Koji Sakiyama

Anatomy and Histology of Rodent and Human Major Salivary Glands: —Overview of the Japan Salivary Gland Society-Sponsored Workshop—

Major salivary glands of both humans and rodents consist of three pairs of macroscopic glands: parotid, submandibular, and sublingual. These glands secrete serous, mucous or mixed saliva via the proper main excretory ducts connecting the glandular bodies with the oral cavity. A series of discoveries about the salivary ducts in the 17th century by Niels Stensen (1638–1686), Thomas Wharton (1614–1673), and Caspar Bartholin (1655–1738) established the concept of exocrine secretion as well as salivary glands. Recent investigations have revealed the endocrine functions of parotin and a variety of cell growth factors produced by salivary glands. The present review aims to describe macroscopic findings on the major salivary glands of rodents and the microscopic differences between those of humans and rodents, which review should be of interest to those researchers studying salivary glands.

Expression of Myostatin and Follistatin in Mdx Mice, an Animal Model for Muscular Dystrophy

Follistatin is a functional antagonist of several members of the TGF-&bgr; family of secreted signaling factors, including myostatin, the most powerful inhibitor of muscle growth characterized to date. Myostatin inhibition offers a novel therapeutic strategy for muscular dystrophy by restoring skeletal muscle mass and suppressing the progression of muscle degeneration. To assess the potential benefits of follistatin in treating muscle degenerative diseases, we examined the expression of myostatin and follistatin in Mdx mice, a model for Duchenne muscular dystrophy, and in B10 mice as a control. Our results demonstrated a temporary and coincident expression of follistatin and myostatin in both mouse strains, but this expression was significantly higher in Mdx mice than in B10 mice. The maximum expression of follistatin and myostatin in the presence of restoring necrotic muscle was detected 4 weeks after birth in Mdx mice. Interestingly, during the stage of complete regeneration, the absence of myostatin and follistatin proteins and a marked decrease in the expression of both genes were observed 9 weeks after birth in both mouse strains. These findings suggest that follistatin not only blocks myostatin but also allows other activators to function in muscle development, emphasizing that follistatin could be a very potent molecule in combating muscle loss during dystrophies and muscle ageing, disuse, or denervation.

Effect of mechanical stretching on expressions of muscle specific transcription factors MyoD, Myf-5, myogenin and MRF4 in proliferated myoblasts.

We examined expression of four important members of myogenic regulatory factors (MRFs) in the myoblasts both at mRNA and protein levels, which were subjected to mechanical stretching in in vitro condition. Our results showed that MyoD expression existed both in the stretch and in the control group at all time periods of the mechanical stimulus. Myf‐5 expressed only at early stage of the stretch group. Although mRNA and protein expressions of myogenin and MRF4 were detected both in the stretch and in the control group at 12 h after the stretching, their expressions were only shown in the stretch group at 24 h after the mechanical stimulus. However, at 36 and 48 h, none of the MRFs examined except MyoD appeared in both groups. Our results suggest that the MRFs are up‐regulated upon mechanical stimulus and each member plays a different major role for either proliferation or differentiation of the myoblasts.

Meckel's Cartilage: Discovery, Embryology and Evolution: —Overview of the Specificity of Meckel's Cartilage—

Abstract Meckels cartilage, discovered by German anatomist J. F. Meckel (1781–1833), is hyaline cartilage formed in the mandibular process of the first branchial arch of vertebrate embryos. An intermediate portion of Meckels cartilage is absorbed by multinucleated cells and disappears during the following developmental stages in mammals. The process of Meckels cartilage disappearance is not accompanied by the apoptosis of chondrocytes. The most posterior portion of the cartilage is changed to auditory ossicles by endochondral ossification, and the rest of the posterior portion transdifferentiates into sphenomandibular ligament. Meckels cartilage is not considered an anlage of the mandible, since bony tissue of the mandible is independently formed by membranous ossification. Chondrocytes are differentiated from mesodermal cells in general, whereas cells forming Meckels cartilage are differentiated from ectodermal mesenchymal cells of neural crest origin. In reptiles, Meckels cartilage ossifies and forms bone in the lower jaws, and cretaceous mammals had calcified Meckels cartilage in addition to a mandible. During the evolution from reptile to mammal, the mode of mastication changed, and the origin of bone in the lower jaw may have been gradually transferred from Meckels cartilage to the mandible.

