Secondary chondrocyte-derived Ihh stimulates proliferation of periosteal cells during chick development

An immunohistochemical study of matrix components in primary and secondary cartilages of embryonic chick skull

OBJECTIVES

This study aimed to compare the extracellular matrix of primary cartilage with the secondary cartilage of chicks using immunohistochemical analyses in order to understand the features of chick secondary chondrogenesis.

METHODS

Immunohistochemical analysis was performed on the extracellular matrix of quadrate (primary), squamosal, surangular, and anterior pterygoid secondary cartilages using various antibodies targeting the extracellular matrix of cartilage and bone.

RESULTS

The localization of collagen types I, II, and X, versican, aggrecan, hyaluronan, link protein, and tenascin-C was identified in the quadrate cartilage, with variations within and between the regions. Newly formed squamosal and surangular secondary cartilages showed simultaneous immunoreactivity for all molecules investigated. However, collagen type X immunoreactivity was not observed, and there was weak immunoreactivity for versican and aggrecan in the anterior pterygoid secondary cartilage.

CONCLUSIONS

The immunohistochemical localization of extracellular matrix in the quadrate (primary) cartilage was comparable to that of long bone (primary) cartilage in mammals. The fibrocartilaginous nature and rapid differentiation into hypertrophic chondrocytes, which are known structural features of secondary cartilage, were confirmed in the extracellular matrix of squamosal and surangular secondary cartilages. Furthermore, these tissues appear to undergo developmental processes similar to those in mammals. However, the anterior pterygoid secondary cartilage exhibited unique features that differed from primary and other secondary cartilages, suggesting it is formed through a distinct developmental process.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

FGF and TGFβ signaling link form and function during jaw development and evolution

How does form arise during development and change during evolution? How does form relate to function, and what enables embryonic structures to presage their later use in adults? To address these questions, we leverage the distinct functional morphology of the jaw in duck, chick, and quail. In connection with their specialized mode of feeding, duck develop a secondary cartilage at the tendon insertion of their jaw adductor muscle on the mandible. An equivalent cartilage is absent in chick and quail. We hypothesize that species-specific jaw architecture and mechanical forces promote secondary cartilage in duck through the differential regulation of FGF and TGFβ signaling. First, we perform transplants between chick and duck embryos and demonstrate that the ability of neural crest mesenchyme (NCM) to direct the species-specific insertion of muscle and the formation of secondary cartilage depends upon the amount and spatial distribution of NCM-derived connective tissues. Second, we quantify motility and build finite element models of the jaw complex in duck and quail, which reveals a link between species-specific jaw architecture and the predicted mechanical force environment. Third, we investigate the extent to which mechanical load mediates FGF and TGFβ signaling in the duck jaw adductor insertion, and discover that both pathways are mechano-responsive and required for secondary cartilage formation. Additionally, we find that FGF and TGFβ signaling can also induce secondary cartilage in the absence of mechanical force or in the adductor insertion of quail embryos. Thus, our results provide novel insights on molecular, cellular, and biomechanical mechanisms that couple musculoskeletal form and function during development and evolution.

Major evolutionary transitions and innovations: the tympanic middle ear

One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear. In this review, we look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved. This starts with an overview of the tympanic (air-filled) middle ear, and how the ear drum (tympanic membrane) and the cavity itself form during development in amniotes. This is followed by an investigation of how the ear is connected to the pharynx and the relationship of the ear to the bony bulla in which it sits. Finally, the novel mammalian jaw joint and versatile dentary bone will be discussed with respect to evolution of the mammalian middle ear.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Cibotium barometz polysaccharides stimulate chondrocyte proliferation in vitro by promoting G1/S cell cycle transition

