Receptor-regulated and inhibitory Smads are critical in regulating transforming growth factor beta-mediated Meckel's cartilage development

Genetic architecture of trait variance in craniofacial morphology

The genetic architecture of trait variance has long been of interest in genetics and evolution. One of the earliest attempts to understand this architecture was presented in Lerner's Genetic Homeostasis (1954). Lerner proposed that heterozygotes should be better able to tolerate environmental perturbations because of functional differences between the alleles at a given locus, with each allele optimal for slightly different environments. This greater robustness to environmental variance, he argued, would result in smaller trait variance for heterozygotes. The evidence for Lerner's hypothesis has been inconclusive. To address this question using modern genomic methods, we mapped loci associated with differences in trait variance (vQTL) on 1,101 individuals from the F34 of an advanced intercross between LG/J and SM/J mice. We also mapped epistatic interactions for these vQTL in order to understand the influence of epistasis for the architecture of trait variance. We did not find evidence supporting Lerner's hypothesis, that heterozygotes tend to have smaller trait variances than homozygotes. We further show that the effects of most mapped loci on trait variance are produced by epistasis affecting trait means and that those epistatic effects account for about a half of the differences in genotypic-specific trait variances. Finally, we propose a model where the different interactions between the additive and dominance effects of the vQTL and their epistatic partners can explain Lerner's original observations but can also be extended to include other conditions where heterozygotes are not the least variable genotype.

The relationship between Meckel's cartilage resorption and incisor tooth germ in mice

Our understanding of the initiation and cellular mechanisms underlying endochondral resorption of Meckel's cartilage (MC) remains limited. Several studies have shown that the resorption site of MC and the mandibular incisor tooth germ are located close to each other. However, whether incisor tooth germ development is involved in MC resorption remains unclear. In this study, we aimed to elucidate the spatio-temporal interaction between the initiation site of MC resorption and the development of incisor tooth germs in an embryonic mouse model. To this effect, we developed a histology-based three-dimensional (3D) reconstruction technique using paraffin-embedded serial sections of various tissues in the jaw. The serial sections were cut in the frontal section and the tissue constituents (e.g., MC, incisor, and mineralized mandible) were studied using conventional and enzyme-based histochemistry. The outline of each component was marked on the frontal sectional images and 3D structures were constructed. To assess the vascular architecture at the site of MC resorption, immunohistochemical staining using anti-laminin, anti-factor VIII, and anti-VEGF antibodies was performed. MC resorption was first observed on the lateral incisor-facing side of the cartilage rods at sites anterior to the mental foramen on E16.0. The 3D analysis suggested that: (a) the posterior region of the clastic cartilage resorption corresponds to the cervical loop of the incisor; (b) the cervical portion of the tooth germ inflates probably due to temporal cellular congestion prior to differentiation into matrix-producing cells; (c) the incisor tooth germ tissue is present in close proximity to MC even in mouse with continuously growing tooth and determines the disappearance of MC as the tooth development.

A proteomic approach reveals biomineralization and immune response for mantle to pearl sac in the freshwater pearl mussel (Hyriopsis cumingii)

In the process of production of freshwater pearl, implanted mantle pieces undergo a series of complex physiological and biochemical processes to form pearl sac, which produce pearl. This is a very important site of occurrence due to immune-induced biomineralization, while its molecular regulatory mechanism is still unclear. Here, we use proteomics to identify differentially expressed proteins (DEPs) of the mantle and pearl sac and examine the biomineralization and immune response of the pearl sac formation process in Hyriopsis cumingii. Using iTRAQ technology and bioinformatics analysis, we obtained DEP profiles between the mantle and pearl sac. A total of 1871 proteins were identified. Of these, 74 DEPs were found between the pearl sac and outer mantle, 112 DEPs between the pearl sac and inner mantle, and 124 DEPs between the outer and inner mantles. Bioinformatics analysis revealed that the screened biomineralization-related DEPs were mainly enriched in signaling pathways associated with calcium signaling, regulation of the actin cytoskeleton and protein processing in the endoplasmic reticulum, while the immune-related DEPs were mainly enriched in the Notch, Hippo, nuclear factor kappa-B (NF-κB), and transforming growth factor-β (TGF-β) signaling pathways. In addition, the expression of six biomineralization-related and four immune-related proteins were verified at the transcriptional level using quantitative real-time PCR. Our findings contribute to furthering the understanding of the mechanisms of pearl formation and immune response, and have long-term implications for future studies on the production of high-quality freshwater pearls and development of the freshwater pearl industry.

Calcification and resorption of mouse Meckel's cartilage analyzed by von Kossa and tartrate-resistant acid phosphatase histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry

Meckel's cartilage is essential for the normal development of the mandible. The fate of the intermediate portion of Meckel's cartilage is unique as most of it disappears soon after birth except for the part that forms the sphenomandibular ligament. The mechanism of the disappearance of Meckel's cartilage is unknown; therefore, this study was designed to investigate the process of Meckel's cartilage degradation, focusing on cartilage matrix calcification and the appearance of chondroclasts. Developing mouse mandibles at embryonic days 15, 16, 17, and 18, and postnatal day 2 were processed for whole-mount staining with alcian blue and alizarin red. The mandibles on embryonic days 15, 16, 17, and 18 were fixed and embedded in paraffin. Adjacent sections were processed for von Kossa and tartrate-resistant acid phosphatase (TRAP) histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS). Calcification and the element concentrations of calcium, phosphorus, and carbon were examined with von Kossa histochemistry and SEM/EDS. The involvement of chondroclasts was investigated using TRAP histochemistry. The results demonstrated that the intermediate portion of Meckel's cartilage is resorbed by chondroclasts after chondrocyte hypertrophy and cartilage matrix calcification and that the mineral concentration of calcified Meckel's cartilage is comparable to that of the surrounding bone. This study contributes to the understanding of the mechanism of Meckel's cartilage resorption and provides useful insights into the development of the mandible.

Sox9-Increased miR-322-5p Facilitates BMP2-Induced Chondrogenic Differentiation by Targeting Smad7 in Mesenchymal Stem Cells

Bone morphogenetic protein 2 (BMP2) induces effective chondrogenesis of mesenchymal stem cells (MSCs) by promoting Sox9 expression. However, BMP2 also induces chondrocyte hypertrophy and endochondral ossification by upregulating Smad7 expression, which leads to the disruption of chondrogenesis. In addition, Smad7 can be inhibited by Sox9. Therefore, the underlying mechanism is not clear. Currently, an increasing number of studies have shown that microRNAs play a pivotal role in chondrogenic and pathophysiological processes of cartilage. The purpose of this study was to determine which microRNA is increased by Sox9 and targets Smad7, thus assisting BMP2 in maintaining stable chondrogenesis. We found that miR-322-5p meets the requirement through next-generation sequencing (NGS) and bioinformatic analysis. The targeting relationship between miR-322-5p and Smad7 was confirmed by dual-luciferase reporter assays, qPCR, and western blotting (WB). The in vitro study indicated that overexpression of miR-322-5p significantly inhibited Smad7 expression, thus causing increased chondrogenic differentiation and decreased hypertrophic differentiation, while silencing of miR-322-5p led to the opposite results. Flow cytometry (FCM) analysis indicated that overexpression of miR-322-5p significantly decreased the rate of early apoptosis in BMP2-stimulated MSCs, while silencing of miR-322-5p increased the rate. A mouse limb explant assay revealed that the expression of miR-322-5p was negatively correlated with the length of the BMP2-stimulated hypertrophic zone of the growth plate. An in vivo study also confirmed that miR-322-5p assisted BMP2 in chondrogenic differentiation. Taken together, our results suggested that Sox9-increased miR-322-5p expression can promote BMP2-induced chondrogenesis by targeting Smad7, which can be exploited for effective tissue engineering of cartilage.

