Scholarly article on topic 'Advances and challenges in stem cell culture'

Advances and challenges in stem cell culture Academic research paper on "Biological sciences"

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{"3-D culture" / Self-renewal / Biomimicking / "Large-scale expansion" / "Self-assembling scaffold" / Hydrogel / Bioreactor}

Abstract of research paper on Biological sciences, author of scientific article — Christina McKee, G. Rasul Chaudhry

Abstract Stem cells (SCs) hold great promise for cell therapy, tissue engineering, and regenerative medicine as well as pharmaceutical and biotechnological applications. They have the capacity to self-renew and the ability to differentiate into specialized cell types depending upon their source of isolation. However, use of SCs for clinical applications requires a high quality and quantity of cells. This necessitates large-scale expansion of SCs followed by efficient and homogeneous differentiation into functional derivatives. Traditional methods for maintenance and expansion of cells rely on two-dimensional (2-D) culturing techniques using plastic culture plates and xenogenic media. These methods provide limited expansion and cells tend to lose clonal and differentiation capacity upon long-term passaging. Recently, new approaches for the expansion of SCs have emphasized three-dimensional (3-D) cell growth to mimic the in vivo environment. This review provides a comprehensive compendium of recent advancements in culturing SCs using 2-D and 3-D techniques involving spheroids, biomaterials, and bioreactors. In addition, potential challenges to achieve billion-fold expansion of cells are discussed.

Academic research paper on topic "Advances and challenges in stem cell culture"

Accepted Manuscript

Title: Advances and challenges in stem cell culture

Authors: Christina McKee, G. Rasul Chaudhry

PII: DOI:

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S0927-7765(17)30463-0 |_

http://dx.doi.org/doi:10.1016/j.colsurfb.2017.07.051 COLSUB 8714

To appear in:

Colloids and Surfaces B: Biointerfaces

Received date: Revised date: Accepted date:

14-4-2017

4-7-2017

22-7-2017

Please cite this article as: Christina McKee, G.Rasul Chaudhry, Advances and challenges in stem cell culture, Colloids and Surfaces B: Biointerfaceshttp://dx.doi.org/10.1016/j.colsurfb.2017.07.051

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Review

Advances and challenges in stem cell culture

Christina McKee1,2, and G. Rasul Chaudhry1,2^

department of Biology, Oakland University, Rochester, MI, USA, 48309

2OU-WB Institute for Stem Cell and Regenerative Medicine, Oakland University, Rochester, MI,

USA, 4830

Corresponding Author: G. Rasul Chaudhry

Department of Biological Sciences

Oakland University, 118 Library Drive, Rochester, MI, USA Phone No.: +1-248-370-3350 Email: chaudhry@oakland.edu

Graphical abstract

Highlights

• Lack of scalability of 2-D culture systems.

• Physiologically relevant human cell culture techniques.

• Techniques mimicking native microenvironment for SC growth and differentiation.

• Higher density and billion fold expansion using 3-D culture systems.

• Challenges and advantages of 3-D culture systems.

Abstract

Stem cells (SCs) hold great promise for cell therapy, tissue engineering, and regenerative medicine as well as pharmaceutical and biotechnological applications. They have the capacity to self-renew and the ability to differentiate into specialized cell types depending upon their source of isolation. However, use of SCs for clinical applications requires a high quality and quantity of cells. This necessitates large-scale expansion of SCs followed by efficient and homogeneous differentiation into functional derivatives. Traditional methods for maintenance and expansion of cells rely on two-dimensional (2-D) culturing techniques using plastic culture plates and xenogenic media. These methods provide limited expansion and cells tend to lose clonal and differentiation capacity upon long-term passaging. Recently, new approaches for the expansion of SCs have emphasized three-dimensional (3-D) cell growth to mimic the in vivo environment. This review provides a comprehensive compendium of recent advancements in culturing SCs using 2-D and 3-D techniques involving spheroids, biomaterials, and bioreactors. In addition, potential challenges to achieve billion-fold expansion of cells are discussed.

Key words: 3-D culture; self-renewal; biomimicking; large-scale expansion; self-assembling scaffold; hydrogel; bioreactor.

Abbreviations

SCs Stem cells

2-D Two-dimensional

3-D Three-dimensional ESCs Embryonic stem cells

iPSCs Induced pluripotent stem cells

ECM Extracellular matrix

MEF Mouse embryonic fibroblast

LIF Leukemia inhibitory factor

MSCs Mesenchymal stem/stromal cells

HA Hyaluronic acid

PEG Polyethelene glycol

PLL Poly-l-lysine

PLA Poly-lactic acid

PGA Poly-glycolic acid

PCL Polycaprolactone

PGLA Poly-dl-lactic acid-co-glycolic acid

Dex-SH Thiol-functionalized dextran

PEG-4-Acr PEG functionalized with four-arm acrylate

PDMS Polydimethylsiloxane

1. Introduction

Stem cells (SCs) have the capacity to self-renew and differentiate into specialized cells and are defined by their origin and degree of potency [1]. Pluripotent SCs are capable of unlimited self-renewal and differentiation into any of the over 200 types of cells in the body [2, 3]. There are two sources of pluripotent SCs. First, embryonic stem cells (ESCs) are derived from the inner cell mass of a pre-implantation blastocyst [4] and pluripotency is controlled by an intrinsic regulatory network of core transcription factors, octamer-binding transcription factor 4 (OCT4), sex determining region Y-box 2 (SOX2), and Nanog homeobox (NANOG) [5]. Second, induced pluripotent stem cells (iPSCs) are derived by the ectopic or elevated expression

of four transcription factors, OCT4, SOX2, Kruppel like factor 4 (KLF4), and MYC proto-oncogene (C-MYC) essential for induction of pluripotency in somatic cells [3].

Another type of SCs, mesenchymal stem/stromal cells (MSCs) are isolated from adult sources such as bone marrow and adipose tissues [6] or perinatal tissues, such as umbilical cord, cord blood, placenta and amniotic fluid [7, 8]. MSCs are characterized by adherent growth to plastic culture plates, exhibiting clonal growth. They are positive for mesenchymal surface markers, CD90, CD73, CD29, and CD105 and negative for hematopoietic lineage markers, CD45, CD34, and HLA-DR [9]. Unlike pluripotent SCs, MSCs are multipotent and differentiate into only limited cell types such as osteogenic, chondrogenic, and adipogenic cells [10]. Also, their self-renewal and differentiation potential is dependent upon the source of isolation [11]. In addition, MSCs derived from adult tissues are also affected by aging and exposure to environmental stresses, which could alter genetic stability [8]. In comparison to adult MSCs, MSCs obtained from perinatal tissues exhibit higher growth and stemness potential [12].

As such, the study of SCs has contributed to the elucidation of basic biochemical and developmental processes [13-17]. They have also shown tremendous potential for in vitro disease models, tissue engineering, regenerative medicine, and cell therapy as well as pharmaceutical applications [18-21]. Furthermore, they can be used for drug discovery and development [22, 23]. All these uses require large-scale production of high quality cells [24].

