The Instructive Role of the Vasculature in Stem Cell Niches

Unlocking the Potential of Stem Cell Microenvironments In Vitro

The field of regenerative medicine has recently witnessed groundbreaking advancements that hold immense promise for treating a wide range of diseases and injuries. At the forefront of this revolutionary progress are stem cells. Stem cells typically reside in specialized environments in vivo, known as microenvironments or niches, which play critical roles in regulating stem cell behavior and determining their fate. Therefore, understanding the complex microenvironments that surround stem cells is crucial for advancing treatment options in regenerative medicine and tissue engineering applications. Several research articles have made significant contributions to this field by exploring the interactions between stem cells and their surrounding niches, investigating the influence of biomechanical and biochemical cues, and developing innovative strategies for tissue regeneration. This review highlights the key findings and contributions of these studies, shedding light on the diverse applications that may arise from the understanding of stem cell microenvironments, thus harnessing the power of these microenvironments to transform the landscape of medicine and offer new avenues for regenerative therapies.

Two-step regulation by matrix Gla protein in brown adipose cell differentiation

OBJECTIVE

Bone morphogenetic protein (BMP) signaling is intricately involved in adipose tissue development. BMP7 together with BMP4 have been implicated in brown adipocyte differentiation but their roles during development remains poorly specified. Matrix Gla protein (MGP) inhibits BMP4 and BMP7 and is expressed in endothelial and progenitor cells. The objective was to determine the role of MGP in brown adipose tissue (BAT) development.

METHODS

The approach included global and cell-specific Mgp gene deletion in combination with RNA analysis, immunostaining, thermogenic activity, and in vitro studies.

RESULTS

The results revealed that MGP directs brown adipogenesis at two essential steps. Endothelial-derived MGP limits triggering of white adipogenic differentiation in the perivascular region, whereas MGP derived from adipose cells supports the transition of CD142-expressing progenitor cells to brown adipogenic maturity. Both steps were important to optimize the thermogenic function of BAT. Furthermore, MGP derived from both sources impacted vascular growth. Reduction of MGP in either endothelial or adipose cells expanded the endothelial cell population, suggesting that MGP is a factor in overall plasticity of adipose tissue.

CONCLUSION

MGP displays a dual and cell-specific function in BAT, essentially creating a "cellular shuttle" that coordinates brown adipogenic differentiation with vascular growth during development.

Mimicking Molecular Pathways in the Design of Smart Hydrogels for the Design of Vascularized Engineered Tissues

Biomaterials are pivotal in supporting and guiding vascularization for therapeutic applications. To design effective, bioactive biomaterials, understanding the cellular and molecular processes involved in angiogenesis and vasculogenesis is crucial. Biomaterial platforms can replicate the interactions between cells, the ECM, and the signaling molecules that trigger blood vessel formation. Hydrogels, with their soft and hydrated properties resembling natural tissues, are widely utilized; particularly synthetic hydrogels, known for their bio-inertness and precise control over cell-material interactions, are utilized. Naturally derived and synthetic hydrogel bases are tailored with specific mechanical properties, controlled for biodegradation, and enhanced for cell adhesion, appropriate biochemical signaling, and architectural features that facilitate the assembly and tubulogenesis of vascular cells. This comprehensive review showcases the latest advancements in hydrogel materials and innovative design modifications aimed at effectively guiding and supporting vascularization processes. Furthermore, by leveraging this knowledge, researchers can advance biomaterial design, which will enable precise support and guidance of vascularization processes and ultimately enhance tissue functionality and therapeutic outcomes.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Mesenchymal stem cell therapy for neurological disorders: The light or the dark side of the force?