The Mandibular and Hyoid Arches-From Molecular Patterning to Shaping Bone and Cartilage

The mandibular and hyoid arches collectively make up the facial skeleton, also known as the viscerocranium. Although all three germ layers come together to assemble the pharyngeal arches, the majority of tissue within viscerocranial skeletal components differentiates from the neural crest. Since nearly one third of all birth defects in humans affect the craniofacial region, it is important to understand how signalling pathways and transcription factors govern the embryogenesis and skeletogenesis of the viscerocranium. This review focuses on mouse and zebrafish models of craniofacial development. We highlight gene regulatory networks directing the patterning and osteochondrogenesis of the mandibular and hyoid arches that are actually conserved among all gnathostomes. The first part of this review describes the anatomy and development of mandibular and hyoid arches in both species. The second part analyses cell signalling and transcription factors that ensure the specificity of individual structures along the anatomical axes. The third part discusses the genes and molecules that control the formation of bone and cartilage within mandibular and hyoid arches and how dysregulation of molecular signalling influences the development of skeletal components of the viscerocranium. In conclusion, we notice that mandibular malformations in humans and mice often co-occur with hyoid malformations and pinpoint the similar molecular machinery controlling the development of mandibular and hyoid arches.

BMP4/ALK3 deficiency leads to Meckel's cartilage truncation mimicking the mandible Tessier 30 cleft

OBJECTIVE

In chondrogenesis, BMP signaling was inferred to exhibit regional specificity during Meckel's cartilage morphogenesis. This study aimed to explore the differences in BMP signaling activity between different parts of Meckel's cartilage and the impacts of BMP4 or ALK3 deficiency on the development of Meckel's cartilage during embryogenesis.

MATERIALS AND METHODS

The BRE-gal reporter mouse line was utilized to gain an overall picture of canonical BMP signaling activity, as assessed by X-gal staining. Mouse models lacking either Bmp4 or Alk3 in neural crest cells (Wnt1-Cre;Bmp4^fl/fl and Wnt1-Cre;Alk3^fl/fl ) were generated to explore the morphogenesis of Meckel's cartilage and the mandibular symphysis, as assessed by skeletal staining, histology, and immunostaining.

RESULTS

Different parts of Meckel's cartilage exhibited activation of different combinations of BMP signaling pathways. In Wnt1-Cre;Bmp4^fl/fl mutants, Sox9⁺ condensation of the chondrogenic rostral process failed to form, and the V-shaped Runx2⁺ tissue was split in the median mandibular symphysis. The Wnt1-Cre;Bmp4^fl/fl and Wnt1-Cre;Alk3^fl/fl mouse models both exhibited truncated Meckel's cartilage, aberrant mandibular intramembranous bone, and tongue muscle abnormalities.

CONCLUSIONS

The central hard-tissue loss of both mutant mouse models led to a mandibular symphysis cleft, mimicking the typical sign of the median mandible Tessier 30 cleft in humans.

Diverse Fate of an Enigmatic Structure: 200 Years of Meckel's Cartilage

Meckel's cartilage was first described by the German anatomist Johann Friedrich Meckel the Younger in 1820 from his analysis of human embryos. Two hundred years after its discovery this paper follows the development and largely transient nature of the mammalian Meckel's cartilage, and its role in jaw development. Meckel's cartilage acts as a jaw support during early development, and a template for the later forming jaw bones. In mammals, its anterior domain links the two arms of the dentary together at the symphysis while the posterior domain ossifies to form two of the three ear ossicles of the middle ear. In between, Meckel's cartilage transforms to a ligament or disappears, subsumed by the growing dentary bone. Several human syndromes have been linked, directly or indirectly, to abnormal Meckel's cartilage formation. Herein, the evolution, development and fate of the cartilage and its impact on jaw development is mapped. The review focuses on developmental and cellular processes that shed light on the mechanisms behind the different fates of this cartilage, examining the control of Meckel's cartilage patterning, initiation and maturation. Importantly, human disorders and mouse models with disrupted Meckel's cartilage development are highlighted, in order to understand how changes in this cartilage impact on later development of the dentary and the craniofacial complex as a whole. Finally, the relative roles of tissue interactions, apoptosis, autophagy, macrophages and clast cells in the removal process are discussed. Meckel's cartilage is a unique and enigmatic structure, the development and function of which is starting to be understood but many interesting questions still remain.

Network architecture and regulatory logic in neural crest development

The neural crest is an ectodermal cell population that gives rise to over 30 cell types during vertebrate embryogenesis. These stem cells are formed at the border of the developing central nervous system and undergo extensive migration before differentiating into components of multiple tissues and organs. Neural crest formation and differentiation is a multistep process, as these cells transition through sequential regulatory states before adopting their adult phenotype. Such changes are governed by a complex gene regulatory network (GRN) that integrates environmental and cell-intrinsic inputs to regulate cell identity. Studies of neural crest cells in a variety of vertebrate models have elucidated the function and regulation of dozens of the molecular players that are part of this network. The neural crest GRN has served as a platform to explore the molecular control of multipotency, cell differentiation, and the evolution of vertebrates. In this review, we employ this genetic program as a stepping-stone to explore the architecture and the regulatory principles of developmental GRNs. We also discuss how modern genomic approaches can further expand our understanding of genetic networks in this system and others. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Biological Mechanisms > Cell Fates Developmental Biology > Lineages Models of Systems Properties and Processes > Cellular Models.

Neural crest and the origin of species-specific pattern

For well over half of the 150 years since the discovery of the neural crest, the special ability of these cells to function as a source of species-specific pattern has been clearly recognized. Initially, this observation arose in association with chimeric transplant experiments among differentially pigmented amphibians, where the neural crest origin for melanocytes had been duly noted. Shortly thereafter, the role of cranial neural crest cells in transmitting species-specific information on size and shape to the pharyngeal arch skeleton as well as in regulating the timing of its differentiation became readily apparent. Since then, what has emerged is a deeper understanding of how the neural crest accomplishes such a presumably difficult mission, and this includes a more complete picture of the molecular and cellular programs whereby neural crest shapes the face of each species. This review covers studies on a broad range of vertebrates and describes neural-crest-mediated mechanisms that endow the craniofacial complex with species-specific pattern. A major focus is on experiments in quail and duck embryos that reveal a hierarchy of cell-autonomous and non-autonomous signaling interactions through which neural crest generates species-specific pattern in the craniofacial integument, skeleton, and musculature. By controlling size and shape throughout the development of these systems, the neural crest underlies the structural and functional integration of the craniofacial complex during evolution.

