Cranial muscles in amphibians: development, novelties and the role of cranial neural crest cells

Head dysgenesis and disruption of cranial neural crest stem cells behaviour by 4-octylphenol in fire-bellied toad Bombina orientalis embryos

Alkylphenolic endocrine disruptors (Eds) have been known to affect development of the descendants of multipotent neural crest cells (NCCs) in amphibian embryos. To unravel the mechanism of head dysgenesis induced by alkylphenols in amphibians, the effect of 4-octylphenol (OP) on the differentiation of cranial NCCs in developing embryos and tadpoles, ex vivo NC explant, and isolated NCCs was examined in fire-bellied toad Bombina orientalis with 0, 1, 2, 5, 10, 25 and 50 μM concentrations. Following OP treatment, head cartilages were frequently absent together with the decreased col2a1 mRNA level in tadpoles. While the lipid hydroperoxide (LPO), endoplasmic reticulum stress (ERS), apoptosis, and DNA fragmentation were significantly increased in stage 22 neulurae and heads of stage 45 tadpoles. In stage 22 neulurae, OP decreased sox9 mRNA, the master transcription factor for chondrogenic differentiation and increased undifferentiated NCC markers. The ectopic NCCs were found in endoderm while mesodermal SOX10(+) cells were decreased. In cranial NCCs isolated from stage 22 embryos, OP treatment decreased cellular survival and increased apoptosis, epithelial-mesenchymal transition (EMT) and cell migration. In chondrogenic induced cranial NC explants, OP treatment decreased SOX9(+) chondrocytes and cartilage development. Together, OP potentiated oxidative damage, apoptosis, EMT, and ectopic migration of NCCs. Considering that tissue differentiation requires stem cells to activate the molecular mechanism of differentiation at the correct location during embryonic development, these changes caused by OP may inhibit sox9-dependent chondrogenic differentiation of cranial NCCs, leading to head dysgenesis in B. orientalis embryos. Therefore, developing multipotent NCCs could be an important target of OP, provides new direction for the estimation of the risk of EDs exposure in human and wildlife animals.

Normal development in Xenopus laevis: A complementary staging table for the skull based on cartilage and bone

BACKGROUND

Xenopus laevis is a widely used model organism in the fields of genetics and development, and more recently evolution. At present, the most widely used staging table for X. laevis is based primarily on external features and does not describe the corresponding skull development in detail. Here, we describe skull development in X. laevis, complete with labelled figures, for each relevant stage in the most widely used staging table.

RESULTS

We find skull development in X. laevis is, for the most part, distinct at each of the previously established stages based on external anatomy. However, variation does exist in the timing of onset of ossification of certain bones in the skull, which results in a range of stages where a skull element first ossifies. The overall sequence of ossification is less variable than the timing of ossification onset.

CONCLUSIONS

While events in skull development vary somewhat between specimens, and in comparison, to external events, this staging table is useful in showing both when bones first appear and for documenting the range of temporal variance in X. laevis skull development more accurately than previously done. Furthermore, when only skull data is available, the approximate stage of a specimen can now be determined. This article is protected by copyright. All rights reserved.

Suckling, Feeding, and Swallowing: Behaviors, Circuits, and Targets for Neurodevelopmental Pathology

All mammals must suckle and swallow at birth, and subsequently chew and swallow solid foods, for optimal growth and health. These initially innate behaviors depend critically upon coordinated development of the mouth, tongue, pharynx, and larynx as well as the cranial nerves that control these structures. Disrupted suckling, feeding, and swallowing from birth onward-perinatal dysphagia-is often associated with several neurodevelopmental disorders that subsequently alter complex behaviors. Apparently, a broad range of neurodevelopmental pathologic mechanisms also target oropharyngeal and cranial nerve differentiation. These aberrant mechanisms, including altered patterning, progenitor specification, and neurite growth, prefigure dysphagia and may then compromise circuits for additional behavioral capacities. Thus, perinatal dysphagia may be an early indicator of disrupted genetic and developmental programs that compromise neural circuits and yield a broad range of behavioral deficits in neurodevelopmental disorders.