Neurological disorders are recognized as major causes of death and disability worldwide. Because of this, they represent one of the largest public health challenges. With awareness of the massive burden associated with these disorders, came the recognition that treatment options were disproportionately scarce and, oftentimes, ineffective. To address these problems, modern research is increasingly looking into novel, more effective methods to treat neurological patients; one of which is cell-based therapies. In this review, we present a critical analysis of the features, challenges, and prospects of one of the stem cell types that can be employed to treat numerous neurological disorders-mesenchymal stem cells (MSCs). Despite the fact that several studies have already established the safety of MSC-based treatment approaches, there are still some reservations within the field regarding their immunocompatibility, heterogeneity, stemness stability, and a range of adverse effects-one of which is their tumor-promoting ability. We additionally examine MSCs' mechanisms of action with respect to in vitro and in vivo research as well as detail the findings of past and ongoing clinical trials for Parkinson's and Alzheimer's disease, ischemic stroke, glioblastoma multiforme, and multiple sclerosis. Finally, this review discusses prospects for MSC-based therapeutics in the form of biomaterials, as well as the use of electromagnetic fields to enhance MSCs' proliferation and differentiation into neuronal cells.

Advances in periodontal stem cells and the regulating niche: From in vitro to in vivo

Periodontium possesses stem cell populations for its self-maintenance and regeneration, and has been proved to be an optimal stem cell source for tissue engineering. In vitro studies have shown that stem cells can be isolated from periodontal ligament, alveolar bone marrow and gingiva. In recent years, more studies have focused on identification of periodontal stem cells in vivo. Multiple genetic markers, including Gli1, Prx1, Axin2, αSMA, and LepR, were identified with the lineage tracing approaches. Characteristics, functions, and regulatory mechanisms of specific populations expressing one of these markers have been investigated. In vivo studies also revealed that periodontal stem cells can be regulafrted by different niche and mechanisms including intercellular interactions, ECM and multiple secreted factors. In this review, we summarized the current knowledge of in vitro characteristics and in vivo markers of periodontal stem cells, and discussed the specific regulating niche.

A Substrate-Mimicking Basement Membrane Drives the Organization of Human Mesenchymal Stromal Cells and Endothelial Cells Into Perivascular Niche-Like Structures

Extracellular matrix-derived products (e.g. Matrigel) are widely used for in vitro cell cultures both as two-dimensional (2D) substrates and as three-dimensional (3D) encapsulation gels because of their ability to control cell phenotypes through biospecific cues. However, batch-to-batch variations, poor stability, cumbersome handling, and the relatively high costs strictly limit their use. Recently, a new substrate known as PhenoDrive-Y has been used as 2D coating of tissue culture plastic showing to direct the bone marrow mesenchymal stromal cells (MSCs) toward the formation of 3D spheroids. When organized into 3D spheroids, the MSCs expressed levels of pluripotency markers and of paracrine angiogenic activity higher than those of the MSCs adhering as fibroblast-like colonies on tissue culture plastic. The formation of the spheroids was attributed to the properties of this biomaterial that resemble the main features of the basement membrane by mimicking the mesh structure of collagen IV and by presenting the cells with orderly spaced laminin bioligands. In this study, PhenoDrive-Y was compared to Matrigel for its ability to drive the formation of perivascular stem cell niche-like structures in 2D co-culture conditions of human endothelial cells and adult bone marrow MSCs. Morphological analyses demonstrated that, when compared to Matrigel, PhenoDrive-Y led endothelial cells to sprout into a more consolidated tubular network and that the MSCs nestled as compact spheroids above the anastomotic areas of this network resemble more closely the histological features of the perivascular stem cell niche. A study of the expressions of relevant markers led to the identification of the pathways linking the PhenoDrive-Y biomimicking properties to the acquired histological features, demonstrating the enhanced levels of stemness, renewal potential, predisposition to migration, and paracrine activities of the MSCs.

