Collagen gene expression during development of avian synovial joints: transient expression of types II and XI collagen genes in the joint capsule

Contribution of neural crest-derived stem cells and nasal chondrocytes to articular cartilage regeneration

Due to poor self-regenerative potential of articular cartilage, stem cell-based regeneration becomes a hopeful approach for the treatment of articular cartilage defects. Recent studies indicate that neural crest-derived cells (NCDCs) have the potential for repairing articular cartilage with even greater chondrogenic capacity than mesoderm-derived cells (MDCs): a conventional stem cell source for cartilage regeneration. Given that NCDCs originate from a different germ layer in the early embryo compared with MDCs that give rise to articular cartilage, a mystery remains regarding their capacity for articular cartilage regeneration. In this review, we summarize the similarities and differences between MDCs and NCDCs including articular and nasal chondrocytes in cell origin, anatomy, and chondrogenic differentiation and propose that NCDCs might be promising cell origins for articular cartilage regeneration.

Epiphyseal Cartilage Formation Involves Differential Dynamics of Various Cellular Populations During Embryogenesis

A joint connects two or more bones together to form a functional unit that allows different types of bending and movement. Little is known about how the opposing ends of the connected bones are developed. Here, applying various lineage tracing strategies we demonstrate that progenies of Gdf5-, Col2-, Prrx1-, and Gli1-positive cells contribute to the growing epiphyseal cartilage in a spatially asymmetrical manner. In addition, we reveal that cells in the cartilaginous anlagen are likely to be the major sources for epiphyseal cartilage. Moreover, Gli1-positive cells are found to proliferate along the skeletal edges toward the periarticular region of epiphyseal surface. Finally, a switch in the mechanism of growth from cell division to cell influx likely occurs in the epiphyseal cartilage when joint cavitation has completed. Altogether, our findings reveal an asymmetrical mechanism of growth that drives the formation of epiphyseal cartilage ends, which might implicate on how the articular surface of these skeletal elements acquires their unique and sophisticated shape during embryonic development.

Computational model of a synovial joint morphogenesis

Joints enable the relative movement between the connected bones. The shape of the joint is important for the joint movements since they facilitate and smooth the relative displacement of the joint's parts. The process of how the joints obtain their final shape is yet not well understood. Former models have been developed in order to understand the joint morphogenesis leaning only on the mechanical environment; however, the obtained final anatomical shape does not match entirely with a realistic geometry. In this study, a computational model was developed with the aim of explaining how the morphogenesis of joints and shaping of ossification structures are achieved. For this model, both the mechanical and biochemical environments were considered. It was assumed that cartilage growth was controlled by cyclic hydrostatic stress and inhibited by octahedral shear stress. In addition, molecules such as PTHrP and Wnt promote chondrocyte proliferation and therefore cartilage growth. Moreover, the appearance of the primary and secondary ossification centers was also modeled, for which the osteogenic index and PTHrP-Ihh concentrations were taken into account. The obtained results from this model show a coherent final shape of an interphalangeal joint, which suggest that the mechanical and biochemical environments are crucial for the joint morphogenesis process.

Cell origin, volume and arrangement are drivers of articular cartilage formation, morphogenesis and response to injury in mouse limbs

Limb synovial joints are composed of distinct tissues, but it is unclear which progenitors produce those tissues and how articular cartilage acquires its functional postnatal organization characterized by chondrocyte columns, zone-specific cell volumes and anisotropic matrix. Using novel Gdf5^CreERT2 (Gdf5-CE), Prg4-CE and Dkk3-CE mice mated to R26-Confetti or single-color reporters, we found that knee joint progenitors produced small non-migratory progenies and distinct local tissues over prenatal and postnatal time. Stereological imaging and quantification indicated that the columns present in juvenile-adult tibial articular cartilage consisted of non-daughter, partially overlapping lineage cells, likely reflecting cell rearrangement and stacking. Zone-specific increases in cell volume were major drivers of tissue thickening, while cell proliferation or death played minor roles. Second harmonic generation with 2-photon microscopy showed that the collagen matrix went from being isotropic and scattered at young stages to being anisotropic and aligned along the cell stacks in adults. Progenitor tracing at prenatal or juvenile stages showed that joint injury provoked a massive and rapid increase in synovial Prg4+ and CD44+/P75+ cells some of which filling the injury site, while neighboring chondrocytes appeared unresponsive. Our data indicate that local cell populations produce distinct joint tissues and that articular cartilage growth and zonal organization are mainly brought about by cell volume expansion and topographical cell rearrangement. Synovial Prg4+ lineage progenitors are exquisitely responsive to acute injury and may represent pioneers in joint tissue repair.

Articular cartilage and joint development from embryogenesis to adulthood

Within each synovial joint, the articular cartilage is uniquely adapted to bear dynamic compressive loads and shear forces throughout the joint's range of motion. Injury and age-related degeneration of the articular cartilage often lead to significant pain and disability, as the intrinsic repair capability of the tissue is extremely limited. Current surgical and biological treatment options have been unable to restore cartilage de novo. Before successful clinical cartilage restoration strategies can be developed, a better understanding of how the cartilage forms during normal development is essential. This review focuses on recent progress made towards addressing key questions about articular cartilage morphogenesis, including the origin of synovial joint progenitor cells, postnatal development and growth of the tissue. These advances have provided novel insight into fundamental questions about the developmental biology of articular cartilage, as well as potential cell sources that may participate in joint response to injury.

Articular Cartilage: Structural and Developmental Intricacies and Questions

Articular cartilage has obvious and fundamental roles in joint function and body movement. Much is known about its organization, extracellular matrix, and phenotypic properties of its cells, but less is known about its developmental biology. Incipient articular cartilage in late embryos and neonates is a thin tissue with scanty matrix and small cells, while adult tissue is thick and zonal and contains large cells and abundant matrix. What remains unclear is not only how incipient articular cartilage forms, but how it then grows and matures into a functional, complex, and multifaceted structure. This review focuses on recent and exciting discoveries on the developmental biology and growth of articular cartilage, frames them within the context of classic studies, and points to lingering questions and research goals. Advances in this research area will have significant relevance to basic science, and also considerable translational value to design superior cartilage repair and regeneration strategies.

Determination of differential gene expression profiles in superficial and deeper zones of mature rat articular cartilage using RNA sequencing of laser microdissected tissue specimens

Superficial zone (SFZ) cells, which are morphologically and functionally distinct from chondrocytes in deeper zones, play important roles in the maintenance of articular cartilage. Here, we established an easy and reliable method for performance of laser microdissection (LMD) on cryosections of mature rat articular cartilage using an adhesive membrane. We further examined gene expression profiles in the SFZ and the deeper zones of articular cartilage by performing RNA sequencing (RNA-seq). We validated sample collection methods, RNA amplification and the RNA-seq data using real-time RT-PCR. The combined data provide comprehensive information regarding genes specifically expressed in the SFZ or deeper zones, as well as a useful protocol for expression analysis of microsamples of hard tissues.