Development of human limb muscles based on whole-mount immunostaining and the links between ontogeny and evolution

We provide the first detailed ontogenetic analysis of human limb muscles using whole-mount immunostaining. We compare our observations with the few earlier studies that have focused on the development of these muscles, and with data available on limb evolution, variations and pathologies. Our study confirms the transient presence of several atavistic muscles - present in our ancestors but normally absent from the adult human - during normal embryonic human development, and reveals the existence of others not previously described in human embryos. These atavistic muscles are found both as rare variations in the adult population and as anomalies in human congenital malformations, reinforcing the idea that such variations/anomalies can be related to delayed or arrested development. We further show that there is a striking difference in the developmental order of muscle appearance in the upper versus lower limbs, reinforcing the idea that the similarity between various distal upper versus lower limb muscles of tetrapod adults may be derived.

Novel tumor-suppressor FOXN3 is downregulated in adult acute myeloid leukemia

Forkhead box protein N3 (FOXN3) is a transcriptional repressor involved in cell cycle regulation and tumorigenesis. Abnormalities in gene structure and epigenetics of FOXN3 are closely associated with the occurrence of hematological malignancies; however, its involvement in the pathogenesis of acute myeloid leukemia (AML) remains unknown. The present study aimed to examine the potential significance of FOXN3 in AML. FOXN3 expression levels were examined in patients with AML and AML cell lines, and its clinical significance in AML was evaluated. FOXN3-overexpressing AML cell lines were established, and the biological function of FOXN3 was detected by flow cytometry and a Cell Counting Kit-8 assay. A significant decrease in FOXN3 expression levels was observed in patients with AML and in the AML cell lines in vitro. FOXN3 expression levels were associated with the number of leukocytes in patients. FOXN3 overexpression may inhibit cell proliferation in AML cell lines, induce cell cycle S-phase arrest and promote apoptosis in OCI-AML3 and THP-AML cells. The present study provided insight into how FOXN3 may serve as a novel tumor suppressor in AML.

Recent Advances in Understanding FOXN3 in Breast Cancer, and Other Malignancies

FOXN3 (forkhead box N3; CHES1: check point suppressor 1) belongs to the forkhead box (FOX) protein family. FOXN3 displays transcriptional inhibitory activity, and is involved in cell cycle regulation and tumorigenesis. FOXN3 is a tumor suppresser and alterations in FOXN3 are found in of a variety of cancers including melanoma, osteosarcoma, and hepatocellular carcinoma. While the roles of FOXN3 role in some cancers have been explored, its role in breast cancer remains unclear. Here we describe current state of knowledge of FOXN3 functions, and focus on its roles (known and potential) in breast cancer.

Identity and novelty in the avian syrinx

In its most basic conception, a novelty is simply something new. However, when many previously proposed evolutionary novelties have been illuminated by genetic, developmental, and fossil data, they have refined and narrowed our concept of biological "newness." For example, they show that these novelties can occur at one or multiple levels of biological organization. Here, we review the identity of structures in the avian vocal organ, the syrinx, and bring together developmental data on airway patterning, structural data from across tetrapods, and mathematical modeling to assess what is novel. In contrast with laryngeal cartilages that support vocal folds in other vertebrates, we find no evidence that individual cartilage rings anchoring vocal folds in the syrinx have homology with any specific elements in outgroups. Further, unlike all other vertebrate vocal organs, the syrinx is not derived from a known valve precursor, and its origin involves a transition from an evolutionary "spandrel" in the respiratory tract, the site where the trachea meets the bronchi, to a target for novel selective regimes. We find that the syrinx falls into an unusual category of novel structures: those having significant functional overlap with the structures they replace. The syrinx, along with other evolutionary novelties in sensory and signaling modalities, may more commonly involve structural changes that contribute to or modify an existing function rather than those that enable new functions.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

The North American bullfrog draft genome provides insight into hormonal regulation of long noncoding RNA

Frogs play important ecological roles, and several species are important model organisms for scientific research. The globally distributed Ranidae (true frogs) are the largest frog family, and have substantial evolutionary distance from the model laboratory Xenopus frog species. Unfortunately, there are currently no genomic resources for the former, important group of amphibians. More widely applicable amphibian genomic data is urgently needed as more than two-thirds of known species are currently threatened or are undergoing population declines. We report a 5.8 Gbp (NG50 = 69 kbp) genome assembly of a representative North American bullfrog (Rana [Lithobates] catesbeiana). The genome contains over 22,000 predicted protein-coding genes and 6,223 candidate long noncoding RNAs (lncRNAs). RNA-Seq experiments show thyroid hormone causes widespread transcriptional change among protein-coding and putative lncRNA genes. This initial bullfrog draft genome will serve as a key resource with broad utility including amphibian research, developmental biology, and environmental research.

