Effects of restriction of fetal jaw movement on prenatal development of the temporalis muscle

Downregulation of SOX9 expression in developing entheses adjacent to intramembranous bone

Entheses are classified into three types: fibrocartilaginous, fibrous, and periosteal insertions. However, the mechanism behind the development of fibrous entheses and periosteal insertions remains unclear. Since both entheses are part of the temporomandibular joint (TMJ), this study analyzes the TMJ entheses. Here, we show that SOX9 expression is negatively regulated during TMJ enthesis development, unlike fibrocartilage entheses which are modularly formed by SCX and SOX9 positive progenitors. The TMJ entheses was adjacent to the intramembranous bone rather than cartilage. SOX9 expression was diminished during TMJ enthesis development. To clarify the functional role of Sox9 in the development of TMJ entheses, we examined these structures in TMJ using Wnt1Cre;Sox9flox/+ reporter mice. Wnt1Cre;Sox9flox/+ mice showed enthesial deformation at the TMJ. Next, we also observed a diminished SOX9 expression area at the enthesis in contact with the clavicle's membranous bone portion, similar to the TMJ entheses. Together, these findings reveal that the timing of SOX9 expression varies with the ossification development mode.

Mechanical stress can regulate temporomandibular joint cavitation via signalling pathways

The temporomandibular joint (TMJ), composed of temporal fossa, mandibular condyle and a fibrocartilage disc with upper and lower cavities, is the biggest synovial joint and biomechanical hinge of the craniomaxillofacial musculoskeletal system. The initial events that give rise to TMJ cavities across diverse species are not fully understood. Most studies focus on the pivotal role of molecules such as Indian hedgehog (Ihh) and hyaluronic acid (HA) in TMJ cavitation. Although biologists have observed that mechanical stress plays an irreplaceable role in the development of biological tissues and organs, few studies have been concerned with how mechanical stress regulates TMJ cavitation. Based on the evidence from human or other animal embryos today, it is implicated that mechanical stress plays an essential role in TMJ cavitation. In this review, we discuss the relationship between mechanical stress and TMJ cavitation from evo-devo perspectives and review the clinical features and potential pathogenesis of TMJ dysplasia.

Developmental characteristics of secondary cartilage in the mandibular condyle and sphenoid bone in mice

OBJECTIVE

Secondary cartilage develops from osteochondral progenitor cells. Hypertrophic chondrocytes in secondary cartilage increase within a very short time and then ossify rapidly. In the present study, we investigated the sequential development process of osteochondral progenitor cells, and the morphology and size of hypertrophic chondrocytes in secondary cartilage.

DESIGN

ICR mice at embryonic days (E) 14.5-17.5 were used. The mandibular condyle and the medial pterygoid process of the sphenoid bone were observed as secondary cartilage, and the cranial base and the lateral pterygoid process of the sphenoid bone, which is primary cartilage, were observed as a control. Thin sections were subjected to immunostaining and alkaline phosphatase (ALP) staining. Using a confocal laser microscope, 3D stereoscopic reconstruction of hypertrophic cells was performed. To evaluate the size of hypertrophic chondrocytes objectively, the cell size was measured in each cartilage.

RESULTS

Hypertrophic chondrocytes of secondary cartilage first expressed type X collagen (Col X) at E15.5. SRY-box 9 (Sox 9) and ALP were co-expressed in the fibroblastic/polymorphic tissue layer of secondary cartilage. This layer was very thick at E15.5, and then rapidly became thin. Hypertrophic cells in secondary cartilage were markedly smaller than those in primary cartilage.

CONCLUSIONS

The small hypertrophic cells present in secondary cartilage may have been a characteristic acquired in order for the cartilage to smoothly promote a marked increase in hypertrophic cells and rapid calcification.

Developmental mechanism of muscle-tendon-bone complex in the fetal soft palate

OBJECTIVE

This study was performed to investigate how the palatine aponeurosis, medial pterygoid process (MPP) of the sphenoid bone, and tensor veli palatini (TVP) muscle form the pulley: muscle-tendon-bone complex.

DESIGN

Mice at embryonic day (ED) 14-17 were used as sample in this study. Azan staining was performed to observe the morphology, and immunohistochemical staining of desmin was performed to closely observe the development of the myotendinous junction. To confirm the bone formation process, immunohistochemical staining of type II collagen (col II), tartrate-resistant acid phosphatase (TRAP), and alkaline phosphatase (ALP) staining were performed. Furthermore, to objectively evaluate bone formation, the major axis and width of the MPP were measured, and osteoclasts that appeared in the MPP were counted.

RESULTS

At ED 14 and 14.5, ALP showed a reaction throughout the MPP. The col II-positive area expanded until ED 16.5, but it was markedly reduced at ED 17. The TVP initially contacted with the palatine aponeurosis at ED 16.5. The major axis and width of the MPP and the number of TRAP-positive osteoclasts were significantly increased as the TVP and palatine aponeurosis joined.