Sox9 augments BMP2-induced chondrogenic differentiation by downregulating Smad7 in mesenchymal stem cells (MSCs)

Cartilage injuries caused by arthritis or trauma pose formidable challenges for effective clinical management due to the limited intrinsic proliferative capability of chondrocytes. Autologous stem cell-based therapies and transgene-enhanced cartilage tissue engineering may open new avenues for the treatment of cartilage injuries. Bone morphogenetic protein 2 (BMP2) induces effective chondrogenesis of mesenchymal stem cells (MSCs) and can thus be explored as a potential therapeutic agent for cartilage defect repair. However, BMP2 also induces robust endochondral ossification. Although the precise mechanisms through which BMP2 governs the divergence of chondrogenesis and osteogenesis remain to be fully understood, blocking endochondral ossification during BMP2-induced cartilage formation may have practical significance for cartilage tissue engineering. Here, we investigate the role of Sox9-donwregulated Smad7 in BMP2-induced chondrogenic differentiation of MSCs. We find that overexpression of Sox9 leads to a decrease in BMP2-induced Smad7 expression in MSCs. Sox9 inhibits BMP2-induced expression of osteopontin while enhancing the expression of chondrogenic marker Col2a1 in MSCs. Forced expression of Sox9 in MSCs promotes BMP2-induced chondrogenesis and suppresses BMP2-induced endochondral ossification. Constitutive Smad7 expression inhibits BMP2-induced chondrogenesis in stem cell implantation assay. Mouse limb explant assay reveals that Sox9 expands BMP2-stimulated chondrocyte proliferating zone while Smad7 promotes BMP2-intitated hypertrophic zone of the growth plate. Cell cycle analysis indicates that Smad7 induces significant early apoptosis in BMP2-stimulated MSCs. Taken together, our results strongly suggest that Sox9 may facilitate BMP2-induced chondrogenesis by downregulating Smad7, which can be exploited for effective cartilage tissue engineering.

Spatial and temporal expression of Smad signaling members during the development of mandibular condylar cartilage

The present study aimed to explore the underlying developmental mechanism of mothers against decapentaplegic homolog (Smad) signaling in the development of mandibular condylar cartilage. To achieve this, the expression levels of Smad2, 3, 4 and 7, and phosphorylated Smad2/3 were investigated at different time points in developing mandibular condylar cartilage. Mandibular condyles from C57BL/6J mice were dissected at the prenatal and postnatal stages. Serial sections were made and the distributions of Smad proteins were examined using immunohistochemical techniques intermittently between day 14.5 of gestation and postnatal day 7. All Smad proteins examined in the present study were expressed in the condylar blastema and during early chondrogenesis. At the postnatal stage, Smad2 and 4 were localized in proliferative and mineralized hypertrophic chondrocytes. Smad3 and 7 were expressed in proliferative and hypertrophic chondrocytes, including pre-hypertrophic and mineralized hypertrophic chondrocytes. Later, positive immunoreactivity of Smad3 reduced at postnatal day 7. A similar expression pattern to Smad3 was observed for p-Smad2/3, but p-Smad2/3 was located in the nuclei of proliferative chondrocytes. These results suggest that Smad signaling members are involved in the development of mandibular condylar cartilage. In addition, the spatial and temporal expression of these Smads indicate that Smad signaling is involved in regulating the differentiation of chondrocytes and endochondral ossification, in order to maintain normal chondrogenesis and morphogenesis of mandibular condylar cartilage.

Bmpr1a Signaling in Cartilage Development and Endochondral Bone Formation

The type IA bone morphogenetic protein receptor (Bmpr1a), encoded by 11 exons and spanning about 40 kb on chromosome 14 in mice and chromosome 10 in human (Derynck & Feng, 1997; Mishina, Hanks, Miura, Tallquist, & Behringer, 2002), is an essential receptor for BMP signaling. This chapter focuses on the current understanding of the role of Bmpr1a in cartilage development and endochondral ossification, including formation of the mesenchymal condensation, chondrocyte differentiation and maturation, and endochondral bone development.

Sonic hedgehog from pharyngeal arch 1 epithelium is necessary for early mandibular arch cell survival and later cartilage condensation differentiation

BACKGROUND

Morphogenesis of vertebrate craniofacial skeletal elements is dependent on a key cell population, the cranial neural crest cells (NCC). Cranial NCC are formed dorsally in the cranial neural tube and migrate ventrally to form craniofacial skeletal elements as well as other tissues. Multiple extracellular signaling pathways regulate the migration, survival, proliferation, and differentiation of NCC.

RESULTS

In this study, we demonstrate that Shh expression in the oral ectoderm and pharyngeal endoderm is essential for mandibular development. We show that a loss of Shh in these domains results in increased mesenchymal cell death in pharyngeal arch 1 (PA1) after NCC migration. This increased cell death can be rescued in utero by pharmacological inhibition of p53. Furthermore, we show that epithelial SHH is necessary for the early differentiation of mandibular cartilage condensations and, therefore, the subsequent development of Meckel's cartilage, around which the dentary bone forms. Nonetheless, a rescue of the cell death phenotype does not rescue the defect in cartilage condensation formation.

CONCLUSIONS

Our results show that SHH produced by the PA1 epithelium is necessary for the survival of post-migratory NCC, and suggests a key role in the subsequent differentiation of chondrocytes to form Meckel's cartilage.

Effects of serial passage on the characteristics and chondrogenic differentiation of canine umbilical cord matrix derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are often known to have a therapeutic potential in the cell-mediated repair for fatal or incurable diseases. In this study, canine umbilical cord MSCs (cUC-MSCs) were isolated from umbilical cord matrix (n = 3) and subjected to proliferative culture for 5 consecutive passages. The cells at each passage were characterized for multipotent MSC properties such as proliferation kinetics, expression patterns of MSC surface markers and self-renewal associated markers, and chondrogenic differentiation. In results, the proliferation of the cells as determined by the cumulative population doubling level was observed at its peak on passage 3 and stopped after passage 5, whereas cell doubling time dramatically increased after passage 4. Expression of MSC surface markers (CD44, CD54, CD61, CD80, CD90 and Flk-1), molecule (HMGA2) and pluripotent markers (sox2, nanog) associated with self-renewal was negatively correlated with the number of passages. However, MSC surface marker (CD105) and pluripotent marker (Oct3/4) decreased with increasing the number of subpassage. cUC-MSCs at passage 1 to 5 underwent chondrogenesis under specific culture conditions, but percentage of chondrogenic differentiation decreased with increasing the number of subpassage. Collectively, the present study suggested that sequential subpassage could affect multipotent properties of cUC-MSCs and needs to be addressed before clinical applications.

