Regulatory changes associated with the head to trunk developmental transition.

BACKGROUND: Development of vertebrate embryos is characterized by early formation of the anterior tissues followed by the sequential extension of the axis at their posterior end to build the trunk and tail structures, first by the activity of the primitive streak and then of the tail bud. Embryological, molecular and genetic data indicate that head and trunk development are significantly different, suggesting that the transition into the trunk formation stage involves major changes in regulatory gene networks. RESULTS: We explored those regulatory changes by generating differential interaction networks and chromatin accessibility profiles from the posterior epiblast region of mouse embryos at embryonic day (E)7.5 and E8.5. We observed changes in various cell processes, including several signaling pathways, ubiquitination machinery, ion dynamics and metabolic processes involving lipids that could contribute to the functional switch in the progenitor region of the embryo. We further explored the functional impact of changes observed in Wnt signaling associated processes, revealing a switch in the functional relevance of Wnt molecule palmitoleoylation, essential during gastrulation but becoming differentially required for the control of axial extension and progenitor differentiation processes during trunk formation. We also found substantial changes in chromatin accessibility at the two developmental stages, mostly mapping to intergenic regions and presenting differential footprinting profiles to several key transcription factors, indicating a significant switch in the regulatory elements controlling head or trunk development. Those chromatin changes are largely independent of retinoic acid, despite the key role of this factor in the transition to trunk development. We also tested the functional relevance of potential enhancers identified in the accessibility assays that reproduced the expression profiles of genes involved in the transition. Deletion of these regions by genome editing had limited effect on the expression of those genes, suggesting the existence of redundant enhancers that guarantee robust expression patterns. CONCLUSIONS: This work provides a global view of the regulatory changes controlling the switch into the axial extension phase of vertebrate embryonic development. It also revealed mechanisms by which the cellular context influences the activity of regulatory factors, channeling them to implement one of several possible biological outputs.

Single-cell transcriptome analysis of the zebrafish embryonic trunk.

During embryonic development, cells differentiate into a variety of distinct cell types and subtypes with diverse transcriptional profiles. To date, transcriptomic signatures of different cell lineages that arise during development have been only partially characterized. Here we used single-cell RNA-seq to perform transcriptomic analysis of over 20,000 cells disaggregated from the trunk region of zebrafish embryos at the 30 hpf stage. Transcriptional signatures of 27 different cell types and subtypes were identified and annotated during this analysis. This dataset will be a useful resource for many researchers in the fields of developmental and cellular biology and facilitate the understanding of molecular mechanisms that regulate cell lineage choices during development.

Novel concept for the epaxial/hypaxial boundary based on neuronal development.

Trunk muscles in vertebrates are classified as either dorsal epaxial or ventral hypaxial muscles. Epaxial and hypaxial muscles are defined as muscles innervated by the dorsal and ventral rami of spinal nerves, respectively. Each cluster of spinal motor neurons passing through dorsal rami innervates epaxial muscles, whereas clusters traveling on the ventral rami innervate hypaxial muscles. Herein, we show that some motor neurons exhibiting molecular profiles for epaxial muscles follow a path in the ventral rami. Dorsal deep-shoulder muscles and some body wall muscles are defined as hypaxial due to innervation via the ventral rami, but a part of these ventral rami has the molecular profile of motor neurons that innervate epaxial muscles. Thus, the epaxial and hypaxial boundary cannot be determined simply by the ramification pattern of spinal nerves. We propose that, although muscle innervation occurs via the ventral rami, dorsal deep-shoulder muscles and some body wall muscles represent an intermediate group that lies between epaxial and hypaxial muscles.

Lineage tracing of axial progenitors using Nkx1-2CreERT2 mice defines their trunk and tail contributions.