E-cigarette aerosol exposure can cause craniofacial defects in Xenopus laevis embryos and mammalian neural crest cells

Since electronic cigarette (ECIG) introduction to American markets in 2007, vaping has surged in popularity. Many, including women of reproductive age, also believe that ECIG use is safer than traditional tobacco cigarettes and is not hazardous when pregnant. However, there are few studies investigating the effects of ECIG exposure on the developing embryo and nothing is known about potential effects on craniofacial development. Therefore, we have tested the effects of several aerosolized e-cigarette liquids (e-cigAM) in an in vivo craniofacial model, Xenopus laevis, as well as a mammalian neural crest cell line. Results demonstrate that e-cigAM exposure during embryonic development induces a variety of defects, including median facial clefts and midface hypoplasia in two of e-cigAMs tested e-cigAMs. Detailed quantitative analyses of the facial morphology revealed that nicotine is not the main factor in inducing craniofacial defects, but can exacerbate the effects of the other e-liquid components. Additionally, while two different e-cigAMs can have very similar consequences on facial appearances, there are subtle differences that could be due to the differences in e-cigAM components. Further assessment of embryos exposed to these particular e-cigAMs revealed cranial cartilage and muscle defects and a reduction in the blood supply to the face. Finally, the expression of markers for vascular and cartilage differentiation was reduced in a mammalian neural crest cell line corroborating the in vivo effects. Our work is the first to show that ECIG use could pose a potential hazard to the developing embryo and cause craniofacial birth defects. This emphasizes the need for more testing and regulation of this new popular product.

The role of folate metabolism in orofacial development and clefting

Folate deficiency has been associated with numerous diseases and birth defects including orofacial defects. However, whether folate has a role in the face during early orofacial development has been unclear. The present study reveals that pharmacological and antisense oligonucleotide mediated inhibition of DHFR, an integral enzyme in the folate pathway, results in specific changes in the size and shape of the midface and embryonic mouth. Such defects are accompanied by a severe reduction in the muscle and cartilage jaw elements without significant change in neural crest pattern or global levels of methylation. We propose that the orofacial defects associated with DHFR deficient function are the result of decreased cell proliferation and increased cell death via DNA damage. In particular, localized apoptosis may also be depleting the cells of the face that express crucial genes for the differentiation of the jaw structures. Folate supplementation is widely known to reduce human risk for orofacial clefts. In the present study, we show that activating folate metabolism can reduce median oral clefts in the primary palate by increasing cell survival. Moreover, we demonstrate that a minor decrease in DHFR function exacerbates median facial clefts caused by RAR inhibition. This work suggests that folate deficiencies could be a major contributing factor to multifactorial orofacial defects.

The emergence of Pax7-expressing muscle stem cells during vertebrate head muscle development

Pax7 expressing muscle stem cells accompany all skeletal muscles in the body and in healthy individuals, efficiently repair muscle after injury. Currently, the in vitro manipulation and culture of these cells is still in its infancy, yet muscle stem cells may be the most promising route toward the therapy of muscle diseases such as muscular dystrophies. It is often overlooked that muscular dystrophies affect head and body skeletal muscle differently. Moreover, these muscles develop differently. Specifically, head muscle and its stem cells develop from the non-somitic head mesoderm which also has cardiac competence. To which extent head muscle stem cells retain properties of the early head mesoderm and might even be able to switch between a skeletal muscle and cardiac fate is not known. This is due to the fact that the timing and mechanisms underlying head muscle stem cell development are still obscure. Consequently, it is not clear at which time point one should compare the properties of head mesodermal cells and head muscle stem cells. To shed light on this, we traced the emergence of head muscle stem cells in the key vertebrate models for myogenesis, chicken, mouse, frog and zebrafish, using Pax7 as key marker. Our study reveals a common theme of head muscle stem cell development that is quite different from the trunk. Unlike trunk muscle stem cells, head muscle stem cells do not have a previous history of Pax7 expression, instead Pax7 expression emerges de-novo. The cells develop late, and well after the head mesoderm has committed to myogenesis. We propose that this unique mechanism of muscle stem cell development is a legacy of the evolutionary history of the chordate head mesoderm.

