The response of primary articular chondrocytes to micrometric surface topography and sulphated hyaluronic acid-based matrices

Micro- and nanotechnology in biomedical engineering for cartilage tissue regeneration in osteoarthritis

Osteoarthritis, which typically arises from aging, traumatic injury, or obesity, is the most common form of arthritis, which usually leads to malfunction of the joints and requires medical interventions due to the poor self-healing capacity of articular cartilage. However, currently used medical treatment modalities have reported, at least in part, disappointing and frustrating results for patients with osteoarthritis. Recent progress in the design and fabrication of tissue-engineered microscale/nanoscale platforms, which arises from the convergence of stem cell research and nanotechnology methods, has shown promising results in the administration of new and efficient options for treating osteochondral lesions. This paper presents an overview of the recent advances in osteochondral tissue engineering resulting from the application of micro- and nanotechnology approaches in the structure of biomaterials, including biological and microscale/nanoscale topographical cues, microspheres, nanoparticles, nanofibers, and nanotubes.

Extracellular Nucleotides Selectively Induce Migration of Chondrocytes and Expression of Type II Collagen

The migration of chondrocytes from healthy to injured tissues is one of the most important challenges during cartilage repair. Additionally, maintenance of the chondrogenic phenotype remains another limitation, especially during monolayer culture in vitro. Using both the differentiated and undifferentiated chondrogenic ATDC5 cell line, we showed that extracellular nucleotides are able to increase the migration rate of chondrocytes without affecting their chondrogenic phenotype. We checked the potency of natural nucleotides (ATP, ADP, UTP, and UDP) as well as their stable phosphorothioate analogs, containing a sulfur atom in the place of one nonbridging oxygen atom in a phosphate group. We also detected P2y1, P2y2, P2y4, P2y6, P2y12, P2y13, and P2y14 mRNA transcripts for nucleotide receptors, demonstrating that P2y1 and P2y13 are highly upregulated in differentiated ATDC5 cells. We showed that ADPβS, UDPβS, and ADP are the best stimulators of migration of differentiated chondrocytes. Additionally, ADP and ADPβS positively affected the expression of type II collagen, a structural component of the cartilage matrix.

Chondrocyte colonisation of a tissue-engineered cartilage substitute under a mechanical stimulus

Cell-free collagen scaffolds as cartilage substitute for small focal defects show promising results in first clinical studies. However, chondrocyte migration between scaffolds and the colonisation process of a cell-free implant is yet to be fully understood. We here focus on mechanobiological interdependencies between cell migration and mechanical stimulus in a 3D environment. We develop an in vitro model composed of a human chondrocyte-seeded collagen base and adjacent cell-free collagen type I scaffolds of varying collagen concentrations. Constructs are either cultured statically or dynamically under the influence of a physiological compression (0.5Hz, 0.5% initial strain). After 20 days we identify vital chondrocytes inside all collagen implants, proving that chondrocytes migrated from the underlying scaffold into the implants. Chondrocytes have not colonised the entire sample and are predominantly found in the bottom of the implant. In static culture conditions, a nearly equal cell number is found inside of all collagen scaffolds. In dynamic culture, the total amount of cells is increased by 30% to 320%, with the highest population in a commercial implant. Differences in cell population between the materials in dynamic culturing can be referred to differences in mechanical properties of the scaffolds, such as strain-rate insensitivity fostering the colonisation process.

Surface chemistry, substrate, and topography guide the behavior of human articular chondrocytes cultured in vitro

Understanding the behavior of chondrocytes in contact with artificial culture surfaces is becoming increasingly important in attaining appropriate ex vivo culture conditions of chondrocytes in cartilage regeneration. Chondrocyte transplantation-based cartilage repair requires efficiently expanded chondrocytes, and the culture surface plays an important role in guiding the behavior of the cell. Micro- and nano-engineered surfaces make it possible to modulate cell behavior. We hypothesized that the combined influence of topography, substrate, and surface chemistry may affect the chondrocyte culturing in terms of proliferation and phenotypic means. Human chondrocytes were cultured on polystyrene fabricated microstructures, flat polydimethylsiloxane (PDMS), or polystyrene treated with fibronectin or oxygen plasma and cultured for 1, 4, 7, and 10 days. The behavior of chondrocytes was evaluated by proliferation, viability, chondrogenic gene expression, and cell morphology. Contrary to our hypothesis, microstructures in polystyrene did not significantly influence the behavior of chondrocytes neither under normoxic- nor hypoxic conditions. However, changes in the substrate stiffness and surface chemistry were found to influence cell viability, gene expression, and morphology of human chondrocytes. Oxygen plasma treatment was the most important parameter followed by the softer substrate type PDMS. The findings indicate the culture of human chondrocytes on softer substratum and surface activation by oxygen plasma may prevent dedifferentiation and may improve chondrocyte transplantation-based cartilage repair. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2805-2816, 2018.