Cibotium barometz polysaccharides (CBPS) are one of the most important bioactive components extracted from the Cibotium barometz plant, which belongs to the Dicksoniaceae family. It has been widely used for the treatment of orthopedic diseases in traditional Chinese medicine. However, the molecular mechanisms behind the therapeutic effects of CBPS remain to be clarified. In the present study, the concentration of CBPS was detected by phenol-vitriol colorimetry. Furthermore, the effects stimulated by CBPS on the viability and G1/S cell cycle transition in primary chondrocytes from Sprague-Dawley rats were investigated. A cell viability assay demonstrated that chondrocyte proliferation may be enhanced by CBPS in a dose- and time-dependent manner. The mechanism underlying the promotion of chondrocyte cell cycle was suggested to involve the stimulation of G1 to S phase transition. To further confirm the results, reverse transcription-quantitative polymerase chain reaction and western blot analyses were used to detect the expression of mRNA and protein levels of cyclin D1, cyclin-dependent kinase 4 and retinoblastoma protein. The results suggested that CBPS may stimulate chondrocyte proliferation via promoting G1/S cell cycle transition. Since osteoarthritis is characterized by deficient proliferation in chondrocytes, the present study indicates that CBPS may potentially serve as a novel method for the treatment of osteoarthritis.

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid

Background

The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear.

Results

By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction.

Conclusions

The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.

An immunohistochemistry study of Sox9, Runx2, and Osterix expression in the mandibular cartilages of newborn mouse

The purpose of this study is to investigate the spacial expression pattern and functional significance of three key transcription factors related to bone and cartilage formation, namely, Sox9, Runx2, and Osterix in cartilages during the late development of mouse mandible. Immunohistochemical examinations of Sox9, Runx2, and Osterix were conducted in the mandibular cartilages of the 15 neonatal C57BL/6N mice. In secondary cartilages, both Sox9 and Runx2 were weakly expressed in the polymorphic cell zone, strongly expressed in the flattened cell zone and throughout the entire hypertrophic cell zone. Similarly, both transcriptional factors were weakly expressed in the uncalcified Meckel's cartilage while strongly expressed in the rostral cartilage. Meanwhile, Osterix was at an extremely low level in cells of the flattened cell zone and the upper hypertrophic cell zone in secondary cartilages. Surprisingly, Osterix was intensely expressed in hypertrophic chondrocytes in the center of the uncalcified Meckel's cartilage while moderately expressed in part of hypertrophic chondrocytes in the rostral process. Consequently, it is suggested that Sox9 is a main and unique positive regulator in the hypertrophic differentiation process of mandibular secondary cartilages, in addition to Runx2. Furthermore, Osterix is likely responsible for phenotypic conversion of Meckel's chondrocytes during its degeneration.

Evolution of the mammalian middle ear and jaw: adaptations and novel structures

Having three ossicles in the middle ear is one of the defining features of mammals. All reptiles and birds have only one middle ear ossicle, the stapes or columella. How these two additional ossicles came to reside and function in the middle ear of mammals has been studied for the last 200 years and represents one of the classic example of how structures can change during evolution to function in new and novel ways. From fossil data, comparative anatomy and developmental biology it is now clear that the two new bones in the mammalian middle ear, the malleus and incus, are homologous to the quadrate and articular, which form the articulation for the upper and lower jaws in non-mammalian jawed vertebrates. The incorporation of the primary jaw joint into the mammalian middle ear was only possible due to the evolution of a new way to articulate the upper and lower jaws, with the formation of the dentary-squamosal joint, or TMJ in humans. The evolution of the three-ossicle ear in mammals is thus intricately connected with the evolution of a novel jaw joint, the two structures evolving together to create the distinctive mammalian skull.

First evidence of dinosaurian secondary cartilage in the post-hatching skull of Hypacrosaurus stebingeri (Dinosauria, Ornithischia)

Bone and calcified cartilage can be fossilized and preserved for hundreds of millions of years. While primary cartilage is fairly well studied in extant and fossilized organisms, nothing is known about secondary cartilage in fossils. In extant birds, secondary cartilage arises after bone formation during embryonic life at articulations, sutures and muscular attachments in order to accommodate mechanical stress. Considering the phylogenetic inclusion of birds within the Dinosauria, we hypothesized a dinosaurian origin for this "avian" tissue. Therefore, histological thin sectioning was used to investigate secondary chondrogenesis in disarticulated craniofacial elements of several post-hatching specimens of the non-avian dinosaur Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). Secondary cartilage was found on three membrane bones directly involved with masticatory function: (1) as nodules on the dorso-caudal face of a surangular; and (2) on the bucco-caudal face of a maxilla; and (3) between teeth as islets in the alveolar processes of a dentary. Secondary chondrogenesis at these sites is consistent with the locations of secondary cartilage in extant birds and with the induction of the cartilage by different mechanical factors - stress generated by the articulation of the quadrate, stress of a ligamentous or muscular insertion, and stress of tooth formation. Thus, our study reveals the first evidence of "avian" secondary cartilage in a non-avian dinosaur. It pushes the origin of this "avian" tissue deep into dinosaurian ancestry, suggesting the creation of the more appropriate term "dinosaurian" secondary cartilage.