Genesis and morphogenesis of limb synovial joints and articular cartilage

Limb synovial joints are intricate structures composed of articular cartilage, synovial membranes, ligaments and an articular capsule. Together, these tissues give each joint its unique shape, organization and biomechanical function. Articular cartilage itself is rather complex and organized in distinct zones, including the superficial zone that produces lubricants and contains stem/progenitor cells. For many years there has been great interest in deciphering the mechanisms by which the joints form and come to acquire such unique structural features and diversity. Decades ago, classic embryologists discovered that the first overt sign of joint formation at each prescribed limb site was the appearance of a dense and compact population of mesenchymal cells collectively called the interzone. Work carried out since then by several groups has provided evidence that the interzone cells actively participate in joint tissue formation over developmental time. This minireview provides a succinct but comprehensive description of the many important recent advances in this field of research. These include studies using various conditional reporter mice to genetically trace and track the origin, fate and possible function of joint progenitor cells; studies on the involvement and roles in signaling pathways and transcription factors in joint cell determination and functioning; and studies using advanced methods of gene expression analyses to uncover novel genetic determinants of joint formation and diversity. The overall advances are impressive, and the findings are not only of obvious interest and importance but also have major implications in the conception of future translational medicine tools to repair and regenerate defective, overused or aging joints.

BMPRIA mediated signaling is essential for temporomandibular joint development in mice

The central importance of BMP signaling in the development and homeostasis of synovial joint of appendicular skeleton has been well documented, but its role in the development of temporomandibular joint (TMJ), also classified as a synovial joint, remains completely unknown. In this study, we investigated the function of BMPRIA mediated signaling in TMJ development in mice by transgenic loss-of- and gain-of-function approaches. We found that BMPRIA is expressed in the cranial neural crest (CNC)-derived developing condyle and glenoid fossa, major components of TMJ, as well as the interzone mesenchymal cells. Wnt1-Cre mediated tissue specific inactivation of BmprIa in CNC lineage led to defective TMJ development, including failure of articular disc separation from a hypoplastic condyle, persistence of interzone cells, and failed formation of a functional fibrocartilage layer on the articular surface of the glenoid fossa and condyle, which could be at least partially attributed to the down-regulation of Ihh in the developing condyle and inhibition of apoptosis in the interzone. On the other hand, augmented BMPRIA signaling by Wnt1-Cre driven expression of a constitutively active form of BmprIa (caBmprIa) inhibited osteogenesis of the glenoid fossa and converted the condylar primordium from secondary cartilage to primary cartilage associated with ectopic activation of Smad-dependent pathway but inhibition of JNK pathway, leading to TMJ agenesis. Our results present unambiguous evidence for an essential role of finely tuned BMPRIA mediated signaling in TMJ development.

The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology

The hooked element in the pes of turtles was historically identified by most palaeontologists and embryologists as a modified fifth metatarsal, and often used as evidence to unite turtles with other reptiles with a hooked element. Some recent embryological studies, however, revealed that this element might represent an enlarged fifth distal tarsal. We herein provide extensive new myological and developmental observations on the hooked element of turtles, and re-evaluate its primary and secondary homology using all available lines of evidence. Digital count and timing of development are uninformative. However, extensive myological, embryological and topological data are consistent with the hypothesis that the hooked element of turtles represents a fusion of the fifth distal tarsal with the fifth metatarsal, but that the fifth distal tarsal dominates the hooked element in pleurodiran turtles, whereas the fifth metatarsal dominates the hooked element of cryptodiran turtles. The term 'ansulate bone' is proposed to refer to hooked elements that result from the fusion of these two bones. The available phylogenetic and fossil data are currently insufficient to clarify the secondary homology of hooked elements within Reptilia.

Structural and functional analysis of intra-articular interzone tissue in axolotl salamanders

OBJECTIVE

Knowledge of mechanisms directing diarthrodial joint development may be useful in understanding joint pathologies and identifying new therapies. We have previously established that axolotl salamanders can fully repair large articular cartilage lesions, which may be due to the presence of an interzone-like tissue in the intra-articular space. Study objectives were to further characterize axolotl diarthrodial joint structure and determine the differentiation potential of interzone-like tissue in a skeletal microenvironment.

DESIGN

Diarthrodial joint morphology and expression of aggrecan, brother of CDO (BOC), type I collagen, type II collagen, and growth/differentiation factor 5 (GDF5) were examined in femorotibial joints of sexually mature (>12 months) axolotls. Joint tissue cellularity was evaluated in individuals from 2 to 24 months of age. Chondrogenic potential of the interzone was evaluated by placing interzone-like tissue into 4 mm tibial defects.

RESULTS

Cavitation reached completion in the femoroacetabular and humeroradial joints, but an interzone-like tissue was retained in the intra-articular space of distal limb joints. Joint tissue cellularity decreased to 7 months of age and then remained stable. Gene expression patterns of joint markers are broadly similar in developing mammals and mature axolotls. When interzone-like tissue was transplanted into critical size skeletal defects, an accessory joint developed within the defect site.

CONCLUSIONS

These experiments indicate that mature axolotl diarthrodial joints are phenotypically similar to developing synovial joints in mammals. Generation of an accessory joint by interzone-like tissue suggests multipotent cellular differentiation potential similar to that of interzone cells in the mammalian fetus. The data support the axolotl as a novel vertebrate model for joint development and repair.

Regeneration of the elbow joint in the developing chick embryo recapitulates development

Synovial joints are among the most important structures that give us complex motor abilities as humans. Degenerative joint diseases, such as arthritis, cause loss of normal joint functioning and affect over 40 million people in the USA and approximately 350 million people worldwide. Therapies based on regenerative medicine hold the promise of effectively repairing or replacing damaged joints permanently. Here, for the first time, we introduce a model for synovial joint regeneration utilizing the chick embryo. In this model, a block of tissue that contains the prospective elbow is excised, leaving a window with strips of anterior and posterior tissue intact (window excision, WE). In contrast, we also slice out the same area containing the elbow and the distal piece of the limb is pinned back onto the stump (slice excision, SE). Interestingly, when the elbow is removed via WE, regeneration of the joint takes place, whereas the elbow joint does not regenerate following SE. In order to investigate whether the regeneration response recapitulates the developmental program of forming joints, we used GDF-5 and Autotaxin (Atx) as joint tissue specific markers, and Sox-9 and Col-9 as cartilage markers for in situ hybridization on sections at different time points after WE and SE surgeries. Re-expression of GDF-5 and Atx is observed in the WE samples by 60h after surgery. In contrast, the majority of the samples that underwent SE surgery did not express GDF-5 and Atx. Also, in SE fusion of cartilage elements takes place and the joint interzone does not form. This is indicated by continuous Col-9 expression in SE limbs, whereas Col-9 is downregulated at the joint interzone in the regenerating WE samples. This order and pattern of gene expression observed in regenerates is similar to the development of a joint suggesting that regeneration recapitulates development at the molecular level. This model defines some of the conditions required for inducing joint regeneration in an otherwise nonregenerating environment. This knowledge can be useful for designing new therapeutic approaches for joint loss or for conditions affecting joint integrity in humans.

