Controlling dynamic mechanical properties and degradation of composites for bone regeneration by means of filler content

Bone tissue is a dynamic composite system that adapts itself, in response to the surrounding daily (cyclic) mechanical stimuli, through an equilibrium between growth and resorption processes. When there is need of synthetic bone grafts, the biggest issue is to support bone regeneration without causing mechanically-induced bone resorption. Apart from biological properties, such degradable materials should initially support and later leave room to bone formation. Further, dynamic mechanical properties comparable to those of bone are required. In this study we prepared composites comprising calcium phosphate and L-lactide/D-lactide copolymer in various content ratios using the extrusion method. We evaluated the effect of the inorganic filler amount on the polymer phase (i.e. on the post-extrusion intrinsic viscosity). We then studied their in vitro degradation and dynamic mechanical properties (in dry and humid conditions). By increasing the filler content, we observed significant decrease of the intrinsic viscosity of the polymer phase during the extrusion process. Composites containing higher amounts of apatite had faster degradation, and were also mechanically stiffer. But, due to the lower intrinsic viscosity of their polymer phase, they had larger damping properties. Besides this, higher amounts of apatite also rendered the composites more hydrophilic letting them absorb more water and causing them the largest decrease in stiffness. These results show the importance of filler content in controlling the properties of such composites. Further, in this study we observed that the viscoelastic properties of the composite containing 50wt% apatite were comparable to those of dry human cortical bone.

Continuous and uninterrupted oxygen tension influences the colony formation and oxidative metabolism of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are an attractive cell source for tissue engineering applications due to their multipotentiality and increased expansion potential compared to mature cells. However, the full potential of MSCs for cellular therapies is not realised, due, in part, to premature proliferative senescence and impaired differentiation capacity following expansion under 20% oxygen. Bone marrow MSCs reside under reduced oxygen levels (4%-7% oxygen), thus this study investigates the effects of uninterrupted physiological oxygen tensions (2%, 5%) on MSC expansion and subsequent differentiation. Expansion potential was evaluated from colony formation efficiency, population-doubling rates, and cellular senescence. Colony formation was significantly reduced under 5% oxygen compared to 2% and 20% oxygen. Population-doubling time was initially shorter with 20% oxygen, but subsequently no significant differences in doubling time were detected between the oxygen conditions. MSCs expanded with 20% oxygen contained a greater proportion of senescent cells than those under physiological oxygen levels, indicated by a three to fourfold increase in β-galactosidase staining. This may be related to the approximately twofold enhanced mitochondrial oxygen consumption under this culture condition. Chondrogenic differentiation was achieved following expansion at each oxygen condition. However, osteogenesis was only achieved for cells expanded and differentiated at 20% oxygen, indicated by alkaline phosphatase activity and alizarin red staining. These studies demonstrate that uninterrupted hypoxia may enhance long-term MSC expansion, but results in a population with impaired osteogenic differentiation potential. Thus, novel differentiation conditions are required to enable differentiation to nonchondrogenic lineages using hypoxia-cultured MSCs.

In vivo performance of microstructured calcium phosphate formulated in novel water-free carriers

Osteoinductive calcium phosphate (CaP) ceramics can be combined with polymeric carriers to make shapeable bone substitutes as an alternative to autologous bone; however, carriers containing water may degrade the ceramic surface microstructure, which is crucial to bone formation. In this study five novel tricalcium phosphate (TCP) formulations were designed from water-free polymeric binders and osteoinductive TCP granules of different particle sizes (500-1000 μm for moldable putty forms, and 150-500 μm for flowable paste forms). The performance of these novel TCP formulations was studied and compared with control TCP granules alone (both 150-500 and 500-1000 μm). In vitro the five TCP formulations were characterized by their carrier dissolution times and TCP mineralization kinetic profiles in simulated body fluid. In vivo the formulations were implanted in the dorsal muscle and a unicortical femoral defect (Ø=5 mm) of dogs for 12 weeks. The TCP formulation based on a xanthan gum-glycerol carrier exhibited fast carrier dissolution (1 h) and TCP mineralization (7 days) in vitro, but induced inflammation and showed little ectopic bone formation. This carrier chemistry was thus found to disrupt the early cellular response related to osteoinduction by microstructured TCP. TCP formulations based on carboxymethyl cellulose-glycerol and Polyoxyl 15-hydroxystearate-Pluronic(®) F127 allowed the in vitro surface mineralization of TCP by day 7 and produced the highest level of orthotopic bone bridging and ectopic bone formation, which was equivalent to the control. These results demonstrate that water-free carriers can preserve the chemistry, microstructure, and performance of osteoinductive CaP ceramics.