An immunohistochemical study on the localization of type II collagen in the developing mouse mandibular condyle

The present chronological investigation assessed the distribution of type II collagen expression in the developing mouse mandibular condyle using immunohistochemical staining with respect to the anatomy of the anlage of the mandibular condyle, the histological characteristics of which were disclosed in our previous investigation. We analyzed fetuses, obtained by cross breeding of ICR strain mice, between 14.0 and 19.0 days post-conception (dpc) and pups on 1, 3, and 5 days post-natal (dpn) using immunohistochemical staining with 2 anti-type II collagen antibodies. The expression of type II collagen was first detected at 15.0 dpc in the lower part of the hypertrophic chondrocyte zone; thereafter, this type II collagen-positive layer was expanded and intensified (P1 layer). At 17.0 dpc, we identified a type II collagen-negative layer (N layer) around the P1 layer and we also identified another newly formed type II collagen-positive layer (P, layer) on the outer surface of the N layer. The most typical and conspicuous 3-layered distribution was observed at 1 dpn; thereafter, there was a reduction in the intensity of expression, and with it, the demarcation between the layers was weakened by 5 dpn. The P1 layer was derived from the central region of the core cell aggregate of the anlage of the mandibular condyle and participated in endochondral bone formation. The N layer was derived from the fringe of the core cell aggregate of the anlage, formed the bone collar at the side of the condyle by intramembranous bone formation, and showed a high level of proliferative activity at the vault. The P2 layer was formed from the outgrowth of the N layer, and could be considered as the secondary cartilage. The intensive expression of type II collagen from 17.0 dpc to 3 dpn was detected in the fibrous sheath covering the condylar head, which is derived from the peripheral cell aggregate of the anlage. Since its expression in the fibrous sheath was not detected in the neighboring section in the absence of hyaluronidase digestion, some changes in the extracellular matrix of the fibrous sheath appear to participate in the generation of the lower joint space. The results of the present investigation indicate that further studies are required to fully characterize the development of the mouse mandibular condyle.

Mesenchymal and mechanical mechanisms of secondary cartilage induction

Secondary cartilage occurs at articulations, sutures, and muscle attachments, and facilitates proper kinetic movement of the skeleton. Secondary cartilage requires mechanical stimulation for its induction and maintenance, and accordingly, its evolutionary presence or absence reflects species-specific variation in functional anatomy. Avians illustrate this point well. In conjunction with their distinct adult mode of feeding via levered straining, duck develop a pronounced secondary cartilage at the insertion (i.e., enthesis) of the mandibular adductor muscles on the lower jaw skeleton. An equivalent cartilage is absent in quail, which peck at their food. We hypothesized that species-specific pattern and a concomitant dissimilarity in the local mechanical environment promote secondary chondrogenesis in the mandibular adductor enthesis of duck versus quail. To test our hypothesis we employed two experimental approaches. First, we transplanted neural crest mesenchyme (NCM) from quail into duck, which produced chimeric "quck" with a jaw complex resembling that of quail, including an absence of enthesis secondary cartilage. Second, we modified the mechanical environment in embryonic duck by paralyzing skeletal muscles, and by blocking the ability of NCM to support mechanotransduction through stretch-activated ion channels. Paralysis inhibited secondary cartilage, as evidenced by changes in histology and expression of genes that affect chondrogenesis, including members of the FGF and BMP pathways. Ion channel inhibition did not alter enthesis secondary cartilage but caused bone to form in place of secondary cartilage at articulations. Thus, our study reveals that enthesis secondary cartilage forms through mechanisms that are distinct from those regulating other secondary cartilage. We conclude that by directing the musculoskeletal patterning and integration of the jaw complex, NCM modulates the mechanical forces and molecular signals necessary to control secondary cartilage formation during development and evolution.

