Human embryonic stem cells as an in vitro model for human vascular development and the induction of vascular differentiation

Early embryonic blood vessels are typically composed of fragile tubes of endothelial cells encircled by vascular smooth muscle cells. Early human vasculogenesis was explored in spontaneous and directed differentiation models derived from human embryonic stem (HES) cells. In a 3-dimensional (3D) model, HES cells were studied for their potential for vascular differentiation during the spontaneous formation of embryoid bodies. Directed differentiation was investigated by means of a 2-dimensional (2D) differentiation method to promote vascular differentiation from HES cells (without the formation of embryoid bodies). Using this latter approach, up-regulation of early lineage markers of endothelial progenitors were induced. Additional culture under strict conditions and exposure to angiogenic growth factors resulted in a prolonged differentiation pathway into mature endothelial cells and up-regulation of vascular smooth muscle cell markers. The use of 3D collagen gels and Matrigel assays for the induction and inhibition of human vascular sprouting in vitro further established the vascular potential of the cells generated by the 2D differentiation system. Our study shows that HES cells can provide useful models to study early differentiation and development of blood vessels. Moreover, the 2D differentiation model facilitates both the production of vascular lineage cells from HES cells for various potential therapeutic applications and also provides a model for studying the mechanisms involved in early human embryonic blood vessel development.     

Neural lineage differentiation of embryonic stem cells within alginate microbeads

Cell replacement therapies, using renewable stem cell sources, hold tremendous potential to treat a wide range of degenerative diseases. Although many studies have established techniques to successfully differentiate stem cells into different mature cell lineages using growth factors or extracellular matrix protein supplementation in both two and three-dimensional configurations, they are often limited by lack of control and low yields of differentiated cells. Previously, we developed a scalable murine embryonic stem cell differentiation environment which maintained cell viability and supported ES cell differentiation to hepatocyte lineage cells. Differentiated hepatocyte function was contingent upon aggregate formation within the alginate microbeads. The present studies were designed to determine the feasibility of adapting the alginate encapsulation technique to neural lineage differentiation. The results of our studies indicate that by incorporating the soluble inducer, retinoic acid (RA), into the permeable microcapsule system, cell aggregation was decreased and neural lineage differentiation enhanced. In addition, we demonstrated that even in the absence of RA, differentiation could be directed away from the hepatocyte and toward the neural lineage by physical cell-cell aggregation blocking. In conjunction with the mechanical and physical characterization of the alginate crosslinking network, we determined that 2.2% alginate microencapsulation can be optimally adapted to ES neural differentiation. This study offers insights into targeting cellular differentiation toward both endodermal and ectodermal cell lineages, and could potentially be adaptable to differentiation of other stem cell types given the correct inducible factors and material properties.     

Collagen IV induces trophoectoderm differentiation of mouse embryonic stem cells

The earliest segregation of lineages in the developing embryo is the commitment of cells to the inner cell mass or the trophoectoderm in preimplantation blastocysts. The exogenous signals that control commitment to a particular cell lineage are poorly understood; however, it has been suggested that extracellular "niche" and extracellular matrix, in particular, play an important role in determining the developmental fate of stem cells. Collagen IV (ColIV) has been reported to direct embryonic stem (ES) cell differentiation to mesodermal lineages in both mouse and human ES cells. To define the effects of ColIV on ES cell differentiation and to identify the resulting heterogeneous cell types, we performed microarray analyses and determined global gene expression. We observed that ColIV induced the expression of mesodermal genes specific to hematopoietic, endothelial, and smooth muscle cells and, surprisingly, also a panel of trophoectoderm-restricted markers. This effect was specific to collagen IV, as no trophoblast differentiation was seen on collagen I, laminin, or fibronectin. Stimulation with basic fibroblast growth factor (FGF) or FGF4 increased the number of trophoectodermal cells. These cells were isolated under clonal conditions and successfully differentiated into a variety of trophoblast derivatives. Interestingly, differentiation of ES cells to trophoblastic lineages was only seen in ES cell lines maintained on embryonic feeder layers and was caudal-type homeobox protein 2 (Cdx2)-dependent, consistent with Cdx2's postulated role in trophoectoderm commitment. Our data suggest that, given the appropriate extracellular stimuli, mouse embryonic stem cells can differentiate into trophoectoderm. Disclosure of potential conflicts of interest is found at the end of this article.     