Three-Dimensional Analysis of the Internal Structure of the Mandibular Condyle in Dentulous and Edentulous Jaws Using Micro-CT

Abstract This study clarifies the internal structure of the mandibular condyle in dentulous and edentulous jaws; three-dimensional observation and morphological measurements were performed using micro-CT. The materials used in this study were 14 mandibular condyles each removed from the dentulous and edentulous cadavers. Internal structures of the mandibular condyle were morphologically analyzed in three anatomical planes. Additionally, morphometrical observations of the trabecular bone structure at five regions of interest and cortical bone thickness were performed. The mandibular condyle of dentulous jaws consisted predominantly of regularly aligned plate-shaped trabeculae. Conversely, edentulous jaws were predominantly irregularly aligned narrow rod-shaped trabeculae. The results of morphometrical analysis showed that trabecular separation (Tb.Sp) in the central region of the mandibular condyle most closely reflected the difference between dentulous and edentulous jaws. These results suggest that, in edentulous jaws, reduced masticatory function due to tooth loss affects the internal structures of mandibular condyle.

Muscle Plasticity: Changes in Oral Function of Muscle Fiber Characteristics

To clarify changes in oral function and its relation to muscle fiber characteristics, we evaluated changes in isoforms of the myosin heavy chain as a muscle contraction protein in the mouse masseter, temporalis, and tongue muscles after weaning. As a result, all muscles showed changes in muscle fiber characteristics immediately after weaning, i. e., at the initiation of mastication, increasing the number of isoforms. This finding was marked in the temporalis muscle. In addition, when mechanical stress was applied to myoblasts, the numbers of myoblasts and nuclei in each myoblast increased, and muscle fiber characteristics also changed.

Origin of the torus mandibularis: An embryological hypothesis

Torus mandibularis, a well‐known protuberance in the dental field, has been defined as a hyperostosis in the lingual aspect of the body of the mandible above the mylohyoid line. However, the origin of the torus mandibularis has not yet been clarified. The aim of this study was to provide a better understanding on the origin of the torus in view of the specific development of Meckels cartilage at the site corresponding to the adult torus. A total of 40 mid‐term human fetuses at 7–16 weeks of gestation were examined. The 10–13 weeks stage corresponded to the critical period in which Meckels cartilage with endochondral ossification underwent a bending at the beginning of the intramandibular course. At the level of mental foramen, which was located between the deciduous canine and the first deciduous molar germs, the medial lamina of the mandible protruded medially to reach Meckels cartilage. Thus, the medial lamina covered the posterior and superior aspect of the bending Meckels cartilage just above the attachment of the developing mylohyoid muscle (i.e., in the oral cavity). We considered a bony prominence, which composed the protruding medial lamina and the bending Meckels cartilage as the fetal origin of the torus mandibularis. A new theory is proposed for the origin of the torus mandibularis based on the existence of an anlage formed during the development of the mandible, variable in morphology and size, but always constant. Clin. Anat. 26:944–952, 2013.

Effects of mechanical stretching on caspase and IGF-1 expression during the proliferation process of myoblasts.