The morphology and functions of articular chondrocytes on a honeycomb-patterned surface

The present study investigated the potential of a novel micropatterned substrate for neocartilage formation. Articular chondrocytes were cultured on poly(ɛ-caprolactone) materials whose surfaces were either flat or honeycomb-patterned. The latter was prepared using a novel self-organization technique, while the former, was prepared by spin-coating. The chondrocytes attached and proliferated on both surfaces. On the honeycomb films, chondrocytes were found at the top surface and encased within the 10 μm pores. Meanwhile, chondrocytes on the spin-coated surface flattened out. Accumulation of DNA and keratin sulphate was comparatively higher on the honeycomb films within the first 7 days. At their respective peaks, DNA concentration increased on the honeycomb and flat surfaces by approximately 210% and 400% of their day 1 values, respectively. However, cultures on the flat surface took longer to peak. Extracellular Matrix (ECM) concentrations peaked at 900% and 320% increases for the honeycomb and flat cultures. Type II collagen was upregulated on the honeycomb and flat surfaces by as much as 28% and 25% of their day 1 values, while aggrecan was downregulated with time, by 3.4% and 7.4%. These initial results demonstrate the potential usefulness of honeycomb-based scaffolds during early cultures neocartilage and soft tissue engineering.

Consequences of Neutralization on the Proliferation and Cytoskeletal Organization of Chondrocytes on Chitosan-Based Matrices

In tissue engineering strategies that seek to repair or regenerate native tissues, adhesion of cells to scaffolds or matrices is essential and has the potential to influence subsequent cellular events. Our focus in this paper is to better understand the impact of cellular seeding and adhesion in the context of cartilage tissue engineering. When scaffolds or surfaces are constructed from chitosan, the scaffolds must be first neutralized with sodium hydroxide and then washed copiously to render the surface, cell compatible. We seek to better understand the effect of surface pretreatment regimen on the cellular response to chitosan-based surfaces. In the present paper, sodium hydroxide concentration was varied between 0.1 M and 0.5 M and two different contacting times were studied: 10 minutes and 30 minutes. The different pretreatment conditions were noted to affect cell proliferation, morphology, and cytoskeletal distribution. An optimal set of experimental parameters were noted for improving cell growth on scaffolds.

Chondrogenic potential of electrospun nanofibres for cartilage tissue engineering

Articular cartilage has a heterogeneous structure, comprising elongated cells at the articulating surface and rounded cells elsewhere. This feature poses a complex challenge when fabricating 3D tissue engineering scaffolds able to mimic the native extracellular matrix (ECM) of cartilage for tissue repair and regeneration. Nanofibre scaffolds can provide an ECM-like structure, but are mechanically weak and typically have subcellular pore geometries. In this study, the use of poly(L,D-lactide) (PLDLA) nanofibre coatings on PLDLA microfibres or films (nanofibre composites) to influence bovine chondrocyte behaviour was investigated. It was demonstrated that electrospun nanofibres facilitated the adhesion of chondrocytes and helped to maintain smaller projected cell areas and a rounded cell phenotype, when compared to PLDLA films or microfibres. Random nanofibre composites were associated with the smallest and most rounded cells and aligned nanofibre composites also demonstrated a similar tendency. Quantitative PCR revealed that nanofibres promoted the expression of chondrogenic markers, such as collagen type IIaI and aggrecan, while maintaining low levels of collagen IaI. It was also found, by water contact angle measurement, that nanofibres were significantly more hydrophobic than cast films. The lower wettability of polymeric nanofibres favoured the maintenance of rounded chondrocyte morphology. To our knowledge this is the first study to confirm the positive influence on preserving chondrogenic phenotype and gene expression at the interface of true nano-microfibrous composites by using individual microfibres coated with aligned nanofibres. Such composites can potentially be fabricated into mechanically durable 3D scaffolds with better cell infiltration throughout the scaffolds.