CONCLUSIONS

Therefore, in addition to the tissue units: muscle, tendon, and bone, the interaction in organogenesis promotes rapid growth of the pulley: muscle-tendon-bone complex.

Histological study of the developing pterygoid process of the fetal mouse sphenoid

The pterygoid process undergoes ossification of both the cartilage and membrane. However, few studies have attempted to explore the sequential development of the pterygoid process. Using histological examination, we performed morphological observations of the pterygoid process and surrounding tissue. ICR mice at embryonic days 13.5-18.0 and postnatal day 0 were used for morphological observations of the pterygoid process. By embryonic day 14.5, a mesenchymal cell condensation forming the anlage of the future medial pterygoid process differentiated into osteoid-like tissue and cartilage. At embryonic days 15.5-16.5, cartilage cells were clearly evident in the medial pterygoid process. In the medial pterygoid process, a bone collar was evident and calcified bone tissue surrounded the cartilage. At this point, a mesenchymal cell condensation formed the anlage of the pterygoid hamulus. At embryonic days 17.0-18.0, the cartilages were located along the lower and posterior border of the medial pterygoid process. A metachromatically stained matrix first became detectable around cells located in the pterygoid hamulus. On the other hand, at embryonic day 13.5, a metachromatically stained matrix was already evident in the space between the flattened cells in the lateral pterygoid process. At embryonic day 17.0, a hypertrophic cell zone had clearly formed in the diaphysis. On the basis of our present investigation, the lateral pterygoid process can be classified as primary cartilage, whereas the medial pterygoid process can be classified as secondary cartilage. Furthermore, it was found that the pterygoid hamulus is formed latest in the medial pterygoid process.

In utero and exo utero fetal surgery on histogenesis of organs in animals

Until recently, fetal surgery was only used for fetuses with very poor prognosis who were likely to die without intervention. With advances in imaging, endoscopic techniques, anesthesia and novel interventions, fetal surgery is becoming a realistic option for conditions with less severe prognoses, where the aim is now to improve quality of life rather than simply allow survival. Until forty years ago, the uterus shielded the fetus from observation and therapy. Rapid changes in the diagnosis and treatment of human fetal anatomical abnormalities are due to improved fetal imaging studies, fetal sampling techniques (e.g., amniocentesis and chorionic villus sampling), and a better understanding of fetal pathophysiology derived from laboratory animals. Fetal therapy is the logical culmination of progress in fetal diagnosis. In other words, the fetus is now a patient. Now-a-days, in utero (IU) and exo utero (EU) surgical methods are popular for experimental analyses of the histogenesis of organ development. Using these surgical methods, developmental anomalies can be created and then repaired. By applying microinjection and/or fetal surgery with these methods, models of developmental anomalies such as neural tube defects, temporomandibular joint defects, hip joint defects, digit amputation, limb and digit development and regeneration, and tooth germ transplantation in the jaw could be created and later observed. After observing different types of anomalies, novel IU and EU surgical techniques would be the best approach for repairing or treating those anomalies or diseases. This review will focus on the rationale for the IU and EU creation of animal models of different organ defects or anomalies and their repair, based on analyses of organ histogenesis and pathologic observations. It will also focus in detail on the surgical techniques of both IU and EU methods.

Fetal jaw movement affects Ihh signaling in mandibular condylar cartilage development: the possible role of Ihh as mechanotransduction mediator

OBJECTIVE

Jaw movement is an important mechanical factor for prenatal development of the condylar cartilage of mandible. Fetal jaw movement restriction has been shown to cause deformity of the mandibular condyle. We hypothesized that this treatment affects the expression of mechanosensitive molecules, namely Indian hedgehog (Ihh) and Parathyroid hormone related protein (PTHrP) in the condyle.

EXPERIMENTAL METHODS

We restrained jaw movement by suturing the jaw of E15.5 mouse embryos and allowed them to develop until E18.5 using exo utero system, and analyzed them by immunohistochemistry and in situ hybridization methods.

RESULTS

Morphological, histomorphometric and immunohistochemical study showed that the mandibular condylar cartilage was reduced and deformed, the volume and total cell numbers in the condylar cartilage were also reduced, and number and/or distribution of 5-bromo-2'-deoxyuridine-positive cells, Ihh-positive cells in the mesenchymal and pre-hypertrophic zones were significantly and correspondingly decreased in the sutured group. Using in situ hybridization, reduced expression of Ihh, PTHrP and their related receptors were observed in condylar cartilage of the sutured embryos.

CONCLUSIONS

Our results revealed that the altered mechanical stress induced by prenatal jaw movement restriction decreased proliferating cells, the amount of cartilage, and altered expression of the Ihh and PTHrP, suggesting that Ihh act as mechanotransduction mediators in the development of mandibular condylar cartilage.

