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IScience Apr 2021Somitogenesis is often described using the clock-and-wavefront (CW) model, which does not explain how molecular signaling rearranges the pre-somitic mesoderm (PSM) cells...
Somitogenesis is often described using the clock-and-wavefront (CW) model, which does not explain how molecular signaling rearranges the pre-somitic mesoderm (PSM) cells into somites. Our scanning electron microscopy analysis of chicken embryos reveals a caudally-progressing epithelialization front in the dorsal PSM that precedes somite formation. Signs of apical constriction and tissue segmentation appear in this layer 3-4 somite lengths caudal to the last-formed somite. We propose a mechanical instability model in which a steady increase of apical contractility leads to periodic failure of adhesion junctions within the dorsal PSM and positions the future inter-somite boundaries. This model produces spatially periodic segments whose size depends on the speed of the activation front of contraction (), and the buildup rate of contractility (Λ). The Λ/ ratio determines whether this mechanism produces spatially and temporally regular or irregular segments, and whether segment size increases with the front speed.
PubMed: 33889816
DOI: 10.1016/j.isci.2021.102317 -
Nature Jan 2023Sequential segmentation creates modular body plans of diverse metazoan embryos. Somitogenesis establishes the segmental pattern of the vertebrate body axis. A molecular...
Sequential segmentation creates modular body plans of diverse metazoan embryos. Somitogenesis establishes the segmental pattern of the vertebrate body axis. A molecular segmentation clock in the presomitic mesoderm sets the pace of somite formation. However, how cells are primed to form a segment boundary at a specific location remains unclear. Here we developed precise reporters for the clock and double-phosphorylated Erk (ppErk) gradient in zebrafish. We show that the Her1-Her7 oscillator drives segmental commitment by periodically lowering ppErk, therefore projecting its oscillation onto the ppErk gradient. Pulsatile inhibition of the ppErk gradient can fully substitute for the role of the clock, and kinematic clock waves are dispensable for sequential segmentation. The clock functions upstream of ppErk, which in turn enables neighbouring cells to discretely establish somite boundaries in zebrafish. Molecularly divergent clocks and morphogen gradients were identified in sequentially segmenting species. Our findings imply that versatile clocks may establish sequential segmentation in diverse species provided that they inhibit gradients.
Topics: Animals; Basic Helix-Loop-Helix Transcription Factors; Body Patterning; Gene Expression Regulation, Developmental; Somites; Zebrafish; Zebrafish Proteins; Biological Clocks; Periodicity; Extracellular Signal-Regulated MAP Kinases
PubMed: 36517597
DOI: 10.1038/s41586-022-05527-x -
Methods in Molecular Biology (Clifton,... 2019The rabbit is a mainstay of regulatory developmental toxicity testing; however, due to the historic absence of experimental tools for this species, there is a dearth of...
The rabbit is a mainstay of regulatory developmental toxicity testing; however, due to the historic absence of experimental tools for this species, there is a dearth of information about its fundamental embryology and the mechanisms underlying developmental toxicity. Relatively recently, there have been advances in the methods of rabbit whole embryo culture (WEC), and this has prompted an increase in understanding of rabbit embryogenesis. Described herein are the methods used to remove early somite-stage embryos (gestation day 9) and sustain their growth for 48 h. Although there are similarities to the well-described rodent WEC, there are also important differences. Akin to rodent WEC, the major phases of organogenesis can be investigated, including neural tube development, cardiac looping, segmentation, and the development of the anlagen of the optic and otic regions, craniofacial development, somites, and early limb bud development. Unlike the rodent, rabbit WEC requires the use of an apparatus that allows for the continuous gassing of embryos, and one may observe the expansion and closure of the visceral yolk sac around the embryo. After completion of the culture period, embryos are examined across several growth and developmental parameters including a quantitative morphological scoring system. Embryonic growth and development in the absence of maternal influences allows for the study of the direct action of agents or their metabolites on the embryo. The use of both rodent and rabbit WEC together is a powerful strategy with which to investigate species-specific vulnerabilities to specific agents.
Topics: Animals; Embryo Culture Techniques; Embryo, Mammalian; Embryonic Development; Female; Male; Models, Animal; Organogenesis; Rabbits; Somites
PubMed: 31069678
DOI: 10.1007/978-1-4939-9182-2_15 -
Journal of Anatomy Feb 2017Segmentation of the vertebrate body axis is established in the embryo by formation of somites, which give rise to the axial muscles (myotome) and vertebrae (sclerotome)....
