Long-term organ culture of embryonic chick femora: a system for investigating bone and cartilage formation at an intermediate level of organization

Bone Regenerative Effect of Injectable Hypoxia Preconditioned Serum-Fibrin (HPS-F) in an Ex Vivo Bone Defect Model

Biofunctionalized hydrogels are widely used in tissue engineering for bone repair. This study examines the bone regenerative effect of the blood-derived growth factor preparation of Hypoxia Preconditioned Serum (HPS) and its fibrin-hydrogel formulation (HPS-F) on drilled defects in embryonic day 19 chick femurs. Measurements of bone-related growth factors in HPS reveal significant elevations of Osteopontin, Osteoprotegerin, and soluble-RANKL compared with normal serum (NS) but no detection of BMP-2/7 or Osteocalcin. Growth factor releases from HPS-F are measurable for at least 7 days. Culturing drilled femurs organotypically on a liquid/gas interface with HPS media supplementation for 10 days demonstrates a 34.6% increase in bone volume and a 52.02% increase in bone mineral density (BMD) within the defect area, which are significantly higher than NS and a basal-media-control, as determined by microcomputed tomography. HPS-F-injected femur defects implanted on a chorioallantoic membrane (CAM) for 7 days exhibit an increase in bone mass of 123.5% and an increase in BMD of 215.2%, which are significantly higher than normal-serum-fibrin (NS-F) and no treatment. Histology reveals calcification, proteoglycan, and collagen fiber deposition in the defect area of HPS-F-treated femurs. Therefore, HPS-F may offer a promising and accessible therapeutic approach to accelerating bone regeneration by a single injection into the bone defect site.

Ex vivo Bone Models and Their Potential in Preclinical Evaluation

PURPOSE OF REVIEW

Novel therapies for damaged and diseased bone are being developed in a preclinical testing process consisting of in vitro cell experiments followed by in vivo animal studies. The in vitro results are often not representative of the results observed in vivo. This could be caused by the complexity of the natural bone environment that is missing in vitro. Ex vivo bone explant cultures provide a model in which cells are preserved in their native three-dimensional environment. Herein, it is aimed to review the current status of bone explant culture models in relation to their potential in complementing the preclinical evaluation process with specific attention paid to the incorporation of mechanical loading within ex vivo culture systems.

RECENT FINDINGS

Bone explant cultures are often performed with physiologically less relevant bone, immature bone, and explants derived from rodents, which complicates translatability into clinical practice. Mature bone explants encounter difficulties with maintaining viability, especially in static culture. The integration of mechanical stimuli was able to extend the lifespan of explants and to induce new bone formation. Bone explant cultures provide unique platforms for bone research and mechanical loading was demonstrated to be an important component in achieving osteogenesis ex vivo. However, more research is needed to establish a representative, reliable, and reproducible bone explant culture system that includes both components of bone remodeling, i.e., formation and resorption, in order to bridge the gap between in vitro and in vivo research in preclinical testing.

Trauma induced tissue survival in vitro with a muscle-biomaterial based osteogenic organoid system: a proof of concept study

BACKGROUND

The translation from animal research into the clinical environment remains problematic, as animal systems do not adequately replicate the human in vivo environment. Bioreactors have emerged as a good alternative that can reproduce part of the human in vivo processes at an in vitro level. However, in vitro bone formation platforms primarily utilize stem cells only, with tissue based in vitro systems remaining poorly investigated. As such, the present pilot study explored the tissue behavior and cell survival capability within a new in vitro skeletal muscle tissue-based biomaterial organoid bioreactor system to maximize future bone tissue engineering prospects.

RESULTS

Three dimensional printed β-tricalcium phosphate/hydroxyapatite devices were either wrapped in a sheet of rat muscle tissue or first implanted in a heterotopic muscle pouch that was then excised and cultured in vitro for up to 30 days. Devices wrapped in muscle tissue showed cell death by day 15. Contrarily, devices in muscle pouches showed angiogenic and limited osteogenic gene expression tendencies with consistent TGF-ß₁, COL4A1, VEGF-A, RUNX-2, and BMP-2 up-regulation, respectively. Histologically, muscle tissue degradation and fibrin release was seen being absorbed by devices acting possibly as a support for new tissue formation in the bioceramic scaffold that supports progenitor stem cell osteogenic differentiation.