One-dimensional self-assembly of mouse embryonic stem cells using an array of hydrogel microstrands

The ability of embryonic stem (ES) cells to self-renew indefinitely and to differentiate into multiple cell lineages holds promise for advances in modeling disease progression, screening drugs and treating diseases. To realize these potentials, it is imperative to study self-assembly in an embryonic microenvironment, as this may increase our understanding of ES cell maintenance and differentiation. In this study, we synthesized an array of one-dimensional alginate gel microstrands and aqueous microstrands through an SU-8 filter device by means of capillary action. Furthermore, we investigated self-assembly behaviors and differentiation potentials of mouse ES cells cultured in microstrands of varying diameters. We found that microstrands with an aqueous interior facilitated high density cell culture and formed compact microtissue structures, while microstrands with gelled interiors promote smaller cell aggregate structures. In particular, we noticed that ES cells collected from one-dimensional aqueous microstrands favored the differentiation towards cell lineages of endoderm and mesoderm, whereas those from gelled microstrands preferred to differentiate into ectoderm and mesoderm lineages. In addition to providing a "liquid-like" tubular microenvironment to understand one-dimensional self-assembly process of ES cells, this alginate hydrogel microstrand system also offers an alternative way to manipulate the stem cell fate-decision using bioengineered microenvironments.     

The differentiation of embryonic stem cells seeded on electrospun nanofibers into neural lineages

Due to advances in stem cell biology, embryonic stem (ES) cells can be induced to differentiate into a particular mature cell lineage when cultured as embryoid bodies. Although transplantation of ES cells-derived neural progenitor cells has been demonstrated with some success for either spinal cord injury repair in small animal model, control of ES cell differentiation into complex, viable, higher ordered tissues is still challenging. Mouse ES cells have been induced to become neural progenitors by adding retinoic acid to embryoid body cultures for 4 days. In this study, we examine the use of electrospun biodegradable polymers as scaffolds not only for enhancing the differentiation of mouse ES cells into neural lineages but also for promoting and guiding the neurite outgrowth. A combination of electrospun fiber scaffolds and ES cells-derived neural progenitor cells could lead to the development of a better strategy for nerve injury repair.     

Biomimetic three-dimensional cultures significantly increase hematopoietic differentiation efficacy of embryonic stem cells

Stem cell-based tissue engineering is a promising technology in the effort to create functional tissues of choice. To establish an efficient approach for generating hematopoietic cell lineages directly from embryonic stem (ES) cells and to study the effects of three-dimensional (3D) biomaterials on ES cell differentiation, we cultured mouse ES cells on 3D, highly porous, biomimetic scaffolds. Cell differentiation was evaluated by microscopy and flow cytometry analysis with a variety of hematopoiesis- specific markers. Our data indicate that ES cells differentiated on porous 3D scaffold structures developed embryoid bodies (EBs) similar to those in traditional two-dimensional (2D) cultures; however, unlike 2D differentiation, these EBs integrated with the scaffold and appeared embedded in a network of extracellular matrix. Most significantly, the efficiency of hematopoietic precursor cell (HPC) generation on 3D, as indicated by the expression of various HPC-specific surface markers (CD34, Sca-1, Flk-1, and c-Kit) and colony-forming cell (CFC) assays, was reproducibly increased (about 2-fold) over their 2D counterparts. Comparison of static and dynamic 3D cultures demonstrated that spinner flask technology also contributed to the higher hematopoietic differentiation efficiency of ES cells seeded on scaffolds. Continued differentiation of 3D-derived HPCs into the myeloid lineage demonstrated increased efficiency (2-fold) of generating myeloid compared with differentiation from 2D-derived HPCs.     

Use of embryonic stem cell-derived endothelial cells as a cell source to generate vessel structures in vitro

Embryonic stem (ES) cells could potentially serve as an excellent cell source for various applications in regenerative medicine and tissue engineering. Our laboratory is particularly interested in generating a reproducible endothelial cell source for the development of prevascularized materials for tissue/organ reconstruction. After developing methods to isolate highly purified (>96%) proliferating populations of endothelial cells from mouse embryonic stem cells, we tested their ability to form three-dimensional (3-D) vascular structures in vitro. The ES cell-derived endothelial cells were embedded in 3-D collagen gel constructs with rat tail collagen type I (2 mg/mL) at a concentration of 10(6) cells/mL of gel. The gels were observed daily with a phase-contrast microscope to analyze the time course for endothelial cell assembly. The first vessels were observed between days 3 and 5 after gel construct formation. The number and complexity of structures steadily increased, reaching a maximum before beginning to regress. By 2 weeks, all vessel-like structures had regressed back to single cells. Histology and fluorescent images of the vessel-like structures verified that tube structures were multicellular and could develop patent lumens. We have shown that endothelial cells derived, purified and expanded in vitro from ES cells sustain an important endothelial cell function, the ability to undergo vasculogenesis in collagen gels, indicating that endothelial products derived in vitro from stem cells could be useful in regenerative medicine applications.     

