The effect of ascorbic acid on the growth of chick bone rudiments in chemically defined medium

Harnessing the purinergic receptor pathway to develop functional engineered cartilage constructs

OBJECTIVE

Mechanical stimulation is a widely used method to enhance the formation and properties of tissue-engineered cartilage. While this approach can be highly successful, it may be more efficient and effective to harness the known underlying mechanotransduction pathways responsible. With this aim, the purpose of this study was to assess the effect of directly stimulating the purinergic receptor pathway through exogenous adenosine 5'-triphosphate (ATP) in absence of externally applied forces.

METHODS

Isolated bovine articular chondrocytes were seeded in high density, 3D culture and supplemented with varying doses of ATP for up to 4 weeks. The effects on biosynthesis, extracellular matrix accumulation and mechanical properties were then evaluated. Experiments were also conducted to assess whether exogenous ATP elicited any undesirable effects, such as: inflammatory mediator release, matrix turn-over and mineralization.

RESULTS

Supplementation with ATP had a profound effect on the growth and maturation of the developed tissue. Exogenous ATP (62.5-250 microM) increased biosynthesis by 80-120%, and when stimulated for a period of 4 weeks resulted in increased matrix accumulation (80% increase in collagen and 60% increase in proteoglycans) and improved mechanical properties (6.5-fold increase in indentation modulus). While exogenous ATP did not stimulate the release of inflammatory mediators or induce mineralization, high doses of ATP (250 microM) elicited a 2-fold increase in matrix metalloproteinase-13 expression suggesting the emergence of a catabolic response.

CONCLUSIONS

Harnessing the ATP-purinergic receptor pathway is a highly effective approach to improve tissue formation and impart functional mechanical properties. However, the dose of ATP needs to be controlled as not to elicit a catabolic response.

A novel method for embedding neonatal murine calvaria in methyl methacrylate suitable for visualizing mineralization, cellular and structural detail

The study of undecalcified bone by histological methods is essential in the field of bone research. Culturing skeletal tissues such as neonatal murine calvaria provides a reliable bridge between assessment of bone formation in vitro and anabolic activity in vivo and contains most of the essential elements of bone for studying bone formation. Neonatal calvarial assay, supported by histological methods, is used to study the anabolic effects of a wide variety of factors and compounds on bone tissue. To optimize visualization and histomorphometric measurements using neonatal calvaria, we developed a method that provides high quality tissue sections suitable for routine and histochemical staining. Undecalcified neonatal mouse calvaria were processed and embedded using a low temperature methyl methacrylate procedure. Various staining methods were performed on deplastisized and floated sections to examine mineralization and to identify cells. The Von Kossa stain counterstained with a modified H & E yielded precise images of unmineralized bone including mineralization sites, and distinct osteoblasts and osteoclasts. Toluidine blue, Ladewig's trichrome, tartrate-resistant acid phosphatase, Goldner, H & E and Villanueva stains also were tested on the undecalcified neonatal calvaria sections.

Long-term intermittent shear deformation improves the quality of cartilaginous tissue formed in vitro

The formation of cartilaginous tissue in vitro is a promising alternative to repair damaged articular cartilage. However, recent attempts to tissue-engineer articular cartilage that has similar properties to the native tissue have proven to be difficult. The in vitro-formed cartilaginous tissue typically has a similar proteoglycan content to native cartilage, but has a reduced collagen content and only a fraction of the mechanical properties. In this study, we investigated whether the intermittent application of cyclic shearing forces during tissue formation would improve the tissue quality. Chondrocyte cultures were stimulated at a 2% shear strain amplitude at a frequency of 1 Hz for 400 cycles every 2nd day. At one week, both collagen and proteoglycan synthesis increased (23+/-6% and 20+/-6%, respectively) over the unstimulated, static controls. At four weeks, an increased amount of tissue formed (stimulated: 1.85+/-0.08, unstimulated: 1.58+/-0.07 mg dry wt.). This tissue contained approximately 40% more collagen (stimulated: 511+/-23, unstimulated: 367+/-24 microg/construct) and 35% more proteoglycans (stimulated: 376+/-21, unstimulated: 279+/-26 microg/construct). Tissues that formed in the presence of shearing forces also displayed a 3-fold increase in compressive load-bearing capacity (stimulated: 16+/-5, unstimulated: 5+/-1 kPa max. equilibrium stress) and a 6-fold increase in stiffness (stimulated: 112+/-36, unstimulated: 20+/-6 kPa max. equilibrium modulus) compared to the static controls. These results demonstrate that intermittent application of dynamic shearing forces over a four-week period improves the quality of cartilaginous tissue formed in vitro. Interestingly, low amplitudes of shear stimulation for short periods of time (6 min of stimulation applied every 2nd day) produced these changes.