Traditionally, SCs are propagated as a monolayer in two-dimensional (2-D) plastic culture plates and often require undefined or xenogenic materials including but not limited to attachment substrates, cytokines and growth factors, as well as serum. Use of xenogenic or animal derived media can potentially transmit pathogens and limit reproducibility between cultures due to lot-to-lot fluctuation of the material used [25]. Monolayer culture necessitates

routine passaging to maintain self-renewal and potency of cells, which is highly inefficient for large-scale expansion of cells. In addition, 2-D attachment alters cell shape and geometry [26], leading to cell flattening and changes in the internal cytoskeleton and nuclear shape [27], which in turn modifies gene and protein expression [28, 29]. Furthermore, studies have shown that the composition and organization of the extracellular matrix (ECM) can also send biochemical and mechanical signals for cell differentiation [25, 30, 31].

2-D culture techniques and applications have been practiced for the majority of primary and established cell lines and standardized for analytical assays ranging from microscopy and counting cells to the study of disease processes and drug testing [32]. Notably, 2-D culture has been used for directed differentiation of SCs into many specialized cells, including chondrocytes, osteocytes, adipocytes, cardiomyocytes, smooth muscle cells and hepatocytes [33, 34]. However, 2-D culture often results in a lack of functional derivatives [35]. Studies have also shown that 2D monolayer culture fails to accurately reproduce animal physiology [36], and proves insufficient to validate drug discovery [37].

Overall, 2-D culture conditions lack the intricacy necessary to mimic the SC niche, dynamic and specialized three-dimensional (3-D) microenvironments, which are responsible for the regulation of SC fate in vivo. Native 3-D niches allow for complex spatial interactions between cells, ECM components, and gradients of nutrients, oxygen, and waste [38]. The effect of the 3-D microenvironment can be easily demonstrated in pluripotent SCs, which retain their pluripotency when injected into the inner cell mass of another embryo, but spontaneously differentiate into all three germ layers when injected elsewhere in animals due to external cues [39]. Furthermore, supportive niches for multipotent SCs, have been identified in the gonads, intestinal crypts, hair follicles, and bone marrow in vivo, whereby anchored cells release signals

that govern continued self-renewal or lineage-specific differentiation [40]. Transplantation of multipotent SCs outside the niche results in differentiation into various cell lineages dependent upon the cues in the surrounding in vivo microenvironment [41].

To address the various problems facing 2-D culture of SCs, 3-D culture methods have been developed to control cell fate by incorporating physiologically relevant and biomimetic microenvironments by mimicking the ECM composition and stiffness of the SC niche in vitro [27, 38, 42, 43]. However, 3-D culture for SC maintenance and expansion still remains a challenge [44], primarily, due to need to optimize culture conditions for different types of cells and analysis. Therefore, is it necessary to elucidate the mechanisms important for SC maintenance, including biomaterial signaling and mechanical forces that would aid in the uniform and reproducible expansion of SCs without loss of genetic stability or differentiation potential.

This review is focused on the recent progress in 3-D culture for expansion of SCs, both in quality and quantity, acceptable for clinical, pharmaceutical and biotechnological applications. In addition, we will report some important recent advancements in 2-D culture, since these methods have historically led to significant developments in SC biology and early studies have been eloquently reviewed previously [22, 45]. Overall, this review assesses the latest advances in proliferation and differentiation of pluripotent and multipotent SCs.

2. Two-dimensional culture techniques

2.1 Maintenance and differentiation of pluripotent SCs

2-D culture studies have focused on the maintenance of stemness, a self-renewing state characteristic of undifferentiated SCs, by extrinsic factors such as mechanical forces between

both cells and their 3-D microenvironment, which are transduced to biological cues that can control cell shape and function [25, 46-49].

Usually, pluripotent SCs are grown on plastic culture dishes coated with ECM components (such as gelatin, Matrigel, or collagen) or a mouse embryonic fibroblast (MEF) feeder layer to aid in attachment [50]. Mouse ESCs can be maintained in their pluripotent state either by culturing them on MEF feeder layers or in medium containing leukemia inhibitory factor (LIF), a cytokine released from MEFs [48, 51]. However, supplementation of LIF is insufficient to maintain the pluripotency of human ESCs, which requires the addition of fibroblast growth factor 2 (FGF2) and transforming growth factor pi (TGFpi) in the culture medium [52, 53]. Recent evidence suggests that these differences are not species specific but rather developmental [54]. For instance, human ESCs and mouse epiblast SCs display flat morphology, and poor clonogenicity, whereas mouse ESCs exhibit a 3-D dome morphology and good clonal growth from single cells. Recent studies have shown that addition of ROCK inhibitors to the culture medium improved their propagation by protecting against apoptosis of single cells [55]. It has been proposed that there are two consecutive states of pluripotency in embryonic development, an early naïve state followed by a primed state, from which mouse ESCs and human ESCs are derived, respectively [56]. Naïve pluripotent SCs are early totipotent cells capable of differentiation into both extraembryonic and embryonic tissues, whereas primed cells are derived from a more advanced state of development and differentiate into only embryonic tissues [57].

In fact, recent studies have demonstrated the ability of human ESCs to revert from the primed to the naïve state of pluripotency [56, 58], which may greatly improve the proliferative capacity of human ESCs in culture. In addition, 2-D expansion of ESCs has been improved using

fully defined and xeno-free culture media and attachment substrates such as E8 media and humanized recombinant protein vitronectin, respectively [45, 53, 59]. However, uniform expansion of pluripotent SCs is still difficult to achieve as 2-D culture methods for propagation of pluripotent SCs are laborious, expensive and require a high level of expertise.

Pluripotent SCs can be differentiated into cell types from all three germ layers [4]. In general, 2-D culture conditions favor non-specific differentiation of pluripotent SCs. However, differentiation can be directed to specific lineages by controlling the composition and organization of the ECM, growth factors, and attachment substrates, which can send mechanical signals for cell fate determination [26]. Both ESCs and iPSCs can be differentiated using similar protocols, however, iPSCs exhibit higher variability due to the nature of somatic cells and the method used for generating iPSCs [60]. ESCs have been directed to differentiate in monolayer culture using specific induction media into ectodermal, mesodermal, and endodermal lineages, including mature neurons and glial cells [61], osteocytes [62], cardiomyocytes [63], and hepatocytes [64]. One prodigious advantage of monolayer culture is that it allows for uniform treatment for differentiation of cells [65]. As such, there has been some progress in 2-D culture using synthetic substrates and stepwise protocols for controlled differentiation of human ESCs [60, 63, 65]. For instance, ESCs were previously differentiated into mesoderm cell types including cardiomyocytes in monolayer culture with approximately 30% efficiency using MEF conditioned medium, followed by treatment of Activin A and BMP4 [63]. However, addition of GSK3 and Wnt inhibitors to the medium resulted in approximately 80-90% differentiation of ESCs into cardiomyocytes [60]. Similarly, stepwise strategies have been applied to differentiate ESCs first into endoderm, hepatic progenitors, and then into hepatocytes using specific growth factors [64]. Likewise, mouse ESCs were differentiated into ectoderm lineage using neural

differentiation medium supplemented with a sonic hedgehog antagonist to form neocortical-like neurons [66]. However, the differentiated neurons were heterogeneous and failed to display specific neuronal characteristics in vitro [66]. Despite great strides, differentiation of ESCs using 2-D culture often results in mixed populations of differentiated cells [67].