Adult stem cell niches for tissue homeostasis

Adult stem cells are fundamental to maintain tissue homeostasis, growth, and regeneration. They reside in specialized environments called niches. Following activating signals, they proliferate and differentiate into functional cells that are able to preserve tissue physiology, either to guarantee normal turnover or to counteract tissue damage caused by injury or disease. Multiple interactions occur within the niche between stem cell‐intrinsic factors, supporting cells, the extracellular matrix, and signaling pathways. Altogether, these interactions govern cell fate, preserving the stem cell pool, and regulating stem cell proliferation and differentiation. Based on their response to body needs, tissues can be largely classified into three main categories: tissues that even in normal conditions are characterized by an impressive turnover to replace rapidly exhausting cells (blood, epidermis, or intestinal epithelium); tissues that normally require only a basal cell replacement, though able to efficiently respond to increased tissue needs, injury, or disease (skeletal muscle); tissues that are equipped with less powerful stem cell niches, whose repairing ability is not able to overcome severe damage (heart or nervous tissue). The purpose of this review is to describe the main characteristics of stem cell niches in these different tissues, highlighting the various components influencing stem cell activity. Although much has been done, more work is needed to further increase our knowledge of niche interactions. This would be important not only to shed light on this fundamental chapter of human physiology but also to help the development of cell‐based strategies for clinical therapeutic applications, especially when other approaches fail.

Spatiotemporal extracellular matrix modeling for in situ cell niche studies

Extracellular matrix (ECM) components govern a range of cell functions, such as migration, proliferation, maintenance of stemness, and differentiation. Cell niches that harbor stem-/progenitor cells, with matching ECM, have been shown in a range of organs, although their presence in the heart is still under debate. Determining niches depends on a range of in vitro and in vivo models and techniques, where animal models are powerful tools for studying cell-ECM dynamics; however, they are costly and time-consuming to use. In vitro models based on recombinant ECM proteins lack the complexity of the in vivo ECM. To address these issues, we present the spatiotemporal extracellular matrix model for studies of cell-ECM dynamics, such as cell niches. This model combines gentle decellularization and sectioning of cardiac tissue, allowing retention of a complex ECM, with recellularization and subsequent image processing using image stitching, segmentation, automatic binning, and generation of cluster maps. We have thereby developed an in situ representation of the cardiac ECM that is useful for assessment of repopulation dynamics and to study the effect of local ECM composition on phenotype preservation of reseeded mesenchymal progenitor cells. This model provides a platform for studies of organ-specific cell-ECM dynamics and identification of potential cell niches.

Biofabrication of vasculature in microphysiological models of bone

Bone contains a dense network of blood vessels that are essential to its homoeostasis, endocrine function, mineral metabolism and regenerative functions. In addition, bone vasculature is implicated in a number of prominent skeletal diseases, and bone has high affinity for metastatic cancers. Despite vasculature being an integral part of bone physiology and pathophysiology, it is often ignored or oversimplified inin vitrobone models. However, 3D physiologically relevant vasculature can now be engineeredin vitro, with microphysiological systems (MPS) increasingly being used as platforms for engineering this physiologically relevant vasculature. In recent years, vascularised models of bone in MPSs systems have been reported in the literature, representing the beginning of a possible technological step change in how bone is modelledin vitro. Vascularised bone MPSs is a subfield of bone research in its nascency, however given the impact of MPSs has had inin vitroorgan modelling, and the crucial role of vasculature to bone physiology, these systems stand to have a substantial impact on bone research. However, engineering vasculature within the specific design restraints of the bone niche is significantly challenging given the different requirements for engineering bone and vasculature. With this in mind, this paper aims to serve as technical guidance for the biofabrication of vascularised bone tissue within MPS devices. We first discuss the key engineering and biological considerations for engineering more physiologically relevant vasculaturein vitrowithin the specific design constraints of the bone niche. We next explore emerging applications of vascularised bone MPSs, and conclude with a discussion on the current status of vascularised bone MPS biofabrication and suggest directions for development of next generation vascularised bone MPSs.