Segmentation of the vertebrate body axis is established in the embryo by formation of somites, which give rise to the axial muscles (myotome) and vertebrae (sclerotome). To allow a muscle to attach to two successive vertebrae, the myotome and sclerotome must be repositioned by half a segment with respect to each other. Two main models have been put forward: 'resegmentation' proposes that each half-sclerotome joins with the half-sclerotome from the next adjacent somite to form a vertebra containing cells from two successive somites on each side of the midline. The second model postulates that a single vertebra is made from a single somite and that the sclerotome shifts with respect to the myotome. There is conflicting evidence for these models, and the possibility that the mechanism may vary along the vertebral column has not been considered. Here we use DiI and DiO to trace somite contributions to the vertebrae in different axial regions in the chick embryo. We demonstrate that vertebral bodies and neural arches form by resegmentation but that sclerotome cells shift in a region-specific manner according to their dorsoventral position within a segment. We propose a 'resegmentation-shift' model as the mechanism for amniote vertebral patterning.
Topics: Animals; Body Patterning; Chick Embryo; Chickens; Models, Anatomic; Somites; Spine
PubMed: 27580767
DOI: 10.1111/joa.12540 -
Results and Problems in Cell... 2015This review will focus on the use of the chicken and quail as model systems to analyze myogenesis and as such will emphasize the experimental approaches that are... (Review)
Review
This review will focus on the use of the chicken and quail as model systems to analyze myogenesis and as such will emphasize the experimental approaches that are strongest in these systems-the amenability of the avian embryo to manipulation and in ovo observation. During somite differentiation, a wide spectrum of developmental processes occur such as cellular differentiation, migration, and fusion. Cell lineage studies combined with recent advancements in cell imaging allow these biological phenomena to be readily observed and hypotheses tested extremely rapidly-a strength that is restricted to the avian system. A clear weakness of the chicken in the past has been genetic approaches to modulate gene function. Recent advances in the electroporation of expression vectors, siRNA constructs, and use of tissue specific reporters have opened the door to increasingly sophisticated experiments that address questions of interest not only to the somite/muscle field in particular but also fundamental to biology in general. Importantly, an ever-growing body of evidence indicates that somite differentiation in birds is indistinguishable to that of mammals; therefore, these avian studies complement the complex genetic models of the mouse.
Topics: Animals; Cell Differentiation; Cell Lineage; Chick Embryo; Gene Expression Regulation, Developmental; Mice; Models, Biological; Muscle Development; Quail; Somites
PubMed: 25344668
DOI: 10.1007/978-3-662-44608-9_5 -
Zoological Letters 2015Somites, blocks of mesoderm tissue located on either side of the neural tube in the developing vertebrate embryo, are derived from mesenchymal cells in the presomitic...
INTRODUCTION
Somites, blocks of mesoderm tissue located on either side of the neural tube in the developing vertebrate embryo, are derived from mesenchymal cells in the presomitic mesoderm (PSM) and are a defining characteristic of vertebrates. In vertebrates, the somite segmental boundary is determined by Notch signalling and the antagonistic relationship of the downstream targets of Notch, Lfng, and Delta1 in the anterior PSM. The presence of somites in the basal chordate amphioxus (Branchiostoma floridae) indicates that the last common ancestor of chordates also had somites. However, it remains unclear how the genetic mechanisms underlying somitogenesis in vertebrates evolved from those in ancestral chordates.
RESULTS
We demonstrate that during the gastrula stages of amphioxus embryos, BfFringe expression in the endoderm of the archenteron is detected ventrally to the ventral limit of BfDelta expression in the presumptive rostral somites along the dorsal/ventral (D/V) body axis. Suppression of Notch signalling by DAPT (a γ-secretase inhibitor that indirectly inhibits Notch) treatment from the late blastula stage reduced late gastrula stage expression of BfFringe in the endodermal archenteron and somite markers BfDelta and BfHairy-b in the mesodermal archenteron. Later in development, somites in the DAPT-treated embryo did not separate completely from the dorsal roof of the archenteron. In addition, clear segmental boundaries between somites were not detected in DAPT-treated amphioxus embryos at the larva stage. Similarly, in vertebrates, DAPT treatment from the late blastula stage in Xenopus (Xenopus laevis) embryos resulted in disruption of somite XlDelta-2 expression at the late gastrula stage. At the tail bud stage, the segmental expression of XlMyoD in myotomes was diminished.
CONCLUSIONS
We propose that Notch signalling and the Fringe/Delta cassette for dorso-ventral boundary formation in the archenteron that separates somites from the gut in an amphioxus-like ancestral chordate were co-opted for anteroposterior segmental boundary formation in the vertebrate anterior PSM during evolution.
PubMed: 26613046
DOI: 10.1186/s40851-015-0033-0 -
Nature May 2022The body axis of vertebrate embryos is periodically segmented into bilaterally symmetric pairs of somites. The anteroposterior length of somites, their position and...
