluripotent stem cells (iPSC)-based therapies could bring new perspectives in the treatment of joint diseases. A number of protocols of in vitro differentiation of iPSC in chondrocytes for regenerative purposes have been recently described. However, in order to use these cells in clinics, the elimination of animal serum and feeder cells is essential. In our study, a strictly defined and controllable protocol was designed for the differentiation of pluripotent stem cells (BG01V, ND 41658*H, GPCCi001-A) in chondrocyte-like cells in serum- and a feeder cell-free system, using the embryoid bodies step. The extension of the protocol and culture conditions (monolayer versus 3D culture) was also tested after the initial 21 days of chondrogenic differentiation. Promotion of the chondrogenic differentiation in 3D culture via the elevated expression of genes related to chondrogenesis was achieved. Using immunofluorescence and immunohistochemistry staining techniques, the increased deposition of the specific extracellular matrix was indicated. As a result, chondrocyte-like cells in the early stages of their differentiation using pellet culture under fully controlled and defined conditions were obtained.

In Vitro Effect of Estradiol and Progesterone on Ovine Amniotic Epithelial Cells

Amniotic epithelial cells (AECs), an emerging source of extrafoetal stem cells, have recently attracted attention for their great regenerative potential. Since AEC amplifications are accompanied by the loss of their native epithelial phenotype and by the progressive reduction of relevant biological properties, the issue to be addressed is the development of effective culture protocols. In this context, recently, it has been demonstrated that progesterone (P₄) supplementation during ovine AEC (oAEC) expansion could prevent the undesirable epithelial-mesenchymal transition (EMT). In contrast, there is no information to date on the role of the other pregnancy steroids in culture. With this aim, the present study has been designed to clarify the impact of estradiol (E₂), alone or in combination with P₄ (12.5 μM and 25 μM), during oAEC amplification. Steroid supplementations were assessed by testing oAEC proliferation, stemness, EMT, and osteogenic or chondrogenic plasticity. The results indicated that EMT can be prevented exclusively in the presence of high doses of P₄, while it occurred rapidly in cells exposed to E₂ as denoted by protein (cytokeratin-8 and alpha-SMA) and gene expression (vimentin and snail) profiles. Moreover, steroid exposure was able to influence highly oAEC plasticity. Particularly, P₄-treated cells displayed a precommitment towards osteogenic lineage, confirmed by the upregulation of OCN, RUNX2, and the greater deposition of calcium nodules. Conversely, P₄ exposure inhibited oAEC chondrogenic differentiation, which was induced in E₂-treated cells as confirmed by the upregulation of chondrogenesis-related genes (SOX9, ACAN, and COL2A1) and by the accumulation of Alcian blue-positive extracellular matrix. Simultaneously, E₂-treated cells remained unresponsive to osteogenic inductive stimuli. In conclusion, media supplementation with high doses of steroids may be adopted to modulate phenotype and plasticity during oAEC amplification. Relevantly, the osteo or chondro steroid-induced precommitment may open unprecedented cell-based therapies to face the unsolved orthopaedic issues related to osteochondral regeneration.

A Non-Enzymatic Method to Obtain a Fat Tissue Derivative Highly Enriched in Adipose Stem Cells (ASCs) from Human Lipoaspirates: Preliminary Results

Adipose tissue possesses phenotypic gene expression characteristics that are similar to human mesenchymal stem cells (hMSCs). Nevertheless, the multilineage potential may be inhibited, and cells may not expand adequately to satisfy the requirements of Good Manufacturing Practice (cGMP). An autologous hMSC-enriched fat product would fulfil the void from a biomedical and clinical perspective. In this study, we suggest a novel mechanism using a closed system without enzymes, additives or other modifications, which will produce non-expanded, accessible material. This decentralized fat product, unlike unprocessed lipoaspirates, adequately encloses the vascular stroma with adipocytes and stromal stalks along with their vascular channels and lumina. This fat product contained hASCs and fewer hematopoietic elements such as lipoaspirates, which were digested enzymatically according to flow cytometric investigations, and molecular analysis also showed significant hASC uniformity within the cells of the stromal vascular tissue. Moreover, the fat product produced a higher quantity of hASCs similar to hMSCs in isolation with the typical characteristics of an osteogenic, chondrogenic and adipogenic lineage. Interestingly, these properties were evident in the non-enzymatic derived adipose tissue, as opposed to hASCs in isolation from the enzymatically digested lipoaspirates, suggesting that the aforementioned procedure may be an adequate alternative to regenerate and engineer tissue for the treatment of various medical conditions and promote efficient patient recovery.