CONCLUSIONS

These results therefore demonstrate that the skeletal muscle pouch-based biomaterial culturing system can support tissue survival over a prolonged culture period and represents a novel organoid tissue model that with further adjustments could generate bone tissue for direct clinical transplantations.

Organotypic Culture Method to Study the Development Of Embryonic Chicken Tissues

The embryonic chicken is commonly used as a reliable model organism for vertebrate development. Its accessibility and short incubation period makes it ideal for experimentation. Currently, the study of these developmental pathways in the chicken embryo is conducted by applying inhibitors and drugs at localized sites and at low concentrations using a variety of methods. In vitro tissue culturing is a technique that enables the study of tissues separated from the host organism, while simultaneously bypassing many of the physical limitations present when working with whole embryos, such as the susceptibility of embryos to high doses of potentially lethal chemicals. Here, we present an organotypic culturing protocol for culturing the embryonic chicken half head in vitro, which presents new opportunities for the examination of developmental processes beyond the currently established methods.

The use of rats and mice as animal models in ex vivo bone growth and development studies

In vivo animal experimentation has been one of the cornerstones of biological and biomedical research, particularly in the field of clinical medicine and pharmaceuticals. The conventional in vivo model system is invariably associated with high production costs and strict ethical considerations. These limitations led to the evolution of an ex vivo model system which partially or completely surmounted some of the constraints faced in an in vivo model system. The ex vivo rodent bone culture system has been used to elucidate the understanding of skeletal physiology and pathophysiology for more than 90 years. This review attempts to provide a brief summary of the historical evolution of the rodent bone culture system with emphasis on the strengths and limitations of the model. It encompasses the frequency of use of rats and mice for ex vivo bone studies, nutritional requirements in ex vivo bone growth and emerging developments and technologies. This compilation of information could assist researchers in the field of regenerative medicine and bone tissue engineering towards a better understanding of skeletal growth and development for application in general clinical medicine.Cite this article: A. A. Abubakar, M. M. Noordin, T. I. Azmi, U. Kaka, M. Y. Loqman. The use of rats and mice as animal models in ex vivo bone growth and development studies. Bone Joint Res 2016;5:610-618. DOI: 10.1302/2046-3758.512.BJR-2016-0102.R2.

Evaluation of skeletal tissue repair, part 1: assessment of novel growth-factor-releasing hydrogels in an ex vivo chick femur defect model

Current clinical treatments for skeletal conditions resulting in large-scale bone loss include autograft or allograft, both of which have limited effectiveness. In seeking to address bone regeneration, several tissue engineering strategies have come to the fore, including the development of growth factor releasing technologies and appropriate animal models to evaluate repair. Ex vivo models represent a promising alternative to simple in vitro systems or complex, ethically challenging in vivo models. We have developed an ex vivo culture system of whole embryonic chick femora, adapted in this study as a critical size defect model to investigate the effects of novel bone extracellular matrix (bECM) hydrogel scaffolds containing spatio-temporal growth factor-releasing microparticles and skeletal stem cells on bone regeneration, to develop a viable alternative treatment for skeletal degeneration. Alginate/bECM hydrogels combined with poly (d,l-lactic-co-glycolic acid) (PDLLGA)/triblock copolymer (10-30% PDLLGA-PEG-PDLLGA) microparticles releasing VEGF, TGF-β3 or BMP-2 were placed, with human adult Stro-1+ bone marrow stromal cells, into 2mm central segmental defects in embryonic chick femurs. Alginate/bECM hydrogels loaded with HSA/VEGF or HSA/TGF-β3 demonstrated a cartilage-like phenotype, with minimal collagen I deposition, comparable to HSA-only control hydrogels. The addition of BMP-2 releasing microparticles resulted in enhanced structured bone matrix formation, evidenced by increased Sirius red-stained matrix and collagen expression within hydrogels. This study demonstrates delivery of bioactive growth factors from a novel alginate/bECM hydrogel to augment skeletal tissue formation and the use of an organotypic chick femur defect culture system as a high-throughput test model for scaffold/cell/growth factor therapies for regenerative medicine.