miRNAs control tracheal chondrocyte differentiation

The specific program that enables the stereotypic differentiation of specialized cartilages, including the trachea, is intrinsically distinct from the program that gives rise to growth plate hypertrophic chondrocytes. For example, Snail1 is an effector of FGF signaling in growth plate pre-hypertrophic chondrocytes, but it derails the normal program of permanent chondrocytes, repressing the transcription of Aggrecan and Collagen type 2a1 (Col2a1). Here we show that miRNA activity is essential for normal trachea development and that miR-125b and miR-30a/c keep Snail1 at low levels, thus enabling full functional differentiation of Col2a1 tracheal chondrocytes. Specific inhibition of miR-125b and miR-30a/c in chondrocytes or Dicer1 knockout in the trachea, de-repress Snail1. As a consequence, the transcription of Aggrecan and Col2a1 is hampered and extracellular matrix deposition is decreased. Our data reveals a new miRNA pathway that is safekeeping the specific genetic program of differentiated and matrix-producing tracheal chondrocytes from acquisition of unwanted signals. This pathway may improve understanding of human primary tracheomalacia and improve protocols for cartilage tissue engineering.

Changes of chondrocyte expression profiles in human MSC aggregates in the presence of PEG microspheres and TGF-β3

Biomaterial microparticles are commonly utilized as growth factor delivery vehicles to induce chondrogenic differentiation of mesenchymal stem/stromal cells (MSCs). To address whether the presence of microparticles could themselves affect differentiation of MSCs, a 3D co-aggregate system was developed containing an equal volume of human primary bone marrow-derived MSCs and non-degradable RGD-conjugated poly(ethylene glycol) microspheres (PEG-μs). Following TGF-β3 induction, differences in cell phenotype, gene expression and protein localization patterns were found when compared to MSC aggregate cultures devoid of PEG-μs. An outer fibrous layer always found in differentiated MSC aggregate cultures was not formed in the presence of PEG-μs. Type II collagen protein was synthesized by cells in both culture systems, although increased levels of the long (embryonic) procollagen isoforms were found in MSC/PEG-μs aggregates. Ubiquitous deposition of type I and type X collagen proteins was found in MSC/PEG-μs cultures while the expression patterns of these collagens was restricted to specific areas in MSC aggregates. These findings show that MSCs respond differently to TGF-β3 when in a PEG-μs environment due to effects of cell dilution, altered growth factor diffusion and/or cellular interactions with the microspheres. Although not all of the expression patterns pointed toward improved chondrogenic differentiation in the MSC/PEG-μs cultures, the surprisingly large impact of the microparticles themselves should be considered when designing drug delivery/scaffold strategies.

Atypical molecular profile for joint development in the avian costal joint

Development of synovial joints involves generation of cartilaginous anlagen, formation of interzones between cartilage anlagen, and cavitation of interzones to produce fluid filled cavities. Interzone development is not fully understood, but interzones are thought to develop from skeletogenic cells that are inhibited from further chondrogenic development by a cascade of gene expression including Wnt and Bmp family members. We examined the development of the rarely studied avian costal joint to better understand mechanisms of joint development. The costal joint is found within ribs, is morphologically similar to the metatarsophalangeal joint, and undergoes cavitation in a similar manner. In contrast to other interzones, Wnt14/9a, Gdf5, Chordin, Barx1, and Bapx1 are absent from the costal joint interzone, consistent with the absence of active β-catenin and phosphorylated Smad 1/5/8. However Autotaxin and Noggin are expressed. The molecular profile of the costal joint suggests there are alternative mechanisms of interzone development.

Synovial joint formation requires local Ext1 expression and heparan sulfate production in developing mouse embryo limbs and spine

Heparan sulfate proteoglycans (HSPGs) regulate a number of major developmental processes, but their roles in synovial joint formation remain unknown. Here we created conditional mouse embryo mutants lacking Ext1 in developing joints by mating Ext1(f/f) and Gdf5-Cre mice. Ext1 encodes a subunit of the Ext1/Ext2 Golgi-associated protein complex responsible for heparan sulfate (HS) synthesis. The proximal limb joints did form in the Gdf5-Cre;Ext1(f/f) mutants, but contained an uneven articulating superficial zone that expressed very low lubricin levels. The underlying cartilaginous epiphysis was deranged as well and displayed random patterns of cell proliferation and matrillin-1 and collagen IIA expression, indicative of an aberrant phenotypic definition of the epiphysis itself. Digit joints were even more affected, lacked a distinct mesenchymal interzone and were often fused likely as a result of local abnormal BMP and hedgehog activity and signaling. Interestingly, overall growth and lengthening of long bones were also delayed in the mutants. To test whether Ext1 function is needed for joint formation at other sites, we examined the spine. Indeed, entire intervertebral discs, normally composed by nucleus pulposus surrounded by the annulus fibrosus, were often missing in Gdf5-Cre;Ext1(f/f) mice. When disc remnants were present, they displayed aberrant organization and defective joint marker expression. Similar intervertebral joint defects and fusions occurred in Col2-Cre;β-catenin(f/f) mutants. The study provides novel evidence that local Ext1 expression and HS production are needed to maintain the phenotype and function of joint-forming cells and coordinate local signaling by BMP, hedgehog and Wnt/β-catenin pathways. The data indicate also that defects in joint formation reverberate on, and delay, overall long bone growth.

3D representation of the developing chick knee joint: a novel approach integrating multiple components

The knee joint has a highly complex 3-dimensional (3D) morphology that is sculpted at the interface of the forming long bones as they are generated in the embryo. Although it is clear that regulatory genes guide joint formation, the mechanisms that are responsible for morphogenesis of the knee are poorly understood. Certainly the process involves integration across several tissues and physical/mechanical influences from neighbouring tissues are important. We describe the acquisition of shape in the chick knee joint in detail and show that by HH34 the joint already displays shape characteristics of the adult structure. Through imaging developing cartilage, tendons, ligaments and muscle across developmental stages from HH28-34 we have built 3D representations of the forming structure including the various components important in knee formation. We describe the timing of muscle and tendon development in parallel with the refinement of cartilage shape, showing when and where (tendon attachment points) muscle forces are applied to the cartilage elements. Shape begins to emerge as the tendons are forming (HH30-32) but is fully refined (HH34) in the presence of tendons. The resulting integrated 3D representations of the developing knee across time will serve as the foundation for computational analysis of the mechanical environment, and experimental approaches to investigating morphogenetic mechanisms.