Isolation and biological characteristics of multipotent mesenchymal stromal cells derived from chick embryo intestine

1. Over the past decade, rapid advancement in isolation methods for identifying markers of the once elusive intestinal stem cell (ISC) populations has laid the foundation for unravelling their complex interrelationships during homeostasis. Study on ISC in avian intestinal tissue might play a pivotal foundation for further studies on the epithelial-to-mesenchymal transition (EMT) in gastrointestinal disease and cell-based therapy as well as intestinal tissue engineering. 2. The following experiment isolated a population of fibroblast-like, plastic adhering cells derived from chick embryo intestine, showing a strong self-renewing and proliferative ability, which was maintained in vitro up to passage 25. The findings included growth characteristics, detected expression of cell surface markers and characterised the capability of these cells to differentiate towards the osteogenic, adipogenic, and chondrogenic cell lineages. 3. RT-PCR analysis showed that these cells from chick embryos expressed mesenchymal stromal cell markers CD44, CD90 and VIMENTIN as well as ISC-specific genes LGR5, MI1, SMOC2, BMI1, and HOPX. Immunofluorescence and flow cytometry confirmed this biology characterisation further. 4. In conclusion, cells were isolated from the intestine of 18-day-old chicken embryos that exhibited the biological characteristics of mesenchymal stromal cells as well as markers of intestinal stem cells. Our findings may provide a novel insight for in vitro cell culture and characteristics of ISCs in avian species, which may also indicate a benefit for obtaining cell source for intestinal tissue engineering as well as cell-based investigation for gastrointestinal disease and treatment.

Induction of chondrogenesis of human placenta-derived mesenchymal stem cells via heparin-grafted human fibroblast derived matrix

Background

Formation of mature and functional articular cartilage is still challenging in cartilage tissue engineering. This study investigates the potential of using heparin-grafted decellularized extracellular matrix (ECM) as a novel growth factor delivery platform towards human placenta-derived mesenchymal stem cells (hPMSCs) chondrogenic differentiation. Human fibroblast-derived extracellular matrix (hFDM) is naturally obtained from in vitro-cultured human lung fibroblasts via a mild decellularization process. hFDM was then conjugated with heparin via N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) chemistry and subject to transforming growth factor (TGF)-β1 immobilization. Once heparin grafted-hFDM (hFDM-hep) and hPMSCs were co-embedded into collagen gel, they were examined for in vitro and in vivo chondrogenesis of hPMSCs for 4 weeks.

Results

We identified heparin moieties on hFDM via toluidine blue O assay and Fourier transform infrared spectroscopy, respectively. We found out that collagen spheroids containing hFDM-hep and TGF-β1 exhibited a sustained release of growth factor for 28 days in vitro. Chondrogenesis of hPMSCs in vitro was supported by accumulated glycosaminoglycan (GAG) content and upregulated chondrogenic specific markers (collagen II, aggrecan, Sox9). Meanwhile, PKH26 - labeled hPMSCs incorporated collagen with either hFDM or hFDM-hep was pre-conditioned in a chondrogenic media for 3 days and subcutaneously implanted in the back of nude mice for 4 weeks. The implanted collagen spheroids containing both hPMSCs and hFDM-hep retained more viable hPMSCs and showed higher level of chondrogenic differentiation, based on immunostaining of collagen type II over collagen alone or Col/hFDM group. In addition, histological examination showed more positive signals of GAG via Safranin-O staining.

Conclusion

TGF-β1-immobilized hFDM-hep can provide an appropriate microenvironment for chondrogenic differentiation of hPMSCs in 3D collagen spheroid.

Electronic supplementary material

The online version of this article (10.1186/s40824-018-0121-2) contains supplementary material, which is available to authorized users.