Developmental cues for bone formation from parathyroid hormone and parathyroid hormone-related protein in an ex vivo organotypic culture system of embryonic chick femora

Enhancement and application of our understanding of skeletal developmental biology is critical to developing tissue engineering approaches to bone repair. We propose that use of the developing embryonic femur as a model to further understand skeletogenesis, and the effects of key differentiation agents, will aid our understanding of the developing bone niche and inform bone reparation. We have used a three-dimensional organotypic culture system of embryonic chick femora to investigate the effects of two key skeletal differentiation agents, parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), on bone and cartilage development, using a combination of microcomputed tomography and histological analysis to assess tissue formation and structure, and cellular behavior. Stimulation of embryonic day 11 (E11) organotypic femur cultures with PTH and PTHrP initiated osteogenesis. Bone formation was enhanced, with increased collagen I and STRO-1 expression, and cartilage was reduced, with decreased chondrocyte proliferation, collagen II expression, and glycosaminoglycan levels. This study demonstrates the successful use of organotypic chick femur cultures as a model for bone development, evidenced by the ability of exogenous bioactive molecules to differentially modulate bone and cartilage formation. The organotypic model outlined provides a tool for analyzing key temporal stages of bone and cartilage development, providing a paradigm for translation of bone development to improve scaffolds and skeletal stem cell treatments for skeletal regenerative medicine.

A novel approach for studying the temporal modulation of embryonic skeletal development using organotypic bone cultures and microcomputed tomography

Understanding the structural development of embryonic bone in a three dimensional framework is fundamental to developing new strategies for the recapitulation of bone tissue in latter life. We present an innovative combined approach of an organotypic embryonic femur culture model, microcomputed tomography (μCT) and immunohistochemistry to examine the development and modulation of the three dimensional structures of the developing embryonic femur. Isolated embryonic chick femurs were organotypic (air/liquid interface) cultured for 10 days in either basal, chondrogenic, or osteogenic supplemented culture conditions. The growth development and modulating effects of basal, chondrogenic, or osteogenic culture media of the embryonic chick femurs was investigated using μCT, immunohistochemistry, and histology. The growth and development of noncultured embryonic chick femur stages E10, E11, E12, E13, E15, and E17 were very closely correlated with increased morphometric indices of bone formation as determined by μCT. After 10 days in the organotpyic culture set up, the early aged femurs (E10 and E11) demonstrated a dramatic response to the chondrogenic or osteogenic culture conditions compared to the basal cultured femurs as determined by a change in μCT morphometric indices and modified expression of chondrogenic and osteogenic markers. Although the later aged femurs (E12 and E13) increased in size and structure after 10 days organotpypic culture, the effects of the osteogenic and chondrogenic organotypic cultures on these femurs were not significantly altered compared to basal conditions. We have demonstrated that the embryonic chick femur organotpyic culture model combined with the μCT and immunohistochemical analysis can provide an integral methodology for investigating the modulation of bone development in an ex vivo culture setting. Hence, these interdisciplinary techniques of μCT and whole organ bone cultures will enable us to delineate some of the temporal, structural developmental paradigms and modulation of bone tissue formation to underpin innovative skeletal regenerative technology for clinical therapeutic strategies in musculoskeletal trauma and diseases.

Buried alive: How osteoblasts become osteocytes

During osteogenesis, osteoblasts lay down osteoid and transform into osteocytes embedded in mineralized bone matrix. Despite the fact that osteocytes are the most abundant cellular component of bone, little is known about the process of osteoblast-to-osteocyte transformation. What is known is that osteoblasts undergo a number of changes during this transformation, yet retain their connections to preosteoblasts and osteocytes. This review explores the osteoblast-to-osteocyte transformation during intramembranous ossification from both morphological and molecular perspectives. We investigate how these data support five schemes that describe how an osteoblast could become entrapped in the bone matrix (in mammals) and suggest one of the five scenarios that best fits as a model. Those osteoblasts on the bone surface that are destined for burial and destined to become osteocytes slow down matrix production compared to neighbouring osteoblasts, which continue to produce bone matrix. That is, cells that continue to produce matrix actively bury cells producing less or no new bone matrix (passive burial). We summarize which morphological and molecular changes could be used as characters (or markers) to follow the transformation process.