The vertebrate body forms by continuous generation of new tissue from progenitors at the posterior end of the embryo. The study of these axial progenitors has proved to be challenging in vivo largely because of the lack of unique molecular markers to identify them. Here, we elucidate the expression pattern of the transcription factor Nkx1-2 in the mouse embryo and show that it identifies axial progenitors throughout body axis elongation, including neuromesodermal progenitors and early neural and mesodermal progenitors. We create a tamoxifen-inducible Nkx1-2CreERT2 transgenic mouse and exploit the conditional nature of this line to uncover the lineage contributions of Nkx1-2-expressing cells at specific stages. We show that early Nkx1-2-expressing epiblast cells contribute to all three germ layers, mostly neuroectoderm and mesoderm, excluding notochord. Our data are consistent with the presence of some self-renewing axial progenitors that continue to generate neural and mesoderm tissues from the tail bud. This study identifies Nkx1-2-expressing cells as the source of most trunk and tail tissues in the mouse and provides a useful tool to genetically label and manipulate axial progenitors in vivo.

The neural crest and evolution of the head/trunk interface in vertebrates.

The migration and distribution patterns of neural crest (NC) cells reflect the distinct embryonic environments of the head and trunk: cephalic NC cells migrate predominantly along the dorsolateral pathway to populate the craniofacial and pharyngeal regions, whereas trunk crest cells migrate along the ventrolateral pathways to form the dorsal root ganglia. These two patterns thus reflect the branchiomeric and somitomeric architecture, respectively, of the vertebrate body plan. The so-called vagal NC occupies a postotic, intermediate level between the head and trunk NC. This level of NC gives rise to both trunk- and cephalic-type (circumpharyngeal) NC cells. The anatomical pattern of the amphioxus, a basal chordate, suggests that somites and pharyngeal gills coexist along an extensive length of the body axis, indicating that the embryonic environment is similar to that of vertebrate vagal NC cells and may have been ancestral for vertebrates. The amniote-like condition in which the cephalic and trunk domains are distinctly separated would have been brought about, in part, by anteroposterior reduction of the pharyngeal domain.

Ancient evolutionary origin of vertebrate enteric neurons from trunk-derived neural crest.

The enteric nervous system of jawed vertebrates arises primarily from vagal neural crest cells that migrate to the foregut and subsequently colonize and innervate the entire gastrointestinal tract. Here we examine development of the enteric nervous system in the basal jawless vertebrate the sea lamprey (Petromyzon marinus) to gain insight into its evolutionary origin. Surprisingly, we find no evidence for the existence of a vagally derived enteric neural crest population in the lamprey. Rather, labelling with the lipophilic dye DiI shows that late-migrating cells, originating from the trunk neural tube and associated with nerve fibres, differentiate into neurons within the gut wall and typhlosole. We propose that these trunk-derived neural crest cells may be homologous to Schwann cell precursors, recently shown in mammalian embryos to populate post-embryonic parasympathetic ganglia, including enteric ganglia. Our results suggest that neural-crest-derived Schwann cell precursors made an important contribution to the ancient enteric nervous system of early jawless vertebrates, a role that was largely subsumed by vagal neural crest cells in early gnathostomes.

Oct4 Is a Key Regulator of Vertebrate Trunk Length Diversity.

Vertebrates exhibit a remarkably broad variation in trunk and tail lengths. However, the evolutionary and developmental origins of this diversity remain largely unknown. Posterior Hox genes were proposed to be major players in trunk length diversification in vertebrates, but functional studies have so far failed to support this view. Here we identify the pluripotency factor Oct4 as a key regulator of trunk length in vertebrate embryos. Maintaining high Oct4 levels in axial progenitors throughout development was sufficient to extend trunk length in mouse embryos. Oct4 also shifted posterior Hox gene-expression boundaries in the extended trunks, thus providing a link between activation of these genes and the transition to tail development. Furthermore, we show that the exceptionally long trunks of snakes are likely to result from heterochronic changes in Oct4 activity during body axis extension, which may have derived from differential genomic rearrangements at the Oct4 locus during vertebrate evolution.

Mechanisms of vertebrate embryo segmentation: Common themes in trunk and limb development.