Biomaterial Strategies to Bolster Neural Stem Cell-Mediated Repair of the Central Nervous System

Stem cell therapies have the potential to not only repair, but to regenerate tissue of the central nervous system (CNS). Recent studies demonstrate that transplanted stem cells can differentiate into neurons and integrate with the intact circuitry after traumatic injury. Unfortunately, the positive findings described in rodent models have not been replicated in clinical trials, where the burden to maintain the cell viability necessary for tissue repair becomes more challenging. Low transplant survival remains the greatest barrier to stem cell-mediated repair of the CNS, often with fewer than 1-2% of the transplanted cells remaining after 1 week. Strategic transplantation parameters, such as injection location, cell concentration, and transplant timing achieve only modest improvements in stem cell transplant survival and appear inconsistent across studies. Biomaterials provide researchers with a means to significantly improve stem cell transplant survival through two mechanisms: (1) a vehicle to deliver and protect the stem cells and (2) a substrate to control the cytotoxic injury environment. These biomaterial strategies can alleviate cell death associated with delivery to the injury and can be used to limit cell death after transplantation by limiting cell exposure to cytotoxic signals. Moreover, it is likely that control of the injury environment with biomaterials will lead to a more reliable support for transplanted cell populations. This review will highlight the challenges associated with cell delivery in the CNS and the advances in biomaterial development and deployment for stem cell therapies necessary to bolster stem cell-mediated repair.

Angiogenic biomaterials to promote therapeutic regeneration and investigate disease progression

The vasculature is a key component of the tissue microenvironment. Traditionally known for its role in providing nutrients and oxygen to surrounding cells, the vasculature is now also acknowledged to provide signaling cues that influence biological outcomes in regeneration and disease. These cues come from the cells that comprise vasculature, as well as the dynamic biophysical and biochemical properties of the surrounding extracellular matrix that accompany vascular development and remodeling. In this review, we illustrate the larger role of the vasculature in the context of regenerative biology and cancer progression. We describe cellular, biophysical, biochemical, and metabolic components of vascularized microenvironments. Moreover, we provide an overview of multidimensional angiogenic biomaterials that have been developed to promote therapeutic vascularization and regeneration, as well as to mimic elements of vascularized microenvironments as a means to uncover mechanisms by which vasculature influences cancer progression and therapy.

Blood vessel formation in cerebral organoids formed from human embryonic stem cells

Current cerebral organoid technology provides excellent in vitro models mimicking the structure and function of the developing human brain, which enables studies on normal and pathological brain; however, further improvements are necessary to overcome the problems of immaturity and dearth of non-parenchymal cells. Vascularization is one of the major challenges for recapitulating processes in the developing human brain. Here, we examined the formation of blood vessel-like structures in cerebral organoids induced by vascular endothelial growth factor (VEGF) in vitro. The results indicated that VEGF enhanced differentiation of vascular endothelial cells (ECs) without reducing neuronal markers in the embryonic bodies (EBs), which then successfully developed into cerebral organoids with open-circle vascular structures expressing an EC marker, CD31, and a tight junction marker, claudin-5, characteristic of the blood-brain barrier (BBB). Further treatment with VEGF and Wnt7a promoted the formation of the outer lining consisting of pericyte-like cells, which surrounded the vascular tubes. RNA sequencing revealed that VEGF upregulated genes associated with tube formation, vasculogenesis, and the BBB; it also changed the expression of genes involved in brain embryogenesis, suggesting a role of VEGF in neuronal development. These results indicate that VEGF treatment can be used to generate vessel-like structures with mature BBB characteristics in cerebral organoids in vitro.