Characterization of cartilagenous tissue formed on calcium polyphosphate substrates in vitro

Successful joint resurfacing by tissue-engineered cartilage has been limited, in part, by an inability to secure the implant to bone. To overcome this, we have developed the methodology to form a cartilage implant in vitro consisting of a layer of cartilagenous tissue overlying a porous, biodegradable calcium polyphosphate (CPP) substrate. As bone will grow into the CPP after implantation, it will result in anchorage of the cartilage. In this study, the cartilagenous tissue formed in vitro after 8 weeks in culture was characterized and compared to native articular cartilage. Light microscopic examination of histological sections showed that there was a continuous layer of cartilagenous tissue on, and integrated with the subsurface of, the CPP substrate. The in vitro-formed tissue achieved a similar thickness to native articular cartilage (mean +/- SEM: in vitro = 0.94 +/- 0.03 mm; ex vivo = 1.03 +/- 0.01 mm). The cells in the in vitro-formed tissue synthesized large proteoglycans (Kav +/- SEM: in vitro = 0.27 +/- 0.01; ex vivo = 0.27 +/- 0.01) and type II collagen similar to the chondrocytes in the ex-vivo cartilage. The in vitro-formed tissue had a similar amount of proteoglycan (GAG microg/mg dry wt.: in vitro = 198 +/- 10; ex vivo = 201 +/- 13) but less collagen than the native cartilage (hydroxyproline microg/mg dry wt.: in vitro = 21 +/- 1; ex vivo = 70 +/- 8). The in vitro-formed tissue had only about 3% of the load-bearing capacity and stiffness of the native articular cartilage, determined from unconfined mechanical compression testing. Although low, this was within the range of properties reported by others for tissue-engineered cartilage. It is possible that the limited load-bearing capacity is the result of the low collagen content and further studies are required to identify the conditions that will increase collagen synthesis.

Periosteum responds to dynamic fluid pressure by proliferating in vitro

Periosteum provides a source of undifferentiated chondrocyte precursor cells for fracture healing that can also be used for cartilage repair. The quantity of cartilage that can be produced, which is a determining factor in fracture healing and cartilage repair, is related to the number of available stem cells in the cambium layer. Cartilage formation during both of these processes is enhanced by motion of the fracture or joint in which periosteum has been transplanted. The effect of dynamic fluid pressure on cell proliferation in periosteal tissue cultures was determined in 452 explants from 60 immature (2-month-old) New Zealand White rabbits. The explants were cultured in agarose suspension for 1-14 days. One group was subjected to cyclic hydrostatic pressure, which is referred to as dynamic fluid pressure, at 13 kPa and a frequency of 0.3 Hz. Control explants were cultured in similar chambers without application of pressure. DNA synthesis ([3H]thymidine uptake) and total DNA were measured. The temporal pattern and distribution of cell proliferation in periosteum were evaluated with autoradiography and immunostaining with proliferating cell nuclear antigen. Dynamic fluid pressure increased proliferation of periosteal cells significantly, as indicated by a significant increase in [3H]thymidine uptake at all time points and a higher amount of total DNA compared with control values. On day 3, when DNA synthesis reached a peak in periosteal explants, [3H]thymidine uptake was 97,000+/-5,700 dpm/microg DNA in the group exposed to dynamic fluid pressure and 46,000+/-6,000 dpm/microg in the controls (p < 0.001). Aphidicolin, which blocks DNA polymerase alpha, inhibited [3H]thymidine uptake in a dose-dependent manner in the group subjected to dynamic fluid pressure as well as in the positive control (treated with 10 ng/ml of transforming growth factor-beta1) and negative control (no added growth factors) groups, confirming that [3H]thymidine measurements represent proliferation and dynamic fluid pressure stimulates DNA synthesis. Total DNA was also significantly higher in the group exposed to dynamic fluid pressure (5,700+/-720 ng/mg wet weight) than in the controls (3,700+/-630) on day 3 (p < 0.01). Autoradiographs with [3H]thymidine revealed that one or two cell cycles of proliferation took place in the fibrous layer prior to proliferation in the cambium layer (where chondrocyte precursors reside). Proliferating cell nuclear antigen immunophotomicrographs confirmed the increased proliferative activity due to dynamic fluid pressure. These findings suggest either a paracrine signaling mechanism between the cells in these two layers of the periosteum or recruitment/migration of proliferating cells from the fibrous to the cambium layer. On the basis of the data presented in this study, we postulate that cells in the fibrous layer respond initially to mechanical stimulation by releasing growth factors that induce undifferentiated cells in the cambium layer to divide and differentiate into chondrocytes. These data indicate that cell proliferation in the early stages of chondrogenesis is stimulated by mechanical factors. These findings are important because they provide a possible explanation for the increase in cartilage repair tissue seen in joints subjected to continuous passive motion. The model of in vitro periosteal chondrogenesis under dynamic fluid pressure is valuable for studying the mechanisms by which mechanical factors might be involved in the formation of cartilage in the early fracture callus and during cartilage repair.