Pleiotrophin/Osteoblast-stimulating factor 1: dissecting its diverse functions in bone formation

OSF-1, more commonly known as pleiotrophin (PTN) or heparin-binding growth-associated molecule (HB-GAM), belongs to a new family of secreted HB proteins, which are structurally unrelated to any other growth factor family. The aims of this study were to dissect the diverse functions of PTN in bone formation. The study showed that PTN was synthesized by osteoblasts at an early stage of osteogenic differentiation and was present at sites of new bone formation, where PTN was stored in the new bone matrix. Low concentrations (10 pg/ml) of PTN stimulated osteogenic differentiation of mouse bone marrow cells and had a modest effect on their proliferation, whereas higher concentrations (ng/ml) had no effect. However, PTN did not have the osteoinductive potential of bone morphogenetic proteins (BMPs) because it failed to convert C2C12 cells, a premyoblastic cell line, to the osteogenic phenotype, whereas recombinant human BMP-2 (rhBMP-2) was able to do so. When PTN was present together with rhBMP-2 during the osteoinductive phase, PTN inhibited the BMP-mediated osteoinduction in C2C12 cells at concentrations between 0.05 pg/ml and 100 ng/ml. However, when added after osteoinduction had been achieved, PTN enhanced further osteogenic differentiation. An unusual effect of PTN (50 ng/ml) was the induction of type I collagen synthesis by chondrocytes in organ cultures of chick nasal cartilage and rat growth plates. Thus, PTN had multiple effects on bone formation and the effects were dependent on the concentration of PTN and the timing of its presence. To explain these multiple effects, we propose that PTN is an accessory signaling molecule, which is involved in a variety of processes in bone formation. PTN enhances or inhibits primary responses depending on the prevailing concentrations, the primary stimulus, and the availability of appropriate receptors.

Glucosamine HCl reduces equine articular cartilage degradation in explant culture

Objective To determine whether glucosamine inhibits experimentally induced degradation of equine articular cartilage explants. Methods Articular cartilage was obtained from the antebrachio-carpal and middle joints of horses (2-8 years old) killed for reasons unrelated to lameness. Cartilage discs were harvested from the weight-bearing region of the articular surface and cultured. Media were exchanged daily and the recovered media stored at 4 degrees C. Explants were maintained in basal media 2 days prior to the start of four treatment days. On days 1-4 lipopolysaccharide (LPS, 10 microg/ml) or recombinant human interleukin-1 (rhIL-1, 50 ng/ml) were added to induce cartilage degradation. To test the potential protective effects of glucosamine, the compound was added in three concentrations (0.25, 2.5, or 25 mg/ml) and treatments were performed in triplicate. Controls included wells without LPS, rhIL-1beta, or glucosamine. Nitric oxide, proteoglycan and matrix metalloproteinases (MMP) released into conditioned media and tissue proteoglycan synthesis were measured as indicators of cartilage metabolism. Results Maximal nitric oxide production, proteoglycan release, and MMP activity were detected 1 day after the addition of LPS or rhIL-1beta to the media. The addition of 25 mg/ml of glucosamine prevented the increase in nitric oxide production, proteoglycan release and MMP activity induced by LPS or rhIL-1. Conclusions These data indicate that glucosamine can prevent experimentally induced cartilage degradation in vitro.

Biochemical characterization of cartilage degradation in embryonic chick tibial explant cultures

The growth plates of birds reared for meat production are susceptible to diseases such as tibial dyschondroplasia (TD). We have modified a tibial explant culture system to study the regulation of growth plate cartilage turnover. The purpose of these experiments was to characterize some of the biochemical changes that occur in cultured tibiae as the cartilage is degraded. Tibiae were dissected from 12-d-old embryos and cultured in medium formulated for chondrocytes. Proteoglycan and nitric oxide concentrations as well as metalloproteinase and lactate dehydrogenase activities were measured in recovered media. Metalloproteinase activity was also measured in cartilage extracts from tibiae collected every 2 d during the culture period. Proteoglycan and nitric oxide concentrations in recovered media increased after 8 d in culture and peaked on Day 14. Lactate dehydrogenase (LDH), an indicator of cell death, increased in media after 10 d in culture. Metalloproteinase activity in the cartilage increased after 6 d, whereas activity in recovered media did not increase until after Day 10. These results suggest that chondrocytes in the tibiae undergo hypertrophy, degrade the extracellular matrix, and die. Further experiments demonstrated that pyrrolidine dithiocarbamate (PDTC), which is from a family of molecules that induce TD, inhibited both nitric oxide production and proteoglycan degradation. Thus, we think our tibial explant culture system can be useful in elucidating molecules that regulate growth plate cartilage turnover as well as predicting what conditions or molecules might lead to bone growth problems in birds.