Microtopographical regulation of adult bone marrow progenitor cells chondrogenic and osteogenic gene and protein expressions

Microtopographic features affect diverse cell behaviors. Adult bone marrow progenitor cells (AMPCs) constitute a multipotent heterogeneous population. We hypothesized that microtopographies could direct AMPCs lineage-specific differentiation. AMPCs isolated from Sprague-Dawley rats were CD45 depleted, expanded, and plated at 10(5) cells/cm2 on epoxy-microfabricated: (1) 60-microm-deep grooves with 95-microm pitch (D60P95), (2) 55-microm-wide and 10-microm-deep squares (W55D10), (3) 30-microm-deep grooves with 45-microm pitch (D30P45), (4) 17-microm-wide and 10-microm-deep pillars (W17D10), and (5) smooth control. AMPCs were cultured using expansion, chondrogenesis, or osteogenesis supporting media. Cell cultures were examined by scanning electron microscopy, qRT-PCR, and immunostaining at 2, 9, 16, and 23 days after plating. Expressions of osteogenesis-related genes, such as Runx-2, alkaline phosphatase, osteopontin, osteocalcin, and parathyroid hormone-related protein receptor (PTHr), and chondrogenesis-associated genes, such as Sox-9, type II collagen, and aggrecan, were determined. In expansion medium, W55D10 induced a transient increase of Sox9 expression. Compared with smooth surfaces, type II collagen mRNA and protein expressions in chondrogenic medium were significantly upregulated on W55D10 by day 23. In contrast, osteocalcin and PTHr expressions were significantly increased on D30P45 in osteogenic medium. We have demonstrated that W55D10 and D30P45 enhanced AMPCs chondrogenic and osteogenic terminal differentiation with appropriate culture conditions.

Migration of periodontal ligament fibroblasts on nanometric topographical patterns: influence of filopodia and focal adhesions on contact guidance

Considered to be the "holy grail" of dentistry, regeneration of the periodontal ligament in humans remains a major clinical problem. Removal of bacterial biofilms is commonly achieved using EDTA gels or lasers. One side effect of these treatment regimens is the etching of nanotopographies on the surface of the tooth. However, the response of periodontal ligament fibroblasts to such features has received very little attention. Using laser interference lithography, we fabricated precisely defined topographies with continuous or discontinuous nanogrooves to assess the adhesion, spreading and migration of PDL fibroblasts. PDL fibroblasts adhered to and spread on all tested surfaces, with initial spreading and focal adhesion formation slower on discontinuous nanogrooves. Cells had a significantly smaller planar area on both continuous and discontinuous nanogrooves in comparison with cells on non-patterned controls. At 24 h post seeding, cells on both types of nanogrooves were highly elongated parallel to the groove long axis. Time-lapse video microscopy revealed that PDL fibroblast movement was guided on both types of grooves, but migration velocity was not significantly different from cells cultured on non-patterned controls. Analysis of filopodia formation using time-lapse video microscopy and labeling of vinculin and F-actin revealed that on nanogrooves, filopodia were highly aligned at both ends of the cell, but with increasing time filopodia and membrane protrusions developed at the side of the cell perpendicular to the cell long axis. We conclude that periodontal ligament fibroblasts are sensitive to nanotopographical depths of 85-100 µm, which could be utilized in regeneration of the periodontal ligament.

