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The role of soluble messengers in directing cellular behaviours has been recognized for decades. However, many cellular processes, including adhesion, migration and stem cell ...differentiation, are also governed by chemical and physical interactions with non-soluble components of the extracellular matrix (ECM). Among other effects, a cell’s perception of nanoscale features such as substrate topography and ligand presentation, and its ability to deform the matrix via the generation of cytoskeletal tension play fundamental roles in these cellular processes. As a result, many biomaterials-based tissue engineering and regenerative medicine strategies aim to harness the cell’s perception of substrate stiffness and nanoscale features to direct particular behaviours. However, since cell–ECM interactions vary considerably between two-dimensional (2-D) and three-dimensional (3-D) models, understanding their influence over normal and pathological cell responses in 3-D systems that better mimic the in vivo microenvironment is essential to translate such insights efficiently into medical therapies. This review summarizes the key findings in these areas and discusses how insights from 2-D biomaterials are being used to examine cellular behaviours in more complex 3-D hydrogel systems, in which not only matrix stiffness, but also degradability, plays an important role, and in which defining the nanoscale ligand presentation presents an additional challenge.
Although understanding how soluble cues direct cellular processes revolutionised the study of cell biology in the second half of the 20th century, over the last two decades, new insights into how ...mechanical cues similarly impact cell fate decisions has gained momentum. During development, extrinsic cues such as fluid flow, shear stress and compressive forces are essential for normal embryogenesis to proceed. Indeed, both adult and embryonic stem cells can respond to applied forces, but they can also detect intrinsic mechanical cues from their surrounding environment, such as the stiffness of the extracellular matrix, which impacts differentiation and morphogenesis. Cells can detect changes in their mechanical environment using cell surface receptors such as integrins and focal adhesions. Moreover, dynamic rearrangements of the cytoskeleton have been identified as a key means by which forces are transmitted from the extracellular matrix to the cell and
vice versa.
Although we have some understanding of the downstream mechanisms whereby mechanical cues are translated into changes in cell behaviour, many of the signalling pathways remain to be defined. This review discusses the importance of intrinsic mechanical cues on adult cell fate decisions, the emerging roles of cell surface mechano-sensors and the cytoskeleton in enabling cells to sense its microenvironment, and the role of intracellular signalling in translating mechanical cues into transcriptional outputs. In addition, the contribution of mechanical cues to fundamental processes during embryogenesis such as apical constriction and convergent extension is discussed. The continued development of tools to measure the biomechanical properties of soft tissues
in vivo
is likely to uncover currently underestimated contributions of these cues to adult stem cell fate decisions and embryogenesis, and may inform on regenerative strategies for tissue repair.
Mineralization is a ubiquitous process in the animal kingdom and is fundamental to human development and health. Dysfunctional or aberrant mineralization leads to a variety of medical problems, and ...so an understanding of these processes is essential to their mitigation. Osteoblasts create the nano-composite structure of bone by secreting a collagenous extracellular matrix (ECM) on which apatite crystals subsequently form. However, despite their requisite function in building bone and decades of observations describing intracellular calcium phosphate, the precise role osteoblasts play in mediating bone apatite formation remains largely unknown. To better understand the relationship between intracellular and extracellular mineralization, we combined a sample-preparation method that simultaneously preserved mineral, ions, and ECM with nano-analytical electron microscopy techniques to examine osteoblasts in an in vitro model of bone formation. We identified calcium phosphate both within osteoblast mitochondrial granules and intracellular vesicles that transported material to the ECM. Moreover, we observed calcium-containing vesicles conjoining mitochondria, which also contained calcium, suggesting a storage and transport mechanism. Our observations further highlight the important relationship between intracellular calcium phosphate in osteoblasts and their role in mineralizing the ECM. These observations may have important implications in deciphering both how normal bone forms and in understanding pathological mineralization.
Abstract Bioactive glasses (BG) which contain strontium have the potential to combine the known bone regenerative properties of BG with the anabolic and anti-catabolic effects of strontium cations. ...Here we created a BG series (SiO2 –P2 O5 –Na2 O–CaO) in which 0–100% of the calcium was substituted by strontium and tested their effects on osteoblasts and osteoclasts in vitro . We show that ions released from strontium-substituted BG enhance metabolic activity in osteoblasts. They also inhibit osteoclast activity by both reducing tartrate resistant acid phosphatase activity and inhibiting resorption of calcium phosphate films in a dose-dependent manner. Additionally, osteoblasts cultured in contact with BG show increased proliferation and alkaline phosphatase activity with increasing strontium substitution, while osteoclasts adopt typical resorption morphologies. These results suggest that similarly to the osteoporosis drug strontium ranelate, strontium-substituted BG may promote an anabolic effect on osteoblasts and an anti-catabolic effect on osteoclasts. These effects, when combined with the advantages of BG such as controlled ion release and delivery versatility, may make strontium-substituted BG an effective biomaterial choice for a range of bone regeneration therapies.
As an intermediary between cells and scaffolding biomaterials, the extracellular matrix secreted by the cells offers challenges and opportunities for the design and fabrication of engineered tissues.