Patterns of amino acid metabolism by proliferating human mesenchymal stem cells

The nutritional requirements of stem cells have not been determined; in particular, the amino acid metabolism of stem cells is largely unknown. In this study, we investigated the amino acid metabolism of human mesenchymal stem cells (hMSCs), with focus on two questions: Which amino acids are consumed and/or secreted by hMSCs and at what rates? To answer these questions, hMSCs were cultured on tissue culture plastic and in a bioreactor, and their amino acid profile was analyzed. The results showed that the kinetics of hMSCs growth and amino acid metabolism were significantly higher for hMSCs in tissue culture plastic than in the bioreactor. Despite differences in culture conditions, 8 essential and 6 nonessential amino acids were consumed by hMSCs in both tissue culture plastic and bioreactor cultures. Glutamine was the most consumed amino acid with significantly higher rates than for any other amino acid. The metabolism of nonessential amino acids by hMSCs deviated significantly from that of other cell lines. The secretion of alanine, glycine, glutamate, and ornithine by hMSCs showed that there is a strong overflow metabolism that can be due to the high concentrations of amino acids provided in the medium. In addition, the data showed that there is a metabolic pattern for proliferating hMSCs, which can contribute to the design of medium without animal serum for stem cells. Further, this study shows how to implement amino acid rates and metabolic principles in three-dimensional stem cell biology.

The metabolism of human mesenchymal stem cells during proliferation and differentiation

Human mesenchymal stem cells (MSCs) reside under hypoxic conditions in vivo, between 4% and 7% oxygen. Differentiation of MSCs under hypoxic conditions results in inhibited osteogenesis, while chondrogenesis is unaffected. The reasons for these results may be associated with the inherent metabolism of the cells. The present investigation measured the oxygen consumption, glucose consumption and lactate production of MSCs during proliferation and subsequent differentiation towards the osteogenic and chondrogenic lineages. MSCs expanded under normoxia had an oxygen consumption rate of ∼98 fmol/cell/h, 75% of which was azide-sensitive, suggesting that these cells derive a significant proportion of ATP from oxidative phosphorylation in addition to glycolysis. By contrast, MSCs differentiated towards the chondrogenic lineage using pellet culture had significantly reduced oxygen consumption after 24 h in culture, falling to ∼12 fmol/cell/h after 21 days, indicating a shift towards a predominantly glycolytic metabolism. By comparison, MSCs retained an oxygen consumption rate of ∼98 fmol/cell/h over 21 days of osteogenic culture conditions, indicating that these cells had a more oxidative energy metabolism than the chondrogenic cultures. In conclusion, osteogenic and chondrogenic MSC cultures appear to adopt the balance of oxidative phosphorylation and glycolysis reported for the respective mature cell phenotypes. The addition of TGF-β to chondrogenic pellet cultures significantly enhanced glycosaminoglycan accumulation, but caused no significant effect on cellular oxygen consumption. Thus, the differences between the energy metabolism of chondrogenic and osteogenic cultures may be associated with the culture conditions and not necessarily their respective differentiation.

Influence of different polymeric gels on the ectopic bone forming ability of an osteoinductive biphasic calcium phosphate ceramic

To evaluate moldable osteoinductive putties for bone repair we combined microstructured biphasic calcium phosphate (BCP) particles with five different polymeric gels, carboxymethyl cellulose (CMC), Pluronic(®) F-127 (PLU), polyvinyl alcohol (PVA), chitosan (CHI) and alginate (ALG). In vitro gel dissolution showed that CMC, PLU and ALG gels dissolved rapidly (within hours), while the CHI gel took several days and the PVA gel did not dissolve within 2 weeks. Implanting the putty formulations into sheep muscle for 12 weeks demonstrated ectopic bone formation in the control BCP group as well as the putties prepared with dissolving gels (CMC, PLU, ALG and CHI). Bone was not seen in the putty comprising PVA. Quantitative data showed that the CMC and PLU gels did not significantly affect the osteoinductivity of BCP granules, while the ALG and CHI gels showed a significant decrease in bone formation. These results suggest that the dissolvability and chemistry of the gels may be factors affecting the osteoinduction of the putties.

