Evolution of skull shape in the family Salamandridae (Amphibia: Caudata)

We carried out a comparative morphometric analysis of 56 species of salamandrid salamanders, representing 19 out of 21 extant genera, with the aim of uncovering the major patterns of skull shape diversification, and revealing possible trends and directions of evolutionary change. To do this we used micro‐computed tomography scanning and three‐dimensional geometric morphometrics, along with a well‐resolved molecular phylogeny. We found that allometry explains a relatively small amount of shape variation across taxa. Congeneric species of salamandrid salamanders are more similar to each other and cluster together producing distinct groups in morphospace. We detected a strong phylogenetic signal and little homoplasy. The most pronounced changes in the skull shape are related to the changes of the frontosquamosal arch, a unique feature of the cranial skeleton for the family Salamandridae, which is formed by processes arising from the frontal and squamosal bones that arch over the orbits. By mapping character states over the phylogeny, we found that a reduction of the frontosquamosal arch occurs independently in three lineages of the subfamily Pleurodelinae. This reduction can probably be attributed to changes in the development and ossification rates of the frontosquamosal arch. In general, our results are similar to those obtained for caecilian amphibians, with an early expansion into the available morphospace and a complex history characterizing evolution of skull shape in both groups. To evaluate the specificity of the inferred evolutionary trajectories and Caudata‐wide trends in the diversity of skull morphology, information from additional groups of tailed amphibians is needed.     

Embryonic skull development in the gecko,Tarentola annularis (Squamata: Gekkota: Phyllodactylidae)

Tarentola annularis is a climbing gecko with a wide distribution in Africa north of the equator. In the present paper, we describe the development of the osteocranium of this lizard, from the first appearance of the cranial elements up to the point of hatching. This is based on a combination of histology and cleared and stained specimens. This is the first comprehensive account of gekkotan pre-hatching skull development based on a comprehensive series of embryos, rather than a few selected stages. Given that Gekkota is now widely regarded as representing the sister group to other squamates, this account helps to fill a significant gap in the literature. Moreover, as many authors have considered features of the gekkotan skull and skeleton to be indicative of paedomorphosis, it is important to know whether this hypothesis is supported by delays in the onset of cranial ossification. In fact, we found the sequence of cranial bone ossification to be broadly comparable to that of other squamates studied to date, with no significant lags in development.     

Skull development during anuran metamorphosis

Summary We examined the role of thyroid hormone (TH) in mediating cranial ossification during metamorphosis in the Oriental fire-bellied toad, Bombina orientalis. Exogenous T₃ (3,3,5-triiodo-L-thyronine) was administered in three treatment dosages (0.025, 0.25, and 2.5 g) plus a control dosage via plastic micropellets implanted within the dermis of tadpoles of three Gosner developmental stages: 28/29, 30/31, 32/33. Tadpoles were recovered after 2, 4, 6, and 8 d, and scored for the presence of three bones —median parasphenoid and paired frontoparietals and exoccipitals—as seen in cleared-and-stained, whole-mount preparations. T₃ induced precocious ossification in both a stage-dependent and a dosage-dependent manner; stage dependence corresponded precisely with the degree of osteogenic differentiation at the time of hormone administration. Precocious ossification thus was due to the T₃-promoted growth and calcified matrix deposition of these centers. Differential TH sensitivity among osteogenic sites may underlie both the temporal cranial ossification sequences characteristic of metamorphosing amphibians as well as sequence differences commonly observed among taxa.     

Development of the amphibian oculomotor complex: Evidences for migration of oculomotor motoneurons across the midline

Summary The development of the oculomotor nucleus in five species of salamanders and one anuran species was investigated with tracing techniques. The data presented support the hypothesis that oculomotor motoneurons innervating the superior rectus muscle migrate across the midline. In the salamander Pleurodeles waltl, only ipsilateral oculomotor motoneurons are labeled in early development. Later, these neurons extend dendrites toward the contralateral side into the ventral tegmental neuropil, after which there is displacement of their nuclei (neuronal somata) across the midline. Cell bodies can be observed directly at the midline. In adult Salamandra salamandra, motoneurons innervating the superior rectus muscle are seen occasionally at the midline and on the ipsilateral side, with dendrites toward the contralateral side. Motoneurons on the ipsilateral side do not display these features. In Pleurodeles, developmental brain processes are slowed down, and the sequence of development of the contralateral subnucleus, which can be clearly observed, supports the migration hypothesis. In Xenopus laevis and most other species of salamanders this process is accelerated.     