2.2 Maintenance and differentiation of MSCs

Unlike pluripotent SCs, MSCs are naturally adherent and do not require xenogenic substrates for attachment to culture plates, although they are typically cultured in the media containing animal serum. However, xenogenic-free defined media have recently become available for expansion of MSCs [68]. While supplementation of media with soluble factors is known to maintain stemness of ESCs, the exact mechanism of MSC self-renewal is not well known [69]. Nevertheless, mimicking the in vivo microenvironment such as hypoxia (2% O2) has been shown to enhance stemness in MSCs, resulting in a significant proliferation, without the loss of multipotent differentiation potential over a 6 week culture period [70].

Expansion of MSCs in 2-D culture is highly inefficient. Usually expansion consists of growth in multiple tissue culture flasks to increase the surface area for large-scale attachment and propagation of MSCs [71, 72]. Uniform distribution, growth, and harvesting processes are important to minimize heterogeneity and improve cell yield [73]. Successful in vitro culture of MSCs requires an understanding of the signaling pathways that influence both the proliferation and guided differentiation of these cells.

Numerous studies have shown MSC differentiation into osteocytes, adipocytes, and chondrocytes [12, 74, 75] as well as cardiomyocytes [76] in 2-D cultures. This is often achieved by using cell-specific differentiation media and assessed by translational and transcriptional gene

expression, as well as ECM deposition. However, expansion of MSCs in monolayers leads to phenotypic changes, whereby spindle shaped MSCs exhibit a broad and flattened morphology upon passaging, which in turn alters cell fate and differentiation potential [12, 77, 78]. In particular, chondrogenic differentiation of MSCs in 2-D culture is less efficient when compared to high-density cell culture methods, utilizing pellet culture or spheroids [79]. Despite the limitations of 2-D culture, MSCs have been reported to differentiate into numerous cell types of not only mesodermal but also ectodermal lineages, including retinal progenitors and photoreceptors [80, 81], and neural cells [82, 83], as well as endodermal lineages, such as hepatocyte-like cells [84, 85], and pancreatic islet-like cells [86, 87]. However, in vitro induction of differentiation is often not sufficient to yield functionally competent cells [88]. For instance, transient differentiation of MSCs into retinal ganglion precursor-like cells was reported [89], suggesting that MSCs may de-differentiate during extended culture. While MSCs are a promising source for cell replacement and immune-modulation, improvements in differentiation protocols are necessary for clinical applications.

3. Three-dimensional culture techniques

Due to the inherent problems with the 2-D culture techniques attempts have been made to design 3-D culture methods to better recapitulate native microenvironments. Therefore, several approaches have been explored to maintain, expand, and differentiate SCs utilizing various 3-D culture systems with or without biomaterials under static and dynamic conditions as summarized in Fig. 1.

3.1 Static three-dimensional culture 3.1.1 Spheroids

One of the simplest methods of 3-D culture was achieved by formation of multicellular aggregates, or spheroids, which allow 3-D interactions with cells and the ECM in the absence of additional substrates [90]. These spheroids have been utilized with a wide range of adherent cell types, formed by forced or spontaneous aggregation techniques including hanging drop, rotating culture, or low-adhesion culture plates in suspension culture [91-93]. Spheroids are comprised of both highly proliferative, non-proliferative, and apoptotic cells with limited diffusion of oxygen and nutrients to the center of the spheroid, leading to an increasing hypoxic environment [94]. Due to their heterogeneous nature, spheroids have been more successfully employed to study complex 3-D cell structures, cell differentiation and cancer biology [90, 95] in comparison with homogenous cell proliferation.

Typically, pluripotent SCs are induced to differentiate by formation of 3-D spheroids called embryoid bodies (EBs) capable of spontaneous differentiation into all three germ layers [96]. This mimics embryonic development and promotes heterogeneous differentiation, whereby induction into various specific cell types can be achieved by addition of cytokines or growth factors to the medium [97-99]. However, the inherent heterogeneity complicates both the reproducibility of directed differentiation as well as high-throughput screening techniques of EBs [100, 101]. The size and number of EBs has been shown to affect differentiation efficiency [102] as observed in cardiomyocyte [103], and hematopoietic differentiation [100]. Studies have indicated that differentiation can also be controlled by varying the size of the EBs. Since EBs are comprised of varied microenvironments with differential availability to oxygen and other soluble factors, cell position in the EB can differentially regulate cell fate determination [104]. Furthermore, large EBs favored differentiation into endoderm and mesoderm cell types, while smaller EBs tended to differentiate into ectoderm [104, 105].

In comparison to 2-D culture conditions, 3-D spheroid culture improved hepatocyte differentiation of ESCs based on phenotypic analysis, expression of hepatocyte genes and proteins, and the ability to excrete biliary metabolic products [106, 107]. 3-D spheroid culture of ESCs also displayed improvement in endothelial cell tube formation [108], functional bile ductal differentiation [109], engraftment potential of hepatic progenitor cells [110], and paracrine signaling in hematopoietic differentiation [100]. Overall, 3-D spheroid culture generated functional derivatives of ESCs. However, long-term suspension culture of spheroids often results in agglomeration of cells, leading to necrotic centers due to limited diffusion of nutrients and oxygen into and waste out of the aggregate [94].

In contrast to ESCs, short-term spheroid culture has been employed for maintenance and expansion of MSCs [90]. When subcultured back to 2-D culture conditions, spheroid grown MSCs displayed an undifferentiated morphology, and enhanced differentiation potential via increased ECM deposition when compared to adherent grown MSCs [111]. MSCs cultured in 3D spheroids exhibited increased clonal growth and multipotency [112], as well as altered miRNA expression, and increased acetylation in the promoter regions of pluripotency genes, OCT4, SOX2, and NANOG [113].

Additionally, MSC spheroids grown in short-term culture for 3 days exhibited higher expression of anti-cancer and anti-inflammatory factors including, IL-24, TNFa-related apoptosis inducing ligand, and CD82 as well as TNFa stimulated gene/protein 6 (TSG-6) and stanniocalcin-1, respectively [114]. These studies suggested that 3-D spheroid culture improves the therapeutic properties of MSCs. Another study showed that spheroid cultured MSCs had improved retention, survival, and integration following transplantation in a porcine model [115]. However, long-term culture of MSCs in spheroid culture has been shown to effectively induce

differentiation [116]. In comparison with monolayer culture, MSCs grown in 3-D spheroids displayed increased chondrogenic differentiation [117], as well as osteogenic differentiation in vitro and in vivo, exhibiting enhanced bone regeneration in calvarial defects in rats [118].

Large-scale expansion of SCs using these methods is difficult due to the inability to control aggregate size, leading to agglomeration, necrosis/apoptosis in spheroids, and inhibition of cell proliferation [114]. This requires minimization of spheroid agglomeration by stirring or incorporation of biomaterials, which will be discussed in Section 3.2.1 and 3.2.2.