Distinct populations within Isl1 lineages contribute to appendicular and facial skeletogenesis through the β-catenin pathway

Isl1 expression marks progenitor populations in developing embryos. In this study, we investigated the contribution of Isl1-expressing cells that utilize the β-catenin pathway to skeletal development. Inactivation of β-catenin in Isl1-expressing cells caused agenesis of the hindlimb skeleton and absence of the lower jaw (agnathia). In the hindlimb, Isl1-lineages broadly contributed to the mesenchyme; however, deletion of β-catenin in the Isl1-lineage caused cell death only in a discrete posterior domain of nascent hindlimb bud mesenchyme. We found that the loss of posterior mesenchyme, which gives rise to Shh-expressing posterior organizer tissue, caused loss of posterior gene expression and failure to expand chondrogenic precursor cells, leading to severe truncation of the hindlimb. In facial tissues, Isl1-expressing cells broadly contributed to facial epithelium. We found reduced nuclear β-catenin accumulation and loss of Fgf8 expression in mandibular epithelium of Isl1(-/-) embryos. Inactivating β-catenin in Isl1-expressing epithelium caused both loss of epithelial Fgf8 expression and death of mesenchymal cells in the mandibular arch without affecting epithelial proliferation and survival. These results suggest a Isl1→β-catenin→Fgf8 pathway that regulates mesenchymal survival and development of the lower jaw in the mandibular epithelium. By contrast, activating β-catenin signaling in Isl1-lineages caused activation of Fgf8 broadly in facial epithelium. Our results provide evidence that, despite its broad contribution to hindlimb mesenchyme and facial epithelium, the Isl1-β-catenin pathway regulates skeletal development of the hindlimb and lower jaw through discrete populations of cells that give rise to Shh-expressing posterior hindlimb mesenchyme and Fgf8-expressing mandibular epithelium.

Inactivation of LAR family phosphatase genes Ptprs and Ptprf causes craniofacial malformations resembling Pierre-Robin sequence

Leukocyte antigen related (LAR) family receptor protein tyrosine phosphatases (RPTPs) regulate the fine balance between tyrosine phosphorylation and dephosphorylation that is crucial for cell signaling during development and tissue homeostasis. Here we show that LAR RPTPs are required for normal development of the mandibular and maxillary regions. Approximately half of the mouse embryos lacking both Ptprs (RPTPσ) and Ptprf (LAR) exhibit micrognathia (small lower jaw), cleft palate and microglossia/glossoptosis (small and deep tongue), a phenotype closely resembling Pierre-Robin sequence in humans. We show that jaw bone and cartilage patterning occurs aberrantly in LAR family phosphatase-deficient embryos and that the mandibular arch harbors a marked decrease in cell proliferation. Analysis of signal transduction in embryonic tissues and mouse embryonic fibroblast cultures identifies an increase in Bmp-Smad signaling and an abrogation of canonical Wnt signaling associated with loss of the LAR family phosphatases. A reactivation of β-catenin signaling by chemical inhibition of GSK3β successfully resensitizes LAR family phosphatase-deficient cells to Wnt induction, indicating that RPTPs are necessary for normal Wnt/β-catenin pathway activation. Together these results identify LAR RPTPs as important regulators of craniofacial morphogenesis and provide insight into the etiology of Pierre-Robin sequence.

Enhanced BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage in mice

Meckel's cartilage is a transient supporting tissue of the embryonic mandible in mammals, and disappears by taking different ultimate cell fate along the distal-proximal axis, with the majority (middle portion) undergoing degeneration and chondroclastic resorption. While a number of factors have been implicated in the degeneration and resorption processes, signaling pathways that trigger this degradation are currently unknown. BMP signaling has been implicated in almost every step of chondrogenesis. In this study, we used Noggin mutant mice as a model for gain-of-BMP signaling function to investigate the function of BMP signaling in Meckel's cartilage development, with a focus on the middle portion. We showed that Bmp2 and Bmp7 are expressed in early developing Meckels' cartilage, but their expression disappears thereafter. In contrast, Noggin is expressed constantly in Meckel's cartilage throughout the entire gestation period. In the absence of Noggin, Meckel's cartilage is significantly thickened attributing to dramatically elevated cell proliferation rate associated with enhanced phosphorylated Smad1/5/8 expression. Interestingly, instead of taking a degeneration fate, the middle portion of Meckel's cartilage in Noggin mutants undergoes chondrogenic differentiation and endochondral ossification contributing to the forming mandible. Chondrocyte-specific expression of a constitutively active form of BMPRIa but not BMPRIb leads to enlargement of Meckel's cartilage, phenocopying the consequence of Noggin deficiency. Our results demonstrate that elevated BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage, and support the idea that withdrawal of BMP signaling is required for normal Meckel's cartilage development and ultimate cell fate.

In vitro proliferation and differentiation of adipose-derived stem cells isolated using anti-CD105 magnetic beads

The present study aimed to investigate the feasibility of isolating adipose-derived stem cells (ADSCs) by selecting cells that express the surface receptor CD105. Surface antigen expression of the unsorted cells was undertaken using FACS analysis. Primary adipose-derived cells were isolated. The second passage cells were incubated with anti-CD105 magnetic beads, and separated using a magnetic separator. Cell growth and colony formation was determined by counting and Giemsa staining, respectively. Cells also underwent histological immunohistochemical, and RT-PCR analyses to determine their chondrogenic, adipogenic and osteogenic potential. Increased cell proliferation and colony formation was observed in CD105-positive (CD105⁺) as compared to the CD105-negative (CD105⁻) cells (P<0.001). Following induction, the expression of type II collagen and the number of calcium deposits and lipid droplets in the CD105⁺ ADCs were markedly higher than in the CD105⁻ ADCs. Furthermore, increased alkaline phosphatase (AKP), leptin and PPARγ2 mRNA expression was detected in the CD105⁺ ADCs (P<0.01). Isolation of CD105⁺ ADSCs by MACS was feasible. Thus, CD105 can be used as a relatively specific marker for the selection of ADSCs. Although the chondrogenic, adipogenic and osteogenic potential of these cells is suggestive of their potential for use in tissue engineering treatments, further in vivo studies are necessary.

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.

Roles of collagen and periostin expression by cranial neural crest cells during soft palate development

The tissue in the palatal region can be divided into the hard and the soft palates, each having a specialized function such as occlusion, speech, or swallowing. Therefore, an understanding of the mechanism of palatogenesis in relation to the function of each region is important. However, in comparison with the hard palate, there is still a lack of information about the mechanisms of soft palate development. In this study, the authors investigated the contribution of cranial neural crest (CNC) cells to development of both hard and soft palates. They also demonstrated a unique pattern of periostin expression during soft palate development, which was closely related to that of collagen type I (Col I) in palatine aponeurosis. Furthermore, organ culture analysis showed that exogenous transforming growth factor-β (TGF-β) induced the expression of both periostin and Col I. These novel patterns of expression in the extracellular matrix (ECM) induced by CNC cells suggest that these cells may help to determine the character of both the hard and soft palates through ECM induction. TGF-β signaling appears to be one of the mediators of Col I and periostin expression in the formation of functional structures during soft palate development.

