Chondroid tissue in the early facial morphogenesis of the chick embryo

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

Human genetic variation within neural crest enhancers: molecular and phenotypic implications

Developmental gene expression programmes are coordinated by the specialized distal cis-regulatory elements called enhancers, which integrate lineage- and signalling-dependent inputs to guide morphogenesis. In previous work, we characterized the genome-wide repertoire of active enhancers in human neural crest cells (hNCC), an embryonic cell population with critical roles in craniofacial development. We showed that in hNCC, co-occupancy of a master regulator TFAP2A with nuclear receptors NR2F1 and NR2F2 correlates with the presence of permissive enhancer chromatin states. Here, we take advantage of pre-existing human genetic variation to further explore potential cooperation between TFAP2A and NR2F1/F2. We demonstrate that isolated single nucleotide polymorphisms affecting NR2F1/F2-binding sites within hNCC enhancers can alter TFAP2A occupancy and overall chromatin features at the same enhancer allele. We propose that a similar strategy can be used to elucidate other cooperative relationships between transcription factors involved in developmental transitions. Using the neural crest and its major contribution to human craniofacial phenotypes as a paradigm, we discuss how genetic variation might modulate the molecular properties and activity of enhancers, and ultimately impact human phenotypic diversity.

Imaging analysis of osteogenic transformation of Meckel's chondrocytes from green fluorescent protein-transgenic mice during intrasplenic transplantation

Our previous studies demonstrated that Meckel's chondrocytes, which are derived from ectomesenchyme, have the potential to transform into osteogenic phenotypes. The present study aimed to clarify the role of cell origin in the phenotypic transformation of chondrocytes. Cell pellets from ectomesenchyme-derived Meckel's cartilage and mesoderm-derived costal cartilage from green fluorescent protein (GFP)-transgenic mice were transplanted into the spleen for up to 4 weeks. Chondrocyte pellets from both cartilages adapted well to the splenic tissues and formed an alizarin red-positive calcified matrix, with increasing duration of transplantation. Following the production of cartilage-specific type II and type X collagens, newly-formed type I collagen appeared in the chondrocyte pellets from Meckel's cartilage during the late stage of transplantation. Although the bone-marker proteins: osteocalcin, osteopontin, osteonectin and bone morphogenetic protein-2, were detected in pellets from both Meckel's and costal cartilage, only type I collagen in Meckel's cartilage was a significant marker protein for detecting transformation. These bone-type protein-producing cells represented osteogenic cells transformed from GFP-expressing cells, rather than from recipient cells. These results indicate that neural crest-derived Meckel's cartilage displays a higher potential for phenotypic switching than mesoderm-derived costal chondrocytes under in vivo conditions.

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.

Two developmental modules establish 3D beak-shape variation in Darwin's finches

Bird beaks display tremendous variation in shape and size, which is closely associated with the exploitation of multiple ecological niches and likely played a key role in the diversification of thousands of avian species. Previous studies have demonstrated some of the molecular mechanisms that regulate morphogenesis of the prenasal cartilage, which forms the initial beak skeleton. However, much of the beak diversity in birds depends on variation in the premaxillary bone. It forms later in development and becomes the most prominent functional and structural component of the adult upper beak/jaw, yet its regulation is unknown. Here, we studied a group of Darwin's finch species with different beak shapes. We found that TGFβIIr, β-catenin, and Dickkopf-3, the top candidate genes from a cDNA microarray screen, are differentially expressed in the developing premaxillary bone of embryos of species with different beak shapes. Furthermore, our functional experiments demonstrate that these molecules form a regulatory network governing the morphology of the premaxillary bone, which differs from the network controlling the prenasal cartilage, but has the same species-specific domains of expression. These results offer potential mechanisms that may explain how the tightly coupled depth and width dimensions can evolve independently. The two-module program of development involving independent regulating molecules offers unique insights into how different developmental pathways may be modified and combined to induce multidimensional shifts in beak morphology. Similar modularity in development may characterize complex traits in other organisms to a greater extent than is currently appreciated.