Ihh signaling regulates mandibular symphysis development and growth

Symphyseal secondary cartilage is important for mandibular development, but the molecular mechanisms underlying its formation remain largely unknown. Here we asked whether Indian hedgehog (Ihh) regulates symphyseal cartilage development and growth. By embryonic days 16.5 to 18.5, Sox9-expressing chondrocytes formed within condensed Tgfβ-1/Runx2-expressing mesenchymal cells at the prospective symphyseal joint site, and established a growth-plate-like structure with distinct Ihh, collagen X, and osteopontin expression patterns. In post-natal life, mesenchymal cells expressing the Ihh receptor Patched1 were present anterior to the Ihh-expressing secondary cartilage, proliferated, differentiated into chondrocytes, and contributed to anterior growth of alveolar bone. In Ihh-null mice, however, symphyseal development was defective, mainly because of enhanced chondrocyte maturation and reduced proliferation of chondroprogenitor cells. Proliferation was partially restored in dual Ihh;Gli3 mutants, suggesting that Gli3 is normally a negative regulator of symphyseal development. Thus, Ihh signaling is essential for symphyseal cartilage development and anterior mandibular growth.

A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Resorption and remodelling of skeletal tissues is required for development and growth, mechanical adaptation, repair, and mineral homeostasis of the vertebrate skeleton. Here we review for the first time the current knowledge about resorption and remodelling of the skeleton in teleost fish, the largest and most diverse group of extant vertebrates. Teleost species are increasingly used in aquaculture and as models in biomedical skeletal research. Thus, detailed knowledge is required to establish the differences and similarities between mammalian and teleost skeletal remodelling, and between distantly related species such as zebrafish (Danio rerio) and medaka (Oryzias latipes). The cellular mechanisms of differentiation and activation of osteoclasts and the functions of teleost skeletal remodelling are described. Several characteristics, related to skeletal remodelling, distinguish teleosts from mammals. These characteristics include (a) the absence of osteocytes in most species; (b) the absence of haematopoietic bone marrow tissue; (c) the abundance of small mononucleated osteoclasts performing non-lacunar (smooth) bone resorption, in addition to or instead of multinucleated osteoclasts; and (d) a phosphorus- rather than calcium-driven mineral homeostasis (mainly affecting the postcranial dermal skeleton). Furthermore, (e) skeletal resorption is often absent from particular sites, due to sparse or lacking endochondral ossification. Based on the mode of skeletal remodelling in early ontogeny of all teleosts and in later stages of development of teleosts with acellular bone we suggest a link between acellular bone and the predominance of mononucleated osteoclasts, on the one hand, and cellular bone and multinucleated osteoclasts on the other. The evolutionary origin of skeletal remodelling is discussed and whether mononucleated osteoclasts represent an ancestral type of resorbing cells. Revealing the differentiation and activation of teleost skeletal resorbing cells, in the absence of several factors that trigger mammalian osteoclast differentiation, is a current challenge. Understanding which characters of teleost bone remodelling are derived and which characters are conserved should enhance our understanding of the process in fish and may provide insights into alternative pathways of bone remodelling in mammals.

Differential gene expression in the perichondrium and cartilage of the neonatal mouse temporomandibular joint