Id2 controls chondrogenesis acting downstream of BMP signaling during maxillary morphogenesis

Maxillofacial dysmorphogenesis is found in 5% of the population. To begin to understand the mechanisms required for maxillofacial morphogenesis, we employed the inhibitors of the differentiation 2 (Id2) knock-out mouse model, in which Id proteins, members of the regulator of basic helix-loop-helix (bHLH) transcription factors, modulate cell proliferation, apoptosis, and differentiation. We now report that spatially-restricted growth defects are localized at the skull base of Id2 KO mice. Curiously, at birth, neither the mutant Id2 KO nor wild-type (WT) mice differed, based upon cephalometric and histological analyses of cranial base synchondroses. In postnatal week 2, a narrower hypertrophic zone and an inhibited proliferative zone in presphenoid synchondrosis (PSS) and spheno-occipital synchondrosis (SOS) with maxillary hypoplasia were identified in the Id2 mutant mice. Complementary studies revealed that exogenous bone morphogenetic proteins (BMPs) enhanced cartilage growth, matrix deposition, and chondrocyte proliferation in the WT but not in the mutant model. Id2-deficient chondrocytes expressed more Smad7 transcripts. Based on our results, we assert that Id2 plays an essential role, acting downstream of BMP signaling, to regulate cartilage formation at the postnatal stage by enhancing BMP signals through inhibiting Smad7 expression. As a consequence, abnormal endochondral ossification was observed in cranial base synchondroses during the postnatal growth period, resulting in the clinical phenotype of maxillofacial dysmorphogenesis.

Developmental Patterns in Mesozoic Evolution of Mammal Ears

Complex structures with significant biological function can arise multiple times in evolution by common gene patterning and developmental pathways. The mammalian middle ear, with its significant hearing function, is such a complex structure and a key evolutionary innovation. Newly discovered fossils have now shown that the detachment of the ear from the jaw, an important transformation of the middle ear in early mammals, has major homoplasies; the morphogenesis of these homoplasies is also illuminated by new genetic studies of ear development in extant mammals. By extrapolating the developmental morphogenesis of genetic studies into the early mammal fossil record, evolution of the middle ear in early mammals provides an integrated case study of how development has impacted, mechanistically, the transformation of a major structural complex in evolution.

CD47 regulates the TGF-β signaling pathway in osteoblasts and is distributed in Meckel's cartilage

Previously, CD47 gene expression has been shown to increase during mandible development using a micro array technique. To determine the function of CD47 in osteoblasts, CD47 was silenced using siRNA in vitro. The TGF-β1 and phosphorylated-Smad2 levels and transcription factor genes related to bone metabolism increased dose-dependently with CD47 silencing. Furthermore, we determined the distribution of CD47 in mouse embryonic E13 and E15 in vivo. The CD47-positive cells were localized in Meckel's cartilage and antenatal mandibular bone. These results suggest that TGF-β1 signaling and mandible development might be regulated by CD47.

Expression of smoothened in mouse embryonic maxillofacial development

Hedgehog (Hh) signaling plays many key roles in the development of Drosophila and vertebrate embryos including regulation of craniofacial development. The seven-transmembrane protein, smoothened (Smo) transduces the Hh signal across the plasma membrane as an essential receptor of PTCHED1/2. There are few studies that evaluate the detailed expression of Smo in mouse embryonic craniofacial development. We investigated the expression patterns of Smo during murine embryonic craniofacial development using in situ hybridization (ISH), studies of whole-mounts and sections, immunohistochemistry, quantitative real time PCR, and Western blot analysis. We found that Smo mRNA was expressed in the face of mouse embryos at 11 and 12.5 days post coitum (dpc). After 13.5 dpc, the expression decreased to a low level and was faintly detected after birth. Smo protein could be detected also in embryos at 11, 12.5, and 14.5 dpc. After 15.5 dpc, the expression was very faint and paralleled the gene expression studies. No expression was detected in whisker follicle during facial development and faint signal was detected in Meckel's cartilage. These findings concerning Smo expression should guide further investigation of sonic Hh signaling pathway gene function during maxillofacial development.

MicroRNA-221 regulates chondrogenic differentiation through promoting proteosomal degradation of slug by targeting Mdm2

MicroRNAs (miRNAs) are small RNAs that fulfill diverse functions by negatively regulating gene expression. Here, we investigated the involvement of miRNAs in the chondrogenic differentiation of chick limb mesenchymal cells and found that the expression of miR-221 increased upon chondrogenic inhibition. Blockade of miR-221 via peanut agglutinin-based antisense oligonucleotides reversed the chondro-inhibitory actions of a JNK inhibitor on the proliferation and migration of chondrogenic progenitors as well as the formation of precartilage condensations. We determined that mdm2 is a relevant target of miR-221 during chondrogenesis. miR-221 was necessary and sufficient to down-regulate Mdm2 expression, and this down-modulation of Mdm2 by miR-221 prevented the degradation of (and consequently up-regulated) the Slug protein, which negatively regulates the proliferation of chondroprogenitors. These results indicate that miR-221 contributes to the regulation of cell proliferation by negatively regulating Mdm2 and thereby inhibiting Slug degradation during the chondrogenesis of chick limb mesenchymal cells.

The intermediate filament vimentin regulates chondrogenesis of adult human bone marrow-derived multipotent progenitor cells

Cytoskeletal proteins play important regulatory roles in a variety of cellular processes, including proliferation, migration, and differentiation. However, whereas actin and tubulin have established roles regulating developmental chondrogenesis, there is no evidence supporting a function for the intermediate filament vimentin in embryonic cartilage formation. We hypothesized that vimentin may regulate the chondrogenic differentiation of adult multipotent progenitor cells (MPCs), such as those involved in cartilage formation during bone fracture repair. As our model of adult progenitor cell chondrogenesis, we employed high-density pellet cultures of human bone marrow-derived MPCs. siRNA-mediated knockdown of vimentin mRNA and protein triggered a reduction in the extent of MPC cartilage formation, as evidenced by depressed accumulation of mRNAs for the cartilage-specific marker genes aggrecan and collagen type II, as well as reduced levels of Alcian blue-stainable proteoglycan and collagen II protein in the extracellular matrix. Moreover, mRNA and protein levels for the chondro-regulatory transcription factors SOX5, SOX6, and SOX9 were diminished by vimentin knockdown. Depleted cellular vimentin also induced a drastic reduction in PKA phosphorylation levels but did not affect the phosphorylation of multiple other chondro-regulatory kinases and transcription factors, including ERK1/2, p38, Smad2, and Smad1/5/8. Importantly, siRNA-mediated knockdown of PKA C-alpha mRNA and protein mimicked the reduction in chondrogenesis caused by diminished cellular vimentin. Finally, overexpression of vimentin in MPCs significantly enhanced the activity of a transfected collagen II promoter-luciferase reporter gene. In conclusion, we describe a novel role for the intermediate filament vimentin as a positive regulator of adult human bone marrow-derived MPC chondrogenesis.