"Cell paralysis" as an intermediate stage in the programmed cell death of epiphyseal chondrocytes during development

The efficient elimination of apoptotic cells depends on heterophagocytosis by other cells, which is difficult or impossible when the dying cells are embedded in an extracellular matrix. This situation is exemplified by the epiphyseal chondrocytes during the development of the chondroepiphyses of long bones. A detailed ultrastructural study identified an unusual type of epiphyseal chondrocyte which contained a very dark nucleus with irregular patches of condensed chromatin and a crenated nuclear membrane. The cytosol consisted of excessively expanded endoplasmic reticulum lumen, containing "islands" of cytoplasm and organelles. Since these cells appeared to be "in limbo," neither viable nor dead, they are referred to as "paralyzed" cells. By studying cells of intermediate morphologies, we were able to demonstrate the sequence of events leading to cell paralysis. It is proposed that the paralysis represents an intermediate state in the physiological cell death of epiphyseal chondrocytes in which destruction is orderly and avoids a inflammatory, potentially locally destructive, reaction. The cell is rendered paralyzed in terms of function but impotent in respect of damaging consequences. Paralysis is compared and contrasted with apoptosis, autophagocytosis, and necrosis and may represent another mode of programmed cell death in situations where cells are immature and/or where phagocytosis by neighboring cells is difficult.

Individual and combined effects of calciotropic hormones and growth factors on mineral metabolism in embryonic chick tibiae

We have investigated single and combined effects of calciotropic hormones and growth factors on the regulation of alkaline phosphatase (ALP) activity and calcium metabolism in an optimized serum-free bone organ culture system of embryonic chick tibiae. Parathyroid hormone PTH(1-34) alone mobilized calcium from bone tissue time- and dose-dependently and inhibited ALP activity. Both the bisphosphonate (BM 21.0955) and to a lesser extent salmon calcitonin alone slightly increased calcium uptake and inhibited the stimulation of bone resorption by PTH(1-34). 1,25(OH)2D3 mobilized calcium and inhibited ALP activity in contrast to 24,25(OH)2D3 which inhibited ALP activity but had no significant effect on calcium metabolism. Interestingly the combination of PTH(1-34) with 1,25(OH)2D3 but not 24,25(OH)2D3 reduced calcium mobilization. The combination of the midregional fragment PTH(28-48), which by itself has no effect on calcium metabolism, with 1,25(OH)2D3 reduced calcium mobilization more efficiently. Several PTH-regulated mediators have been assayed in this system. Of the tested growth factors, IGF-I at high concentrations caused bone resorption with no effect on ALP activity. TGF-beta 1 (transforming growth factor beta) and BMP-2 had no significant effect on calcium metabolism; however, ALP activity was inhibited by TGF-beta 1 and induced dose dependently by BMP-2. Of the other factors known to be present in bone, platelet-derived growth factor (PDGFA/B) and epidermal growth factor (EGF) had a small effect on calcium mobilization but had no effect on ALP activity. bFGF reduced ALP activity slightly without an effect on calcium metabolism. Our results show that this in vitro system can mimic some interactions of calciotropic hormones in vivo and allows the assaying of mediators in terms of regulation of ALP activity and of calcium metabolism.

New aspects of endochondral ossification in the chick: chondrocyte apoptosis, bone formation by former chondrocytes, and acid phosphatase activity in the endochondral bone matrix

A detailed histological study of the growth plates from 9- to 20-day-old embryonic chick long bones was carried out with the aim of clarifying the long-debated question of the fate of the hypertrophic chondrocytes. Since resorption in chick bones does not occur synchronously across the plate as it does in mammals, specialized regions develop and the fate of the chondrocyte depends on its location within the growth plate. Where resorption took place, as at the sites of primary vascular invasion or at the main cartilage/marrow interface, chondrocytes underwent apoptosis before the lacunae were opened. In addition, spontaneous apoptosis of chondrocytes occurred at apparently random sites throughout all stages of chondrocyte differentiation. In older chick bones, a thick layer of endochondral bone matrix covered the cartilage edge. This consisted of type I collagen and the typical noncollagenous bone proteins but, in addition, contained tartrate-resistant acid phosphatase in the mineralized matrix. Where such matrix temporarily protected the subjacent cartilage from resorption, chondrocytes differentiated to bone-forming cells and deposited bone matrix inside their lacunae. At sites of first endochondral bone formation, some chondrocytes underwent an asymmetric cell division resulting in one daughter cell which underwent apoptosis, while the other cell remained viable and re-entered the cell cycle. This provided further support for the notion that chondrocytes as well as marrow stromal cells give rise to endochondral osteoblasts.