The lateral plate mesoderm: a novel source of skeletal muscle

It has been established in the last century that the skeletal muscle cells of vertebrates originate from the paraxial mesoderm. However, recently the lateral plate mesoderm has been identified as a novel source of the skeletal muscle. The branchiomeric muscles, such as masticatory and facial muscles, receive muscle progenitor cells from both the cranial paraxial mesoderm and lateral plate mesoderm. At the occipital level, the lateral plate mesoderm is the sole source of the muscle progenitors of the dorsolateral neck muscle, such as trapezius and sternocleidomastoideus in mammals and cucullaris in birds. The lateral plate mesoderm requires a longer time for generating skeletal muscle cells than the somites. The myogenesis of the lateral plate is determined early, but not cell autonomously and requires local signals. Lateral plate myogenesis is regulated by mechanisms controlling the cranial myogenesis. The connective tissue of the lateral plate-derived muscle is formed by the cranial neural crest. Although the cranial neural crest cells do not control the early myogenesis, they regulate the patterning of the branchiomeric muscles and the cucullaris muscle. Although satellite cells derived from the cranial lateral plate show distinct properties from those of the trunk, they can respond to local signals and generate myofibers for injured muscles in the limbs. In this review, we key feature in detail the muscle forming properties of the lateral plate mesoderm and propose models of how the myogenic fate may have arisen.

Quantitative analysis of orofacial development and median clefts in Xenopus laevis

Xenopus has become a useful tool to study the molecular mechanisms underlying orofacial development. However, few quantitative analyses exist to describe the anatomy of this region. In this study we combine traditional facial measurements with geometric morphometrics to describe anatomical changes in the orofacial region during normal and abnormal development. Facial measurements and principal component (PC) analysis indicate that during early tadpole development the face expands primarily in the midface region accounting for the development of the upper jaw and primary palate. The mouth opening correspondingly becomes flatter and wider as it incorporates the jaw elements. A canonical variate analysis of orofacial and mouth opening shape emphasized that changes in the orofacial shape occur gradually. Orofacial anatomy was quantified after altered levels of retinoic acid using all-trans retinoic acid or an inhibitor of retinoic acid receptors or by injecting antisense oligos targeting RALDH2. Such perturbations resulted in major decreases in the width of the midface and the mouth opening illustrated in facial measurements and a PC analysis. The mouth opening shape also had a gap in the primary palate resulting in a median cleft in the mouth opening that was only illustrated quantitatively in the morphometric analysis. Finally, canonical and discriminant function analysis statistically distinguished the orofacial and mouth opening shape changes among the different modes used to alter retinoic acid signaling levels. By combining quantitative analyses with molecular studies of orofacial development we will be better equipped to understand the complex morphogenetic processes involved in palate development and clefting.

Cranial muscle development in the model organism ambystoma mexicanum: implications for tetrapod and vertebrate comparative and evolutionary morphology and notes on ontogeny and phylogeny

There is still confusion about the homology of several cranial muscles in salamanders with those of other vertebrates. This is true, in part, because of the fact that many muscles present in early ontogeny of amphibians disappear during development and specifically during metamorphosis. Resolving this confusion is important for the understanding of the comparative and evolutionary morphology of vertebrates and tetrapods because amphibians are the phylogenetically most plesiomorphic tetrapods, concerning for example their myology, and include two often used model organisms, Xenopus laevis (anuran) and Ambystoma mexicanum (urodele). Here we provide the first detailed report of the cranial muscle development in axolotl from early ontogenetic stages to the adult stage. We describe different and complementary types of general muscle morphogenetic gradients in the head: from anterior to posterior, from lateral to medial, and from origin to insertion. Furthermore, even during the development of neotenic salamanders such as axolotls, various larval muscles become indistinct, contradicting the commonly accepted view that during ontogeny the tendency is mostly toward the differentiation of muscles. We provide an updated comparison between these muscles and the muscles of other vertebrates, a discussion of the homologies and evolution, and show that the order in which the muscles appear during axolotl ontogeny is in general similar to their appearance in phylogeny (e.g. differentiation of adductor mandibulae muscles from one anlage to four muscles), with only a few remarkable exceptions, as for example the dilatator laryngis that appears evolutionary later but in the development before the intermandibularis.