Various ultradian rhythms ensure proper temporal regulations during embryo development. The embryo molecular clock, which was first identified in the presomitic mesoderm (PSM) underlying periodic somite formation, is one among them. Somites are the earliest manifestation of the segmented vertebrate body and they are formed with strict temporal precision. The tetrapod limb is also a segmented structure and the formation of limb bone elements have also been associated with a molecular clock, operating in the distal limb mesenchyme. In both the PSM and the distal limb mesenchyme, the molecular clock (MC) is influenced by FGF, SHH and RA, which are also the key regulators of the development of these tissues. While somitogenesis has been continuously scrutinized to understand the mechanisms of the MC, the limb bud has served as an outstanding paradigm to study how a cohort of undifferentiated cells is organized into functional 3D structures. The fact that both the trunk and limb development are shaped by the MC and by common signaling molecules has prompted the exciting possibility of establishing parallelisms between somitogenesis and limb development. Systematically correlating various parameters during trunk and limb development will help us to appreciate the common principles underlying segmented structure formation and allow the rise of new questions in order to fill the gaps in our present understanding. In this review we have established the parallelisms between somitogenesis and limb development at the level of gene expression patterns and their regulation. Finally, we have also discussed the most evident new avenues this exercise could open to the scientific community.

Single-cell analysis of endothelial morphogenesis in vivo.

Vessel formation has been extensively studied at the tissue level, but the difficulty in imaging the endothelium with cellular resolution has hampered study of the morphogenesis and behavior of endothelial cells (ECs) in vivo. We are using endothelial-specific transgenes and high-resolution imaging to examine single ECs in zebrafish. By generating mosaics with transgenes that simultaneously mark endothelial nuclei and membranes we are able to definitively identify and study the morphology and behavior of individual ECs during vessel sprouting and lumen formation. Using these methods, we show that developing trunk vessels are composed of ECs of varying morphology, and that single-cell analysis can be used to quantitate alterations in morphology and dynamics in ECs that are defective in proper guidance and patterning. Finally, we use single-cell analysis of intersegmental vessels undergoing lumen formation to demonstrate the coexistence of seamless transcellular lumens and single or multicellular enclosed lumens with autocellular or intercellular junctions, suggesting that heterogeneous mechanisms contribute to vascular lumen formation in vivo. The tools that we have developed for single EC analysis should facilitate further rigorous qualitative and quantitative analysis of EC morphology and behavior in vivo.

Cdon promotes neural crest migration by regulating N-cadherin localization.

Neural crest cells (NCCs) are essential embryonic progenitor cells that are unique to vertebrates and form a remarkably complex and coordinated system of highly motile cells. Migration of NCCs occurs along specific pathways within the embryo in response to both environmental cues and cell-cell interactions within the neural crest population. Here, we demonstrate a novel role for the putative Sonic hedgehog (Shh) receptor and cell adhesion regulator, cdon, in zebrafish neural crest migration. cdon is expressed in developing premigratory NCCs but is downregulated once the cells become migratory. Knockdown of cdon results in aberrant migration of trunk NCCs: crestin positive cells can emigrate out of the neural tube but stall shortly after the initiation of migration. Live cell imaging analysis demonstrates reduced directedness of migration, increased velocity and mispositioned cell protrusions. In addition, transplantation analysis suggests that cdon is required cell-autonomously for directed NCC migration in the trunk. Interestingly, N-cadherin is mislocalized following cdon knockdown suggesting that the role of cdon in NCCs is to regulate N-cadherin localization. Our results reveal a novel role for cdon in zebrafish neural crest migration, and suggest a mechanism by which Cdon is required to localize N-cadherin to the cell membrane in migratory NCCs for directed migration.

From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate.