Spiral and square microstructured surfaces: the effect of the decreasing size of photo-immobilized hyaluronan domains on cell growth

Spiral and squared micropatterned surfaces of decreasing dimensions were realized by photo-immobilizing a photoreactive hyaluronan (Hyal) derivative on silanized glass substrates. The microstructured surfaces were observed by atomic force microscope and scanning electron microscope. Scanning electron microscope analysis revealed the presence of a spiral (ranging from 100 microm down to 1 microm in the central part) and a square pattern consisting of a central square of 100 microm x 100 microm and squares of different dimensions decreasing from the centre to the edges of the micropatterned area (2 microm x 1 microm). Three cell types were tested on all the microstructured surfaces: human coronary artery endothelial cells (HCAEC), human dermal fibroblasts (C54), and NIH 3T3 fibroblasts. Cell adhesion analysis demonstrated that HCAEC and C54 did not adhere to the immobilized Hyal on silanized glass but adapted their shape to the different sizes of the square and spiral patterns. Also, in both geometric patterns, the reduction of the adhesive glass width induced C54 to create bonds amongst themselves. NIH 3T3 cells adhered inside the squares and the spiral but reducing the adhesive glass width induced NIH 3T3 to adhere to immobilized Hyal. This fact is explained by the interactions between the cells and the immobilized Hyal as a consequence of the CD44/Hyal binding.

A 3D hybrid model for tissue growth: the interplay between cell population and mass transport dynamics

To provide theoretical guidance for the design and in vitro cultivation of bioartificial tissues, we have developed a multiscale computational model that can describe the complex interplay between cell population and mass transport dynamics that governs the growth of tissues in three-dimensional scaffolds. The model has three components: a transient partial differential equation for the simultaneous diffusion and consumption of a limiting nutrient; a cellular automaton describing cell migration, proliferation, and collision; and equations that quantify how the varying nutrient concentration modulates cell division and migration. The hybrid discrete-continuous model was parallelized and solved on a distributed-memory multicomputer to study how transport limitations affect tissue regeneration rates under conditions encountered in typical bioreactors. Simulation results show that the severity of transport limitations can be estimated by the magnitude of two dimensionless groups: the Thiele modulus and the Biot number. Key parameters including the initial seeding mode, cell migration speed, and the hydrodynamic conditions in the bioreactor are shown to affect not only the overall rate, but also the pattern of tissue growth. This study lays the groundwork for more comprehensive models that can handle mixed cell cultures, multiple nutrients and growth factors, and other cellular processes, such as cell death.

Directional change produced by perpendicularly-oriented microgrooves is microtubule-dependent for fibroblasts and epithelium

Anisotropic substrata such as micromachined grooves can control cell shape, orientation, and the direction of cell movement, a phenomena termed topographic guidance. Although many types of cells exhibit topographic guidance, little is known regarding cell responses to conflicting topographic cues. We employed a substratum with intersecting grooves in order to present fibroblasts and epithelial cells with conflicting topographic cues. Using time-lapse and confocal microscopy, we examined cell behavior at groove intersections. Migrating fibroblasts and epithelial cells typically extended a cell process into the intersection ahead of the cell body. After travelling along the "X" groove to enter the intersection, the leading lamellipodia of the cell body encountered the perpendicular "Y" groove, and spread latterly along the "Y" groove. The formation of lateral lamellipodia resulted in cells forming "T" or "L" morphologies, which were characterized by the formation of phosphotyrosine-rich focal adhesions at the leading edges. The "Y" groove did not prove an absolute barrier to cell migration, particularly for epithelial cells. Analysis of cytoskeletal distribution revealed that F-actin bundles did not adapt closely to the groove patterns, but typically did align to either the "X" or "Y" grooves. In contrast microtubules (MT) adapted closely to the walls. Inhibition of microtubule nucleation attenuated fibroblast and epithelial cell orientation within the intersection of the perpendicular grooves. We conclude that MT may be the prime determinant of fibroblast and epithelial cell conformation to conflicting topographies.

In vitro generation of cartilage-carrier-constructs on hydroxylapatite ceramics with different surface structures

Tissue engineering approaches for healing cartilage defects are partly limited by the inability to fix cartilage to bone during implantation. To overcome this problem, cartilage can be - already in vitro - generated on a ceramic carrier which serves as bone substitute. In this study, the influence of a hydroxylapatite carrier and its surface structure on the quality of tissue engineered cartilage was investigated. Application of the carrier reduced significantly biomechanical and biochemical properties of the generated tissue. In addition, slight changes in the quality of the formed matrix, in the adhesive strength between cartilage and biomaterial and in attachment and proliferation of a chondrocyte monolayer could be observed for commercial grade carriers, with respect to modified topographies obtained by smooth grinding/polishing. These first results demonstrated an influence of the carrier and its surface structure, but further research is needed for explaining the described effects and for optimization of cartilage-carrier-constructs.