Our goal was to discover genes differentially expressed in the perichondrium (PC) of the mandibular condylar cartilage (MCC) that might enhance regenerative medicine or orthopaedic therapies directed at the tissues of the temporomandibular joint. We used targeted gene arrays (osteogenesis, stem cell) to identify genes preferentially expressed in the PC and the cartilaginous (C) portions of the MCC in 2-day-old mice. Genes with higher expression in the PC sample related to growth factor ligand-receptor interactions [FGF-13 (6.4x), FGF-18 (4x), NCAM (2x); PGDF receptors, transforming growth factor (TGF)-beta and IGF-1], the Notch isoforms (especially Notch 3 and 4) and their ligands or structural proteins/proteoglycans [collagen XIV (21x), collagen XVIII (4x), decorin (2.5x)]. Genes with higher expression in the C sample consisted mostly of known cartilage-specific genes [aggrecan (11x), procollagens X (33x), XI (14x), IX (4.5x), Sox 9 (4.4x) and Indian hedgehog (6.7x)]. However, the functional or structural roles of several genes that were expressed at higher levels in the PC sample are unclear [myogenic factor (Myf) 9 (9x), tooth-related genes such as tuftelin (2.5x) and dentin sialophosphoprotein (1.6x), VEGF-B (2x) and its receptors (3-4x) and sclerostin (1.7x)]. FGF, Notch and TGF-beta signalling may be important regulators of MCC proliferation and differentiation; the relatively high expression of genes such as Myf6 and VEGF-B and its receptors suggests a degree of unsuspected plasticity in PC cells.

Cartilage differentiation in cephalopod molluscs

Amongst the various metazoan lineages that possess cartilage, tissues most closely resembling vertebrate hyaline cartilage in histological section are those of cephalopod molluscs. Although elements of the adult skeleton have been described, the development of these cartilages has not. Using serial histology of sequential developmental stages of the European cuttlefish, Sepia officinalis, we investigate these skeletal elements and offer the first description of the formation of any cellular invertebrate cartilage. Our data reveal that cuttlefish cartilage most often differentiates from uncondensed mesenchymal cells near the end of embryonic development, but that the earliest-forming cartilages differentiate from a cellular condensation which goes through a protocartilage stage in a manner typical of vertebrate primary cartilage formation. We further investigate the distribution and degree of differentiation of cartilages at the time of hatching in an additional four cephalopod species. We find that the timing of cartilage development varies between elements within a single species, as well as between species. We identify a tendency towards cartilage differentiation from uncondensed connective tissue in elements that form at the end of embryogenesis or after hatching. These data suggest a form of metaplasia from connective tissue is the ancestral mode of cartilage formation in this lineage.

Dense collagen matrix accelerates osteogenic differentiation and rescues the apoptotic response to MMP inhibition

Bone is distinguished from other tissues by its mechanical properties, in particular stiffness. However, we know little of how osteoblasts react to the stiffness of their microenvironment; in this study we describe their response to a dense (>10 wt.%) collagenous 3D environment. Primary pre-osteoblasts were seeded within a novel form of native collagen, dense collagen, and cultured for up to 14 days in the presence and absence of osteogenic supplements: analysis was via Q-PCR, histology, fluorescent in situ zymography, MMP loss-of-function and tensile testing. Differentiation as measured through the up-regulation of Bsp (247-fold), Alp (14.2-fold), Col1A1 (4.5-fold), Mmp-13 (8.0-fold) and Runx2 (1.2-fold) transcripts was greatly accelerated compared to 2D plastic at 7 and 14 days in the same medium. The scale of this enhancement was confirmed through the use of growth factor stimulation on 2D via the addition of BMP-6 and the Hedgehog agonist purmorphamine. In concert, these molecules were capable of the same level of osteo-induction (measured by Bsp and Alp expression) as the dense collagen alone. Mineralisation was initially localised to remodelled pericellular regions, but by 14 days embedded cells were discernible within regions of apatite (confirmed by MicroRaman). Tensile testing of the matrices showed that this had resulted in a significant increase in Young's modulus at low strain values, consistent with a stiffening of the matrix. To determine the need for matrix remodelling in the mineralisation event the broad spectrum MMP Inhibitor Ilomastat was used. It was found that in its presence mineralisation could still occur (though serum-specific) and the apoptosis associated with MMP inhibition in hydrated collagen gels was abrogated. Analysis of gene expression indicated that this was due to the up-regulation of Mmp-13 in the presence of Ilomastat in dense collagen (400-fold), demonstrating a powerful feedback loop and a potential mechanism for the rescue from apoptosis. Osteoid-like matrix (dense collagen) is therefore a potent stimulant of osteoblast differentiation in vitro and provides an environment that enables survival and differentiation in the presence of MMP inhibition.