During vertebrate development, the central (CNS) and peripheral nervous systems (PNS) arise from the neural plate. Cells at the margin of the neural plate give rise to neural crest cells, which migrate extensively throughout the embryo, contributing to the majority of neurons and all of the glia of the PNS. The rest of the neural plate invaginates to form the neural tube, which expands to form the brain and spinal cord. The emergence of molecular cloning techniques and identification of fluorophores like Green Fluorescent Protein (GFP), together with transgenic and electroporation technologies, have made it possible to easily visualize the cellular and molecular events in play during nervous system formation. These lineage-tracing techniques have precisely demonstrated the migratory pathways followed by neural crest cells and increased knowledge about their differentiation into PNS derivatives. Similarly, in the spinal cord, lineage-tracing techniques have led to a greater understanding of the regional organization of multiple classes of neural progenitor and post-mitotic neurons along the different axes of the spinal cord and how these distinct classes of neurons assemble into the specific neural circuits required to realize their various functions. Here, we review how both classical and modern lineage and marker analyses have expanded our knowledge of early peripheral nervous system and spinal cord development.

Divergent and conserved roles of Dll1 signaling in development of craniofacial and trunk muscle.

Craniofacial and trunk skeletal muscles are evolutionarily distinct and derive from cranial and somitic mesoderm, respectively. Different regulatory hierarchies act upstream of myogenic regulatory factors in cranial and somitic mesoderm, but the same core regulatory network - MyoD, Myf5 and Mrf4 - executes the myogenic differentiation program. Notch signaling controls self-renewal of myogenic progenitors as well as satellite cell homing during formation of trunk muscle, but its role in craniofacial muscles has been little investigated. We show here that the pool of myogenic progenitor cells in craniofacial muscle of Dll1(LacZ/Ki) mutant mice is depleted in early fetal development, which is accompanied by a major deficit in muscle growth. At the expense of progenitor cells, supernumerary differentiating myoblasts appear transiently and these express MyoD. The progenitor pool in craniofacial muscle of Dll1(LacZ/Ki) mutants is largely rescued by an additional mutation of MyoD. We conclude from this that Notch exerts its decisive role in craniofacial myogenesis by repression of MyoD. This function is similar to the one previously observed in trunk myogenesis, and is thus conserved in cranial and trunk muscle. However, in cranial mesoderm-derived progenitors, Notch signaling is not required for Pax7 expression and impinges little on the homing of satellite cells. Thus, Dll1 functions in satellite cell homing and Pax7 expression diverge in cranial- and somite-derived muscle.

Primary cilia function regulates the length of the embryonic trunk axis and urogenital field in mice.

The issues of whether and how some organs coordinate their size and shape with the blueprint of the embryo axis, while others appear to regulate their morphogenesis autonomously, remain poorly understood. Mutations in Ift144, encoding a component of the trafficking machinery of primary cilia assembly, result in a range of embryo patterning defects, affecting the limbs, skeleton and neural system. Here, we show that embryos of the mouse mutant Ift144(twt) develop gonads that are larger than wild-type. Investigation of the early patterning of the urogenital ridge revealed that the anterior-posterior domain of the gonad/mesonephros was extended at 10.5 dpc, with no change in the length of the metanephros. In XY embryos, this extension resulted in an increase in testis cord number. Moreover, we observed a concomitant extension of the trunk axis in both sexes, with no change in the length of the tail domain or somite number. Our findings support a model in which: (1) primary cilia regulate embryonic trunk elongation; (2) the length of the trunk axis determines the size of the urogenital ridges; and (3) the gonad domain is partitioned into a number of testis cords that depends on the available space, rather than being divided a predetermined number of times to generate a specific number of cords.

Evolutionarily conserved morphogenetic movements at the vertebrate head-trunk interface coordinate the transport and assembly of hypopharyngeal structures.