In vivo analysis of Arg-Gly-Asp sequence/integrin α5β1-mediated signal involvement in embryonic enchondral ossification by exo utero development system

Enchondral ossification is a fundamental mechanism for longitudinal bone growth during vertebrate development. In vitro studies suggested that functional blockade with RGD peptides or with an antibody that interferes with integrin α5β1-ligand interactions inhibited pre-hypertrophic chondrocyte differentiation. The purpose of this study is to elucidate in vivo the roles of the integrin α5β1-mediated signal through the Arg-Gly-Asp (RGD) sequence in the cell-extracellular matrix (ECM) interaction in embryonic enchondral ossification by an exo utero development system. We injected Arg-Gly-Asp-Ser (RGDS) peptides and anti-integrin α5β1 antibody (α5β1 ab) in the upper limbs of mouse embryos at embryonic day (E) 15.5 (RGDS-injected limbs, α5β1 ab-injected limbs), and compared the effects on enchondral ossification with those found in the control limbs (Arg-Gly-Glu-Ser peptide-, mouse IgG-, or vehicle-injected, and no surgery) at E16.5. In the RGDS-injected limbs, the humeri were shorter and there were fewer BrdU-positive cells than in the control limbs. The ratios of cartilage length and area to those of the humerus were higher in the RGDS-injected limbs. The ratios of type X collagen to type 2 collagen mRNA and protein (Coll X/Coll 2) were significantly lower in the RGDS-injected limbs. In those limbs, TUNEL-positive cells were hardly observed, and the ratios of fractin to the Coll X/Coll 2 ratio were lower than in the control limbs. Furthermore, the α5β1 ab-injected limbs showed results similar to those of RGDS-injected limbs. The present in vivo study by exo utero development system showed that RGDS and α5β1 ab injection decreased chondrocyte proliferation, differentiation, and apoptosis in enchondral ossification, and suggested that the integrin α5β1-mediated ECM signal through the RGD sequence is involved in embryonic enchondral ossification.

Role of Wnt5a-Ror2 signaling in morphogenesis of the metanephric mesenchyme during ureteric budding

Development of the metanephric kidney begins with the induction of a single ureteric bud (UB) on the caudal Wolffian duct (WD) in response to GDNF (glial cell line-derived neurotrophic factor) produced by the adjacent metanephric mesenchyme (MM). Mutual interaction between the UB and MM maintains expression of GDNF in the MM, thereby supporting further outgrowth and branching morphogenesis of the UB, while the MM also grows and aggregates around the branched tips of the UB. Ror2, a member of the Ror family of receptor tyrosine kinases, has been shown to act as a receptor for Wnt5a to mediate noncanonical Wnt signaling. We show that Ror2 is predominantly expressed in the MM during UB induction and that Ror2- and Wnt5a-deficient mice exhibit duplicated ureters and kidneys due to ectopic UB induction. During initial UB formation, these mutant embryos show dysregulated positioning of the MM, resulting in spatiotemporally aberrant interaction between the MM and WD, which provides the WD with inappropriate GDNF signaling. Furthermore, the numbers of proliferating cells in the mutant MM are markedly reduced compared to the wild-type MM. These results indicate an important role of Wnt5a-Ror2 signaling in morphogenesis of the MM to ensure proper epithelial tubular formation of the UB required for kidney development.

Adrenocorticotropic tumor cells transplanted into mouse embryos affect pancreatic histogenesis

A wide range of individual differences exist in the total number of functional and structural units in each organ, such as β cells in pancreatic islands, and these units are the basis of the organ's overall function, including its functional reserve. The endocrine environment may influence organ histogenesis, during which functional and structural units are formed and increase in number. We analyzed the effects of a continuous high level of adrenocorticotropic hormone (ACTH) and/or secondarily induced glucocorticoid on histogenesis of the pancreas in mouse embryos. Pituitary tumor-derived AtT20 cells, which secrete ACTH continuously, were injected subcutaneously into mouse embryos at embryonic day (E) 12.5, and the embryos were allowed to develop exo utero until E18.5 (AtT20 group). E18.5 AtT20 group embryos with high ACTH levels (23.74 ± 6.19 ng/mL vs control group, 0.48 ± 0.40 ng/mL, P < 0.05) were examined for the effects on histogenesis of the pancreas. Using serial sections of the E18.5 pancreas, we stereologically measured the volumes, and counted total cell numbers and numbers of mitotic or pyknotic cells of the whole pancreas, endocrine and exocrine cells, and glucagon-immunopositive α cells and insulin-immunopositive β cells in the endocrine part. Although the volumes of the whole pancreas and exocrine part did not change significantly, in the AtT20 group the endocrine part was significantly larger, with fewer pyknotic cells and lower ratios of α and β cells than in the control group. These results suggest that the high level of ACTH and/or glucocorticoid affects histogenesis of the pancreas.