Regulation of the chondrogenic phenotype in culture

In recent years, there has been a great deal of interest in the development of regenerative approaches to produce hyaline cartilage ex vivo that can be utilized for the repair or replacement of damaged or diseased tissue. It is clinically imperative that cartilage engineered in vitro mimics the molecular composition and organization of and exhibits biomechanical properties similar to persistent hyaline cartilage in vivo. Experimentally, much of our current knowledge pertaining to the regulation of cartilage formation, or chondrogenesis, has been acquired in vitro utilizing high-density cultures of undifferentiated chondroprogenitor cells stimulated to differentiate into chondrocytes. In this review, we describe the extracellular matrix molecules, nuclear transcription factors, cytoplasmic protein kinases, cytoskeletal components, and plasma membrane receptors that characterize cells undergoing chondrogenesis in vitro and regulate the progression of these cells through the chondrogenic differentiation program. We also provide an extensive list of growth factors and other extracellular signaling molecules, as well as chromatin remodeling proteins such as histone deacetylases, known to regulate chondrogenic differentiation in culture. In addition, we selectively highlight experiments that demonstrate how an understanding of normal hyaline cartilage formation can lead to the development of novel cartilage tissue engineering strategies. Finally, we present directions for future studies that may yield information applicable to the in vitro generation of hyaline cartilage that more closely resembles native tissue.

Potent in vitro chondrogenesis of CD105 enriched human adipose-derived stem cells

Adipose-derived stem cells (ASCs) are considered as a promising cell source for cartilage regeneration. However, the heterogeneity of this cell source may affect their ability in cartilage formation. It is therefore necessary to establish an efficient method for isolating the cells that have chondrogenic potential. To date, no specific markers have been reported to be able to isolate such a cell population from human adipose tissue. In recent studies, endoglin (CD105) has been known as a relatively specific marker for identifying mesenchymal stem cells, but no studies show it is related to chondrogenic potential of human ASCs. In this study, human cells from adipose tissue were isolated, cultured, and sorted according to CD105 expression. The sorted cells were then subjected to adipogenic, osteogenic, and chondrogenic induction to confirm their multi-potentiality. In adipogenic conditions, CD105- cells showed stronger Oil Red staining and higher expression of adipose-specific genes compared to CD105+ cells. By contrast, CD105+ cells exhibited better osteogenic potential with stronger Alizarin Red staining and higher expression of osteogenic specific genes than CD105- cells. Noticeably, CD105+ cells also exhibited a much stronger chondrogenic potential than CD105- cells, with stronger collagen II staining and higher gene expression of collagen II and aggrecan. Most importantly, CD105+ cells could form a homogeneous cartilage-like tissue when seeded into a biodegradable scaffold and cultured in chondrogenic media for 8 weeks. These results indicate that sorting of ASC subpopulation with CD105 as a marker may allow better in vitro chondrogenesis and thus provide an important implications for cartilage regeneration and reconstruction using autologous cells from adipose tissue.

Immunolocalization of proteoglycans in Meckel's cartilage of the rat

The aim of this study was to investigate the presence and distribution of proteoglycans within Meckel’s cartilage of rat embryos. A standard indirect immunoperoxidase technique was used on paraffin sections of rat heads. Sections were incubated with monoclonal antibodies recognising core protein epitopes in the proteoglycans versican and CD44. Polyclonal antibodies localized the proteoglycans decorin, biglycan and lumican. Versican was expressed by chondrocytes, but very weekly by the extracellular matrix. Decorin was strongly expressed by both of chondrocytes and the ECM. Both of biglycan and lumican were moderately expressed by chondrocytes, but weakly by the extracellular matrix. CD44 was weakly expressed by chondrocytes only, without staining of the ECM. It is concluded that Meckel’s cartilage chondrocytes express the proteoglycans versican, decorin, biglycan, lumican and CD44 at variable levels during development in the rat. Such data are important for a greater understanding of the changes that take place during mandibular development. Further studies are needed to elucidate the exact role of proteoglycans during Meckel’s cartilage and mandibular organogenesis.

Congenital deformities and developmental abnormalities of the mandibular condyle in the temporomandibular joint

The temporomandibular joint (TMJ) consists of the mandibular condyle and the articular eminence of the temporal bone. The morphological development of the TMJ during prenatal life lags behind other joints in terms of both the timing of its appearance and its progress. At birth, the joint is still largely underdeveloped. There are many causes of the various growth disturbances and abnormalities of the mandibular condyle and related structures. Growth disturbances in the development of the mandibular condyle may occur in utero late in the first trimester and may result in disorders such as aplasia or hypoplasia of the mandibular condyle. Meanwhile, hyperplasia of the mandibular condyle is not visible at birth and seems to be gradually acquired during growth. In the present review article, the congenital abnormalities of the mandibular condyle are classified morphologically into three major groups and two subgroups from a clinical standpoint: (1) hypoplasia or aplasia of the mandibular condyle, including (i) primary condylar aplasia and hypoplasia, (ii) secondary condylar hypoplasia; (2) hyperplasia; and (3) bifidity. In addition, the molecular-based etiology of anomalies of the mandibular condyle is also discussed.

Regulation of cartilage formation and maturation by mitogen-activated protein kinase signaling

The majority of bones comprising the adult vertebrate skeleton are generated from hyaline cartilage templates that form during embryonic development. A process known as endochondral ossification is responsible for the conversion of these transient cartilage anlagen into mature, calcified bone. Endochondral ossification is a highly regulated, multistep cell specification program involving the initial differentiation of prechondrogenic mesenchymal cells into hyaline chondrocytes, terminal differentiation of hyaline chondrocytes into hypertrophic chondrocytes, and finally, apoptosis of hypertrophic chondrocytes followed by bone matrix deposition. Recently, extensive research has been carried out describing roles for the three major mitogen-activated protein kinase (MAPK) signaling pathways, the extracellular signal-regulated kinase 1/2 (ERK1/2), p38, and c-jun N-terminal kinase (JNK) pathways, in the successive stages of chondrogenic differentiation. In this review, we survey this research examining the involvement of ERK1/2, p38, and JNK pathway signaling in all aspects of the chondrogenic differentiation program from embryonic through postnatal stages of development. In addition, we summarize evidence from in vitro studies examining MAPK function in immortalized chondrogenic cell lines and adult mesenchymal stem cells. We also provide suggestions for future studies that may help ameliorate existing confusion concerning the specific roles of MAPK signaling at different stages of chondrogenesis.