3.1.2 Biomaterials

Natural and synthetic biocompatible and biodegradable materials can provide various biological signals and differing degrees of mechanical strength. Biomaterials have been increasingly integrated into in vitro culture to mimic the biochemical and biophysical properties of SC niches to help direct self-renewal or differentiation of cells into specific lineages [119]. Incorporation of natural biomaterials may result in varying levels of inductive signals, cell attachment, and scaffold integrity. Natural biomaterials, including agarose, fibronectin, collagen type 1 (Col1), hyaluronic acid (HA), chitosan, and alginate, can also be functionalized but are more difficult to control in vitro as they often transduce biological signals to cells [120]. As a result, biomaterials can aid in cell maintenance and differentiation. However, problems such as the lot-to-lot variability and the potential of xenogenic media components to cause disease limit their use [120]. Synthetic polymers such as polyethelene glycol (PEG), poly-l-lysine (PLL), poly-lactic acid (PLA), poly-glycolic acid (PGA), polycaprolactone (PCL), and poly-dl-lactic acid-co-glycolic acid (PGLA), are often end-functionalized with the addition of crosslinking groups [48]. PEG polymers are arguably the most commonly studied since they are non-toxic, easily functionalized, and exhibit low protein adsorption, making them safe for biomedical

applications in humans [121].

Natural biomaterials and synthetic polymers are increasingly being used in 3-D scaffolds for maintenance, expansion and differentiation of SCs. Scaffolds with tunable mechanical properties allow modulation of cell viability, proliferation and differentiation [122, 123]. The scaffold constructs are able to mitigate cell aggregation that negatively affects SC viability and growth in suspension culture, by limiting the availability of oxygen and nutrients to cells at the center of aggregates resulting in quiescence and ultimately necrosis/apoptosis [22, 124, 125]. Scaffolds can also incorporate soluble factors such as cytokines or growth factors or be further functionalized with adhesive ligands for anchorage dependent proliferation, providing a framework for ECM production for SC self-renewal and differentiation [120]. They are formed using various techniques such as electrospinning, solvent casting, gas foaming, 3-D printing, and self-assembly resulting in scaffolds with distinctly different pore size, elasticity, adhesion, and tensile strength [126, 127].

Generally, two types of scaffolds are used for 3-D culturing of SCs. First, prefabricated or rigid scaffolds require cell seeding, or migration of cells into the scaffold. These scaffolds are frequently used for differentiation protocols, since substrate stiffness, and mechanical and biomaterial signaling are important for SC differentiation. The second type of scaffolds self-assemble encapsulating cells at the time of scaffold formation. The self-assembly of biomaterials is usually promoted by various crosslinking methods. Physical crosslinked scaffolds are often called reversible gels and can be formed via reversible changes in pH or temperature, resulting in mechanically weak scaffolds [128]. Whereas scaffolds formed via chemical crosslinking resulting in covalent bonds tend to yield mechanically strong scaffolds [129, 130]. Hydrogel scaffolds made of hydrophilic biomaterials are often used for SC culture because they mimic the

fully hydrated ECM of natural soft tissue [131]. Components of the hydrogels can also be modified with drugs, cytokines, and growth factors, to support growth and differentiation of cells in vitro as well as integration of cells in vivo [132-135]. Scale up of 3-D culture is limited by the size of hydrogel scaffolds due to nutrient diffusion in static culture.

Generally, maintenance of ESCs in 3-D hydrogel scaffolds requires routine passaging akin to 2-D cultures via scaffold degradation [44, 136-138]. Human pluripotent SCs cultured in thermoresponsive hydrogels and passaged every 5 days for over 50 passages maintained pluripotency in about 95% of the cell population [136]. Similarly, iPSCs grown as aggregates on highly porous electrospun polystyrene 3-D scaffolds maintained their pluripotency and differentiation potential for numerous passages under xeno-free conditions [137]. However, ideal 3-D culture systems should support both the expansion of pluripotent SCs while minimizing the need for routine passaging, thus limiting cell manipulation.

HA based substrates have been frequently used to support the growth of pluripotent SCs [44, 139, 140], since HA is present during embryogenesis and highly expressed in undifferentiated cells [141]. In addition, modified HA hydrogels prepared at various rigidities have been used to assess the effect of mechanical strength on ESC stemness in 3-D culture; ESCs grown on softer hydrogels best supported the proliferation and pluripotency of ESCs in both feeder-free and MEF conditioned media by mimicking the microenvironment of the inner cell mass of the pre-implantation blastocyst [140, 142]. Prolonged 3-D culture of human ESCs in photopolymerized HA hydrogels maintained the pluripotency and self-renewal potential without passaging for 20 days [139] signifying that the 3-D hydrogels provided a biomimetic microenvironment supportive of compact clonal growth of ESCs.

We recently reported that self-assembling 3-D hydrogels, made of thiol-functionalized dextran (Dex-SH) and PEG functionalized with tetra-acrylate (PEG-4-Acr), maintained mouse ESCs for over 6 weeks without passaging [143]. The schematic of the formation of self-assembling scaffolds with simultaneous encapsulation of ESCs is depicted in Fig. 2A and the clonal growth of the encapsulated cells is shown in Fig. 2C-E. Interestingly, growth of ESCs was anchor independent since the scaffolds lack adhesive ligands. Furthermore, expression of three pluripotency markers, OCT4, NANOG and KLF4, was enhanced in ESCs grown in 3-D hydrogels during an extended period of culture. Similar observations have been reported in several studies suggesting that both dimensionality and inductive biomaterial signaling affects the mechanism of self-renewal in ESCs [144, 145]. For instance, differential regulation of stemness genes was observed in ESCs cultured in three different scaffolds prepared using PGLA, collagen, and chitosan, resulting in upregulation of OCT4 expression but downregulation of SOX2; whereas NANOG was only highly upregulated in chitosan scaffolds [144, 145]. Taken together, it can be speculated that the scaffold microenvironments play an important role in influencing cell-matrix communication. The mechanism of upregulation of pluripotent genes in ESCs grown in 3-D culture warrants future investigation. Although 3-D culture of ESCs in hydrogel scaffolds holds great promise, further optimization incorporating adhesion ligands and growth factors, might aid in the proliferation of human ESCs.

Differentiation of ESC derivatives using 3-D scaffolds have been shown to improve the processes of chondrogenesis [146, 147], osteogenesis [148], hematopoiesis [149], neurogenesis [150], cardiomyocyte differentiation [151] and hepatocyte proliferation [152] by mimicking the biochemical and mechanical signaling in vivo [153]. For instance, Col1 scaffolds combined with fibronectin induced vascularization and endothelial differentiation [151], whereas incorporation

of laminin resulted in cardiomyocyte differentiation [151]. 3-D culture in synthetic self-assembling peptide scaffolds [154] and nanofibrous scaffolds [155] promoted ESC differentiation into osteoblast-like cells. While nanofibrous PCL scaffolds supplemented with gelatin supported chondrogenic differentiation of iPSCs [156]. PEG hydrogels conjugated with N-Cadherin derived peptides (His-Ala-Val-Asp-Lle) induced neural differentiation of mouse ESCs [157]. Modification of alginate scaffolds with adhesive cue, RGD (Arg-Gly-Asp) sequences supported retinal differentiation of ESCs and derivatization of retinal tissue [158].