Muscle contraction is necessary to maintain joint progenitor cell fate

During embryogenesis, organ development is dependent upon maintaining appropriate progenitor cell commitment. Synovial joints develop from a pool of progenitor cells that differentiate into various cell types constituting the mature joint. The involvement of the musculature in joint formation has long been recognized. However, the mechanism by which the musculature regulates joint formation has remained elusive. In this study, we demonstrate, utilizing various murine models devoid of limb musculature or its contraction, that the contracting musculature is fundamental in maintaining joint progenitors committed to their fate, a requirement for correct joint cavitation and morphogenesis. Furthermore, contraction-dependent activation of beta-catenin, a key modulator of joint formation, provides a molecular mechanism for this regulation. In conclusion, our findings provide the missing link between progenitor cell fate determination and embryonic movement, two processes shown to be essential for correct organogenesis.

Col2a1 lineage tracing reveals that the meniscus of the knee joint has a complex cellular origin

The knee joint consists of multiple interacting tissues that are prone to injury- and disease-related degeneration. Although much is known about the structure and function of the knee's constituent tissues, relatively little is known about their cellular origin and the mechanisms governing their segregation. To investigate the origin and segregation of knee tissues in vivo we performed lineage tracing using a Col2a1-Cre/R26R mouse model system and compared the data obtained with actual Col2a1 expression. These studies demonstrated that at E13.5 the interzone at the presumptive joint site forms when cells within the Col2a1-expressing anlagen cease expression of Col2a1 and not through cellular invasion into the anlagen. Later in development these interzone cells form the cruciate ligament and inner medial meniscus of the knee. At E14.5, after interzone formation, cells that had never expressed Col2a1 appeared in the joint and formed the lateral meniscus. Furthermore, cells with a Col2a1-positive expression history combined with the negative cells to form the medial meniscus. The invading cells started to express Col2a1 1 week after birth, resulting in all cells within the meniscus synthesizing collagen II. These findings support a model of knee development in which cells present in the original anlagen combine with invading cells in the formation of this complex joint.

FGF9 monomer-dimer equilibrium regulates extracellular matrix affinity and tissue diffusion

The spontaneous dominant mouse mutant, Elbow-knee-synostosis (Eks), exhibits elbow and knee joint synosotsis, and premature fusion of cranial sutures. Here we identify a missense mutation in the Fgf9 gene that is responsible for the Eks mutation. Through investigation of the pathogenic mechanisms of joint and suture synostosis in Eks mice, we identify a key molecular mechanism that regulates FGF9 signaling in developing tissues. We show that the Eks mutation prevents homodimerization of the FGF9 protein and that monomeric FGF9 binds to heparin with a lower affinity than dimeric FGF9. These biochemical defects result in increased diffusion of the mutant FGF9 protein (FGF9^Eks) through developing tissues, leading to ectopic FGF9 signaling and repression of joint and suture development. We propose a mechanism in which the range of FGF9 signaling in developing tissues is limited by its ability to homodimerize and its affinity for extracellular matrix heparan sulfate proteoglycans.

Gene expression patterns underlying proximal-distal skeletal segmentation in late-stage zebrafish, Danio rerio

Timing and pattern of expression of ten candidate segmentation genes or gene pairs were reviewed or examined in developing median fins of late-stage zebrafish, Danio rerio. We found a general correspondence in timing and pattern of expression between zebrafish fin radial segmentation and tetrapod joint development, suggesting that molecular mechanisms underlying radial segmentation have been conserved over 400 million years of evolution. Gene co-expression during segmentation (5.5-6.5 mm SL) is similar between tetrapods and zebrafish: bmp2b, bmp4, chordin, and gdf5 in interradial mesenchyme and ZS; bapx1, col2a1, noggin3, and sox9a in chondrocytes. Surprisingly, wnt9a is not expressed in the developing median fins, though wnt9b is detected. In contrast to all other candidate segmentation genes we examined, bapx1 is not expressed in the caudal fin, which does not segment. Together, these data suggest a scenario of gene interactions underlying radial segmentation based on the patterns and timing of candidate gene expression.

Incudomalleal joint formation: the roles of apoptosis, migration and downregulation

Background

The middle ear of mammals is composed of three endochondrial ossicles, the stapes, incus and malleus. Joints link the malleus to the incus and the incus to the stapes. In the mouse the first arch derived malleus and incus are formed from a single Sox9 and Type II collagen expressing condensation that later subdivides to give rise to two separate ossicles. In contrast the stapes forms from a separate condensation derived from the second branchial arch. Fusion of the malleus and incus is observed in a number of human syndromes and results in conductive hearing loss. Understanding how this joint forms during normal development is thus an important step in furthering our understanding of such defects.

Results

We show that the developing incudomalleal joint is characterised by a lack of proliferation and discrete areas of apoptosis. Apoptosis has been suggested to aid in the removal of pre-cartilaginous cells from the joint region, allowing for the physical separation of the cartilaginous elements, however, we show that joint initiation is unaffected by blocking apoptosis. There is also no evidence of cell migration out of the presumptive joint region, as observed by labelling of joint and ossicle cells in culture. Using Type II collagen lacZ reporter mice, however, it is evident that cells in the presumptive joint region remain in place and downregulate cartilage markers.

Conclusion

The malleus and incus first appear as a single united condensation expressing early cartilage markers. The incudomalleal joint region forms by cells in the presumptive joint region switching off cartilage markers and turning on joint markers. Failure in this process may result in fusion of this joint, as observed in human syndromes such as Branchio-Oto-Renal Syndrome or Treacher Collins Syndrome.

Expression of two novel alternatively spliced COL2A1 isoforms during chondrocyte differentiation

Alternative splicing of the type II procollagen gene (COL2A1) is developmentally regulated during chondrogenesis. Type IIA procollagen (+ exon 2) is synthesized by chondroprogenitor cells while type IIB procollagen (- exon 2) is synthesized by differentiated chondrocytes. Here, we report expression of two additional alternatively spliced COL2A1 isoforms during chondrocyte differentiation of bone marrow-derived mesenchymal stem cells (MSCs). One isoform, named IIC, contains only the first 34 nucleotides of exon 2 by the use of an alternative 5' splice site, resulting in a premature termination codon and possible nonsense-mediated decay of IIC mRNA. Low levels of the IIC isoform were detected by RT-PCR and Southern analysis of COL2A1 cDNA amplified from differentiating rabbit and human MSCs. A second novel transcript, named IID, arises by the use of another 5' alternative splice site in intron 2. The IID isoform contains exon 2 plus 3 nucleotides, resulting in the insertion of an additional amino acid. The IID isoform was co-expressed with the IIA isoform during chondrogenesis, and was approximately one-third as abundant. Deletion of the IIC alternative 5' splice site from a COL2A1 mini-gene construct resulted in a significant increase in the IIA:IIB ratio. A mutant mini-gene that inhibited production of the IID isoform, however, had differential effects on the production of the IIA and IIB isoforms: this was apparently related to the differentiation status of the cell type used. These data suggest that COL2A1 mRNA abundance and other aspects of chondrocyte differentiation may be regulated by the use of these previously undetermined alternative splice sites.