The body axis of vertebrate embryos is periodically segmented into bilaterally symmetric pairs of somites. The anteroposterior length of somites, their position and left-right symmetry are thought to be molecularly determined before somite morphogenesis. Here we show that, in zebrafish embryos, initial somite anteroposterior lengths and positions are imprecise and, consequently, many somite pairs form left-right asymmetrically. Notably, these imprecisions are not left unchecked and we find that anteroposterior lengths adjust within an hour after somite formation, thereby increasing morphological symmetry. We find that anteroposterior length adjustments result entirely from changes in somite shape without change in somite volume, with changes in anteroposterior length being compensated by corresponding changes in mediolateral length. The anteroposterior adjustment mechanism is facilitated by somite surface tension, which we show by comparing in vivo experiments and in vitro single-somite explant cultures using a mechanical model. Length adjustment is inhibited by perturbation of molecules involved in surface tension, such as integrin and fibronectin. By contrast, the adjustment mechanism is unaffected by perturbations to the segmentation clock, therefore revealing a distinct process that influences morphological segment lengths. We propose that tissue surface tension provides a general mechanism to adjust shapes and ensure precision and symmetry of tissues in developing embryos.
Topics: Animals; Body Patterning; Embryonic Development; Morphogenesis; Somites; Surface Tension; Zebrafish; Zebrafish Proteins
PubMed: 35477753
DOI: 10.1038/s41586-022-04646-9 -
Developmental Biology May 2015The zebrafish extracellular matrix (ECM) is a dynamic and pleomorphic structure consisting of numerous proteins that together regulate a variety of cellular and... (Review)
Review
The zebrafish extracellular matrix (ECM) is a dynamic and pleomorphic structure consisting of numerous proteins that together regulate a variety of cellular and morphogenetic events beginning as early as gastrulation. The zebrafish genome encodes a similar complement of ECM proteins as found in other vertebrate organisms including glycoproteins, fibrous proteins, proteoglycans, glycosaminoglycans, and interacting or modifying proteins such as integrins and matrix metalloproteinases. As a genetic model system combined with its amenability to high-resolution microscopic imaging, the zebrafish allows interrogation of ECM protein structure and function in both the embryo and adult. Accumulating data have identified important roles for zebrafish ECM proteins in processes as diverse as cell polarity, migration, tissue mechanics, organ laterality, muscle contraction, and regeneration. In this review, I highlight recently published data on these topics that demonstrate how the ECM proteins fibronectin, laminin, and collagen contribute to zebrafish development and adult homeostasis.
Topics: Animals; Body Patterning; Cell Movement; Cell Polarity; Collagen; Extracellular Matrix Proteins; Fibronectins; Gastrula; Heart; Laminin; Morphogenesis; Muscle Contraction; Neural Crest; Somites; Zebrafish
PubMed: 25553981
DOI: 10.1016/j.ydbio.2014.12.022 -
Seminars in Cell & Developmental Biology Mar 2016Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and... (Review)
Review
Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.
Topics: Animals; Body Patterning; Gene Expression Regulation, Developmental; Humans; Muscle Development; Muscle, Skeletal; Myogenic Regulatory Factors; Regeneration; Somites; Xenopus Proteins; Xenopus laevis
PubMed: 26853935
DOI: 10.1016/j.semcdb.2016.02.006 -
Developmental Biology Jan 2017Somitogenesis and subsequent axial skeletal development is regulated by the interaction of pathways that determine the periodicity of somite formation, rostrocaudal...
Somitogenesis and subsequent axial skeletal development is regulated by the interaction of pathways that determine the periodicity of somite formation, rostrocaudal somite polarity and segment identity. Here we use a hypomorphic mutant mouse line to demonstrate that Supt20 (Suppressor of Ty20) is required for development of the axial skeleton. Supt20 hypomorphs display fusions of the ribs and vertebrae at lower thoracic levels along with anterior homeotic transformation of L1 to T14. These defects are preceded by reduction of the rostral somite and posterior shifts in Hox gene expression. While cycling of Notch target genes in the posterior presomitic mesoderm (PSM) appeared normal, expression of Lfng was reduced. In the anterior PSM, Mesp2 expression levels and cycling were unaffected; yet, expression of downstream targets such as Lfng, Ripply2, Mesp1 and Dll3 in the prospective rostral somite was reduced accompanied by expansion of caudal somite markers such as EphrinB2 and Hes7. Supt20 interacts with the Gcn5-containing SAGA histone acetylation complex. Gcn5 hypomorphic mutant embryos show similar defects in axial skeletal development preceded by posterior shift of Hoxc8 and Hoxc9 gene expression. We demonstrate that Gcn5 and Supt20 hypomorphs show similar defects in rostral-caudal somite patterning potentially suggesting shared mechanisms.
Topics: Animals; Body Patterning; Bone Development; Cell Movement; Cell Polarity; Gene Expression Regulation, Developmental; Mesoderm; Mice; Mutation; Neural Crest; Phenotype; Receptors, Notch; Signal Transduction; Somites; Spine; Transcription Factors
PubMed: 27894818
DOI: 10.1016/j.ydbio.2016.11.009