To determine the effects of mechanical properties on differentiation, ESCs were seeded on prefabricated polydimethylsiloxane (PDMS) scaffolds, capable of transducing mechanical signals to cells, due to their high elasticity [30]. As such PDMS scaffolds can be used to study both the effects of matrix elasticity as well as the application of compressive or tensile forces (Fig. 3). 3-D culture of mouse ESCs in 3-D PDMS scaffolds resulted in selective chondrogenic differentiation due to both PDMS matrix elasticity and transduction of compressive stress. PDMS grown ESCs expressed decreased levels of pluripotent markers genes, concurrent with an increase in chondroprogenitor gene markers [147]. We assessed the effect of 3-D culture of mouse ESCs grown in PDMS scaffolds for 7 days, after which a compressive stress (0.05 MPa) was applied for 24 hours. PDMS grown ESCs exhibited differentiated fibroblastoid morphology upon compressive stress (Fig. 3C-E). As such biomimetic scaffolds are useful for both the study of differentiation processes and the generation of ESC derivatives for tissue engineering and transplantation applications.

MSC fibroblastoid morphology strongly correlates with the organization of actin cytoskeleton, and distribution of focal adhesion [159]. Although one of the characteristic features

of MSCs is their adherent growth in 2-D culture conditions, there have been conflicting reports on the necessity of attachment in 3-D culture. MSCs photoencapsulated in nondegradable PEG hydrogels showed improved viability when scaffolds were conjugated with adhesive ligands, RGDSP (Arg-Gly-Asp-Ser-Pro), and to a lesser extent IKVAV (Ile-Lys-Val-Ala-Val) sequences, derived from fibronectin and laminin respectively, similar to findings on MSC adhesion in 2-D culture [42]. The effect of cell spreading and proliferation of MSCs in hydrogel scaffolds was studied using 3-D HA hydrogels supplemented with and without RGD sequences. MSCs proliferated in both hydrogels; however, cell spreading was only observed in scaffolds modified with RGD sequences [160]. Proliferation was limited by matrix stiffness, crosslinking density, RGD concentration, and the rate scaffold degradation in a concentration dependent manner [160, 161]. However, results indicated that hydrogel degradation significantly increased MSC viability, even in the absence of cell adhesion ligands [42].

Similarly, we observed that umbilical cord derived MSCs (Fig.4A) in Dex-SH/PEG-4-Acr self-assembling scaffolds did not show significant proliferation or MSC fibroblastoid morphology (Fig. 4B-C). When Dex-SH/PEG-4-Acr self-assembling scaffolds were prepared with Col1 fibers, Cy3-labeled MSCs adhered, displayed fibroblastoid morphology, and proliferated as visualized by confocal microscopy (Fig. 4D-E).

3-D culture of MSCs may improve adaptability upon transplantation in vivo and enhance the secretion of indoleamine 2,3-dioxygenase, IL-6, M-CSF, IL-10, TGFp, HGF, and PGE2 factors that are involved in immunosuppression [162, 163].

3-D scaffold grown MSCs have been reported to improve the outcome of cell implantation [164-166]. For instance, MSCs grown in 3-D collagen-pullulan hydrogels induced secretion of angiogenic cytokines, vascular endothelial growth factor-a (Vegf-a) and monocyte chemoattractant protein-1 (Mcp-1). Hydrogel grown MSC grafts accelerated wound healing and neovascularlization by differentiating into fibroblasts and endothelial cells [165]. Co-culture of encapsulated MSCs in 3-D HA hydrogels with macrophages, resulted in lower CD16 and HLA-DR, and higher CD206 expression, suggesting that they could be more suitable for transplantation [167]. Interestingly, 3-D collagen-pullulan hydrogels grown bone-marrow MSCs displayed enhanced self-renewal as well as expression of pluripotent genes, OCT4, SOX2, and KLF4, when compared to MSCs grown in 2-D culture conditions [165].

3-D culture conditions mimicking in vivo microenvironments have also been investigated to differentiate MSCs. Mechanical stiffness and loading properties of the scaffolds promoted differentiation of MSCs into various specific lineages [168]. For instance, softer substrates induced MSC differentiation into neural and beta islet cells [169], as well as differentiation into chondrocytes and adipocytes [170-172], whereas increase in substrate stiffness supported MSC differentiation into myoblasts, and osteoblasts [173]. Stiffer substrates also enhanced integrin binding and induced osteogenic differentiation [170, 172]. Compressive forces mimicking joint action also induced chondrogenic differentiation of MSCs via mechanotransductive scaffolds increasing chondrogenic gene expression in MSCs [174-176].

Scaffolds incorporating collagen, a principal component of connective tissue ECM, have been utilized to differentiate MSCs into mesodermal cell lineages including osteoblasts [177, 178], chondrocytes [179], and cardiomyocytes [180]. MSCs differentiated into chondrogenic lineage when cultured in 3-D scaffolds composed of HA to mimic cartilage ECM [181, 182].

MSCs cultured in 3-D scaffolds composed of chitosan, a mechanically strong polysaccharide, promoted osteogenic differentiation [183, 184]. In contrast, umbilical cord derived MSCs differentiated into neurons in soft 3-D alginate scaffolds [185]. Therefore, it is important to optimize the composition and mechanical properties of 3-D scaffolds to direct MSC differentiation into specific cell lineages.

Overall, 3-D culture for maintenance and expansion of MSCs could improve upon current culture techniques by utilizing mechanical signals, spatial gradients of soluble factors, and cell-cell as well as cell-ECM interactions. These studies can shed light on basic cell biology processes and mechanisms. Large-scale cell expansion using individual scaffold constructs in static culture remains challenging. If scaffold size is increased, cell growth in the center of the scaffold may be limited due to diffusion of oxygen, nutrients, and waste into and out of the scaffold. Additional methods of dynamic culture, including agitation and flow perfusion systems, are currently being utilized to control culture conditions for more uniform growth in vitro.

3.2 Dynamic 3-D Culture

Large quantities of homogeneous high quality cells are needed for therapeutic, pharmaceutical, and biotechnological applications, which is not feasible by traditional methods using 2-D culturing techniques. Dynamic bioreactors are helpful in culturing cells at high densities, allowing reproducible cell growth and minimizing variability witnessed with expansion of cells in 2-D culture [72]. Large scale expansion of cells in bioreactors requires well defined and regulated culture conditions, including pH, temperature, and dissolved oxygen concentrations as well as removal of metabolite and waste products [36]. Agglomeration of cells may occur in static culture when cells are seeded at high concentration, reducing oxygen and

nutrients transfer and increasing cell death in large cell aggregates [69].