Expression of cartilage-related genes in bovine synovial tissue

The synovium contains mesenchymal stem cells with chondrogenic potential. Although synovial and articular cartilage tissue develop from a common pool of mesenchymal cells, little is known about their genetic commonalities. In the present study, the mRNA levels for several cartilage-related proteins, namely, cartilage oligomeric matrix protein (COMP), Sox9, aggrecan, and collagen types I, II, IX, X, and XI, were measured using the real-time polymerase chain reaction. Our data reveal the synovium of calf metacarpal joints to physiologically express not only type I collagen but also COMP, Sox9, aggrecan, and collagen types X and XI. The mRNA levels for the latter five proteins lie between 2% and 15% of those in articular cartilage. We speculate that these genes are being expressed by chondroprogenitor cells, whose presence in the synovium reflects a common ontogenetic phase in the fetal development of this tissue and of articular cartilage.

Lineage tracing using matrilin-1 gene expression reveals that articular chondrocytes exist as the joint interzone forms

We have developed a mouse in which the Cre recombinase gene has been targeted to exon 1 of the matrilin-1 gene (Matn1) to investigate the origins of articular chondrocytes and the development of the knee joint. Analysis of joints from offspring of Matn1-Cre/R26R crosses demonstrated that articular chondrocytes are derived from cells that have never expressed matrilin-1 whereas the remainder of the chondrocytes in the cartilage anlagen expresses matrilin-1. A band of chondrocytes adjacent to the developing interzone in the E13.5 day knee joint became apparent because these chondrocytes did not turn on expression of matrilin-1 in contrast to the other chondrocytes of the anlagen. The chondrocytes of the presumptive articular surface therefore appear to arise directly from a subpopulation of early chondrocytes that do not activate matrilin-1 expression rather than by redifferentiation from the flattened cells of the interzone. In addition, lineage tracing using both Matn1-Cre/R26R and Col2a1-Cre/R26R lines indicated that non-cartilaginous structures in the knee such as cruciate ligament, synovium and some blood vessels are formed by cells derived from the early chondrocytes of the anlagen.

Developmental failure of the intra-articular ligaments in mice with absence of growth differentiation factor 5

OBJECTIVE

To show the phenotypic characteristics of the knee joints in brachypodism mice (bp mice), which carry a functional null mutation of the growth differentiation factor 5 (GDF5) gene, we investigated the adult and embryonic bp mice.

METHOD

Radiographic and macroscopic examinations of the knee joint of adult bp mice were performed. A histological examination of the knee joint of bp mice from E12.5 to E18.5 was also performed.

RESULTS

Radiographic and macroscopic examinations of the adult bp mice showed anterior dislocation, hypoplastic condyles, and absence of the intra-articular ligaments. Safranin O staining of knee joints of the embryonic bp mice showed severe hypoplasty of the chondroepiphyses and intra-articular ligaments at E16.5. There was no difference in the number and location of 5-bromo-2'-deoxyuridine (BrdU)-positive cells between wild-type and bp mice through E12.5 to E14.5. A terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) study showed excessive cell death of mesenchymal cells of the future knee joint in bp mice at E12.5 and E13.5.

CONCLUSION

bp mice exhibit developmental failure of the condyles and intra-articular ligament of the knee joints.

Notch and HES5 are regulated during human cartilage differentiation

The molecular mechanisms of cartilage differentiation are poorly understood. In a variety of tissues other than cartilage, members of the basic helix-loop-helix (bHLH) family of transcription factors have been demonstrated to play critical roles in differentiation. We have characterized the human bHLH gene HES5 and investigated its role during chondrogenesis. Blockage of the Notch signaling pathway with a gamma-secretase inhibitor has demonstrated that the human HES5 gene is a downstream marker of Notch signaling in articular chondrocytes. Markers for the Notch signaling pathway significantly decrease during cartilage differentiation in vitro. Cell proliferation assayed by using BrdU has revealed that blockage of Notch signaling is associated with significantly decreased proliferation. Northern blot and reverse transcription/polymerase chain reaction of a panel of various tissues have shown that HES5 is transcribed as a 5.4-kb mRNA that is ubiquitously expressed in diverse fetal and adult tissues. Articular cartilage from HES5(-/-) and wild-type mice has been analyzed by using various histological stains. No differences have been detected between the wild-type and HES5(-/-) mice. Our data thus indicate that the human HES5 gene is coupled to the Notch receptor family, that expression of Notch markers (including HES5) decreases during cartilage differentiation, and that the blockage of Notch signaling is associated with significantly decreased cell proliferation.

Wnt9a signaling is required for joint integrity and regulation of Ihh during chondrogenesis

Joints, which separate skeleton elements, serve as important signaling centers that regulate the growth of adjacent cartilage elements by controlling proliferation and maturation of chondrocytes. Accurate chondrocyte maturation is crucial for endochondral ossification and for the ultimate size of skeletal elements, as premature or delayed maturation results predominantly in shortened elements. Wnt9a has previously been implicated as being a player in joint induction, based on gain-of function experiments in chicken and mouse. We show that loss of Wnt9a does not affect joint induction, but results to synovial chondroid metaplasia in some joints. This phenotype can be enhanced by removal of an additional Wnt gene, Wnt4, suggesting that Wnts are playing a crucial role in directing bi-potential chondro-synovioprogenitors to become synovial connective tissue, by actively suppressing their chondrogenic potential. Furthermore, we show that Wnt9a is a temporal and spatial regulator of Indian hedgehog (Ihh), a central player of skeletogenesis. Loss of Wnt9a activity results in transient downregulation of Ihh and reduced Ihh-signaling activity at E12.5-E13.5. The canonical Wnt/beta-catenin pathway probably mediates regulation of Ihh expression in prehypertrophic chondrocytes by Wnt9a, because embryos double-heterozygous for Wnt9a and beta-catenin show reduced Ihh expression, and in vivo chromatin immunoprecipitation demonstrates a direct interaction between the beta-catenin/Lef1 complex and the Ihh promoter.