Role of oxygen tension during cartilage formation by periosteum

Tissue engineering makes regeneration of cartilage possible but requires optimization of culture conditions. The effects of oxygen tension on cartilage metabolism are controversial in the literature, and we could find no information detailing the optimal oxygen concentration for growing new cartilage (neochondrogenesis). Periosteal cells and tissues can be used to grow cartilage in vivo and in vitro. In this study, using a standard periosteal organ culture model, we found that cartilage formation by periosteal explants is affected by the ambient oxygen concentrations. A total of 480 periosteal explants from 30 2-month-old New Zealand White rabbits were cultured in agarose suspension at different oxygen concentrations (1-90%) for 6 weeks. Chondrogenesis, which was analyzed by histomorphometry and quantitative collagen typing, was maximal at 12-15% oxygen. There were no significant differences in chondrogenesis in the range of 12-45%. There was inhibition of cartilage and type-II collagen formation at very high (90%) and very low (1-5%) oxygen concentrations. However, contrary to what some have thought, chondrogenesis is maximal under aerobic conditions. If this is true for systems other than periosteal implants, it would have important implications for growing cartilage in vitro.

Estrogens and antiestrogens stimulate release of bone resorbing activity by cultured human breast cancer cells

Patients with advanced breast cancer may develop acute, severe hypercalcemia when treated with estrogens or antiestrogens. In this study, we examined the effects of estrogens and related compounds on the release of bone resorbing activity by cultured human breast cancer cells in vitro. We found that the estrogen receptor positive breast cancer cell line MCF-7 releases bone resorbing activity in response to low concentrations of 17 beta-estradiol. Bone resorbing activity was also released in response to the antiestrogen nafoxidine. Other steroidal compounds had no effect on the release of bone resorbing activity. Estrogen-stimulated release of bone resorbing activity occurred with live bone cultures, but not with devitalized bones, indicating that the effect was bone cell mediated. The breast cancer cell line MDA-231, which does not have estrogen receptors, did not release bone resorbing activity in response to 17 beta-estradiol or nafoxidine. Release of the bone resorbing activity by MCF-7 cells incubated with 17 beta-estradiol was inhibited by indomethacin (10 microM) and flufenamic acid (50 microM), two structurally unrelated compounds that inhibit prostaglandin synthesis. Concentrations of 17 beta-estradiol and nafoxidine that caused increased release of bone resorbing activity by the breast cancer cells caused a four- to fivefold increase in release of prostaglandins of the E series by MCF-7 cells. These data may explain why some patients with advanced breast cancer develop acute hypercalcemia when treated with estrogens or antiestrogens, and why bone metastases are more common in patients with estrogen receptor positive tumors.