Epigenetic selection as a possible component of transdifferentiation. Further study of the commitment of hypertrophic chondrocytes to become osteocytes

Transdifferentiation of hypertrophic chondrocytes into osteogenic cells was induced in 14 day chick embryo femurs by cutting through the region of hypertrophic cartilage. The process was studied in organ culture, using electron microscopy, staining for alkaline phosphatase, immunocytochemistry of collagen type I and proliferative cell nuclear antigen, and in situ localization of DNA strand-breaks. In addition, DNA and RNA synthesis were studied by 3[H]-T and 3[H]-U radioautography. Loss of ECM components from the cut edge occurred in culture. During the 12 day period necessary for transdifferentiation we observed phenotypic instability and bi-potentiality, the death of some cells and the gradual promotion of the osteoblastic phenotype in the survivors. Transition from chondrocytic to osteoblastic phenotype progressed stepwise, through variable mosaic intermediates, and involved a few cell cycles including asymmetric (differential) divisions. Proliferating and apoptotic cells were found in close proximity. As judged by the relative proportion of apoptotic cells and composition of the surrounding intralacunar matrix, negative selection of intermediate cell types displaying chondrocytic and altered mosaic phenotypes occurred. When the osteoblastic lineage was finally established, apoptotic cells were no longer present. Our hypothesis is that after disruption of cell-cell or cell-matrix interactions and lack of growth factors certain cells are selected and channelled through proliferation into the new stable phenotype. This process is targeted by the environment through a set of pre-determined steps.

The systemic effect of intraarticular administration of corticosteroid on markers of bone formation and bone resorption in patients with rheumatoid arthritis

OBJECTIVE

To assess the effects of intraarticular (IA) corticosteroid use on bone metabolism in patients with rheumatoid arthritis (RA).

METHODS

Levels of the bone turnover markers, serum osteocalcin (BGP) and urinary pyridinoline (PYD), were monitored in RA patients for 4 weeks following a single IA administration of xylocaine alone or in combination with triamcinolone acetonide.

RESULTS

Levels of the bone resorption marker, PYD, did not show any significant change, whereas BGP levels were drastically decreased 1 day after IA administration of corticosteroid, and then returned to pretreatment levels by day 14. The efficacy of IA corticosteroid treatment lasted for 4 weeks.

CONCLUSION

Our results suggest that IA administration of corticosteroid has no net effects on bone resorption and only a transient systemic effect on bone formation. IA corticosteroid administration may be better for bone metabolism than continuous use of orally administered corticosteroid.

The phenotypic switch from chondrocytes to bone-forming cells involves asymmetric cell division and apoptosis

We have investigated the early cellular events that take place during the phenotypic switch from hypertrophic chondrocytes to bone-forming cells in a) chondrocytes located inside intact lacunae after embryonic chick femurs had been cut through the hypertrophic cartilage and cultured for 1-15 days; and b) at the cartilage/marrow interface of femurs after short-term culture. Ultrastructural studies were combined with in situ methods localizing proliferating and apoptotic cells, and 3D-reconstructions of confocal images of the cartilage/marrow edge. The crucial event in the phenotypic switch was an asymmetric cell division which resulted in one daughter cell which underwent apoptosis and another viable daughter cell which subsequently differentiated to an osteogenic cell, i.e to a smaller basophilic cell that was positive for alkaline phosphatase, type I collagen, osteonectin, osteopontin, bone sialoprotein and osteocalcin and that, after 12-15 days in culture, could synthesize a mineralized bone matrix within intact lacunae. The present results suggest a mechanism whereby differentiated cells can change their phenotype. At least one mitotic division seems to be required to fix the commitment to the new phenotype.