Imaging of the cell surface interface using objective coupled widefield surface plasmon microscopy

We report on the development and on the first use of the widefield surface plasmon (WSPR) microscope in the examination of the cell surface interface at submicron lateral resolutions. The microscope is Kohler illuminated and uses either a 1.45 numerical aperture (NA) oil immersion lens, or a 1.65 NA oil immersion lens to excite surface plasmons at the interface between a thin gold layer and a glass or sapphire cover slip. Like all surface plasmon microscope systems the WSPR has been proven in previous studies to also be capable of nanometric z-scale resolutions. In this study we used the system to image the interface between HaCaT cells and the gold layer. Imaging was performed in air using fixed samples and the 1.45 NA objective based system and also using live cells in culture media using the 1.65 NA based system. Imaging in air enabled the visualisation of high resolution and high-contrast submicron features identified by vinculin immunostaining as component of focal contacts and focal adhesions. In comparison, imaging in fluid enabled cell surface interfacial interactions to be tracked by time-lapse video WSPR microscopy. Our results indicate that the cell surface interface and thus cell signalling mechanisms may be readily interrogated in live cells without the use of labelling techniques.

The effect of chondrocyte cryopreservation on cartilage engineering

Chondrocytes were collected from the stifle joints of four pigs to study the effect of cryopreservation on the chondrogenic potential of chondrocytes. Half of the cells were cryopreserved for 3months. Polyglycolic acid scaffolds were cultured with fresh or cryopreserved chondrocytes for 4weeks. Cell morphology and the quality of engineered tissue were evaluated by scanning electron microscopy, histopathology and biochemical methods. More cells attached to scaffolds at 48h when fresh chondrocytes were seeded. At 4weeks, the numbers of cells, DNA and collagen II were greater in constructs engineered by fresh cells. However, the collagen II/DNA ratio did not differ between the two groups. More matrix was identified on a scanning electron microscope and by histopathology in the fresh group. Cartilage engineered with cryopreserved chondrocytes may contain less matrix and fewer cells. These findings most likely resulted from a lack of cell attachment on the matrix secondary to cryopreservation. Future studies are needed to further evaluate the mechanism by which cryopreservation may affect chondrocyte attachment.

Chondrocyte moves: clever strategies?

GOALS

To review the literature on chondrocyte movements and to develop plausible hypothesis for further work.

DESIGN

Chondrocyte movements are herein defined as translocations of the cell body. A brief overview of cell migration in other cell types is presented to set the stage for a discussion of chondrocyte moves; this includes a discussion of the challenges that cells find when moving within tissues. Reports of isolated chondrocyte migration in vitro (isolated cell systems) and ex vivo (cartilage organ cultures) are then summarized, followed by a discussion of recent studies that infer chondrocyte movements in vivo.

RESULTS

Investigators from different laboratories have observed chondrocyte motility in vitro. I became interested in the question of whether articular chondrocytes retained their phenotype during their migratory excursions. We devised a simple method to separate migratory and stationary chondrocytes and then showed that migratory chondrocytes synthesized collagen II but not I--consistent with a differentiated phenotype. Our time-lapse video microscopy studies showed that the cells displayed appropriate movement kinetics, albeit with low speed and directionality. Similarly, others have presented data consistent with slow movement of chondrocytes out of cartilage explants. It is important to decipher whether these in vitro movements reflect physiological states and if so, which events are simulated. Examples of in vivo studies that have inferred chondrocyte movements include those describing rotational or gliding movements of chondrocytes in the proliferative zone of the growth plate and its importance in the growth process; and the notion that chondrocytes move from the cartilage endplates to the nucleus pulposus (NP) in the spine of rabbits and rats during development. Such studies are consistent with the hypothesis that chondrocytes exhibit highly controlled and specialized movements during tissue growth and remodeling in vivo. On the other hand, the cartilage explant studies elicit interest in the possibility that matrix injuries resulting in disruption of the collagen network of adult cartilages provide a permissive environment for chondrocyte motility.