Ultrastructure of the Mineralized Tissue/Calcium Phosphate Interface in Vitro

An in vitro rat bone marrow cell (RBMC) system was used to examine the structure of the interface established between calcium phosphates (Ca-P) and mineralized tissue. The Ca-P used, varied either in chemical structure or crystallinity. Therefore, not only the influence of chemical composition, but also the effect of degradation of Ca-P ceramics could be studied. The interfaces were examined with scanning and transmission electron microscopy (SEM and TEM).

SEM showed that deposition of mineralized extracellular matrix on the different materials examined varied both in time and morphology. Mineralization started with the formation of afibrillar globules with which collagen fibres became integrated. With TEM, three distinctly different interfacial structural arrangements were observed which were dependent on the presence or absence of an electron dense layer and/or an amorphous zone. The former was considered to be at least partially caused by protein adsorption, which would precede biological mineralization events, whereas the latter was considered to represent partial degradation of the ceramic surfaces.

The results of this study showed that interfacial reactions were not only influenced by the chemical structure, but also by the crystallinity of Ca-P ceramics. Thus, characterisation of Ca-P implant materials is of critical importance in achieving a better understanding of the phenomena that occur at the bone-biomaterial interface.

Goat bone tissue engineering: comparing an intramuscular with a posterolateral lumbar spine location

The aim of this study was to investigate the effect of implant location on bone formation in goats using autologous bone marrow-derived stromal cells in porous calcium phosphate scaffolds. Intramuscular locations were compared to posterolateral spine fusion locations in eight goats. As scaffolds, we used biphasic calcium phosphate porous blocks of 5 x 5 x 5 mm. Cell-seeded implants were compared to empty controls. Bone marrow-derived stromal cells were seeded at 8 million cells per cm(3) scaffold and cultured for 1 week. The follow-up time was 12 weeks. Fluorochromes were administered intravenously at 4, 6, and 8 weeks. Ectopic implants showed 21 +/- 3.6% bone formation for the cell seeded and 2.0 +/- 3.0% for the controls (p < 0.001). Paraspinal implants, however, showed 0.10 +/- 0.13% in the cell seeded compared to 0.023 +/- 0.027% in the control group (p = 0.09). A benefit of the cells was only found in the area closest to the paraspinal muscles (p < 0.01). Bone formation in the control samples was of later onset compared to the cell-seeded implants. In conclusion, cell-based bone tissue engineering in an ectopic environment was clearly effective. Similar constructs implanted in a posterolateral spine fusion location hardly showed any effect.

Growth, metabolism, and growth inhibitors of mesenchymal stem cells

Most therapeutic applications of bone marrow stromal cells (MSCs), or mesenchymal stem cells, require expansion of these cells. This study aimed to obtain more information about human MSCs regarding their expansion characteristics: growth, metabolism, and growth inhibitors. In addition, the same expansion factors were examined for (model species) goat and rat MSCs to evaluate differences between MSCs of mammalian species. MSC proliferation, nutrient consumption, and metabolite production were determined for five donors per species. In addition, the growth inhibitory concentrations of lactate and ammonia (NH3) were established. Results showed that goat MSCs grew significantly faster than human and rat MSCs and that goat cells metabolized glucose more efficiently into energy (Ylac/glc=0.8) than human (Ylac/glc=2.0) and rat MSCs (Ylac/glc=1.9). In addition, human (qGlc= -9.2pmol cell(-1) day(-1) and rat MSCs (qGlc= -5.9pmol cell(-1) day(-1)) consumed more glucose than goat MSCs (qGlc= -2.6pmol cell(-1) day(-1)). Glutamine was shown not to be important as energy source for human, goat, and rat MSCs. Regarding growth inhibition by metabolites, rat MSCs were more sensitive to lactate and NH3 (growth inhibiting at 16mM lactate and at 1.9mM NH3) than goat (lactate: 28.4mM, NH3: 2.9mM) and human MSCs (lactate: 35.4mM, NH3: 2.4mM). Human MSCs did not lose their differentiation potential when their growth was inhibited by lactate or NH3.