The vertebrate head-trunk interface (occipital region) has been heavily remodelled during evolution, and its development is still poorly understood. In extant jawed vertebrates, this region provides muscle precursors for the throat and tongue (hypopharyngeal/hypobranchial/hypoglossal muscle precursors, HMP) that take a stereotype path rostrally along the pharynx and are thought to reach their target sites via active migration. Yet, this projection pattern emerged in jawless vertebrates before the evolution of migratory muscle precursors. This suggests that a so far elusive, more basic transport mechanism must have existed and may still be traceable today. Here we show for the first time that all occipital tissues participate in well-conserved cell movements. These cell movements are spearheaded by the occipital lateral mesoderm and ectoderm that split into two streams. The rostrally directed stream projects along the floor of the pharynx and reaches as far rostrally as the floor of the mandibular arch and outflow tract of the heart. Notably, this stream leads and engulfs the later emerging HMP, neural crest cells and hypoglossal nerve. When we (i) attempted to redirect hypobranchial/hypoglossal muscle precursors towards various attractants, (ii) placed non-migratory muscle precursors into the occipital environment or (iii) molecularly or (iv) genetically rendered muscle precursors non-migratory, they still followed the trajectory set by the occipital lateral mesoderm and ectoderm. Thus, we have discovered evolutionarily conserved morphogenetic movements, driven by the occipital lateral mesoderm and ectoderm, that ensure cell transport and organ assembly at the head-trunk interface.

Position of the conus medullaris in fetuses with skeletal dysplasia.

OBJECTIVES: To examine whether reduced conus distance (CD) measurement may be a marker for impaired growth of the fetal trunk in fetuses with certain types of skeletal dysplasia. METHODS: Two hundred fifty-four normal cases and 24 pregnancies with skeletal dysplasia were included. The fetuses with skeletal dysplasia were subdivided into two groups based on the specific diagnosis: skeletal dysplasias with a short trunk and skeletal dysplasias with a normal size trunk. Linear regression analysis was used to examine the relationship of the CD measurement with the biparietal diameter. CD measurements in the two groups of fetuses with skeletal dysplasia were compared using z-scores. RESULTS: In fetuses with skeletal dysplasia and shortened trunk, the CD measurement was significantly smaller than in the normal population (mean z-score -3.7, p < 0.0001). CD measurements in fetuses with skeletal dysplasia but a normal trunk size were similar to the normal population (mean z-score -0.1, p = 0.997). CONCLUSIONS: Short CD is associated with certain types of skeletal dysplasia. As such, measuring the CD may be helpful in diagnosing these conditions.

The differentiation and movement of presomitic mesoderm progenitor cells are controlled by Mesogenin 1.

Somites are formed from the presomitic mesoderm (PSM) and give rise to the axial skeleton and skeletal muscles. The PSM is dynamic; somites are generated at the anterior end, while the posterior end is continually renewed with new cells entering from the tailbud progenitor region. Which genes control the conversion of tailbud progenitors into PSM and how is this process coordinated with cell movement? Using loss- and gain-of-function experiments and heat-shock transgenics we show in zebrafish that the transcription factor Mesogenin 1 (Msgn1), acting with Spadetail (Spt), has a central role. Msgn1 allows progression of the PSM differentiation program by switching off the progenitor maintenance genes ntl, wnt3a, wnt8 and fgf8 in the future PSM cells as they exit from the tailbud, and subsequently induces expression of PSM markers such as tbx24. msgn1 is itself positively regulated by Ntl/Wnt/Fgf, creating a negative-feedback loop that might be crucial to regulate homeostasis of the progenitor population until somitogenesis ends. Msgn1 drives not only the changes in gene expression in the nascent PSM cells but also the movements by which they stream out of the tailbud into the PSM. Loss of Msgn1 reduces the flux of cells out of the tailbud, producing smaller somites and an enlarged tailbud, and, by delaying exhaustion of the progenitor population, results in supernumerary tail somites. Through its combined effects on gene expression and cell movement, Msgn1 (with Spt) plays a key role both in genesis of the paraxial mesoderm and in maintenance of the progenitor population from which it derives.

The effects of cobalt on the development, oxidative stress, and apoptosis in zebrafish embryos.