Expression of osteogenic proteins during the intrasplenic transplantation of Meckel's chondrocytes: A histochemical and immunohistochemical study

Meckel's chondrocytes, derived from the ectomesenchyme, have the potential to transform into other phenotypes. In this study, we transplanted cell pellets of Meckel's chondrocytes into isogenic mouse spleens and analyzed their phenotypic transformation into osteogenic cells using histological and immunohistochemical methods. With the increasing duration of transplantation, chondrocytes were incorporated into splenic tissues and formed a von Kossa-positive calcified matrix containing calcium and phosphoric acid, similar to that of intact bone. Type I, II, and X collagens, and the bone-marker proteins osteocalcin, osteopontin, osteonectin, and bone morphogenetic protein-2 (BMP-2) were immunolocalized in the matrix formed by the transplanted chondrocytes. Osteopontin and osteonectin were detected in the calcified matrix at earlier stages than osteocalcin and BMP-2. Type II collagen was expressed during the first week of transplantation, and type X collagen-positive cells appeared scattered during the initial stage of calcification, these collagens being later replaced by type I collagen formed by osteocyte-like cells. Electron microscopic observations revealed that chondrocytes surrounded by the calcified matrix transformed into spindle-shaped osteocytic cells accompanying the formation of bone-type thick-banded collagen fibrils. These results suggest that phenotypic switching of Meckel's chondrocytes can occur under in vivo conditions at a cellular morphological level.

Skeletal histology of the dermal armor of Placodontia: the occurrence of 'postcranial fibro-cartilaginous bone' and its developmental implications

Placodontia (Reptilia: Sauropterygia) is a group of enigmatic armored marine reptiles restricted to the Triassic time period. Only a single row of osteoderms dorsal to the spine is present in the basal placodontoid Placodus gigas, whereas derived cyamodontoids superficially resemble turtles in enclosing their body in an armor shell. Despite the extensive occurrence of the dermal armor in the derived cyamodontoid group, little research has focused on its bone histology and development. Here, I present an overview of the bone microstructures that reveals the unique presence of cartilaginous tissue in the postcranial armor plates. Placodont armor plates stand in contrast to osteoderms of other tetrapods that develop intramembraneously or through metaplastic ossification without cartilaginous preformation. The different developmental pathways leading to this 'postcranial fibro-cartilaginous bone' tissue found in placodont plates compared to the dermal bone tissues of most other tetrapod osteoderms indicate the non-homology of these structures. A resulting morphogenetic model of histogenesis is given to exemplify how the derived armor morphologies (i.e. spiked, flat polygonal and hexagonal, and rhomboidal shapes) together with the peculiar bone histologies could have developed through differential growth. In accordance with the pachyostotic limb bones of placodonts, the presence of the compact 'postcranial fibro-cartilaginous bone' is interpreted as an osteosclerotic trend in the armor plates which aids in buoyancy control and affects maneuverability and swimming speed.

Origin-associated features of chondrocytes in mouse Meckel's cartilage and costal cartilage: an in vitro study

Using a cell culture method, we histochemically and immunohistochemically investigated whether chondrocytes deriving from different origins, such as Meckel's or costal cartilages, express similar phenotypic characteristics. Chondrocytes isolated enzymatically from Meckel's and costal cartilages of 17-day embryonic mice both actively proliferated and formed cartilage nodules consisting of toluidine blue-positive proteoglycans and type II collagen. Both deposited calcified cartilaginous matrix as revealed by alkaline phosphatase (ALPase) activity and alizarin red staining throughout 3 weeks in culture. Immunostaining for osteopontin (OP), osteocalcin (OC), and osteonectin (ON) revealed that chondrocytes from both cartilages were positive for their proteins, but type I collagen was detected only in cells transforming from Meckel's chondrocytes late in the culture. Electron microscopy demonstrated that although costal and Meckel's chondrocytes had typical chondrocytic features during 2 weeks in culture, Meckel's chondrocytes transformed into osteocytic cells that produced thick, banded type I collagen fibrils. In contrast, costal chondrocytes maintained typical hypertrophic morphology throughout the final stage of culture. The present study suggests that Meckel's chondrocytes derived from neural crest-ectomesenchyme retain osteogenic potential, and differ from costal chondrocytes originating from mesoderm.