Biocompatibility and gelation of chitosan-glycerol phosphate hydrogels

The aim of this study was to evaluate the cytotoxicity and gelation of thermosensitive chitosan-beta-glycerol phosphate (GP) solutions, which undergo sol-gel transition around body temperature. Chitosan 0.5-2% (w/v) mixed with GP 5-20% (w/v) solutions all gel at 37 degrees C and possess pH around the physiological range. High GP and chitosan concentrations result in faster gelation time. Extracts of all chitosan concentrations mixed with or without 5% (w/v) GP and 2% (w/v) chitosan combined with 10% (w/v) GP demonstrated up to 34% increase in proliferation rate of goat bone marrow derived mesenchymal stem cells when compared with control medium. Extracts from all other chitosan-GP combinations resulted in reduced cell proliferation relative to control medium. Increasing GP content in the gel resulted in a linear increase in the osmolality of the extracts in contact with the gels. The results of this study indicate that chitosan-GP is a biocompatible hydrogel, extracts of which can stimulate mesenchymal stem cell proliferation at certain concentrations. This material is therefore a promising vehicle for cell encapsulation and injectable tissue-engineering applications.

A rapid and efficient method for expansion of human mesenchymal stem cells

During the past decade, there has been much interest in the use of human mesenchymal stem cells (hMSCs) in bone tissue engineering. HMSCs can be obtained relatively easily and expanded rapidly in culture, but for clinical purposes large numbers are often needed and the cost should be kept to a minimum. A rapid and efficient culturing protocol would therefore be beneficial. In this study, we examined the effect of different medium compositions on the expansion and osteogenic differentiation of bone marrow-derived hMSCs from 19 donors. We also investigated the effect of low seeding density and dexamethasone on both hMSCs expansion and their in vitro and in vivo osteogenic differentiation capacity. HMSCs seeded at a density of 100 cells/cm2 had a significantly higher growth rate than at 5000 cell/cm2, which was further improved by the addition of dexamethasone. Expanded hMSCs were characterized in vitro on the basis of positive staining for CD29, CD44, CD105, and CD166. The in vitro osteogenic potential of expanded hMSCs was assessed by flow cytometric staining for alkaline phosphatase. In vivo bone-forming potential of the hMSCs was assessed by seeding the cells in ceramic scaffolds, followed by subcutaneous implantation in nude mice and histopathologic assessment of de novo bone formation after 6-week implantation. Expanded hMSCs from all donors displayed similar osteogenic potential independent of the culture conditions. On the basis of these results we have developed an efficient method to culture hMSCs by seeding the cells at 100 cells/cm2 in an alpha-minimal essential medium-based medium containing dexamethasone.

Ectopic bone formation in cell-seeded poly(ethylene oxide)/poly(butylene terephthalate) copolymer scaffolds of varying porosity

Scaffolds from poly(ethylene oxide) and poly(butylene terephthalate), PEOT/PBT, with a PEO molecular weight of 1,000 and a PEOT content of 70 weight% (1000PEOT70PBT30) were prepared by leaching salt particles (425-500 microm). Scaffolds of 73.5, 80.6 and 85.0% porosity were treated with a CO(2) gas plasma and seeded with rat bone marrow stromal cells (BMSCs). After in vitro culture for 7 days (d) in an osteogenic medium the scaffolds were subcutaneously implanted for 4 weeks in nude mice. Poly(D, L-lactide) (PDLLA) and biphasic calcium phosphate (BCP) scaffolds were included as references. After 4 weeks (wks) all scaffolds showed ectopic formation of bone and bone marrow. For the scaffolds of different porosities, no significant differences were observed in the relative amounts of bone (7-9%) and bone marrow (6-11%) formed, even though micro computed tomography (mu-CT) data showed considerable differences in accessible pore volume and surface area. 1000PEOT70PBT30 scaffolds with a porosity of 85% could not maintain their original shape in vivo. Surprisingly, 1000PEOT70PBT30 scaffolds with a porosity of 73.5% showed cartilage formation. This cartilage formation is most likely due to poorly accessible pores in the scaffolds, as was observed in histological sections. mu-CT data showed a considerably smaller accessible pore volume (as a fraction of the total volume) than in 1000PEOT70PBT30 scaffolds of 80.6 and 85.0% porosity. BMSC seeded PDLLA (83.5% porosity) and BCP scaffolds (29% porosity) always showed considerably more bone and bone marrow formation (bone marrow formation is approximately 40%) and less fibrous tissue ingrowth than the 1000PEOT70PBT30 scaffolds. The scaffold material itself can be of great influence. In more hydrophobic and rigid scaffolds like the PDLLA or BCP scaffolds, the accessibility of the pore structure is more likely to be preserved under the prevailing physiological conditions than in the case of hydrophilic 1000PEOT70PBT30 scaffolds. Scaffolds prepared from other PEOT/PBT polymer compositions, might prove to be more suited.