Anatomical Assessment of the Adult Skeleton of Zebrafish Reared Under Different Thyroid Hormone Profiles

Thyroid hormone (TH) directs the growth and maintenance of tissues throughout the body during development and into adulthood, and plays a particularly important role in proper ossification and homeostasis of the skeleton. To better understand the roles of TH in the skeletogenesis of a vertebrate model, and to define areas of the skeleton that are particularly sensitive to developmental TH, we examined the effects of hypo- and hyperthyroidism on skeletal development in zebrafish. Performing a bone-by-bone anatomical assessment on the entire skeleton of adult fish, we found that TH is required for proper ossification, growth, morphogenesis, and fusion of numerous bones. We showed that the pectoral girdle, dermatocranium, Weberian apparatus, and dentary are particularly sensitive to TH, and that TH affects development of skeletal element regardless of bone type and developmental origin. Indeed, the hormone does not universally promote ossification: we found that developmental TH prevents ectopic ossification in multiple thin bones and within connective tissue of the jaw. In all, we found that TH regulates proper morphogenesis and ossification in the majority of zebrafish bones, and that the requirement for the hormone extends across bone types and developmental profiles. Anat Rec, 302:1754–1769, 2019. © 2019 American Association for Anatomy     

Occipital Ossification of Balaenopteroid Mysticetes

The bones of the posterior portion of the mammalian skull often exhibit incomplete ossification of the joints between the bones at the time of birth, with complete ossification at some point after birth. The sequence of ossification of these joints in mysticetes can be used to characterize the relative age in the calf and early juvenile ontogenetic stages. This study examined occipital joints ossification of 38 dry prepared neonate specimens in four mysticete species from two families (Eschrichtiidae: Eschrichtius robustus; Balaenopteridae: Balaenoptera acutorostrata, Balaenoptera physalus, and Megaptera novaeangliae). Each of the joints responsible for the fusion of the occiput were examined and rated for degree of ossification. The cranial ossification analysis indicates that E. robustus calves have open occipital joints until ～6 months of age and are born at a less mature stage than closely related balaenopterids. All of the species followed the same sequence of ossification: basioccipital/exoccipital joint, followed by the basioccipital/basisphenoid joint, and completed by the supraoccipital/exoccipital joint. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.     

Nasal Septal and Premaxillary Developmental Integration: Implications for Facial Reduction in Homo

The influence of the chondrocranium in craniofacial development and its role in the reduction of facial size and projection in the genus Homo is incompletely understood. As one component of the chondrocranium, the nasal septum has been argued to play a significant role in human midfacial growth, particularly with respect to its interaction with the premaxilla during prenatal and early postnatal development. Thus, understanding the precise role of nasal septal growth on the facial skeleton is potentially informative with respect to the evolutionary change in craniofacial form. In this study, we assessed the integrative effects of the nasal septum and premaxilla by experimentally reducing facial length in Sus scrofa via circummaxillary suture fixation. Following from the nasal septal‐traction model, we tested the following hypotheses: (1) facial growth restriction produces no change in nasal septum length; and (2) restriction of facial length produces compensatory premaxillary growth due to continued nasal septal growth. With respect to hypothesis 1, we found no significant differences in septum length (using the vomer as a proxy) in our experimental (n = 10), control (n = 9) and surgical sham (n = 9) trial groups. With respect to hypothesis 2, the experimental group exhibited a significant increase in premaxilla length. Our hypotheses were further supported by multivariate geometric morphometric analysis and support an integrative relationship between the nasal septum and premaxilla. Thus, continued assessment of the growth and integration of the nasal septum and premaxilla is potentially informative regarding the complex developmental mechanisms that underlie facial reduction in genus Homo evolution. Anat Rec, 2010. © 2010 Wiley‐Liss, Inc.     