Several designs of bioreactors exist and can be used for a large range of applications, from expansion or differentiation of single cells to whole tissue culture [36]. Spinner flasks are most commonly employed due to their low cost and ease of use, whereby an internal impeller allows for continuous agitation of suspended cells by varying the rate of stirring [186]. This method results in hydrodynamic shear stress, mixing the cells with oxygen and nutrients and leading to a more homogenous environment, however, excessive agitation leads to cell death [187]. For this reason, rotating wall vessels were designed to limit shear stress by rotating horizontally, allowing for culture mixing without an internal stirring mechanism [188]. Culture parameters can be controlled by flow perfusion systems to allow continuous exchange of nutrients and waste [189], in contrast to fed-batch bioreactor systems which are limited both in scale and length of culture due to a buildup of metabolites and waste that occurs over time [190]. As such media must then be changed at regular intervals to limit the inhibitory effect of waste accumulation or cells would have to be inoculated in multiple bioreactors to achieve large scale cell expansion. Lastly, mechanical force bioreactors have been developed to mimic tissue physiology, utilizing compressive and tensile forces, and are increasingly being used in conjuncture with tissue engineered constructs for cell differentiation [146, 191]. These techniques can be further modified by the use of biomaterials (i.e. microcarriers and microcapsules) allowing for growth of cells at high concentration of cells in suspension culture, as stated below [69]. Overall, cell growth and fate is dependent on culture conditions, including cell seeding density, media composition, and incorporation of biomaterials.

3.2.1 Microcarriers

Microcarriers, small beads approximately 100-300 p,m, are composed of various materials, including polystyrene, gelatin, dextran, and collagen, synthesized with varying porosities, and topographies [192]. They provide an adhesive surface for anchorage-dependent cells in suspension culture [193, 194]. Adhesion of cells on microcarriers can be optimized to promote expansion or differentiation. Microcarriers also limit aggregation of cells and provide a large surface area for cell growth to high densities [192], whereby the concentrated cells can then be collected by enzymatic dissociation [195].

Dynamic culture techniques involving spinner flasks and microcarriers have been shown to support human ESC proliferation to high densities [196-198]. Attachment of pluripotent SCs has been achieved by coating the surface of microcarriers with adhesive proteins [72, 199]. Cellulose microcarriers coated with Matrigel, an assortment of ECM proteins, have been used for long-term expansion and maintenance of human ESCs [200]. Microcarriers coated with ECM proteins such as vitronectin, laminin or PLL, have been shown to promote pluripotent SC growth [201]. In fact, a 12-fold increase in the growth of human ESCs adherent to microcarriers was observed in continuous perfusion bioreactor systems under xeno-free conditions, lacking animal derived materials [198].

One of the main advantages of 3-D culture using microcarriers allows for expansion and subsequent differentiation into specific cell lineages using differentiation media. Higher quality of human ESCs expanded using microcarriers was demonstrated by their ability to form EBs 10 times more efficiently than cells grown under 2-D conditions [202]. Microcarriers were also found to support differentiation of EBs into hemangioblasts, which were capable of differentiating into hematopoietic and endothelial cells [203]. Both expansion and differentiation of ESCs into cardiomyocytes were achieved using the microcarrier spinner culture technique

[201]. The ESC derived cardiomyocytes expressed cell-specific transcription factors, ion channel genes, and sarcomeric proteins. Likewise, ESCs were first expanded to 45-fold using collagen coated microcarriers and then differentiated into an endoderm lineage, specifically pancreatic islets and liver cells, using differentiation media [204]. However, it is not always clear if ESCs differentiated using microcarriers are functional. In one of the studies, hepatic cells derived from ESCs grown using microcarriers in stirred bioreactor secreted proteins at lower levels than those of primary hepatocyte [205].

Microcarriers have also been investigated to scale up MSC proliferation in suspension culture. MSCs derived from bone marrow and placenta have been expanded on microcarriers grown in dynamic bioreactors. However, they exhibited lower cell viability and proliferation compared to expansion in static culture flasks [206, 207]. MSCs isolated from different bone marrow donors showed various rates of proliferation when grown in dynamic bioreactors and expanded on Cytodex 3, collagen coated dextran based microcarriers, despite exhibiting similar growth characteristics in 2-D culture [72]. Similar studies were performed testing a variety of microcarriers, and reproducible growth of MSCs derived from different bone marrow samples was observed when cultured on SoloHill plastic beads, implying that biomaterial signaling played a large role in MSC proliferation [192]. Therefore, determination of a suitable microcarrier is important for MSC expansion. In addition, various culture parameters, including oxygen concentration, agitation rate, and nutrient exchange, are important consideration to achieve improved proliferation of MSCs.

Considerable interest has been shown in the differentiation of MSCs using microcarriers [195, 208-210]. Several reports demonstrated that microcarriers aided in MSC differentiation into osteogenic [195], and chondrogenic [208, 209] lineages. In fact, biodegradable PCL

microcarriers supported differentiation of fetal MSCs into osteogenic cells in vitro, expressing higher levels of osteogenic genes and calcium deposition and equivalent bone formation upon transplantation in comparison to 2-D cultured cells [210]. Microcarriers, which co-released TGFP3, were shown to improve chondrogenic differentiation of MSCs [208]. In another study, MSCs seeded on microcarriers showed improved chondrogenic regeneration upon transplantation in a mouse model of osteoarthritis [209]. Despite some success in the use of microcarriers for growth and differentiation of MSCs, the functional status of the differentiated cells should be further investigated.

3.2.2 Microencapsulation

In contrast to microcarriers, which primarily aid in attachment of cells to the surface of beads, microencapsulation is a process in which cells are immobilized within a semi-permeable material or membrane that allows the diffusion of nutrients, oxygen, and growth factors essential for cell growth. The process of microencapsulation of cells inside spherical capsules can be used not only to protect against agglomeration and shear forces in suspension culture, but also to prevent immune reactions upon transplantation [211]. The microcapsule microenvironment can be modified to maintain stemness or induce differentiation of SCs dependent on the composition of the biomaterials used and the incorporation of growth factors [212]. Cells are often encapsulated by emulsification or extrusion processes forming protective capsules around the cells and allowing growth within the capsule [213]. Alginate and agarose are two of the most commonly used biomolecules for encapsulation of cells by photo-crosslinking and temperature dependent gelation, respectively [214, 215]. Soft alginate capsules are often layered with mechanically strong polycations (i.e. chitosan, PEG, PLL) to improve capsule integrity [69]. Additionally, since agarose capsules lack adhesive properties, they can be modified by

incorporation of additional biomaterials, such as collagen or gelatin, to promote cell attachment and growth [214].

Maintenance of viability and pluripotency as well as homogeneity of cell populations are essential for large-scale expansion of ESCs. It is important to prevent cell aggregation and ameliorate conditions that allow diffusion of nutrients, oxygen, and growth factors. Microencapsulation in agarose prevented aggregation of cells and allowed higher growth of ESCs without necrosis [216]. In general, encapsulation of pluripotent SCs have been shown to protect against chemical and mechanical stresses, including hydrodynamic shear forces that negatively affect cell viability in suspension culture [22, 124, 125]. ESCs encapsulated in calcium-alginate hydrogel microcapsules grew for an extended culture period in xeno-free conditions of without affecting cell viability or pluripotency [217].

ESCs encapsulated in alginate beads differentiated into beta cells capable of producing insulin [218]. Furthermore, ESCs were also differentiated into hepatocytes capable of secreting albumin and urea [219], suggesting that microencapsulation aided ESC differentiation into functional endodermal derivatives. Similarly, mesodermal differentiation of ESCs was facilitated by microencapsulation. ESCs grown in PLL coated alginate capsules, differentiated into cardiomyocytes expressing high levels of Nkx2.5 and GATA4 [220]. Whereas, ESCs encapsulated in PEGDA microcapsules, modified with RGD sequences [221], and HA hydrogel microcapsules [222] supported differentiation into chondrogenic cells. Osteogenic differentiation of ESCs was promoted by culture in calcium phosphate cement microcapsules, enhancing bone regeneration [223]. Ectodermal differentiation was facilitated by 3-D culture in alginate and alginate modified with HA to improve differentiation of ESCs into neural cells expressing synaptic markers [224, 225]. Overall, 3-D culture by microencapsulation of ESCs for

amplification and differentiation may prove to be integral for tissue engineering applications.