An in situ hybridization study of Runx2, Osterix, and Sox9 in the anlagen of mouse mandibular condylar cartilage in the early stages of embryogenesis

Mandibular condylar cartilage is the best-studied mammalian secondary cartilage, differing from primary cartilage in that it originates from alkaline phosphatase-positive progenitor cells. We previously demonstrated that three transcription factors related to bone and cartilage formation, namely Runx2, Osterix and Sox9, are simultaneously expressed in the anlage of mandibular condylar cartilage (condylar anlage) at embryonic day (E)14. In this study, expression of these transcription factors was investigated in the anlagen of mandibular bone (mandibular anlagen) from E11.0 to 14.0. Runx2 mRNA was first expressed in the mandibular anlage at E11.5. Osterix mRNA was first expressed at E12.0, and showed a different expression pattern from that of Runx2 from E12.5 to E14.0, confirming that Osterix acts downstream of Runx2. Sox9 mRNA was expressed in Meckel's cartilage and its anlagen throughout the experimental period, but not clearly in the mandibular anlagen until E13.0. At E13.5, the condylar anlage was morphologically identified at the posterior end of the mandibular anlage, and enhanced Sox9 mRNA expression was detected here. At this stage, Runx2 and Osterix mRNA were simultaneously detected in the condylar anlage. These results indicate that the Sox9 mRNA-expressing condylar anlage is derived from Runx2/Osterix mRNA-expressing mandibular anlage, and that upregulation of Sox9 in this region acts as a trigger for subsequent condylar cartilage formation.

Tissue engineering and developmental biology: going biomimetic

This article contains the collective views expressed at the first session of the workshop "Tissue Engineering--The Next Generation," which was devoted to the interactions between developmental biology and tissue engineering. Donald Ingber discussed the chasms between developmental biology and tissue engineering from the perspective of a cell biologist who has had interest in tissue engineering since its early days. Van C. Mow shared a historical perspective on the development of tissue engineering as one of the first engineers involved in the field. David Butler offered an assessment of functional tissue engineering, a new area he helped establish and promote. Laura Niklason discussed how to be more effective in developing cellular therapies for large numbers of patients. Johnny Huard described his approach to tissue engineering, based on the use of muscle-derived cells. Jeremy Mao focused on cell homing and cell density in the context of native development and relevance to tissue engineering. Ioannis Yannas proposed a set of "rules" in organ regeneration. Collectively, the faculty expressed a remarkable level of enthusiasm for bridging the gaps between developmental biology and tissue engineering and offered new ideas on how to facilitate the interaction between the two fields.

Bone morphogenetic protein rescues the lack of secondary cartilage in Runx2-deficient mice

Secondary cartilages including mandibular condylar cartilage have unique characteristics. They originate from alkaline phosphatase (ALP)-positive progenitor cells of the periosteum, and exhibit characteristic modes of differentiation. They also have a unique extracellular matrix, and coexpress type I, II and X collagens. We have previously shown that there is a total absence of secondary cartilages in Runx2-deficient (Runx2-/-) mice. To clarify whether Runx2 is essential for chondrocytic differentiation of secondary cartilages, we performed an organ culture system using mandibular explants derived from Runx2-/- mice at embryonic day 18.0. Since mRNA for bone morphogenetic protein 2 (BMP2) was strongly expressed in osteoblasts of condylar anlagen in wild-type mice, and was down-regulated in those of Runx2-/- mice, we chose to investigate BMP2 effects on secondary cartilage formation. Condensed mesenchymal cells of mandibular condylar anlagen in precultured explants were ALP-positive and expressed type I collagen and Sox9. After culture with recombinant human (rh) BMP2, chondrocytic cells showing ALP activity and expressing Sox5, Sox9, and type I and II collagens, appeared from mesenchymal condensation. This expression profile was comparable with the reported pattern of chondrocytes in mouse secondary cartilages. However, chondrocyte hypertrophy was not observed in the explants. These findings indicate that BMP2 partially rescued chondrocyte differentiation but not chondrocyte hypertrophy in secondary cartilage formation in Runx2-/- mice. Runx2 is required for chondrocyte hypertrophy in secondary cartilage formation, and it is likely that BMP2, which is abundantly secreted by osteoblasts in condylar anlagen, contributes to the early process of secondary cartilage formation.