Ellis Van Creveld2 is Required for Postnatal Craniofacial Bone Development

Ellis‐van Creveld (EvC) syndrome is a genetic disorder with mutations in either EVC or EVC2 gene. Previous case studies reported that EvC patients underwent orthodontic treatment, suggesting the presence of craniofacial bone phenotypes. To investigate whether a mutation in EVC2 gene causes a craniofacial bone phenotype, Evc2 knockout (KO) mice were generated and cephalometric analysis was performed. The heads of wild type (WT), heterozygous (Het) and homozygous Evc2 KO mice (1‐, 3‐, and 6‐week‐old) were prepared and cephalometric analysis based on the selected reference points on lateral X‐ray radiographs was performed. The linear and angular bone measurements were then calculated, compared between WT, Het and KO and statistically analyzed at each time point. Our data showed that length of craniofacial bones in KO was significantly lowered by ～20% to that of WT and Het, the growth of certain bones, including nasal bone, palatal length, and premaxilla was more affected in KO, and the reduction in these bone length was more significantly enhanced at later postnatal time points (3 and 6 weeks) than early time point (1 week). Furthermore, bone‐to‐bone relationship to cranial base and cranial vault in KO was remarkably changed, i.e. cranial vault and nasal bone were depressed and premaxilla and mandible were developed in a more ventral direction. Our study was the first to show the cause‐effect relationship between Evc2 deficiency and craniofacial defects in EvC syndrome, demonstrating that Evc2 is required for craniofacial bone development and its deficiency leads to specific facial bone growth defect. Anat Rec, 299:1110–1120, 2016. © 2016 Wiley Periodicals, Inc.     

Wortmannin enhances lipopolysaccharide-induced inducible nitric oxide synthase expression in microglia in the presence of astrocytes in rats

There is evidence that morphological alterations concerning deficiency or abundance of thyroid hormones (TH) may be mediated by brain-derived neurotrophic factor (BDNF). It has been demonstrated that the mRNA-expression of BDNF is increased after TH-treatment during the first postnatal weeks. After transient treatment of newborn rats with thyroxine mRNA expression, protein concentration and number and size of BDNF-immunopositive neurons were quantified in the medial septum/vertical diagonal Band of Broca (MS/vDB). The number and size of BDNF-immunopositive neurons were estimated in young (P10) and adult (4 months). The amount of mRNA and protein are significantly increased in TH-treated rats at P10 compared to control animals. TH-treated animals showed a significant decrease of BDNF-immunopositive cell numbers in the adulthood. The results demonstrate a correlated increase of BDNF mRNA and protein in the septum at P10 which is an important stage of differentiation processes in the septohippocampal system. These results provide further evidence that BDNF is a possible candidate for the mediation the TH effects in the MS/vDB.     

The premaxilla: Embryological evidence that it does not exist in man

Many accounts of the human premaxilla describe its ossification centers and time of fusion with the maxilla, but that such a bone even exists in the human has long been questioned. Very few specimens undergoing initial phases of ossification have been reported and no convincing photographs of separate centers have been published. This report is based on 90 serially sectioned human embryos whose ages (Streeter's Horizons XVIII to XXIII) were closely grouped around the age when ossification of the upper jaw begins. There is but one ossification center bilaterally which, although it appears first in the cuspid region, rapidly involves an area extending from the molar to the central incisor region. Not one of the specimens showed an independent center for the “premaxilla” nor an “incisive suture” in the area of ossification. The premaxilla does not exist as an independent bone in man.     

Clinical significance of the sphenoidal process of the cartilaginous nasal septum: A preliminary morphological evaluation

The bony septum ossifies from cranial to caudal and from ventral to dorsal, thereby forming the perpendicular plate of ethmoid bone and vomer. A small strip of cartilage from the cartilaginous septum remains between these parts, the so‐called sphenoidal process of the cartilaginous septum. This sphenoidal process is usually seen at the top of a deviated nasal septum during septoplasty. This study evaluated the clinical significance of the sphenoidal process of the cartilaginous septum as a cause of nasal septal deviation. We studied 37 patients with septal deviation who underwent septoplasty. The complex consisting of the sphenoidal process of the cartilaginous septum, perpendicular plate of the ethmoid bone (PPE), and vomer was removed at surgery. The sphenoidal process was measured and the surgical specimen were evaluated histologically and compared to controls. The mean length of the sphenoidal process in patients with a deviated nasal septum was 26.05 ± 5.32 mm versus 11.95 ± 2.38 mm in controls. The sphenoidal process was significantly longer in the patients with a deviated nasal septum (P < 0.05). The sphenoidal process of the cartilaginous septum was connected to the PPE and vomer in the patients with a deviated nasal septum and differed histologically from that of the controls. The sphenoidal process of the cartilaginous septum was long and prominent in the patients with septal deviation, implying that delayed nasal septal ossification may be one cause of nasal septal deviation. Clin. Anat. 23:265–269, 2010. © 2010 Wiley‐Liss, Inc.     