Age-related oxidative stress confines damage-responsive Bmi1+ cells to perivascular regions in the murine adult heart

Adult progenitor cells reside in specialized microenvironments which maintain their undifferentiated cell state and trigger regenerative responses following injury. Although these environments are well described in several tissues, the cellular components that comprise the cardiac environment where progenitor cells are located remain unknown. Here we use Bmi1^CreERT and Bmi1^GFP mice as genetic tools to trace cardiac damage-responsive cells throughout the mouse lifespan. In adolescent mice, Bmi1⁺ damage-responsive cells are broadly distributed throughout the myocardium. In adult mice, however, Bmi1⁺ cells are confined predominately in perivascular areas with low levels of reactive oxygen species (ROS) and their number decline in an age-dependent manner. In vitro co-culture experiments with endothelial cells supported a regulatory role of the endothelium in damage-responsive cell behavior. Accordingly, in vivo genetic decrease of ROS levels in adult heart disengaged Bmi1⁺ cells from the cardiovascular network, recapitulating an adolescent-like Bmi1 expression profile. Thus, we identify cardiac perivascular regions as low-stress microenvironments that favor the maintenance of adult damage-responsive cells.

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Bmi1⁺ cardiac damage-responsive cells are sheltered in areas with low ROS levels.

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Aging-related oxidative damage confines cardiac Bmi1⁺ cells to perivascular regions.

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Microvasculature-derived signals regulate adult Bmi1⁺ damage-responsive cell behavior.

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Genetic ROS levels manipulation modifies the percentage and identity of Bmi1⁺ cells.

Mesenchymal stem cells to treat type 1 diabetes

What is clear is we are in the era of the stem cell and its potential in ameliorating human disease. Our perspective is generated from an in vivo model in a large animal that offers significant advantages (complete transplantation tolerance, large size and long life span). This review is an effort to meld our preclinical observations with others for the reader and to outline potential avenues to improve the present outlook for patients with diabetes. This effort exams the history or background of stem cell research in the laboratory and the clinic, types of stem cells, pluripotency or lack thereof based on a variety of pre-clinical investigations attempting endocrine pancreas recovery using stem cell transplantation. The focus is on the use of hematopoietic and mesenchymal stem cells. This review will also examine recent clinical experience following stem cell transplantation in patients with type 1 diabetes.

Inspired by Nature: Hydrogels as Versatile Tools for Vascular Engineering

Significance: Diseases related to vascular malfunction, hyper-vascularization, or lack of vascularization are among the leading causes of morbidity and mortality. Engineered, vascularized tissues as well as angiogenic growth factor-releasing hydrogels could replace defective tissues. Further, treatments and testing of novel vascular therapeutics will benefit significantly from models that allow for the study of vascularized tissues under physiological relevant in vitro conditions. Recent Advances: Inspired by fibrin, the provisional matrix during wound healing, naturally derived and synthetic hydrogel scaffolds have been developed for vascular engineering. Today, engineers and biologists use commercially available hydrogels to pre-vascularize tissues, to control the delivery of angiogenic growth factors, and to establish vascular diseases models. Critical Issue: For clinical translation, pre-vascularized tissue constructs must be sufficiently large and stable to substitute function-relevant tissue defects and integrate with host vascular perfusion. Moreover, the continuous integration of knowhow from basic vascular biology with innovative, tailorable materials and advanced manufacturing technologies is key to achieving near-physiological tissue models and new treatments to control vascularization. Future Directions: For transplantation, engineered tissues must comprise hierarchically organized vascular trees of different caliber and function. The development of novel vascularization-promoting or -inhibiting therapeutics will benefit from physiologically relevant vessel models. In addition, tissue models representing treatment-relevant vascular tissue functions will increase the capacity to screen for therapeutic compounds and will significantly reduce the need for animals for their validation.

Ectopic bone formation by aggregated mesenchymal stem cells from bone marrow and adipose tissue: A comparative study