An in situ hybridization study of Runx2, Osterix, and Sox9 at the onset of condylar cartilage formation in fetal mouse mandible

Mandibular condylar cartilage is the principal secondary cartilage, differing from primary cartilage in its rapid differentiation from progenitor cells (preosteoblasts/skeletoblasts) to hypertrophic chondrocytes. The expression of three transcription factors related to bone and cartilage formation, namely Runx2, Osterix and Sox9, was investigated at the onset of mouse mandibular condylar cartilage formation by in situ hybridization. Messenger RNAs for these three molecules were expressed in the condylar anlage, consisting of preosteoblasts/skeletoblasts, at embryonic day (E)14. Hypertrophic chondrocytes appeared at E15 as soon as cartilage tissue appeared. Runx2 mRNA was expressed in the embryonic zone at the posterior position of the newly formed cartilage, in the bone collar and in the newly formed cartilage, but expression intensity in the newly formed cartilage was slightly weaker. Osterix mRNA was also expressed in the embryonic zone and in the bone collar, but was at markedly lower levels in the newly formed cartilage. Sox9 mRNA was continuously expressed from the embryonic zone to the newly formed cartilage. At this stage, Sox5 mRNA was expressed only in the newly formed cartilage. These results suggest that reduced expression of Osterix in combination with Sox9-Sox5 expression is important for the onset of condylar (secondary) cartilage formation.

Muscling in on stem cells

Skeletal muscle is one of the few adult tissues that possesses the capacity for regeneration (restoration of lost functional tissue) as opposed to repair. This capacity is due to the presence of 'muscle stem cells' known as satellite cells. Detailed investigation of these cells over the past 50 years has revealed that both these and other cells within the skeletal muscle complex are capable of regenerating both muscle and other cell types as well. Here, we review this information, and suggest that skeletal muscle is an exciting reservoir of cells for regenerating skeletal muscle itself, as well as other cell types.

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.

Characterization of two new zebrafish members of the hedgehog family: atypical expression of a zebrafish indian hedgehog gene in skeletal elements of both endochondral and dermal origins

We have characterized two new members of the Hedgehog (Hh) family in zebrafish, ihha and dhh, encoding for orthologues of the tetrapod Indian Hedgehog (Ihh) and Desert Hedgehog (Dhh) genes, respectively. Comparison of ihha and Type X collagen (col10a1) expression during skeletal development show that ihha transcripts are located in hypertrophic chondrocytes of cartilaginous elements of the craniofacial and fin endoskeleton. Surprisingly, col10a1 expression was also detected in cells forming intramembranous bones of the head and in flat cells surrounding cartilaginous structures. The expression of col10a1 in both endochondral and intramembranous bones reflects an atypical composition of the extracellular matrix of the zebrafish craniofacial skeleton. In addition, during fin ray regeneration, both ihha and col10a1 are detected in scleroblasts, osteoblast-like cells secreting the matrix of the dermal bone fin ray. The presence of cartilage markers suggests that the dermal fin ray possesses an intermediate phenotype between cartilage and bone.

Canonical Wnt/beta-catenin signaling prevents osteoblasts from differentiating into chondrocytes

Osteoblasts and chondrocytes are involved in building up the vertebrate skeleton and are thought to differentiate from a common mesenchymal precursor, the osteo-chondroprogenitor. Although numerous transcription factors involved in chondrocyte and osteoblast differentiation have been identified, little is known about the signals controlling lineage decisions of the two cell types. Here, we show by conditionally deleting beta-catenin in limb and head mesenchyme that beta-catenin is required for osteoblast lineage differentiation. Osteoblast precursors lacking beta-catenin are blocked in differentiation and develop into chondrocytes instead. In vitro experiments demonstrate that this is a cell-autonomous function of beta-catenin in an osteoblast precursor. Furthermore, detailed in vivo and in vitro loss- and gain-of-function analyses reveal that beta-catenin activity is necessary and sufficient to repress the differentiation of mesenchymal cells into Runx2- and Sox9-positive skeletal precursors. Thus, canonical Wnt/beta-catenin signaling is essential for skeletal lineage differentiation, preventing transdifferentiation of osteoblastic cells into chondrocytes.