The effects of pulsed magnetic fields on chick embryo cartilaginous skeletal rudiments in vitro

The biological response of cultured 7-day embryonic chick tibiae to small alternating currents induced by pulsed magnetic fields (PMFs) was investigated. It was found that continuous exposure to PMFs over 7 days did not alter the overall DNA content of rudiments nor the incorporation of 3H-thymidine when compared with control tibiae. The overall collagen content of explants was slightly reduced by PMF exposure whilst the incorporation of 3H-proline was significantly suppressed. The synthesis of sulphated glycosaminoglycans was also measured in terms of 35SO4--incorporation, but PMF treatment failed to alter the levels of isotope incorporation. These results suggest that, whereas noncollagenous, and possibly collagenous, protein synthesis is affected by PMF exposure, glycosaminoglycan synthesis is not. Histological and electron microscopical observations substantiated this interpretation and revealed selective inhibition of matrix secretion in the periphery of the proliferative epiphyseal zones in experimental explants. High-power electron microscope examination of these zones showed that PMF-exposed matrix was sparsely invested with fibrous protein while elements of the stellate reticulum had formed large aggregates which were often clumped about the cell membrane. The results are discussed in terms of the possible role of naturally occurring potentials in the development and maintenance of connective tissue systems such as cartilage and bone.

In vitro action of 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol on matrix organization and mineral distribution in rabbit growth plate

Growth plates of 18-day-old rabbits were incubated in a protein-free synthetic medium, either without any additive, with 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] (10(-10) M), with 24,25-dihydroxycholecalciferol [24,25-(OH)2D3] (10(-10) M and 10(-9) M), with both metabolites, or with the ethanol solvent alone. Cartilages, before and after 5 days of incubation, were studied by light and electron microscopy. The intracellular calcium distribution was analyzed by the potassium pyroantimonate method, and the calcium content was verified by x-ray microprobe analysis. When compared to nonincubated samples the cartilages incubated for 5 days without any additive as well as the cartilages incubated with the solvent alone showed excessive hydratation and hypertrophy of the chondrocytes, which had lost their columnar arrangement. The matrix and the cells were devoid of mineral. The ultrastructure of the cells was well preserved. These changes were largely prevented by the presence of both vitamin D3 metabolites. With regard to calcium distribution, 1,25-(OH)2D3 maintained calcium in mitochondria and crystals in matrix vesicles, whereas 24,25-(OH)2D3 only partly maintained mitochondrial mineral. In the chondrocytes incubated with this latter metabolite, small calcium granules were seen in the cytoplasm; most vesicles were devoid of crystals, and amorphous precipitates were seen in the matrix. These data demonstrate the in vitro influence of vitamin D3 metabolites on the organization and mineralization of the cartilage matrix and on the distribution of intracellular calcium in chondrocytes. Furthermore, they support the hypothesis that the in vitro action of 1,25-(OH)2D3 is different from that of 24,25-(OH)2D3 in that 1,25-(OH)2D3 may influence calcium storage in mitochondria and matrix vesicles, whereas 24,25-(OH)2D3 is likely to be involved in calcium transport and release.

Interaction of 24,25-dihydroxyvitamin D3 and parathyroid hormone on bone enzymes in vitro

The in vitro effects of vitamin D3 metabolites, parathyroid extract (PTE), purified parathyroid hormone (bPTH), vitamin A, and heparin on acid and alkaline phosphatases in rat or mouse calvaria in culture were investigated. Results show that: (a) when compared to values found in half calvaria incubated for 24 h in control medium, the bone acid and alkaline phosphatase content is significantly higher in paired halves incubated with PTE (L USP/ml), bPTH (4 x 10(-8)M), heparin (5 USP/ml), vitamin A (23 USP/ml), 25-(OH)D3 (2.5 x 10(-11) to 2.5 x 10(-8)M), 24,25-(OH)2D3, and 1,25-(OH)2D3 (2.5 x 10(-12) to 2.5 x 10(-7M); (b) the presence of 24,25-(OH)2D3 at low concentrations in the incubation medium decreases significantly the PTE, bPTH, vitamin A, or heparin induced stimulation of the phosphatase activities. This interaction is also observed when measuring beta glucuronidase and glucose-6-phosphatase activities and 45Ca release from previously labeled mouse calvaria; (c) a similar activity could not be found with 1,25-(OH)2D3 suggesting that 24,25-(OH)2D3 may have a specific role in bone metabolism.