Early effects of embryonic movement: 'a shot out of the dark'

It has long been appreciated that studying the embryonic chick in ovo provides a variety of advantages, including the potential to control the embryo's environment and its movement independently of maternal influences. This allowed early workers to identify movement as a pivotal factor in the development of the locomotor apparatus. With an increasing focus on the earliest detectable movements, we have exploited this system by developing novel models and schemes to examine the influence of defined periods of movement during musculoskeletal development. Utilizing drugs with known neuromuscular actions to provoke hyperactivity (4-aminopyridine, AP) and either rigid (decamethonium bromide, DMB) or flaccid (pancuronium bromide, PB) paralysis, we have examined the role of movement in joint, osteochondral and muscle development. Our initial studies focusing on the joint showed that AP-induced hyperactivity had little, if any, effect on the timing or scope of joint cavity elaboration, suggesting that endogenous activity levels provide sufficient stimulus, and additional mobilization is without effect. By contrast, imposition of either rigid or flaccid paralysis prior to cavity formation completely blocked this process and, with time, produced fusion of cartilaginous elements and formation of continuous single cartilaginous rods across locations where joints would ordinarily form. The effect of these distinct forms of paralysis differed, however, when treatment was initiated after formation of an overt cavity; rigid, but not flaccid, paralysis partly conserved precavitated joints. This observation suggests that 'static' loading derived from 'spastic' rigidity can act to preserve joint cavities. Another facet of these studies was the observation that DMB-induced rigid paralysis produces a uniform and specific pattern of limb deformity whereas PB generated a diverse range of fixed positional deformities. Both also reduced limb growth, with different developmental periods preferentially modifying specific osteochondral components. Changes in cartilage and bone growth induced by 3-day periods of flaccid immobilization, imposed at distinct developmental phases, provides support for a diminution in cartilage elaboration at an early phase and for a relatively delayed influence of movement on osteogenesis, invoking critical periods during which the developing skeleton becomes receptive to the impact of movement. Immobilization also exerts differential impact along the proximo-distal axis of the limb. Finally, our preliminary results support the possibility that embryonic hyperactivity influences the potential for postnatal muscle growth.

Mechanisms of synovial joint and articular cartilage formation: recent advances, but many lingering mysteries

Synovial joints are elegant, critically important, and deceptively simple biomechanical structures. They are comprised of articular cartilage that covers each end of the opposing skeletal elements, synovial fluid that lubricates and nourishes the tissues, ligaments that hold the skeletal elements in check, and a fibrous capsule that insulates the joints from surrounding tissues. Joints also exhibit an exquisite arrays of shapes and sizes, best exemplified by the nearly spherical convex femoral head articulating into a nearly spherical concave hip acetabulum, or a phalangeal joint with two condyles on the distal side articulating in reciprocally-shaped sockets on the opposing proximal side. Though few in number, joint tissues are highly specialized in structure and function. This is illustrated by articular cartilage with its unique extracellular matrix, unique biomechanical resilience, its largely avascular nature, and its ability to persist through life with minimal turnover of its cells and components. The fact that interest in synovial joints has remained unabated for decades is a reflection of their fundamental importance for organism function and quality of life, and for their susceptibility to a variety of acquired and congenital conditions, most importantly arthritis. This has led to many advances in this field that encompass molecular genetics to biomechanics to medicine. Regrettably, what continues to be poorly understood are the mechanisms by which synovial joints actually form in the developing embryo. If available, this information would be not only of indisputable biological interest, but would also have significant biomedical ramifications, particularly in terms of designing novel tissue regeneration or reconstruction therapies. This review focuses on recent advances in understanding the mechanisms of synovial joint formation in the limbs, and places and discusses the information within the context of classic studies and the many mysteries and questions that remain unanswered.

Development of the articular cavity in the rat temporomandibular joint with special reference to the behavior of endothelial cells and macrophages

Previous developmental studies on the temporomandibular joint (TMJ) have proposed several hypotheses on the formation of its articular cavity. However, detailed information is meager. The present study examined the formation process of the articular cavity in the rat TMJ by immunocytochemistry for CD31, RECA-1, and ED1, which are useful cellular markers for endothelial cells and monocyte/macrophage lineages, respectively. The upper articular cavity formation had begun by embryonic day 21 (E21) and was completed at postnatal day 1 (P1) in advance of the lower cavitation; the latter took place from P1 to P3. The occurrence and distribution pattern of the CD31-, RECA-1-, and ED1-positive cells differed between the upper and lower articular cavity-forming areas: the ED1-positive cells exclusively occurred in the area of the prospective upper articular cavity prior to its formation, while no ED1-positive cell appeared in the lower cavity-forming area. In contrast, the CD31- and RECA-1-positive endothelial cells were restricted to the lower cavity-forming area (never the prospective upper cavity) at E19 and diminished thereafter. Throughout the cavity formation, we failed to find any apoptotic cells in the cavity formation area, indicating no involvement of apoptosis in the cavity formation in TMJ. The present findings on the behaviors of endothelial cells and ED1-positive cells show a possibility of different mechanism in the cavity formation between the upper and lower articular cavities in the rat TMJ. The appearance of ED1-reactive cells and temporal vascularization may play crucial roles in the upper and lower articular cavity formation, respectively.

Biology of developmental and regenerative skeletogenesis

Embryonic skeletal development involves the recruitment, commitment, differentiation, and maturation of mesenchymal cells into those in the skeletal tissue lineage, specifically cartilage and bone along the intramembranous and endochondral ossification pathways. The exquisite control of skeletal development is regulated at the level of gene transcription, cellular signaling, cell-cell and cell-matrix interactions, as well as systemic modulation. Mediators include transcription factors, growth factors, cytokines, metabolites, hormones, and environmentally derived influences. Understanding the mechanisms underlying developmental skeletogenesis is crucial to harnessing the inherent regenerative potential of skeletal tissues for wound healing and repair, as well as for functional skeletal tissue engineering. In this review, a number of key issues are discussed concerning the current and future challenges of the scientific investigation of developmental skeletogenesis in the embryo, specifically limb cartilage development, and how these challenges relate to regenerative or reparative skeletogenesis in the adult. Specifically, a more complete understanding the biology of skeletogenic progenitor cells and the cellular and molecular mechanisms governing tissue patterning and morphogenesis should greatly facilitate the development of regenerative approaches to cartilage repair.