Regenerative medicine aims to tackle a panoply of challenges from repairing focal damage to articular cartilage to preventing pathological tissue remodeling after myocardial infarction. Hydrogels are ...water‐swollen networks formed from synthetic or naturally derived polymers and are emerging as important tools to address these challenges. Recent advances in hydrogel chemistries are enabling researchers to create hydrogels that can act as 3D ex vivo tissue models, allowing them to explore fundamental questions in cell biology by replicating tissues' dynamic and nonlinear physical properties. Enabled by cutting edge techniques such as 3D bioprinting, cell‐laden hydrogels are also being developed with highly controlled tissue‐specific architectures, vasculature, and biological functions that together can direct tissue repair. Moreover, advanced in situ forming and acellular hydrogels are increasingly finding use as delivery vehicles for bioactive compounds and in mediating host cell response. Here, advances in the design and fabrication of hydrogels for regenerative medicine are reviewed. It is also addressed how controlled chemistries are allowing for precise engineering of spatial and time‐dependent properties in hydrogels with a look to how these materials will eventually translate to clinical applications.
Hydrogels have proven to be versatile, important tools in regenerative medicine. Recent advances in hydrogel chemistries and 3D bioprinting technologies have enabled researchers to create hydrogels that can act as sophisticated 3D ex vivo tissue models and form materials with highly controlled tissue‐specific architectures, vasculature, and biological functions. Here, these advances and how these materials will eventually translate to clinical applications are discussed.
Modifiable hydrogels have revealed tremendous insight into how physical characteristics of cells' 3D environment drive stem cell lineage specification. However, in native tissues, cells do not ...passively receive signals from their niche. Instead they actively probe and modify their pericellular space to suit their needs, yet the dynamics of cells' reciprocal interactions with their pericellular environment when encapsulated within hydrogels remains relatively unexplored. Here, we show that human bone marrow stromal cells (hMSC) encapsulated within hyaluronic acid-based hydrogels modify their surroundings by synthesizing, secreting and arranging proteins pericellularly or by degrading the hydrogel. hMSC's interactions with this local environment have a role in regulating hMSC fate, with a secreted proteinaceous pericellular matrix associated with adipogenesis, and degradation with osteogenesis. Our observations suggest that hMSC participate in a bi-directional interplay between the properties of their 3D milieu and their own secreted pericellular matrix, and that this combination of interactions drives fate.
The intestine performs functions central to human health by breaking down food and absorbing nutrients while maintaining a selective barrier against the intestinal microbiome. Key to this barrier ...function are the combined efforts of lumen‐lining specialized intestinal epithelial cells, and the supportive underlying immune cell‐rich stromal tissue. The discovery that the intestinal epithelium can be reproduced in vitro as intestinal organoids introduced a new way to understand intestinal development, homeostasis, and disease. However, organoids reflect the intestinal epithelium in isolation whereas the underlying tissue also contains myriad cell types and impressive chemical and structural complexity. This review dissects the cellular and matrix components of the intestine and discusses strategies to replicate them in vitro using principles drawing from bottom‐up biological self‐organization and top‐down bioengineering. It also covers the cellular, biochemical and biophysical features of the intestinal microenvironment and how these can be replicated in vitro by combining strategies from organoid biology with materials science. Particularly accessible chemistries that mimic the native extracellular matrix are discussed, and bioengineering approaches that aim to overcome limitations in modelling the intestine are critically evaluated. Finally, the review considers how further advances may extend the applications of intestinal models and their suitability for clinical therapies.
The intestinal epithelium can be reproduced in vitro as intestinal organoids; however, the native intestine also contains myriad stromal cell types and impressive biochemical and structural complexity. This review dissects the cellular and physical components of the intestine and discusses bioengineering approaches for replicating them in vitro to create advanced models of the intestine for disease modelling and regenerative therapies.
Articular cartilage lesions, which can progress to osteoarthritis, are a particular challenge for regenerative medicine strategies, as cartilage function stems from its complex depth-dependent ...microstructural organization, mechanical properties, and biochemical composition. Fibrous scaffolds offer a template for cartilage extracellular matrix production; however, the success of homogeneous scaffolds is limited by their inability to mimic the cartilage's zone-specific organization and properties. We fabricated trilaminar scaffolds by sequential electrospinning and varying fiber size and orientation in a continuous construct, to create scaffolds that mimicked the structural organization and mechanical properties of cartilage's collagen fibrillar network. Trilaminar composite scaffolds were then compared to homogeneous aligned or randomly oriented fiber scaffolds to assess in vitro cartilage formation. Bovine chondrocytes proliferated and produced a type II collagen and a sulfated glycosaminoglycan-rich extracellular matrix on all scaffolds. Furthermore, all scaffolds promoted significant upregulation of aggrecan and type II collagen gene expression while downregulating that of type I collagen. Compressive testing at physiological strain levels further demonstrated that the mechanical properties of trilaminar composite scaffolds approached those of native cartilage. Our results demonstrate that trilaminar composite scaffolds mimic key organizational characteristics of native cartilage, support in vitro cartilage formation, and have superior mechanical properties to homogenous scaffolds. We propose that these scaffolds offer promise in regenerative medicine strategies to repair articular cartilage lesions.
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