Tissue engineered constructs (TECs) based on spheroids of bone marrow mesenchymal stromal cells (BM-MSCs) combined with calcium phosphate microparticles and enveloped in a platelet-rich plasma hydrogel showed that aggregation of MSCs improves their ectopic bone formation potential. The stromal vascular fraction (SVF) and adipose-derived MSCs (ASCs) have been recognized as an interesting MSC source for bone tissue engineering, but their ectopic bone formation is limited. We investigated whether aggregation of ASCs could similarly improve ectopic bone formation by ASCs and SVF cells. The formation of aggregates with BM-MSCs, ASCs and SVF cells was carried out and gene expression was analysed for osteogenic, chondrogenic and vasculogenic genes in vitro. Ectopic bone formation was evaluated after implantation of TECs in immunodeficient mice with six conditions: TECs with ASCs, TECs with BM-MSC, TECs with SVF cells (with and without rhBMP2), no cells and no cells with rhBMP2. BM-MSCs showed consistent compact spheroid formation, ASCs to a lesser extent and SVF showed poor spheroid formation. Aggregation of ASCs induced a significant upregulation of the expression of osteogenic markers like alkaline phosphatase and collagen type I, as compared with un-aggregated ASCs. In vivo, ASC and SVF cells both generated ectopic bone in the absence of added morphogenetic proteins. The highest incidence of bone formation was seen with BM-MSCs (7/9) followed by SVF + rhBMP2 (4/9) and no cells + rhBMP2 (2/9). Aggregation can improve ectopic bone tissue formation by adipose-derived cells, but is less efficient than rhBMP2. A combination of both factors should now be tested to investigate an additive effect.

Stemness in Cancer: Stem Cells, Cancer Stem Cells, and Their Microenvironment

Stemness combines the ability of a cell to perpetuate its lineage, to give rise to differentiated cells, and to interact with its environment to maintain a balance between quiescence, proliferation, and regeneration. While adult Stem Cells display these properties when participating in tissue homeostasis, Cancer Stem Cells (CSCs) behave as their malignant equivalents. CSCs display stemness in various circumstances, including the sustaining of cancer progression, and the interaction with their environment in search for key survival factors. As a result, CSCs can recurrently persist after therapy. In order to understand how the concept of stemness applies to cancer, this review will explore properties shared between normal and malignant Stem Cells. First, we provide an overview of properties of normal adult Stem Cells. We thereafter elaborate on how these features operate in CSCs. We then review the organization of microenvironment components, which enables CSCs hosting. We subsequently discuss Mesenchymal Stem/Stromal Cells (MSCs), which, although their stemness properties are limited, represent essential components of the Stem Cell niche and tumor microenvironment. We next provide insights of the therapeutic strategies targeting Stem Cell properties in tumors and the use of state-of-the-art techniques in future research. Increasing our knowledge of the CSCs microenvironment is key to identifying new therapeutic solutions.

Combination of low-energy shock-wave therapy and bone marrow mesenchymal stem cell transplantation to improve the erectile function of diabetic rats

Stem cell transplantation and low-energy shock-wave therapy (LESWT) have emerged as potential and effective treatment protocols for diabetic erectile dysfunction. During the tracking of transplanted stem cells in diabetic erectile dysfunction models, the number of visible stem cells was rather low and decreased quickly. LESWT could recruit endogenous stem cells to the cavernous body and improve the microenvironment in diabetic cavernous tissue. Thus, we deduced that LESWT might benefit transplanted stem cell survival and improve the effects of stem cell transplantation. In this research, 42 streptozotocin-induced diabetic rats were randomized into four groups: the diabetic group (n = 6), the LESWT group (n = 6), the bone marrow-derived mesenchymal stem cell (BMSC) transplantation group (n = 15), and the combination of LESWT and BMSC transplantation group (n = 15). One and three days after BMSC transplantation, three rats were randomly chosen to observe the survival numbers of BMSCs in the cavernous body. Four weeks after BMSC transplantation, the following parameters were assessed: the surviving number of transplanted BMSCs in the cavernous tissue, erectile function, real-time polymerase chain reaction, and penile immunohistochemical assessment. Our research found that LESWT favored the survival of transplanted BMSCs in the cavernous body, which might be related to increased stromal cell-derived factor-1 expression and the enhancement of angiogenesis in the diabetic cavernous tissue. The combination of LESWT and BMSC transplantation could improve the erectile function of diabetic erectile function rats more effectively than LESWT or BMSC transplantation performed alone.