Long bone histology of the stem salamander Kokartus honorarius (Amphibia: Caudata) from the Middle Jurassic of Kyrgyzstan

Kokartus honorarius from the Middle Jurassic (Bathonian) of Kyrgyzstan is one of the oldest salamanders in the fossil record, characterized by a mixture of plesiomorphic morphological features and characters shared with crown-group salamanders. Here we present a detailed histological analysis of its long bones. The analysis of a growth series demonstrates a significant histological maturation during ontogeny, expressed by the progressive appearance of longitudinally oriented primary vascular canals, primary osteons, growth marks, remodelling features in primary bone tissues, as well as progressive resorption of the calcified cartilage, formation of endochondral bone and development of cartilaginous to bony trabeculae in the epiphyses. Apart from the presence of secondary osteons, the long bone histology of Kokartus is very similar to that of miniaturized temnospondyls, other Jurassic stem salamanders, miniaturized seymouriamorphs and modern crown-group salamanders. We propose that the presence of secondary osteons in Kokartus honorarius is a plesiomorphic feature, and the loss of secondary osteons in the long bones of crown-group salamanders as well as in those of miniaturized temnospondyls is the result of miniaturization processes. Hitherto, all stem salamander long bong histology (Kokartus, Marmorerpeton and ‘salamander A’) has been generally described as having paedomorphic features (i.e. the presence of Katschenko''s Line and a layer of calcified cartilage), these taxa were thus most likely neotenic forms. The absence of clear lines of arrested growth and annuli in long bones of Kokartus honorarius suggests that the animals lived in an environment with stable local conditions.     

Postnatal development of dependence on thyroid hormones for growth and differentiation of rat skeletal structures

We have studied the development of thyroid hormone dependence for growth and differentiation of rat skeletal tissues. The radius, tail tip or xiphoid process from 1-day-old rats was transplanted in its entirety under the kidney capsule of euthyroid or hypothyroid (TX) adult syngeneic virgin rats and grown there for 3, 6, 9, 12 or 15 days. Pieces of the sternum or calvarium were treated similarly. Growth of transplanted radius and sternum in TX hosts showed significant retardation by day 3, and that of transplanted calvarium, tail tip and xiphoid showed significant inhibition by day 6 post transplantation. Tissue differentiation of all of the transplants was inhibited in TX hosts and the appearance of epiphyseal ossification centers was significantly delayed. Injection of Propylthiouracil (PTU) in solvent into the neonatal rats from day 1 to day 15 post partum inhibited tail length increase by day 7, and body weight gain by day 9, as compared to the growth of solvent injected littermate pups. Our results indicate that the growth and differentiation of endochondral bones are more dependent on TH levels than are those of membranous bones and cartilage. Endochondral bones develop TH dependence for growth and differentiation before day 4 post partum, while the membranous bones and cartilage develop this dependence at the end of the first week.     

Delayed membranous cranial ossification in a mother and child

We report on a girl and her mother with delayed intramembranous ossification of the cranial vault. The 11-month-old girl had a large ossification defect involving parietal bones, squamous portion of temporal bones, and interparietal region of occipital bone, while the mother showed a complete ossified cranial vault with flat posterior parietal region and prominent occiput. Both had a similar face characterized by frontal bossing, hypertelorism, downward slant of palpebral fissures, flat nasal bridge, and short midface. On reviewing the literature, we concluded that these cases may be a dominant transmitted ossification defect with characteristic face, different from the cranium bifidum-parietal foramina entity. © 1992 Wiley-Liss, Inc.     

Ontogeny of the Shell Bones of Embryos of Podocnemis unifilis (Troschel, 1848) (Testudines,Podocnemididae)

The aim of the present study was to investigate the sequence of shell bone formation in the embryos of the Pleurodira, Podocnemis unifilis. Their bones and cartilage were collected and cleared before staining. The shell was also examined by obtaining a series of histological slices. All the bony elements of the plastron have independent ossification centers, which subsequently join together and retain two fontanelles until the period of hatching. This turtle has a mesoplastra, which is characteristic of the Podocnemididae. The carapace begins to form concurrently with the ossification of the ribs at the beginning of stage 20. All the plates, except the suprapygal, initiate ossification during the embryonic period. The main purpose of the histological investigation was to highlight the relationship between the formation of the carapace and ribs with that of the neural plates. The costal and neural plates were found not to independent ossification centers, but to be closely related to components of the endoskeleton, originating as expansions of the perichondral collar of the ribs and the neural arches, respectively. Considering the ribs as an endoskeletal element of the carapace, the carapace and plastron begin ossification at the same stage in P. unifilis. This pattern reveals similarities with other Pleurodira, as well as evident variations, such as the presence of the seven neural bones and the presence of only one ossification center in the nuchal plate. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.