Phenotypic switching of in vitro mandibular condylar cartilage during matrix mineralization

In order to analyze the phenotypic conversion of chondrocytes, mandibular condyles of mice and rabbits were cultured under cell and organ culture systems, and then examined by a combination of morphological and biochemical procedures. In organ culture, mandibular condylar cartilage (MCC) obtained from newborn mice began to mineralize from the central zone and then progressively widened towards the peripheral zone. Electron microscopic observations showed that with the increasing duration of the organ culture, chondrocytes at the central zone converted into spindle-shaped osteoblastic cells accompanying the formation of the bone type of thick-banded collagen fibrils. To obtain a better understanding of the chondrocytic conversion, immunolocalizations for type I and type X collagens and osteocalcin (OC) were examined in mouse MCC cells in cell culture. Type X collagen and OC were expressed almost simultaneously at the late stage of culture, and type I collagen was detected along the calcified nodules after the production of these proteins. Northern blot analysis in cell cultures of rabbit MCC indicated that type II collagen and alkaline phosphatase (ALPase) messenger ribonucleic acids (mRNAs) were highly expressed at day 7, but subsequently decreased. In contrast, mRNA for type I collagen was expressed at a low level on day 7 and peaked on day 12. The present results suggest that, morphologically and biochemically, cellular modification in MCC cells under culture conditions occurs at a cellular morphological level and also at marker-gene-expression level.

Transient chondrogenic phase in the intramembranous pathway during normal skeletal development

Calvarial and facial bones form by intramembranous ossification, in which bone cells arise directly from mesenchyme without an intermediate cartilage anlage. However, a number of studies have reported the emergence of chondrocytes from in vitro calvarial cell or organ cultures and the expression of type II collagen, a cartilage-characteristic marker, in developing calvarial bones. Based on these findings we hypothesized that a covert chondrogenic phase may be an integral part of the normal intramembranous pathway. To test this hypothesis, we analyzed the temporal and spatial expression patterns of cartilage characteristic genes in normal membranous bones from chick embryos at various developmental stages (days 12, 15 and 19). Northern and RNAse protection analyses revealed that embryonic frontal bones expressed not only the type I collagen gene but also a subset of cartilage characteristic genes, types IIA and XI collagen and aggrecan, thus resembling a phenotype of prechondrogenic-condensing mesenchyme. The expression of cartilage-characteristic genes decreased with the progression of bone maturation. Immunohistochemical analyses of developing embryonic chick heads indicated that type II collagen and aggrecan were produced by alkaline phosphatase activity positive cells engaged in early stages of osteogenic differentiation, such as cells in preosteogenic-condensing mesenchyme, the cambium layer of periosteum, the advancing osteogenic front, and osteoid bone. Type IIB and X collagen messenger RNAs (mRNA), markers for mature chondrocytes, were also detected at low levels in calvarial bone but not until late embryonic stages (day 19), indicating that some calvarial cells may undergo overt chondrogenesis. On the basis of our findings, we propose that the normal intramembranous pathway in chicks includes a previously unrecognized transient chondrogenic phase similar to prechondrogenic mesenchyme, and that the cells in this phase retain chondrogenic potential that can be expressed in specific in vitro and in vivo microenvironments.

Embryonic origin and fate of chondroid tissue and secondary cartilages in the avian skull

BACKGROUND

Chondroid tissue is an intermediate calcified tissue, mainly involved in desmocranial morphogenesis. Often associated with secondary cartilages, it remained of unprecise embryonic origin.

METHODS

The latter was studied by performing isotopic isochronic grafts of quail encephalon onto 30 chick embryos. The so-obtained chimeras were sacrificed at the 9th, 12th, and 14th day of incubation. The contribution of graft- and host-derived cells to the histogenesis of chondroid tissue, bone, and secondary cartilages was analyzed on both microradiographs of thick undecalcified sections and on classical histological sections after several DNA or ECM specific staining procedures.

RESULTS

Chondroid tissue is deposited in the primitive anlage of all membranous bones of the avian skull. Also present on their sutural edges, it uniformly arises from the neural crest. In the face, bone and secondary cartilages share this mesectodermal origin. However, secondary cartilages located along the basal chondrocranium and bone formed on the chondroid primordium of the cranial vault, originate from the cephalic mesoderm.

CONCLUSIONS

These facts provide evidence that chondroid tissue arises from a specific differentiation of neural crest derived cells and that this original skeletogenic program differs from that of secondary chondrogenesis. Moreover, they obviously indicate that in membraneous bone ontogenesis, chondroid tissue replaces functions devoted to mesodermal primary cartilages of the cranial base, and so corroborates at the tissue level, the dual embryonic and phyletic origin of the skull.