Exercise protects against methamphetamine-induced aberrant neurogenesis

While no effective therapy is available for the treatment of methamphetamine (METH)-induced neurotoxicity, aerobic exercise is being proposed to improve depressive symptoms and substance abuse outcomes. The present study focuses on the effect of exercise on METH-induced aberrant neurogenesis in the hippocampal dentate gyrus in the context of the blood-brain barrier (BBB) pathology. Mice were administered with METH or saline by i.p. injections for 5 days with an escalating dose regimen. One set of mice was sacrificed 24 h post last injection of METH, and the remaining animals were either subjected to voluntary wheel running (exercised mice) or remained in sedentary housing (sedentary mice). METH administration decreased expression of tight junction (TJ) proteins and increased BBB permeability in the hippocampus. These changes were preserved post METH administration in sedentary mice and were associated with the development of significant aberrations of neural differentiation. Exercise protected against these effects by enhancing the protein expression of TJ proteins, stabilizing the BBB integrity, and enhancing the neural differentiation. In addition, exercise protected against METH-induced systemic increase in inflammatory cytokine levels. These results suggest that exercise can attenuate METH-induced neurotoxicity by protecting against the BBB disruption and related microenvironmental changes in the hippocampus.

Dual Role of Mesenchymal Stem Cells Allows for Microvascularized Bone Tissue-Like Environments in PEG Hydrogels

In vitro engineered tissues which recapitulate functional and morphological properties of bone marrow and bone tissue will be desirable to study bone regeneration under fully controlled conditions. Among the key players in the initial phase of bone regeneration are mesenchymal stem cells (MSCs) and endothelial cells (ECs) that are in close contact in many tissues. Additionally, the generation of tissue constructs for in vivo transplantations has included the use of ECs since insufficient vascularization is one of the bottlenecks in (bone) tissue engineering. Here, 3D cocultures of human bone marrow derived MSCs (hBM-MSCs) and human umbilical vein endothelial cells (HUVECs) in synthetic biomimetic poly(ethylene glycol) (PEG)-based matrices are directed toward vascularized bone mimicking tissue constructs. In this environment, bone morphogenetic protein-2 (BMP-2) or fibroblast growth factor-2 (FGF-2) promotes the formation of vascular networks. However, while osteogenic differentiation is achieved with BMP-2, the treatment with FGF-2 suppressed osteogenic differentiation. Thus, this study shows that cocultures of hBM-MSCs and HUVECs in biological inert PEG matrices can be directed toward bone and bone marrow-like 3D tissue constructs.

Relationship of Grafted FGF-2-Overexpressing Neural Stem/Progenitor Cells With the Vasculature in the Cerebral Cortex

Neural progenitor cells (NPCs) overexpressing fibroblast growth factor 2 (FGF-2) have the distinct tendency to associate with the vasculature and establish multiple proliferative clusters in the perivascular environment after transplantation into the cerebral cortex. Strikingly, the vascular clusters of progenitor cells give rise to immature neurons after ischemic injury, raising prospects for the formation of ectopic neurogenic niches for repair. We investigated the spatial relationship of perivascular clusters with the host vascular structures. FGF-2-GFP-transduced NPCs were transplanted into the intact somatosensory rat cortex. Confocal microscopic analysis revealed that grafted cells preferentially contacted venules at sites with aquaporin-4-positive astrocytic endfeet and avoided contacts with desmin-positive pericytes. Electron microscopic analysis confirmed that grafted cells preferentially made contact with astroglial endfeet, and only a minority of them reached the endothelial basal lamina. These results provide new insights into the fine structural and anatomical relationship between grafted FGF-2-transduced NPCs and the host vasculature.

Bone-marrow mimicking biomaterial niches for studying hematopoietic stem and progenitor cells

This review discusses the considerations and approaches that have been employed for designing biomaterial based cultures for replicating the hematopoietic stem and progenitor cell niche.