Smad7 Inhibits chondrocyte differentiation at multiple steps during endochondral bone formation and down-regulates p38 MAPK pathways

Bone morphogenetic proteins (BMPs) play critical roles at various stages in endochondral bone formation. In vitro studies have demonstrated that Smad7 regulates transforming growth factor-beta and BMP signals by inhibiting Smad pathways in chondrocytes. However, the in vivo roles of Smad7 during cartilage development are unknown. To investigate distinct effects of Smad7 at different stages during chondrocyte differentiation, we generated a series of conditional transgenic mice that overexpress Smad7 in chondrocytes at various steps of differentiation by using the Cre/loxP system. We generated Col11a2-lacZ(floxed)-Smad7 transgenic mice and mated them with three types of Cre transgenic mice to obtain Smad7(Prx1), Smad7(11Enh), and Smad7(11Prom) conditional transgenic mice. Smad7(Prx1) mice overexpressing Smad7 in condensing mesenchymal cells showed disturbed mesenchymal condensation associated with decreased Sox9 expression, leading to poor cartilage formation. Smad7(11Enh) mice overexpressing Smad7 in round chondrocytes showed decreased chondrocyte proliferation rates. Smad7(11Prom) mice overexpressing Smad7 in flat chondrocytes showed inhibited maturation of chondrocytes toward hypertrophy. Micromass culture of mesenchymal cells showed that BMP-induced cartilaginous nodule formation was down-regulated by overexpression of Smad7, but not Smad6. Overexpression of Smad7, but not Smad6, down-regulated the phosphorylation of p38 MAPKs. Our data provide in vivo evidence for distinct effects of Smad7 at different stages during chondrocyte differentiation and suggest that Smad7 in prechondrogenic cells inhibits chondrocyte differentiation possibly by down-regulating BMP-activated p38 MAPK pathways.

TGF-beta mediated Dlx5 signaling plays a crucial role in osteo-chondroprogenitor cell lineage determination during mandible development

Transforming growth factor-beta (TGF-beta) signaling is crucial for mandible development. During its development, the majority of the mandible is formed through intramembranous ossification whereas the proximal region of the mandible undergoes endochondral ossification. Our previous work has shown that TGF-beta signaling is required for the proliferation of cranial neural crest (CNC)-derived ectomesenchyme in the mandibular primordium where intramembranous ossification takes place. Here we show that conditional inactivation of Tgfbr2 in CNC cells results in accelerated osteoprogenitor differentiation and perturbed chondrogenesis in the proximal region of the mandible. Specifically, the appearance of chondrocytes in Tgfbr2(fl/fl);Wnt1-Cre mice is delayed and they are smaller in size in the condylar process and completely missing in the angular process. TGF-beta signaling controls Sox9 expression in the proximal region, because Sox9 expression is delayed in condylar processes and missing in angular process in Tgfbr2(fl/fl);Wnt1-Cre mice. Moreover, exogenous TGF-beta can induce Sox9 expression in the mandibular arch. In the angular processes of Tgfbr2(fl/fl);Wnt1-Cre mice, osteoblast differentiation is accelerated and Dlx5 expression is elevated. Significantly, deletion of Dlx5 in Tgfbr2(fl/fl);Wnt1-Cre mice results in the rescue of cartilage formation in the angular processes. Finally, TGF-beta signaling-mediated Scleraxis expression is required for tendonogenesis in the developing skeletal muscle. Thus, CNC-derived cells in the proximal region of mandible have a cell intrinsic requirement for TGF-beta signaling.

Concepts in gene therapy for cartilage repair

Once articular cartilage is injured, it has a very limited capacity for self repair. Although current surgical therapeutic procedures for cartilage repair are clinically useful, they cannot restore a normal articular surface. Current research offers a growing number of bioactive reagents, including proteins and nucleic acids, that may be used to augment various aspects of the repair process. As these agents are difficult to administer effectively, gene-transfer approaches are being developed to provide their sustained synthesis at sites of repair. To augment regeneration of articular cartilage, therapeutic genes can be delivered to the synovium or directly to the cartilage lesion. Gene delivery to the cells of the synovial lining is generally considered more suitable for chondroprotective approaches, based on the expression of anti-inflammatory mediators. Gene transfer targeted at cartilage defects can be achieved by either direct vector administration to cells located at or surrounding the defects, or by transplantation of genetically modified chondrogenic cells into the defect. Several studies have shown that exogenous cDNAs encoding growth factors can be delivered locally to sites of cartilage damage, where they are expressed at therapeutically relevant levels. Furthermore, data is beginning to emerge indicating that efficient delivery and expression of these genes is capable of influencing a repair response toward the synthesis of a more hyaline cartilage repair tissue in vivo. This review presents the current status of gene therapy for cartilage healing and highlights some of the remaining challenges.

Cytomegalovirus induces abnormal chondrogenesis and osteogenesis during embryonic mandibular development

Background

Human clinical studies and mouse models clearly demonstrate that cytomegalovirus (CMV) disrupts normal organ and tissue development. Although CMV is one of the most common causes of major birth defects in humans, little is presently known about the mechanism(s) underlying CMV-induced congenital malformations. Our prior studies have demonstrated that CMV infection of first branchial arch derivatives (salivary glands and teeth) induced severely abnormal phenotypes and that CMV has a particular tropism for neural crest-derived mesenchyme (NCM). Since early embryos are barely susceptible to CMV infection, and the extant evidence suggests that the differentiation program needs to be well underway for embryonic tissues to be susceptible to viral infection and viral-induced pathology, the aim of this study was to determine if first branchial arch NCM cells are susceptible to mCMV infection prior to differentiation of NCM derivatives.

Results

E11 mouse mandibular processes (MANs) were infected with mouse CMV (mCMV) for up to 16 days in vitro. mCMV infection of undifferentiated embryonic mouse MANs induced micrognathia consequent to decreased Meckel's cartilage chondrogenesis and mandibular osteogenesis. Specifically, mCMV infection resulted in aberrant stromal cellularity, a smaller, misshapen Meckel's cartilage, and mandibular bone and condylar dysmorphogenesis. Analysis of viral distribution indicates that mCMV primarily infects NCM cells and derivatives. Initial localization studies indicate that mCMV infection changed the cell-specific expression of FN, NF-κB2, RelA, RelB, and Shh and Smad7 proteins.

Conclusion

Our results indicate that mCMV dysregulation of key signaling pathways in primarily NCM cells and their derivatives severely disrupts mandibular morphogenesis and skeletogenesis. The pathogenesis appears to be centered around the canonical and noncanonical NF-κB pathways, and there is unusual juxtaposition of abnormal stromal cells and surrounding matrix. Moreover, since it is critically important that signaling molecules are expressed in appropriate cell populations during development, the aberrant localization of components of relevant signaling pathways may reveal the pathogenic mechanism underlying mandibular malformations.