Osteogenic differentiation of hypertrophic chondrocytes involves asymmetric cell divisions and apoptosis

We have investigated the early cellular events that take place during the change in lineage commitment from hypertrophic chondrocytes to osteoblast-like cells. We have induced this osteogenic differentiation by cutting through the hypertrophic cartilage of embryonic chick femurs and culturing the explants. Immunocytochemical characterization, [3H]thymidine pulse-chase labeling, in situ nick translation or end labeling of DNA breaks were combined with ultrastructural studies to characterize the changing pattern of differentiation. The first responses to the cutting, seen after 2 d, were upregulation of alkaline phosphatase activity, synthesis of type I collagen and single-stranded DNA breaks, probably indicating a metastable state. Associated with the change from chondrogenic to osteogenic commitment was an asymmetric cell division with diverging fates of the two daughter cells, where one daughter cell remained viable and the other one died. The available evidence suggests that the viable daughter cell then divided and generated osteogenic cells, while the other daughter cell died by apoptosis. The results suggest a new concept of how changes in lineage commitment of differentiated cells may occur. The concepts also reconcile previously opposing views of the fate of the hypertrophic chondrocyte.

Doxycycline disrupts chondrocyte differentiation and inhibits cartilage matrix degradation

OBJECTIVE

The effects of doxycycline were tested in an in vitro system in which the cartilages of embryonic avian tibias are completely degraded.

METHODS

Tibias were cultured with 5, 20, or 40 microgram/ml doxycycline. Control tibias were cultured without doxycycline. Conditioned media and tissue sections were examined for enzyme activity and matrix loss.

RESULTS

Cartilages were not resorbed in the presence of doxycycline, whereas control cartilages were completely degraded. Collagen degradation was reduced in association with treatment with doxycycline at all doses studied. Higher concentrations of doxycycline reduced collagenase and gelatinase activity and prevented proteoglycan loss, cell death, and deposition of type X collagen in the cartilage matrix; in addition, treatment with doxycycline at higher concentrations caused increases in the length of the hypertrophic region.

CONCLUSION

The effects of doxycycline extend beyond inhibition of the proteolytic enzymes by stimulating cartilage growth and disrupting the terminal differentiation of chondrocytes.

Recombinant human transforming growth factor beta 1 modulates bone remodeling in a mineralizing bone organ culture

TGF-beta 1 increases cell proliferation and collagen synthesis in osteoblast-like cells and bone organ cultures. However, the effects of TGF-beta 1 on bone resorption remain contradictory. Therefore, the exact role that this growth factor plays in the process of bone remodeling is still not clear. We studied the effects of recombinant human TGF-beta 1 (rhTGF-beta 1) on bone formation and resorption in a mineralizing bone organ culture system. Parietal bones from 20-day-old fetal rat calvariae were cultured up to 7 days in serum-free BGJb medium. They responded to a 1 day pulse or continuous treatment of rhTGF-beta 1 with dose-dependent increases in dry weight, [3H]thymidine ([3H]TdR) incorporation, and collagen synthesis. In contrast, rhTGF-beta 1 reduced the calcium content of the bones. This is not due to increased bone resorption but rather to failure of calcium deposition. The following responses occurred at 1 nM rhTGF-beta 1. Dry weight was increased 25-50% after 6 days in culture. DNA synthesis was increased to a maximum at day 1, reaching twofold of the control level. Adding hydroxyurea at day 0 reduced [3H]TdR incorporation in rhTGF-beta 1 treated bones to 20% of the control and indomethacin abrogated the increase in [3H]TdR stimulated by rhTGF-beta 1 to the control level. Both treatments completely blocked the increase in dry weight induced by rhTGF-beta 1 at day 6. rhTGF-beta 1 stimulated collagen synthesis to reach its maximum at day 2, with a twofold increase in [3H]proline incorporation. Basal alkaline phosphatase activity fell continuously in culture, reaching 35% of day 0 level at day 6. Enzyme activity was not altered by rhTGF-beta 1. Morphologic observations by light and electron microscopy confirmed these findings. In summary, rhTGF-beta 1 altered bone remodeling by increasing organic components and decreasing calcification in a mineralizing bone organ culture system.

Trans-differentiation of hypertrophic chondrocytes into cells capable of producing a mineralized bone matrix