CONCLUSIONS

The case for in vivo chondrocyte motility remains to be proven. However, the in vitro and in vivo data on chondrocyte movements present an argument for further thought and studies in this area.

A new experimental system for the extended application of cyclic hydrostatic pressure to cell culture

Mechanical forces have been shown to be important stimuli for the determination and maintenance of cellular phenotype and function. Many cells are constantly exposed in vivo to cyclic pressure, shear stress, and/or strain. Therefore, the ability to study the effects of these stimuli in vitro is important for understanding how they contribute to both normal and pathologic states. While there exist commercial as well as custom-built devices for the extended application of cyclic strain and shear stress, very few cyclic pressure systems have been reported to apply stimulation longer than 48 h. However, pertinent responses of cells to mechanical stimulation may occur later than this. To address this limitation, we have designed a new cyclic hydrostatic pressure system based upon the following design variables: minimal size, stability of pressure and humidity, maximal accessibility, and versatility. Computational fluid dynamics (CFD) was utilized to predict the pressure and potential shear stress within the chamber during the first half of a 1.0 Hz duty cycle. To biologically validate our system, we tested the response of bone marrow progenitor cells (BMPCs) from Sprague Dawley rats to a cyclic pressure stimulation of 120/80 mm Hg, 1.0 Hz for 7 days. Cellular morphology was measured using Scion Image, and cellular proliferation was measured by counting nuclei in ten fields of view. CFD results showed a constant pressure across the length of the chamber and no shear stress developed at the base of the chamber where the cells are cultured. BMPCs from Sprague Dawley rats demonstrated a significant change in morphology versus controls by reducing their size and adopting a more rounded morphology. Furthermore, these cells increased their proliferation under cyclic hydrostatic pressure. We have demonstrated that our system imparts a single mechanical stimulus of cyclic hydrostatic pressure and is capable of at least 7 days of continuous operation without affecting cellular viability. Furthermore, we have shown for the first time that BMPCs respond to cyclic hydrostatic pressure by alterations in morphology and increased proliferation.

Cell motility on micropatterned treadmills and tracks

Surfaces micropatterned with disjointed cell adhesive/non-adhesive regions allow for precise control of cell shape, internal organization and function. In particular, substrates prepared by the reaction-diffusion ASoMic (nisotropic lid roetching) method localize cells onto transparent micro-islands or tracks surrounded by an opaque, adhesion-resistant background. ASoMic is compatible with several important imaging modalities ( wide-field, fluorescent, TIRF and confocal microscopies), and can be used to study and quantify various intracellular and cellular processes related to cell motility. For cells constrained on the islands, the imposed geometry controls spatial organization of the cytoskeleton, while the transparency of the islands allows for real-time analysis of cytoskeletal dynamics. For cells on transparent, linear tracks, the high optical contrast between these adhesive regions and the surrounding non-adhesive background allows for straightforward quantification of the key parameters describing cell motility. Both types of systems provide analytical-quality data that can assist fundamental studies of cell locomotion and can provide a technological basis for cell motility microassays.

Chondrocyte aggregation on micrometric surface topography: a time-lapse study

Cellular aggregation or mesenchymal "condensation" is a prerequisite in the process of chondrogenesis. It has been observed that during in vitro engineering of cartilage, chondrocytes form aggregates in the initial cell seeding process of polymer scaffolds. However, the exact mechanism behind this aggregation has yet to be elucidated, although cell collision has been implicated. As all polymers have a surface topography, we hypothesized that topography may play a role in chondrocyte aggregation. 1(st) and 2(nd) passage chondrocytes were seeded on micrometric topography ranging from 0.75 to 8 microm in depth and 5 to 12.5 microm in width. Both 1(st) and 2(nd) passage cells formed aggregates as cells collided, and larger aggregates formed as aggregates collided with each other on the grooves. Furthermore, aggregates migrated parallel to the direction of the groove long axis. F-actin organization was altered only in the cellular layer in direct contact with the surface; stress fibers oriented in the direction of the groove long axis. Chondrocytes maintained type II collagen expression on all groove depths. This study shows that micrometric grooves could be an effective means for studying chondrocyte aggregation, and could additionally be utilized in the regeneration of cartilage.