Abstract It has been reported that the synthesis, degradation, and metabolism of muscle proteins in myoblasts, as well as the proliferation and differentiation of cells, are influenced by various related to extracellular signaling molecules, such as neural transmitters, growth factors, and hormones, when muscle tissue has been exposed to mechanical stimulation. However, reports regarding the expression of growth factors during mechanical stimulation of myoblasts are few, and many questions remain unanswered. We examined the mRNA expression of insulin-like growth factor 1 (IGF-1) in myoblasts subjected to mechanical stretching in vitro. In addition, apoptosis caused by intracellular stress has been reported to occur during muscle development at the embryonic stage. To clarify the expression of intracellular stress factors, we here investigated related gene expression. Expression of IGF-1 increased in the early stage of cell stretching, followed by a decrease in the late stage. This suggests that mechanical stimulation resulted in an immediate increase in IGF-1 expression, followed by a decrease as cells acclimated to the inducing environment. Caspase was significantly expressed in a stretch group at 12 hours after the beginning of mechanical stimulation, compared with a control group. This suggests that cellular proliferation is also regulated by intracellular stress factors involving the endoplasmic reticulum, mitochondria, and other organelles during the process of muscle proliferation and differentiation

Changes in the Myosin Heavy Chain 2a and 2b Isoforms of the Anterior Belly of the Digastric Muscle Before and After Weaning in Mice

During the process of growth and development, the digastric muscle is subjected to marked functional changes, including the change from suckling to mastication. In particular, because the anterior belly of the digastric muscle, which is one of the suprahyoid muscles, plays an important role in mastication. Therefore, this muscle seems to undergo a marked functional change before and after weaning. However, the details remain unknown. Here, to clarify the changes in the muscle fibre characteristics of the anterior belly of the digastric muscle before and after weaning, we examined myosin heavy chain isoforms at the protein (immunohistochemistry) and mRNA (transcription) levels. As a control, the changes in the muscle fibre characteristics of the sternohyoid muscle, which is anatomically aligned in the same direction as the anterior belly of the digastric muscle, were analyzed. The results showed that, in the anterior belly of the digastric muscle that is involved in mandibular movements in mice, the ratio of a fast‐contraction isoform with strong contractile force increased after weaning. We believe that this occurred in response to a functional change from suckling to mastication. On the other hand, there was little change in the composition of sternohyoid muscle.

Localization of Heat Shock Protein 27 (Hsp27) in the Rat Gingiva and its Changes with Tooth Eruption

Heat shock protein 27 kDa (Hsp27) functions as a molecular chaperon to prevent apoptosis as well as to contribute to the regulation of cell proliferation and differentiation during development. In the present study, the localization of Hsp27 in the oral epithelium of rats and its expression change during formation of the gingiva with the tooth eruption were examined immunohistochemically to elucidate the roles of Hsp27 in the oral mucosa. In adult rats, Hsp27-immunoreactivity was localized in the prickle and granular layers but absent in the basal and horny layers of the oral epithelium. On the other hand, in the outer and sulcular epithelia of the free gingival, Hsp27-immunoreactivity was detected in the whole layers, while it was not found in the proliferation zone of the junctional epithelium immunoreactive for Ki67. In immature rats on 10th postnatal day, Hsp27-immunoreactivity was intense in the prickle and granular layers of the oral epithelium, but was not detected in its basal layer. In rats at the eruptive phase on 15th postnatal day, Hsp27-immunoreactivity was detected in sites of the basal layer adjacent to where the dental cusps penetrated through the oral epithelium. Although the immunoreactivity for Ki67 was found in the basal layer of the oral epithelium, it was not localized in the Hsp27-immunopositive sites of tooth-penetration in the basal layer. Just after the tooth-eruption on 20th postnatal day, Hsp27-immunoreactivity was not found in the stratified squamous epithelium at the dentogingival junction, whereas it was intense in a single layer of cuboidal epithelial cells attached to the tooth neck. Ki67-positive cells were scattered in the stratified squamous epithelium at the dentogingival junction, whereas no positive cells were found in the portion of a single layer of cuboidal epithelial cells. These findings suggest that the outer and sulcular epithelia of the free gingiva have a relatively slower rate of proliferation than other gingival and oral epithelia, and that Hsp27 might inhibit the proliferation of the basal cells. Such specific phenomenon in the free gingiva occurred immediately after the dental cusps were exposed to the oral cavity.