Spontaneous differentiation of mesenchymal stem cells obtained from fetal rat circulation

Mesenchymal stem cells (MSCs) are thought to be multipotential, capable of differentiating into multiple lineages. We attempted to characterize rat cells derived from fetal circulating blood (FCBCs) that displayed a fibroblastic morphology and differentiated into osteoblastic and chondrocytic lineages. Notably, they differentiated into a chondrocyte-specific phenotype on plastic culture dishes in medium supplemented only with 10% fetal bovine serum (FBS) without the use of a three-dimensional culture substrate. Bone marrow-derived cells did not convey such phenotypic expression under the same conditions. The characteristic features of these cells were analyzed by reverse transcription polymerase chain reaction, immunohistological and von Kossa staining, and by immuno-dot blotting. In one population, expression of collagen types II and X was detected in differentiated cells at the same levels as observed in chondrocytes derived from rat rib cartilage. In another population, parathyroid hormone receptor, alkaline phosphatase, and osteocalcin were also expressed at levels almost equal to those observed in long bone-derived osteoblasts. After 3 weeks in culture, extensively condensed cell masses, stained with anti-type II collagen antibody, could be distinguished histologically from small, multilayered, von Kossa-positive nodules, which stained with anti-osteocalcin, but not with anti-type II collagen antibody. In addition, the FCBCs differentiated into adipogenic cells in the presence of methyl-isobutyl xanthine, dexamethasone, insulin, and indomethacin. These cells expressed PPARgamma2 mRNA and accumulated lipid vesicles detectable by Oil red-O staining. Our findings suggest that FCBCs have the potential to readily differentiate into multiple lineages and that they are distinct from mesenchymal stem cells derived from bone marrow or circulating blood from more mature and adults in their spontaneous differentiation in the absence of specific factors such as transforming growth factor-beta (TGF-beta) or dexamethasone, or a three-dimensional culture environment.

Wnt/beta-catenin signaling is sufficient and necessary for synovial joint formation

A critical step in skeletal morphogenesis is the formation of synovial joints, which define the relative size of discrete skeletal elements and are required for the mobility of vertebrates. We have found that several Wnt genes, including Wnt4, Wnt14, and Wnt16, were expressed in overlapping and complementary patterns in the developing synovial joints, where beta-catenin protein levels and transcription activity were up-regulated. Removal of beta-catenin early in mesenchymal progenitor cells promoted chondrocyte differentiation and blocked the activity of Wnt14 in joint formation. Ectopic expression of an activated form of beta-catenin or Wnt14 in early differentiating chondrocytes induced ectopic joint formation both morphologically and molecularly. In contrast, genetic removal of beta-catenin in chondrocytes led to joint fusion. These results demonstrate that the Wnt/beta-catenin signaling pathway is necessary and sufficient to induce early steps of synovial joint formation. Wnt4, Wnt14, and Wnt16 may play redundant roles in synovial joint induction by signaling through the beta-catenin-mediated canonical Wnt pathway.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Development of synovial joints

Synovial joints arise through two main processes. In long bone elements, cartilaginous differentiation occurs across the locations of the prospective joints that then segment secondarily. This process occurs through the development of a noncartilaginous region known as the interzone. The interzone becomes an important signaling center to the opposing elements, which can regulate growth through such factors as GDF-5. The interzone also expresses bone morphogenetic proteins (BMPs) and their antagonists, such as noggin. Overexpression of BMPs, or the loss of noggin leads to joint fusions. The interzone also expresses Wnt-14, which appears to be specific for this region in the developing anlagen, and regulates its nonchondrogenic nature. Cavitation of the joint follows, driven by selective high-level synthesis of hyaluronan by interzone cells and presumptive synovial cells. In addition, as the interzone disperses during cavity enlargement, data are now accruing that suggest that both the synovium and articular cartilage develop from this population. Finally, the development of articular cartilage progresses through appositional growth driven by a progenitor/stem cell subpopulation that resides in the articular surface. The individual elements of the skeleton are connected together at regions termed joints or articulations. Classically, there are three broad categories of joints: immovable joints (syntharthroses); mixed articulations, in which the range of movement is limited (amphiarthroses); and the movable, or synovial, joints (diarthroses). This review concentrates on the development of the synovial joints.

Defining boundaries during joint cavity formation: going out on a limb

Whilst factors controlling the site at which joints form within the developing limb are recognised, the mechanisms by which articular element separation occurs during the formation of the joint cavity have not been determined. Herein, we review the relationships between early limb patterning, embryonic movement, extracellular matrix composition, local signalling events and the process of joint cavity formation. We speculate that a pivotal event in this process involves the demarcation of signalling boundaries, established by local mechano-dependent modifications in glycosaminoglycan synthesis. In our opinion, studies that examine early patterning and also focus on local developmental alterations in tissue architecture are required in order to help elucidate the fundamental principals regulating joint formation.

Branching, segmentation and the metapterygial axis: pattern versus process in the vertebrate limb

Explanations of the patterns of vertebrate fin and limb evolution are improving as specific hypotheses based on molecular and developmental data are proposed and tested. Comparative analyses of gene expression patterns and functions in developing limbs, and morphological patterns in embryonic, adult and fossil limbs point to digit specification as a key developmental innovation associated with the origin of tetrapods. Digit development during the fin-to-limb transition involved sustained proximodistal outgrowth and a new phase of Hox gene expression in the distal fin bud. These patterning changes in the distal limb have been explained by the linked concepts of the metapterygial axis and the digital arch. These have been proposed to account for the generation of limb pattern by sequential branching and segmentation of precartilagenous elements along the proximodistal axis of the limb. While these ideas have been very fruitful, they have become increasingly difficult to reconcile with experimental and comparative studies of fin and limb development. Here we argue that limb development does not involve a branching mechanism, and reassess the concept of a metapterygial axis in limb development and evolution.

Bone morphogenetic proteins in the development and healing of synovial joints

OBJECTIVES

To review current knowledge of the role of bone morphogenetic proteins (BMPs) in joint formation and how this may be relevant to healing in adult joints.

METHOD

Review of published literature using a search of the PubMed database (1966 to 2000) made available by the National Library of Medicine. Additional articles of historical interest were identified from the bibliographies of published literature.

RESULTS

BMPs and a related family, the growth and differentiation factors (GDFs), are stimulators of bone and cartilage formation in the developing skeleton. They, together with their antagonists, play key roles in the specification of the joint site and cavitation of synovial joints during embryonic development. Disruption of the GDF-5 gene in mice and humans is associated with abnormal joint formation. In situ hybridization studies have shown that BMPs are expressed during formation of synovial joints in the embryo. However, excessive BMP activity leads to obliteration of joints because of cartilage overgrowth. BMPs are being considered as therapeutic agents to stimulate healing of articular cartilage after damage. Evidence suggests that BMPs are present in adult joints and have roles in healing and maintenance. However, inflammatory cytokines and growth factors present in damaged joints modulate the actions of BMPs.

CONCLUSIONS

BMPs, and in particular GDF-5, are involved in synovial joint formation. They may also have effects on the maintenance and healing of adult joints, but factors present after damage may alter their effectiveness.

RELEVANCE

Articular cartilage heals poorly after damage. BMPs may be useful therapeutically to stimulate healing of damaged articular cartilage. Increased knowledge of their role in joint formation will improve understanding of how to use them. Semin Arthritis Rheum 31:33-42.