Differentiation of Mouse Embryonic Stem Cells to Lympho-Hematopoietic Lineagesin Vitro

Mouse embryonic stem (ES) cells can differentiate in culture to late stages of many cell lineages. have found culture conditions that are favorable for development in vitro of ES cells into hematopoietic cells at a stage equivalent to day 11-14 of fetal liver development. describe here: (1) the growth conditions necessary for maintenance of ES cells in an undifferentiated state, and the conditions that allow differentiation of cystic embryoid bodies that contain precursors of most hematopoietic cell lineages, including lymphoid cells; (2) the development of lymphoid vessels from ES fetusesin vivo; (3) the characterization of lymphoid, erythroid, megakaryoid, and myeloid cells from ES fetuses; and (4) the cloning of cell lines representing lymphoid, myeloid lineage cells from differentiated ES cells.     

Derivation and spontaneous differentiation of human embryonic stem cells

Embryonic stem (ES) cells are unique cells derived from the inner cell mass of the mammalian blastocyst. These cells are immortal and pluripotent, retain their developmental potential after prolonged culture, and can be continuously cultured in an undifferentiated state. Many in vitro differentiation systems have been developed for mouse ES cells, including reproducible methods for mouse ES cell differentiation into haematopoietic and neural precursors, cardiomyocytes, insulin‐secreting cells, endothelial cells and various other cell types. The derivation of new human ES cell lines provides the opportunity to develop unique models for developmental research and for cell therapies. In this review we consider the derivation and spontaneous differentiation of human ES cells.     

Embryoid body-mediated differentiation of mouse embryonic stem cells along a hepatocyte lineage: insights from gene expression profiles

Pluripotent embryonic stem (ES) cells represent a promising renewable cell source for the generation of functional differentiated cells. Previous studies incorporating embryoid body (EB)-mediated stem cell differentiation have, either spontaneously or after growth factor and extracellular matrix protein supplementation, yielded populations of hepatocyte lineage cells expressing mature hepatocyte markers such as albumin (ALB). In an effort to promote ES cell commitment to the hepatocyte lineage, we have evaluated the effects of four culture conditions on albumin and gene expression in differentiating ES cells. Quantitative in situ immunofluorescence and cDNA microarray analyses were used to describe not only lineage specificity but also to provide insights into the effects of disparate culture environments on the mechanisms of differentiation. The results of these studies suggest that spontaneous and collagen-mediated differentiation induce cells with the highest levels of ALB expression but mature liver specific genes were only expressed in the spontaneous condition. Further analysis of gene expression profiles indicated that two distinct mechanisms may govern spontaneous and collagen-mediated differentiation.     

Systematic analysis of reportedly distinct populations of multipotent bone marrow-derived stem cells reveals a lack of distinction

Adult human bone marrow-derived stem cells, having the ability to differentiate into cells of multiple lineages, have been isolated and propagated by varied protocols, including positive (CD105(+))/negative (CD45(-)GlyA(-)) selection with immunomagnetic beads, or direct plating into selective culture media. Each substratum-adherent cell population was subjected to a systematic analysis of their cell surface markers and differentiation potential. In the initial stages of culture, each cell population proliferated slowly, reaching confluence in 10-14 days. Adherent cells proliferated at similar rates whether cultured in serum-free medium supplemented with basic fibroblast growth factor, medium containing 2% fetal bovine serum (FBS) supplemented with epidermal growth factor and platelet-derived growth factor, or medium containing 10% FBS alone. Cell surface marker analysis revealed that more than 95% of the cells were positive for CD105/endoglin, a putative mesenchymal stem cell marker, and negative for CD34, CD31, and CD133, markers of hematopoietic, endothelial, and neural stem cells, respectively, regardless of cell isolation and propagation method. CD44 expression was variable, apparently dependent on serum concentration. Functional similarity of the stem cell populations was also observed, with each different cell population expressing the cell type-specific markers beta-tubulin, type II collagen, and desmin, and demonstrating endothelial tube formation when cultured under conditions favoring neural, cartilage, muscle, and endothelial cell differentiation, respectively. On the basis of these data, adult human bone marrow-derived stem cells cultured in adherent monolayer are virtually indistinguishable, both physically and functionally, regardless of the method of isolation or proliferative expansion.     

Differentiation trapping screen in live culture for genes expressed in cardiovascular lineages.