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.

Consideration of the neural crest and its skeletal derivatives in the context of novelty/innovation

I examine the neural crest and skeletal tissues derived from neural crest cells in the context of novelty/innovation by asking whether the neural crest is a novel tissue and whether the evolutionary origin of the neural crest required innovative developmental processes. As a vertebrate autapomorphy, the neural crest is a novel structure. I equate novelty with innovation and take a hierarchical approach. Some other workers separate the two, using novelty for new structures not found in an ancestor and not homologous with a feature in an ancestor, and innovation for the new processes required to generate the novel structure. While development clearly evolves, I do not separate those processes that result in the production of novel features from those that lead to change in existing structures, whether that change is a transition or transformation from one homologous feature to another (fins-->tetrapod limbs or locomotory appendages-->crustacean maxilliped feeding appendages). The existence of novelties causes us to consider the concept of latent homology. Neural crest cells form cartilage, dentine and bone. Cartilage is found in invertebrates and so is not a vertebrate innovation. No invertebrate cartilage mineralizes in vivo, although some can be induced to mineralize in vitro. Mineralization of cartilage in vivo is a vertebrate innovation. Dentine is a novel tissue that only forms from neural crest cells. Bone is a vertebrate innovation but not one exclusive to the neural crest. The developmental processes responsible for the neural crest and for these skeletal tissues did not arise de novo with the vertebrates. Novelty/innovation results from tinkering with existing processes, from the flexibility that arises from modifications of existing gene networks, and from the selective advantage provided by gene duplications or modifications.

Hierarchy revealed in the specification of three skeletal fates by Sox9 and Runx2

Across vertebrates, there are three principal skeletal tissues: bone, persistent cartilage, and replacement cartilage. Although each tissue has a different evolutionary history and functional morphology, they also share many features. For example, they function as structural supports, they are comprised of cells embedded in collagen-rich extracellular matrix, and they derive from a common embryonic stem cell, the osteochondroprogenitor. Occasionally, homologous skeletal elements can change tissue type through phylogeny. Together, these observations raise the possibility that skeletal tissue identity is determined by a shared set of genes. Here, we show that misexpression of either Sox9 or Runx2 can substitute bone with replacement cartilage or can convert persistent cartilage into replacement cartilage and vice versa. Our data also suggest that these transcription factors function in a molecular hierarchy in which chondrogenic factors dominate. We propose a binary molecular code that determines whether skeletal tissues form as bone, persistent cartilage, or replacement cartilage. Finally, these data provide insights into the roles that master regulatory genes play during evolutionary change of the vertebrate skeleton.

Heparan sulfate proteoglycans: Coordinators of multiple signaling pathways during chondrogenesis

Heparan sulfate proteoglycans are abundantly expressed in the pericellular matrix of both developing and mature cartilage. Increasing evidence indicates that the action of numerous chondroregulatory molecules depends on these proteoglycans. This review summarizes the current understanding of the interactions of heparan sulfate chains of cartilage proteoglycans with both soluble and nonsoluble ligands during the process of chondrogenesis. In addition, the consequences of mutating genes encoding heparan sulfate biosynthetic enzymes or heparan sulfate proteoglycan core proteins on cartilage development are discussed.

In search of evolutionary developmental mechanisms: The 30-year gap between 1944 and 1974

The approach I have elected in this retrospective of how I became a student of evo-devo is both biographical and historical, a case study along the lines of Waddington's The Evolution of an Evolutionist ('75), although in my case it is the Evolution of an Evo-devoist. What were the major events that brought me to developmental biology and from there to evo-devo? They were, of course, specific to my generation, to the state of knowledge at the time, and to my own particular circumstances. Although exposed to evolution and embryology as an undergraduate in the 1960s, my PhD and post-PhD research programme lay within developmental biology until the early 1970s. An important formative influence on my studies as an undergraduate was the work of Conrad Hal Waddington (1905-1975), whose writings made me aware of genetic assimilation and gave me an epigenetic approach to my developmental studies. The switch to evo-devo (and my discovery of the existence of the neural crest), I owe to an ASZ (now SICB) symposium held in 1973.