Type IIA procollagen in development of the human intervertebral disc: regulated expression of the NH(2)-propeptide by enzymic processing reveals a unique developmental pathway

Type II collagen can be synthesized in two forms generated by alternative splicing of the precursor mRNA. Type IIA procollagen, which contains a cysteine-rich domain in the NH(2)-propeptide (exon 2), is produced by precartilage and noncartilage epithelial and mesenchymal cells, and type IIB procollagen, without the cysteine-rich domain, is characteristic of chondrocytes. Mice lacking type II collagen fail to develop intervertebral discs. We have previously shown that the human intervertebral disc and notochord synthesize primarily the type IIA form of procollagen. Therefore, we investigated the distribution of type IIA procollagen during early disc development in humans. By processes of radioactive in situ hybridization and fluorescence immunohistochemistry, we localized mRNA and protein of type IIA procollagen, type I collagen, and type III collagen in fetal intervertebral disc specimens ranging from day 42 (embryonic stage 17) to day 101 (week 14.5) of gestation. Antibodies to the three distinct domains of type IIA procollagen: the NH(2)-propeptide, the fibrillar domain, and the COOH-propeptide were used. The earliest stage of developing intervertebral disc (42 days, stage 17) was characterized by diffuse synthesis of types I and III collagens in the dense zone (intervertebral area) and synthesis of type IIA procollagen by the chondrocyte progenitor cells surrounding the disc. The notochord cells synthesized and deposited into the notochordal sheath all three fibrillar collagens. By 54 days (stage 22), the developing disc was clearly divided into three regions: 1.) the outer annulus, characterized by synthesis and deposition of types I and III collagens; 2.) the inner annulus, characterized by synthesis and deposition of type IIA collagen containing the NH(2)-propeptide but devoid of the COOH-propeptide (pN-procollagen); and 3.) the notochord, the cells of which synthesized and deposited of all three fibrillar collagens. In later stages of fetal development (72-101 days), a change in type IIA procollagen processing was observed in the cells of the inner annulus: even though these cells continued to synthesize type IIA procollagen, they deposited into the extracellular matrix (ECM) only the processed fibrillar domain, with the NH(2)-propeptide removed. This finding indicates that there is a developmentally regulated change in the processing of type IIA procollagen NH(2)-propeptide in the cells of the inner annulus. This mechanism is in contrast to previously shown developmental regulation of the cysteine-rich domain of the NH(2)-propeptide by alternative splicing of the precursor mRNA. Although the cells of the inner annulus have been identified as chondrocytes, based on their shape and synthesis of characteristic ECM components, they appear to represent a distinct developmental pathway characterized by their synthesis and differential processing of type IIA procollagen. This developmental pattern may prove important for disc regeneration.

Wnt-14 plays a pivotal role in inducing synovial joint formation in the developing appendicular skeleton

The long bones of the vertebrate appendicular skeleton arise from initially continuous condensations of mesenchymal cells that subsequently segment and cavitate to form discrete elements separated by synovial joints. Little is known, however, about the molecular mechanisms of joint formation. We present evidence that Wnt-14 plays a central role in initiating synovial joint formation in the chick limb. Wnt-14 is expressed in joint-forming regions prior to the segmentation of the cartilage elements, and local misexpression of Wnt-14 induces morphological and molecular changes characteristic of the first steps of joint formation. Induction of an ectopic joint-like region by Wnt-14 suppresses the formation of the immediately adjacent endogenous joint, potentially providing insight into the spacing of joints.

Growth-factor-induced healing of partial-thickness defects in adult articular cartilage

OBJECTIVE

We have previously shown (Hunziker and Rosenberg, J Bone Joint Surg 1996;78A:721-33) that synovial cells can be induced to migrate into partial-thickness articular cartilage defects, therein to proliferate and subsequently to deposit a scar-like tissue. We now wished to ascertain whether these synovial cells could be stimulated to transform into chondrocytes, and thus to lay down cartilage tissue, by the timely introduction of a differentiation factor.

DESIGN

Partial-thickness defects were created in the knee-joint cartilage of adult miniature pigs. These were then filled with a fibrin matrix containing a free chemotactic/mitogenic factor and a liposome-encapsulated chondrogenic differentiation one. Tissue was analyzed (immuno)histochemically at 2, 6 and 12 months.

RESULTS

Defects became filled with cartilage-like tissue which registered positive for all major cartilage-matrix components; it remained compositionally stable throughout the entire follow-up period.

CONCLUSION

Although still requiring considerable refinement, our one-step, growth-factor-based treatment strategy has the basic potential to promote intrinsic healing of partial-thickness articular cartilage defects, thus obviating the need for transplanting cells or tissue.

Morphological and biochemical re-evaluation of the process of cavitation in the rat knee joint: cellular and cell strata alterations in the interzone

To assess the contribution of apoptosis to the mechanism of synovial joint cavitation, and to clarify morphological cellular changes during cavitation, we investigated the development of the rat knee joint by light and electron microscopy, TUNEL methods, and electrophoresis of DNA fragments. Although cavitation occurred within the interzone, which consists of 2 outer and a middle layer termed the intermediate zone, no morphological or biochemical signs of cell death, in particular apoptosis, were seen in the interzone at any embryonic stage. Microscopic and ultrastructural alterations affecting cell differentiation were clearly observed in the interzone, i.e. mesenchymal cells gradually showed elongation, cytoplasmic vacuolation and pyknosis in the intermediate zone where the elongated cells were arranged in parallel in some strata. Some of these cells were further flattened into spindle cells and the number of strata decreased to 2. The rest of the cells were incorporated secondarily into the outer layers, becoming chondroblasts. Collagen fibrils were arranged in a network structure in the outer layers, which obviously differed from the directional pattern parallel to the long axis of elongated cells in the intermediate zone. In addition, the density of collagen fibrils was higher in the outer layers than in the intermediate zone. During cavitation, the initial separation was detected between the elongated cells in the intermediate zone in paraffin sections at E16.5 and the spindle cells in epoxy sections at E18.5. The spindle cells lining the cavity, namely, the surfaces of the epiphysis and meniscus, finally became chondrocytes. The diminution of proteoglycans and collagen fibrils and the synthesis of hyaluronan in the extracellular matrix are now generally believed to be parts of the mechanism for cavitation based on the concept of 'loss of cohesion'. The microscopic and ultrastructural alterations in the interzone seemed to reflect differences in the arrangement and density of collagen fibrils and the developmental condition of the extracellular matrix between layers. Also it did not seem likely that these alterations inhibit the synthesis of hyaluronan at the presumptive joint line because this synthesis takes place at the plasma membrane. Separation between spindle cells should therefore represent the mechanism for developmentally programmed cavitation. Reorganization of the extracellular matrix is probably necessary for the cellular metamorphoses in the interzone involved in the process of cavitation.