Microencapsulation of MSCs is often used for immune modulation [226], resulting in decreased fibrosis [227], and inflammation [228] upon transplantation. Since expansion of MSCs is more reliant on adhesion, modification of microcapsules with adhesive ligands and or ECM components improved cell expansion and differentiation [229]. Dynamic culture improved proliferation of MSCs encapsulated in alginate microcapsules at when compared to static culture [230]. However, MSCs encapsulated in alginate supplemented with adherence ligands, remained viable but did not proliferate [231], suggesting that attachment alone is not sufficient for cell growth. When compressive but not sheer stress was applied to the alginate encapsulated MSCs, they differentiated into chondrogenic progenitors [230]. Alginate microcapsules modified with RGD sequences, allowed for sustained release of TGFpi and induce MSC differentiation with increased chondrogenic gene expression and matrix deposition in vitro and cartilage regeneration in vivo [232]. Preferential MSC differentiation into chondrogenic lineage with enhanced ECM production was also observed in 3-D culture in calcium modified alginate microcapsules [233]. Microencapsulation techniques have also been utilized to promote osteogenic differentiation of MSCs in dynamic 3-D culture via mineralized alginate [234] and PLL coated alginate [235], as well as collagen [236], and collagen modified with agarose [237] and chitosan [238] microcapsules. In addition to differentiation, microencapsulation of MSCs could prove to be an advantageous method for cell delivery of MSC derivatives for repair of degenerative tissues in vivo, while minimizing immune response.

Dynamic bioreactors used in conjuncture with microcarriers and microcapsules have shown potential to improve the quality and quantity of cells. However, many challenges still persist including the potential for differentiation upon agglomeration of cells, altered gene

expression, production of hydrodynamic shear forces, and variability in culture. Therefore, further research is needed, combining dynamic bioreactors with biological and engineered components for optimal expansion and differentiation of SCs.

3.2.3 Microfluidics

Microfluidics involving small-scale systems, both in terms of volume and size, are increasingly being investigated for culturing cells. They can be used to mimic the in vivo microenvironment via perfusion media exchange and chemical gradients of soluble factors for the analysis of single cells [239]. They have been used to study the effects of cell-secreted signals and microenvironment on SC fate [240]. These advancements have led to the development of devices for whole organ systems such as 'organ-on-chips', which mimics the body's microenvironments [32].

Microfluidics can control the soluble microenvironment, which in turn can modulate autocrine/paracrine signaling between cells, and shear stress due to hydrodynamic flow rates [240]. Mouse ESCs cultured in microfluidic perfusion for 4 days, proliferated at higher flow rates but not under the slowest flow rate [241]. When cultured in microfluidic chambers in the absence of feeder layers, mouse ESCs formed colonies and exhibited increased LIF expression, nearly 140 times than that secreted in macro-culture conditions, suggesting that endogenous signaling may play a large role in SC fate determination [242]. When LIF secretion was inhibited, ESCs preferentially differentiated into mesodermal lineages [242]. Microfluidic culture systems can also be used to induce differentiation of ESCs by varying culture conditions, flow rate, and augmentation of exogenous growth factors [243]. Slow-flow microfluidics supported differentiation of mouse ESCs into neuron-like and Schwann-like cells [244].

Additionally, variation in the flow rate in discontinuous perfusion culture allowed for direct differentiation of ESCs into cardiomyocytes and hepatocytes, showing functional phenotypes and responses to drug treatments in microfluidic culture systems [245].

Similar to pluripotent SCs, microfluidic culture systems can be used for small scale proliferation and controlled differentiation of MSCs. Continuous perfusion using microfluidic micromass culture allowed for 34-fold proliferation of MSCs [246]. Subsequent addition of inductive growth factors resulted in homogenous chondrogenic differentiation of MSCs [246]. Constant microfluidic perfusion culture differentiated MSCs into hepatocyte-like cells that expressed cell-specific genes and were capable of low-density lipoprotein uptake, suggestive of functional cells [247]. The application of extremely low fluidic shear stress in microfluidic devices induced cell migration and osteogenic differentiation in MSCs [248, 249]. Mimicking the vascular mechanobiology in vivo via dynamic stretch and circumferential strain in microfluidic devices, resulted in differentiation of MSCs into blood vessel-like cells [250]. MSCs subjected to both IBMX (3-isobutyl-1-methylxanthine) and fluid flow shear stress resulted in a threefold increase in the neuronal cell differentiation [251]. Microfluidic culture techniques are still in their infancy but are promising for elucidation of biological processes, tissue engineering, and pharmaceutical applications.

In summary, numerous types of 3-D culture techniques show improvement in both proliferation and differentiation of SCs. There are advantages and disadvantages for each 3-D culture system based on the desired application as summarized in Fig. 5. In general, these techniques differ in both complexity and controllability. For instance, spheroid formation is a very simple technique that allows for cell-ECM interaction in 3-D, without the incorporation of biomaterials. However, spheroid culture results in heterogeneous cell populations and

microenvironments, including the formation of necrotic/hypoxic cores. The inclusion of biomaterials can improve cell growth and differentiation via inductive biochemical and mechanical signaling by mimicking the in vivo ECM. Determination of the appropriate biomaterials and mechanical properties of 3-D scaffolds must be optimized for each cell type and application. Additionally, scaffolds derived from biological materials may result in poor reproducibility due to lot-to-lot variability. Bioreactors support large-scale growth and/or differentiation of SCs. Static bioreactors are limited by agglomeration of cells in the absence of biomaterials (either microcarriers or microcapsules) and by need for batch media changes. In contrast, dynamic bioreactors can be highly controllable, via agitation and media perfusion allowing for complex homogenous media and cell dispersion. However, these methods are costly and are also affected by the introduction of hydrodynamic stress on cells, which can be limited by the use of biomaterials. Additional optimization is needed to determine the ideal culture parameters, and means of cell analysis during large-scale culture.

4. Challenges

Since the isolation of human ESCs [4], significant progress has been made in understanding the mechanisms of self-renewal, pluripotency, and differentiation using selective media. Likewise, MSCs have been isolated from various tissues and cultured as well as differentiated into mesodermal lineages. Furthermore, new xeno-free media have been developed to culture SCs that can be used for clinical purposes. However, progress regarding the propagation of other SCs such as hematopoietic SCs and progenitors remains sparse. Even the expansion and differentiation of ESCs and MSCs face a number of challenges. These challenges include:

4.1 Heterogeneity of cultured cells

2-D culture techniques are inherently inefficient, labor intensive and expensive, and yield limited expansion of SCs. Moreover, 2-D cultures yield heterogeneous populations of SCs and derivatives. Both ESCs and MSCs tend to undergo non-specific differentiation under 2-D culture conditions. Even directed differentiation of SCs into specific lineages is often only partially achieved and the differentiated derivatives lack functionality. Furthermore, survival, integration, and function of augmented cells grown in 2-D culture conditions are limited. Overall, scale-up of 2-D cultures is difficult and cells cultured under 2-D conditions often fail to mimic the natural microenvironment in vivo for both maintenance and differentiation of SCs.