We have developed a gene trap vector that transduces an EGFP-neo fusion gene (Eno) to monitor the expression of trapped genes in living cells and embryos. Upon in vitro differentiation, most gene-trapped embryonic stem (ES) cell clones exhibited detectable green fluorescence in various specialized cell types, which can be followed in the live culture in real time. Populations of ES cell-derived cardiomyocytes, smooth muscle cells, vascular endothelial cells, and hematopoietic cells were readily recognized by their distinctive morphologies coupled with unique activities, allowing efficient screening for clones with trapped genes expressed in cardiovascular lineages. Applying G418 selection in parallel differentiation cultures further increased detection sensitivity and screening throughput by enriching reporter-expressing cells with intensified green fluorescent protein signals. Sequence analyses and chimera studies demonstrated that the expression of trapped genes in vivo closely correlated with the observed lineage specificity in vitro. This provides a strategy to identify and mutate genes expressed in lineages of interest for further functional studies.     

The effect of matrix composition of 3D constructs on embryonic stem cell differentiation

The use of embryonic stem (ES) cells as unlimited cell source in tissue engineering has ignited the hope of regenerating any kind of tissue in vitro. However, the role of the material in control and guidance of their development and commitment into complex and viable three-dimensional (3D) tissues is still poorly understood. In this work, we investigate the role of material composition and structure on promoting ES cells growth and differentiation, by culturing mouse ES cell-derived embryoid bodies (EBs) in various semi-interpenetrating polymer networks (SIPNs), made of collagen, fibronectin (FN) and laminin (LM). We show that both composition and strength of the supportive matrix play an important role in EBs development. High collagen concentrations inhibit EBs cavitation and hence the following EBs differentiation, by inhibiting apoptosis. The presence of FN in 3D collagen constructs strongly stimulates endothelial cell differentiation and vascularization. Conversely, LM increases the ability of ES cells to differentiate into beating cardiomyocytes. Our data suggest that matrix composition has an important role in EBs development and that it is possible to influence stem cell differentiation toward preferential pattern, by modulating the physical and biochemical properties of the scaffold.     

Bone morphogenetic protein-mediated modulation of lineage diversification during neural differentiation of embryonic stem cells

Embryonic stem cells (ES cells) can give rise to a broad spectrum of neural cell types. The biomedical application of ES cells will require detailed knowledge on the role of individual factors modulating fate specification during in vitro differentiation. Bone morphogenetic proteins (BMPs) are known to exert a multitude of diverse differentiation effects during embryonic development. Here, we show that exposure to BMP2 at distinct stages of neural ES cell differentiation can be used to promote specific cell lineages. During early ES cell differentiation, BMP2-mediated inhibition of neuroectodermal differentiation is associated with an increase in mesoderm and smooth muscle differentiation. In fibroblast growth factor 2-expanded ES cell-derived neural precursors, BMP2 supports the generation of neural crest phenotypes, and, within the neuronal lineage, promotes distinct subtypes of peripheral neurons, including cholinergic and autonomic phenotypes. BMP2 also exerts a density-dependent promotion of astrocyte differentiation at the expense of oligodendrocyte formation. Experiments involving inhibition of the serine threonine kinase FRAP support the notion that these effects are mediated via the JAK/STAT pathway. The preservation of diverse developmental BMP2 effects in differentiating ES cell cultures provides interesting prospects for the enrichment of distinct neural phenotypes in vitro.     

Crucial roles of mesodermal cell lineages in a murine embryonic stem cell-derived in vitro liver organogenesis system

Recent studies in the field of regenerative medicine have exploited the pluripotency of embryonic stem (ES) cells to generate a variety of cell lineages. However, the target has always been only a single lineage, which was isolated from other differentiated cell populations. In the present study, we selected sublines with a high capability for differentiation to contracting cardiomyocytes and also produced germ-line chimeric mice from a parent ES line. We also succeed in establishing embryoid bodies prepared from the ES cells that differentiated into not only hepatocytes but also at least two mesodermal lineages: cardiomyocytes that supported liver development and endothelial cells corresponding to sinusoids. This allowed the development of an in vitro system using murine ES cells that approximated the events of liver development in vivo. The expression of albumin was significantly higher in cardiomyocytes that had arisen in differentiated ES cells than in those that had not. Our in vitro system for liver organogenesis consists of a blood/sinusoid vascular-like network and hepatocyte layers and shows higher levels of hepatic function, such as albumin production and ammonia degradation, than hepatic cell lines and primary cultures of murine adult hepatocytes. This innovative system will lead to the development of second-generation regenerative medicine techniques using ES cells and is expected to be useful for the development of bioartificial liver systems and drug-metabolism assays.