Mesenchyme-dependent BMP signaling directs the timing of mandibular osteogenesis

To identify molecular and cellular mechanisms that determine when bone forms, and to elucidate the role played by osteogenic mesenchyme, we employed an avian chimeric system that draws upon the divergent embryonic maturation rates of quail and duck. Pre-migratory neural crest mesenchyme destined to form bone in the mandible was transplanted from quail to duck. In resulting chimeras, quail donor mesenchyme established significantly faster molecular and histological programs for osteogenesis within the relatively slower-progressing duck host environment. To understand this phenotype, we assayed for changes in the timing of epithelial-mesenchymal interactions required for bone formation and found that such interactions were accelerated in chimeras. In situ hybridization analyses uncovered donor-dependent changes in the spatiotemporal expression of genes, including the osteo-inductive growth factor Bmp4. Mesenchymal expression of Bmp4 correlated with an ability of quail donor cells to form bone precociously without duck host epithelium, and also relied upon epithelial interactions until mesenchyme could form bone independently. Treating control mandibles with exogenous BMP4 recapitulated the capacity of chimeras to express molecular mediators of osteogenesis prematurely and led to the early differentiation of bone. Inhibiting BMP signaling delayed bone formation in a stage-dependent manner that was accelerated in chimeras. Thus, mandibular mesenchyme dictates when bone forms by temporally regulating its interactions with epithelium and its own expression of Bmp4. Our findings offer a developmental mechanism to explain how neural crest-derived mesenchyme and BMP signaling underlie the evolution of species-specific skeletal morphology.

Major biological obstacles for persistent cell-based regeneration of articular cartilage

Hyaline articular cartilage, the load-bearing tissue of the joint, has very limited repair and regeneration capacities. The lack of efficient treatment modalities for large chondral defects has motivated attempts to engineer cartilage constructs in vitro by combining cells, scaffold materials and environmental factors, including growth factors, signaling molecules, and physical influences. Despite promising experimental approaches, however, none of the current cartilage repair strategies has generated long lasting hyaline cartilage replacement tissue that meets the functional demands placed upon this tissue in vivo. The reasons for this are diverse and can ultimately result in matrix degradation, differentiation or integration insufficiencies, or loss of the transplanted cells and tissues. This article aims to systematically review the different causes that lead to these impairments, including the lack of appropriate differentiation factors, hypertrophy, senescence, apoptosis, necrosis, inflammation, and mechanical stress. The current conceptual basis of the major biological obstacles for persistent cell-based regeneration of articular cartilage is discussed, as well as future trends to overcome these limitations.

Cell autonomous requirement for TGF-beta signaling during odontoblast differentiation and dentin matrix formation

TGF-beta subtypes are expressed in tissues derived from cranial neural crest cells during early mouse craniofacial development. TGF-beta signaling is critical for mediating epithelial-mesenchymal interactions, including those vital for tooth morphogenesis. However, it remains unclear how TGF-beta signaling contributes to the terminal differentiation of odontoblast and dentin formation during tooth morphogenesis. Towards this end, we generated mice with conditional inactivation of the Tgfbr2 gene in cranial neural crest derived cells. Odontoblast differentiation was substantially delayed in the Tgfbr2(fl/fl);Wnt1-Cre mutant mice at E18.5. Following kidney capsule transplantation, Tgfbr2 mutant tooth germs expressed a reduced level of Col1a1 and Dspp and exhibited defects including decreased dentin thickness and absent dentinal tubules. In addition, the expression of the intermediate filament nestin was decreased in the Tgfbr2 mutant samples. Significantly, exogenous TGF-beta2 induced nestin and Dspp expression in dental pulp cells in the developing tooth organ. Our data suggest that TGF-beta signaling controls odontoblast maturation and dentin formation during tooth morphogenesis.

Effects of Dose of Recombinant Human BMP-2 on Bone Formation at Palatal Sites in Young and Old Rats

This study was designed to examine the effects of dose of recombinant human BMP-2 (rhBMP-2) on bone formation at palatal sites in 10-week-old (10w) and 70-week-old (70w) rats, when combined with a polylactate-polyglycolate copolymer/gelatine sponge (PGS). New bone formation was observed at six weeks after implantation. In the 10w rats, thickness of new bone (TNB) increased as the dosage increased from 0 microg to 4 microg, and decreased significantly as the dosage increased from 8 microg to 24 microg. In contrast, in the 70w rats, TNB increased as the dosage increased from 0 microg to 16 microg, and did not significantly change as the dosage increased from 16 microg to 24 microg. These results suggest that the most effective dosage of rhBMP-2 for induction of bone formation varies according to age.

The role of TGF-beta signaling in regulating chondrogenesis and osteogenesis during mandibular development

During craniofacial development, Meckel's cartilage and the mandible bone derive from the first branchial arch, and their development depends upon the contribution of cranial neural crest (CNC) cells. We previously demonstrated that conditional inactivation of Tgfbr2 in the neural crest of mice (Tgfbr2(fl/fl);Wnt1-Cre) results in severe defects in mandibular development, although the specific cellular and molecular mechanisms by which TGF-beta signaling regulates the fate of CNC cells during mandibular development remain unknown. We show here that loss of Tgfbr2 does not affect the migration of CNC cells during mandibular development. TGF-beta signaling is specifically required for cell proliferation in Meckel's cartilage and the mandibular anlagen and for the formation of the coronoid, condyle and angular processes. TGF-beta-mediated connective tissue growth factor (CTGF) signaling is critical for CNC cell proliferation. Exogenous CTGF rescues the cell proliferation defect in Meckel's cartilage of Tgfbr2(fl/fl);Wnt1-Cre mutants, demonstrating the biological significance of this signaling cascade in chondrogenesis during mandibular development. Furthermore, TGF-beta signaling controls Msx1 expression to regulate mandibular osteogenesis as Msx1 expression is significantly reduced in Tgfbr2(fl/fl);Wnt1-Cre mutants. Collectively, our data suggest that there are differential signal cascades in response to TGF-beta to control chondrogenesis and osteogenesis during mandibular development.

BMP signaling in the cartilage growth plate

Transforming growth factor-beta (TGF-beta) superfamily members play diverse roles in all aspects of cartilage development and maintenance. It is well established that TGF-betas and bone morphogenetic proteins (BMPs) play distinct roles in the growth plate. This chapter discusses key experiments and experimental approaches that have revealed these roles, and progress toward the identification of previously unsuspected roles. Current understanding of the mechanisms by which different TGF-beta and BMP pathways exert their functions is discussed. Finally attempts to utilize this information to promote cartilage regeneration, and important issues for future research, are outlined.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

Evidence for TGF-ß1 and bleomycin intracellular signaling through autocrine regulation of Smad 3 binding to the proximal promoter of theSmad 7 gene

Both Bleomycin and TGF-beta1 increase the transcription of the COL1A1 gene. Bleomycin acts through TGF-beta1. Bleomycin stimulates the COL1A1 promoter through the distal TGF-beta response element by intracellular and extracellular signaling. As demonstrated in this manuscript, Bleomycin's intracellular signaling can be explained by a decrease of Smad 3 transcription factor binding to the SBE located in the proximal promoter of the inhibitory Smad 7 gene. This would result in TGF-beta1-induced activated SMADS, which would result in more collagen. Bleomycin's extracellular signaling results from the secretion of more latent TGF-beta produced by lung fibroblasts and cleaved to active TGF-beta extracellularly. Since the TGF-beta genes are auto-induced in human embryonic IMR-90 lung fibroblasts, this study indicates an autocrine mechanism to maintain homeostasis in vivo for fibroblasts and other cell types, which produce TGF-beta1 to limit the fibrogenic response to TGF-beta1 and Bleomycin.