Trans-differentiation of hypertrophic chondrocytes into bone-forming cells was observed when femurs from 14-day-old chick embryos were cut through the region of hypertrophic cartilage and the separated pieces were cultured for 2-18 days. Inside many chondrocytic lacunae a new matrix was present which had the staining characteristics of bone matrix including birefringence and the capacity to mineralize. The cells within the lacunae had the characteristics of osteoblasts, such as alkaline phosphatase activity and positive immunocytochemical staining for osteocalcin, osteonectin, osteopontin and type I collagen. Chondrocyte necrosis and empty lacunae were only observed immediately at the cut edge, and in that region no bone-forming cells were present inside the lacunae. Where bone-matrix was present, the lacunae had remained intact, the cells were viable and no evidence of cell migration was observed. This suggested that the bone-forming cells had originated from the hypertrophic chondrocytes. The temporal sequence of events was followed closely. Two days following the cut only a few chondrocytes showed a positive reaction for osteocalcin, osteonectin, osteopontin and the type I collagen. At that time no such reaction product was observed in the chondrocytes of uncut femurs. Many hypertrophic chondrocytes divided, as shown by tritiated thymidine incorporation. The rate of cell division increased between 2-6 days, when several smaller basophilic cells were present inside the lacuna instead of the single hypertrophic chondrocyte. These cells expressed alkaline phosphatase activity, were positive for fibronectin, the above non-collagenous bone proteins and type I collagen. The bone matrix that was observed after 6-18 days was initially confined to the inside of the chondrocytic lacunae, but later spread beyond the lacunar confines. The bone proteins were still associated with the bone-forming cells, but fibronectin was absent when matrix formation was evident. Mineralization of the intra-lacunar osteoid took place after 12-18 days. It is speculated that the trans-differentiation was initiated by disruptions of the normal cell-cell associations.

Avian osteoblast conditioned media stimulate bone resorption by targeting multinucleating osteoclast precursors

Osteoblasts are thought to secrete factors that regulate the rate of osteoclastic bone resorption. We studied the effect of osteoblast conditioned medium on bone degradation by multinucleated osteoclast-like cells generated in vitro from mononuclear precursors and found that the medium stimulates bone degradation primarily through interactions with osteoclast precursors. The conditioned medium also stimulates expression of the osteoclast-specific antigen 121F. The increased bone degradation, but not increased 121F expression, is due to the conditioned medium maintaining activity of the osteoclast precursors. Although the osteoclast precursors exhibit the DNA fragmentation characteristic of apoptosis, the osteoblast conditioned medium does not prevent such fragmentation. Chicken macrophage growth factor neither mimics nor augments the ability of the conditioned medium to stimulate bone degradation. Studies of osteoclast generation or function should carefully consider whether the effects are dependent on the viability of the resorbing cells.

Induction of normal and dystrophic mineralization by glycerophosphates in long-term bone organ culture

The effectiveness of Na-beta-, and Ca-glycerophosphates (GPs) in inducing mineralization was tested during long-term organ culture of femurs from 14-day-old chick embryos. When bones were incubated with Na-GP, a 66% rise in inorganic phosphate level was measured in the medium, supporting the notion that provision of a substrate for alkaline phosphatase (ALP) increased available phosphate. On the other hand, if the concentrations of Ca2+ were raised, available inorganic phosphate was decreased. Similarly, increases in inorganic phosphate decreased available calcium. Both GPs induced mineralization in bone and cartilage, but more matrix was mineralized with Ca-GP. However, the induction of mineralization by GPs was accompanied by dystrophic calcification, reduction of matrix formation and ALP activity, and increased release of lactate dehydrogenase into the culture medium. The new osteoid, which formed during culture, mineralized in the absence of GPs without the above adverse effects provided the culture period was longer than 15 days. The described organ culture system therefore facilitates studies of the mechanism of bone mineralization without the disadvantages of GP addition.

The influence of bone and marrow on cartilage hypertrophy and degradation during 30-day serum-free culture of the embryonic chick tibia

In this study, an organ culture system is defined which demonstrates complete loss of cartilage matrix from embryonic chick tibiae. Efficient loss of the cartilage matrix occurs within 30 days of serum-free culture only when the intact tibiae containing bone, marrow, and cartilage tissue are cultured. During organ culture nonhypertrophic chondrocytes become hypertrophic and stain positively for type X collagen and alkaline phosphatase. The cartilage loses Safranin O staining, and finally all cartilage matrix disappears leaving the bony collar and marrow cells. If the tibial cartilage is separated from the bony collar and cultured alone in serum-free medium, the nonhypertrophic chondrocytes also hypertrophy; the matrix loses Safranin O staining; however, some components of the matrix including type X collagen still remain after 30 days. In the presence of serum, the chondrocytes will hypertrophy but cartilage degradation is not evident. The results of this study support the conclusions that 1) hypertrophy is inherently programmed in the chondrocyte and 2) while Safranin O staining of cartilage cultured alone is diminished in serum-free organ culture, the degradation of cartilage is complete only when bone and marrow are also present.