Analysis of ectopic and orthotopic bone formation in cell-based tissue-engineered constructs in goats

Despite decades of extensive research, the application of cell-based bone tissue engineering in clinically relevant models remains challenging. To improve effectiveness, a better understanding of how the technique should work is crucial. In the current study, we investigated the onset time, rate, location and direction of bone formation in ectopically and orthotopically implanted clinically sized tissue-engineered constructs to gain insight the mechanism behind it. Bone marrow stromal cells (BMSCs) were obtained from 10 goats, culture expanded and cryopreserved. Porous biphasic calcium phosphate (BCP) disks of 17mmx6mm were per-operatively seeded with BMSCs or left empty. Both conditions were implanted intramuscularly and in bilateral critical-sized iliac wing defects. Fluorochromes were administered at 3, 5 and 7 weeks and samples were retrieved after 9 weeks. Histology showed abundant and homogeneous bone formation throughout the intramuscular BMSC samples and little bone in the controls. Histomorphometry and measurements of the fluorochrome labels of the ectopical BMSC samples indicated that osteogenesis started at the periphery and subsequent osteoconduction filled the whole scaffold within 7 weeks. In the orthotopically implanted disks, there was good integration with the surrounding bone, but minimal bone in the center of the implants, in both conditions. Bone was only derived from the interface with the surrounding bone, there was no early bone at the surfaces in contact to soft tissue as was seen in the ectopical samples. Apparently cell survival was minimal and insufficient for relevant additional bone formation. However, the speed of integration with surrounding bone and subsequent bone apposition on the BMSC-seeded orthotopic scaffolds were found to be significantly enhanced, which may be relevant especially in challenging environments.

Cell-based bone tissue engineering

The authors review the available data on bone tissue engineering and discuss possible new research areas that could help to make bone tissue engineering a clinical success.

The effect of cell-based bone tissue engineering in a goat transverse process model

A disadvantage of traditional posterolateral spinal fusion models is that they are highly inefficient for screening multiple conditions. We developed a multiple-condition model that concentrates on the initial process of bone formation from the transverse process and not on a functional fusion. The effect of bone marrow stromal cells (BMSCs) in four different porous ceramic scaffolds was investigated in this setting. Polyacetal cassettes were designed to fit on the goat transverse process and house four different ceramic blocks, i.e: hydroxyapatite (HA) sintered at 1,150 degrees and 1,250 degrees; biphasic calcium phosphate (BCP) and tricalcium phosphate (TCP). Goat BMSCs (n=10) were cultured and per-operatively seeded autologeously on one of two cassettes implanted per animal. The cassettes were bilaterally mounted on the dorsum of decorticated L2-processes for 9 weeks. To asses the dynamics of bone formation, fluorochrome labels were administered and histomorphometry focused on the distribution of bone in the scaffolds. A clear difference in the extent of bone ingrowth was determined for the different scaffold types. An obvious effect of BMSC seeding was observed in three of four scaffold types, especially in scaffold regions adjacent to the overlying muscle. Generally, the BCP and TCP scaffolds showed better osteoconduction and an increased response to BMSCs administration. In conclusion the model provides a reliable and highly efficient method to study bone formation in cell-based tissue engineering. An effect of cell administration was obvious in three of the four scaffold materials.

Cross-species comparison of ectopic bone formation in biphasic calcium phosphate (BCP) and hydroxyapatite (HA) scaffolds

Material-induced bone formation reported in canine, bovid, suid, and primate species does not often occur in lagomorph or rodent models. In this study, we test biphasic calcium phosphate and hydroxyapatite- induced bone formation in subcutaneous pockets of mice and intramuscular pockets in rats, rabbits, and dogs. All scaffolds are of similar size, and all animals were sacrificed at 90 days post-implantation. In dogs (N = 8), all implants showed bone formation with significantly more bone formed in biphasic calcium phosphates (30 +/- 6%, N = 8) as compared to hydroxyapatite (14 +/- 5%, N = 8) (p = 0.003). Hydroxyapatite implants did not induce bone formation in mice, rats, or rabbits. Biphasic calcium phosphate induced bone in 6 of 8 scaffolds implanted in 4 rabbits and 3 of 16 scaffolds implanted in 16 mice, whereas it did not induce bone formation in any of the 8 rats. The results presented herein suggest that the incidence of material-induced bone formation varies with animal species and is related to the implant material used.