The effect of uraemic metabolites on parathyroid extract-induced bone resorption in vitro

A study is reported of the effects of both pre- and post-dialysis serum and some of the known uraemic metabolites on parathyroid extract-induced bone resorption using an in vitro organ culture system. Serum from uraemic patients prior to haemodialysis was shown to inhibit the action of parathyroid hormone on bone and this inhibitory effect was not present after dialysis. Some of the uraemic metabolites studied and also phosphate exerted an inhibitory effect but the metabolites caused their inhibition at higher concentrations than those which are known to occur in uraemic patients. It is concluded that if uraemic metabolites play a role in vivo then it is probably due to a cumulative phenomenon.

A histochemical study on the differentiating vertebral column of chick

1. The differentiation of the vertebral elements of the cervical region of the chick has been studied from 4 to 23 day age group counted from the day of the commencement of incubation. 2. From the histochemical tests it appears that chondrogenesis does not take place simultaneously in all the areas of the vertebra. 3. Histochemical changes usually follow histological differentiation. 4. The findings have been discussed in the light of inducing principles by the spinal cord.

The effects of ascorbic acid deficiency on calcium and collagen metabolism in cultured fetal rat bones

The effects of ascorbic acid deficiency on growth and calcification of bone were studied in whole 18-day fetal rat radii and ulnae cultured in a chemically defined medium. Ascorbic acid deficiency decreased the formation of labeled hydroxyporline from labeled proline in both bone shafts and cartilage ends while incorporation of tryptophan was maintained. Dry weights and collagen content of bone and cartilage were decreased, but calcification was not affected. The optimun initial concentration of ascorbic acid for collagen synthesis was 200 mug/ml. The effect of ascorbic acid was not antagonized by glucoascorbic acid or replaced by dithiothreitol. Decreased collagen synthesis in ascorbic acid deficiency could not be ascribed to loss of available peptidyl proline hydorxylase. Formation of underhydroxylated collagen and its release into the medium accounted for much of the decrease in hydroxylated collagen in ascorbic acid deficient bones. Nevertheless, the total newly synthesized collagen, as measured by collagenase digestion, was still decreased. Similar effects were exerted by alpha, alpha'-dipyridyl which also inhibited general protein synthesis. Ascorbic acid did not stimulate proline incorporation into collagen in the presence of alpha, alpha'-dipyridyl.

Phenytoin inhibition of parathyroid hormone induced bone resorption in vitro

1 A study is reported of the effects of phenytoin and phenobarbitone on bone calcium mobilization by parathyroid hormone in vitro.2 In a therapeutic concentration (15 mug/ml), phenytoin significantly inhibited parathyroid hormone-induced calcium release from bone.3 The inhibitory effect of phenytoin on bone calcium mobilization could play a role in the maintenance of hypocalcaemia in epileptic patients on long-term anticonvulsant drug therapy.

The effect of scurvy on glycosaminoglycans of granulation tissue and costal cartilage

1. The effect of ascorbic acid deficiency on glycosaminoglycans of granulation tissue and cartilage of guinea pigs was investigated by determination of the changes in the glucosamine and galactosamine contents 12 days after tendonectomy. 2. In normal granulation tissue, the glucosamine and galactosamine contents rose to a peak at 5 and 10 days respectively, whereas the hydroxyproline and proline contents continued to rise throughout the 20 days after tendonectomy. 3. The galactosamine in scorbutic granulation tissue, but not in that of pair-fed controls, decreased significantly in absolute amount and relatively to glucosamine, which remained practically unchanged; the cartilage galactosamine did not decrease during the 22 days of deficiency owing to the presence of excess of preformed galactosaminoglycans, which masked the small amount of newly formed glycosaminoglycans. 4. The chemical results were confirmed by radioactivity studies in vivo of incorporation of [U-(14)C]glucose into galactosamine and glucosamine of scorbutic granulation tissue and cartilage. The incorporation of (14)C into galactosamine decreased significantly in scurvy in both tissues. 5. The results indicated in both tissues a decreased formation of galactosamine during scurvy, although an increased degradation of polymerized glycosaminoglycans could not be entirely ruled out. It is concluded that, if lack of ascorbic acid causes an impaired galactosamine formation, the most likely position for the block may be in the UDP-N-acetylglucosamine 4-epimerase reaction.