4.2 Clonal growth

Clonal growth is correlated to the self-renewal of SCs. Decreased clonal growth of SCs is indicative of the reduction of self-renewal capacity and limited SC expansion, particularly for adult SCs such as MSCs. Passaging of MSCs have been shown to reduce clonal growth, proliferative capacity, and self-renewal potential. Human ESCs survive poorly and lose clonogenicity after single cell dissociation, limiting their proliferative potential. Synchronized clonal growth of single cells is important to obtain a homogenous population of cells. Therefore, improvements in the culturing of MSCs and pluripotent SCs are necessary for large scale and long-term expansion of these cells.

4.3 Biomaterials

Often 3-D culture is facilitated by incorporation of biomaterials to mimic the biochemical and biophysical properties of the native 3-D microenvironment. Numerous natural and synthetic biomaterials have been used to develop suitable scaffolds to mimic in vivo conditions for cell

growth. However, it is often difficult to determine which biomaterial would be most appropriate for the growth and differentiation of a particular SC type. For instance, different biomaterials induce variability and reduce reproducibility when used in culture. Biomaterials are also known to play role in signaling that could affect maintenance and differentiation of SCs. Therefore, further studies are needed to identify appropriate biomaterials that could improve quality and quantity of cultured cells.

4.4 Optimization of culture conditions

Despite the rapid increase in the volume of literature regarding the growth and differentiation of SCs, standardized methods for culturing cells are lacking. 2-D culturing methods are simple and commonly practiced but are less efficient. Efforts are needed to develop 3-D culture systems, which while more efficient involve a number of parameters, including media composition, biomaterials, mechanical stress, oxygen content, pH, and media perfusion, which influence both the quality and quantity of cultured cells. In addition, agglomeration of cells and diffusion of nutrient/waste exchange are also important for standardization of the culture methods. This can result in heterogeneous cell populations, necrosis, and inhibition of cell proliferation.

Recent advances in the use of various bioreactors to expand or differentiate SCs are encouraging. Dynamic bioreactors have shown greater promise compared to static bioreactors. However, dynamic bioreactors, which employ agitation or media perfusion could introduce hydrodynamic shear stress, negatively effecting cell viability.

4.5 Functional activity

One of the major considerations of any culturing method is that the cultured cells should have optimal viability and exhibit functional activity comparable to cells found in vivo. These

cells should also be able to survive and integrate into the tissue in vivo. Cells exposed to hypoxic stress have been shown to be more apt to survive upon transplantation. Therefore, cell conditioning should be an important consideration prior to their use for clinical purposes. Furthermore, development of efficient methods yielding homogenously differentiated cells from SCs may help to yield reproducible results but are challenging. Future studies should be focused on investigating the temporal factors that emulate in vivo conditions to devise microenvironments using appropriate biomaterials specific for growth and differentiation of SCs.

4.6 Culture scale up

There is a great need to produce large quantities of high quality cells for various biomedical applications. Current methods for cell expansion yield millions of cells, which have limited autologous usage for individual patients. Even that may not be effective as recent studies have shown that higher concentrations of transplanted cells are more efficacious. Allogenic cell therapy for multi-center clinical trials requires billions to trillions of cells. Scale up expansion of cells to such levels would require substantial advancements in the media, culturing devices and techniques. One advantage of large scale culturing of cells is that they can be used for cell therapy for multiple patients in a clinical trial and results can be correlated. However, large-scale production systems involving bioreactors and GMP facilities necessitate collaboration between biomedical researchers and engineers. In addition, a combination of techniques, including biomaterials modified with growth factors or ligands as well as other growth parameters to optimally grow and differentiate SCs into specific cell types would be essential to further advance the fields of cell therapy, tissue engineering and regenerative medicine.

5. Conclusion

Recently, significant advancements in the culturing of SCs, particularly using biomaterials and bioreactors, have been reported. These 3-D culture methods not only overcome the limitations associated with 2-D culture but also have the potential to expand cells to billion fold as would be needed for clinical trials, pharmaceutical or biotechnology applications. 3-D culturing of cells using bioinductive, biodegradable and biocompatible polymers mimicking the ECM, dimensionality and spatial gradients found in the native microenvironments is not only helpful in expansion and improving functionality but are also amenable for specific differentiation of SCs. Scale-up production of cells requiring bioreactors and GMP facilities would benefit from the multidisciplinary collaborations among the biomedical researchers and engineers. Despite challenges, advances in 3-D culture systems will provide impetus for accelerating the therapeutic uses of SCs to treat many diseases and disorders that have no cure.

Acknowledgments

This research was supported by Oakland University, the Oakland University William Beaumont Institute for Stem Cell and Regenerative Medicine (OU-WB ISCRM), and the Michigan Head and Spine Institute (MHSI). C. McKee was supported by the Oakland University Provost Graduate Student Research Award.

Disclosure Statement

No competing financial interests exist.

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Fig. 1. Schematic of approaches used for three-dimensional culture of stem cells.

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Fig. 2. Three-dimensional growth of green fluorescent protein (GFP) labeled mouse ESCs in self-assembling scaffolds. (A) Schematic of self-assembling scaffold formation and encapsulation of mouse ESCs (B) Image of the self-assembled scaffold, and (C-E) representative composite confocal images of fluorescent green labeled ESCs grown in Dex-SH/PEG-4-Acr self-assembling hydrogels at 2, 7 and 21 days, respectively. 3-D cultured ESCs exhibited undifferentiated clonal growth. Scale bars = 100 p,m.

Fig. 3. Three-dimensional growth of GFP labeled mouse ESCs in prefabricated PDMS scaffolds. (A) Schematic of ESCs seeding in prefabricated PDMS scaffolds subjected to mechanical stress (compression or tension) (B) Macroscopic images of the PDMS scaffolds, and (C-E) representative confocal images of green fluorescent labelled ESCs grown in prefabricated PDMS scaffolds at 2, 7 and 21 days, respectively. ESC seeded scaffolds were compressed at 0.05 MPa for 24 hours on day 7 and further cultured for 2 weeks. Compression of the PDMS scaffolds induced differentiation of ESCs into chondrogenic lineage as evident from the fibroblastoid cell morphology. Scale bars = 100 p,m.

Fig. 4. Human umbilical cord derived Cy3 labeled MSCs grown in three-dimensional self-assembling scaffolds. (A) Light micrograph of 2-D cultured MSCs. (B-E) Representative confocal images of red fluorescent labeled MSCs in Dex-SH/PEG-4-Acr self-assembling scaffolds in the absence and presence of collagen type 1 (Coll) fibers grown for 1 (B and D) and 7 (C and E) days, respectively. MSCs exhibited fibroblastoid morphology when cultured in 3-D scaffolds supplemented with Coll. Scale bars = 100 p,m.

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Fig. 5. Advantages and disadvantages of various 3-D culture systems.