Towards Injectable Cell-based Tissue-Engineered Bone: The Effect of Different Calcium Phosphate Microparticles and Pre-Culturing

Bone tissue engineering by combining bone marrow stromal cells (BMSCs) with a porous scaffold is a promising technology. Current major challenges are to upscale the technique for clinical application and to improve the handling characteristics. With respect to minimal invasive surgery, moldable and/or injectable formulations are highly preferable. Ceramic microparticles of different HA/TCP formulations (100/0, 70/30, 60/40, 40/60, and 0/100) with varying surface roughness were sieved to select 200 microg aliquots of the 212-300 microm fraction. Goat BMSCs were seeded on different aliquots one week prior to in vivo implantation. These constructs and remaining cells were cultured for one week. By then, the remaining cells were harvested and resuspended in a specific binder: hyaluronic acid, alginate, or blood plasma, combined with aliquots of 60/40 microparticles peroperatively. All constructs were implanted in nude rats (n = 10) and analyzed for their bone yield histomorphometrically after 6 weeks. All precultured constructs showed consistent bone formation of comparable quantity. No significant differences were observed between the different material compositions. Peroperatively prepared constructs hardly showed any bone formation. The present study demonstrated the osteogenic potential of a tissue- engineered bone substitute made of microparticles of various HA/TCP compositions. There was an obvious advantage when the constructs were pre-cultured.

A new in vivo screening model for posterior spinal bone formation: Comparison of ten calcium phosphate ceramic material treatments

This study presents a new screening model for evaluating the influence of multiple conditions on the initial process of bone formation in the posterior lumbar spine of a large animal. This model uses cages designed for placement on the decorticated transverse process of the goat lumbar spine. Five conduction channels per cage, each be defined by a different material treatment, are open to both the underlying bone and overlying soft tissue. The model was validated in ten adult Dutch milk goats, with each animal implanted with two cages containing a total of ten calcium phosphate material treatments according to a randomized complete block design. The ten calcium phosphate ceramic materials were created through a combination of material chemistry (BCP, TCP, HA), sintering temperature (low, medium, high), calcination and surface roughness treatments. To monitor the bone formation over time, fluorochrome markers were administered at 3, 5 and 7 weeks and the animals were sacrificed at 9 weeks after implantation. Bone formation in the conduction channels was investigated by histology and histomorphometry of non-decalcified sections using traditional light and epifluorescent microscopy. According to both observed and measured bone formation parameters, materials were ranked in order of increasing magnitude as follows: low sintering temperature BCP (rough and smooth) approximately medium sintering temperature BCP approximately = TCP > calcined low sintering temperature HA > non-calcined low sintering temperature HA > high sintering temperature BCP (rough and smooth) > high sintering temperature HA (calcined and non-calcined). These results agree closely with those obtained in previous studies of osteoconduction and bioactivity of ceramics thereby validating the screening model presented in this study.

Bone tissue engineering on amorphous carbonated apatite and crystalline octacalcium phosphate-coated titanium discs

Poor fixation of bone replacement implants, e.g. the artificial hip, in implantation sites with inferior bone quality and quantity may be overcome by the use of implants coated with a cultured living bone equivalent. In this study, we tested, respectively, amorphous carbonated apatite (CA)- and crystalline octacalcium phosphate (OCP)-coated discs for their use in bone tissue engineering. Subcultured rat bone marrow cells were seeded on the substrates and after 7 days of culture, the implants were subcutaneously implanted in nude mice for 4 weeks. After 7 days of culture, the cells had formed a continuous multi-layer that covered the entire surface of the substrates. The amount of cells was visually higher on the crystalline OCP-coated discs compared to the amorphous CA-coated discs. Furthermore, the amorphous CA-coated discs exhibited a visually higher amount of mineralized extracellular matrix compared to the crystalline OCP-coated discs. After 4 weeks of implantation, clear de novo bone formation was observed on all discs with cultured cells. The newly formed bone on the crystalline OCP-coated discs was more organized and revealed a significantly higher volume compared to the amorphous CA-coated discs. The percentage of bone contact with the discs was also significantly higher on the OCP-coated discs. Overall, the results suggest that a crystalline OCP coating is more suitable for bone tissue engineering than an amorphous CA coating.