Metal-on-metal hip arthroplasty has been performed with increasing frequency throughout the world, particularly in younger and more active patients, including women of childbearing age. The potential toxicity of cobalt exposure on fetus is concerned since cobalt ions generated by metal-on-metal bearings can traverse the placenta and be detected in fetal blood and amniotic fluid. This study examined the effects of cobalt exposure on early embryonic development and the mechanisms underlying its toxicity. Zebrafish embryos were exposed to a range of cobalt concentrations (0-100 mg/L) between 1 and 144 h postfertilization. The survival and early development of embryos were not significantly affected by cobalt at concentrations <100 µg/L. However, embryos exposed to higher concentrations (>100 µg/L) displayed reduced survival rates and abnormal development, including delayed hatching, aberrant morphology, retarded growth, and bradycardia. Furthermore, this study examined oxidative stress and apoptosis in embryos exposed to cobalt at concentrations of 0-500 µg/L. Lipid peroxidation levels were increased in cobalt-treated embryos at concentrations of 100 and 500 µg/L. The mRNA levels of catalase, superoxide dismutase 2, p53, caspase-3, and caspase-9 genes were upregulated in a dose-dependent manner. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays also revealed abnormal apoptotic signals in the brain, trunk, and tail when treated with 500 µg/L cobalt. These data suggest that oxidative stress and apoptosis are associated with cobalt toxicity in zebrafish embryos.

'In parallel' interconnectivity of the dorsal longitudinal anastomotic vessels requires both VEGF signaling and circulatory flow.

Blood vessels deliver oxygen, nutrients, hormones and immunity factors throughout the body. To perform these vital functions, vascular cords branch, lumenize and interconnect. Yet, little is known about the cellular, molecular and physiological mechanisms that control how circulatory networks form and interconnect. Specifically, how circulatory networks merge by interconnecting 'in parallel' along their boundaries remains unexplored. To examine this process we studied the formation and functional maturation of the plexus that forms between the dorsal longitudinal anastomotic vessels (DLAVs) in the zebrafish. We find that the migration and proliferation of endothelial cells within the DLAVs and their segmental (Se) vessel precursors drives DLAV plexus formation. Remarkably, the presence of Se vessels containing only endothelial cells of the arterial lineage is sufficient for DLAV plexus morphogenesis, suggesting that endothelial cells from the venous lineage make a dispensable or null contribution to this process. The discovery of a circuit that integrates the inputs of circulatory flow and vascular endothelial growth factor (VEGF) signaling to modulate aortic arch angiogenesis, together with the expression of components of this circuit in the trunk vasculature, prompted us to investigate the role of these inputs and their relationship during DLAV plexus formation. We find that circulatory flow and VEGF signaling make additive contributions to DLAV plexus morphogenesis, rather than acting as essential inputs with equivalent contributions as they do during aortic arch angiogenesis. Our observations underscore the existence of context-dependent differences in the integration of physiological stimuli and signaling cascades during vascular development.

Competence of failed endocrine progenitors to give rise to acinar but not ductal cells is restricted to early pancreas development.

During mouse pancreas development, the transient expression of Neurogenin3 (Neurog3) in uncommitted pancreas progenitors is required to determine endocrine destiny. However it has been reported that Neurog3-expressing cells can eventually adopt acinar or ductal fates and that Neurog3 levels were important to secure the islet destiny. It is not known whether the competence of Neurog3-induced cells to give rise to non-endocrine lineages is an intrinsic property of these progenitors or depends on pancreas developmental stage. Using temporal genetic labeling approaches we examined the dynamic of endocrine progenitor differentiation and explored the plasticity of Neurog3-induced cells throughout development. We found that Neurog3(+) progenitors develop into hormone-expressing cells in a fast process taking less then 10h. Furthermore, fate-mapping studies in heterozygote (Neurog3(CreERT/+)) and Neurog3-deficient (Neurog3(CreERT/CreERT)) embryos revealed that Neurog3-induced cells have different potential over time. At the early bud stage, failed endocrine progenitors can adopt acinar or ductal fate, whereas later in the branching pancreas they do not contribute to the acinar lineage but Neurog3-deficient cells eventually differentiate into duct cells. Thus these results provide evidence that the plasticity of Neurog3-induced cells becomes restricted during development. Furthermore these data suggest that during the secondary transition, endocrine progenitor cells arise from bipotent precursors already committed to the duct/endocrine lineages and not from domain of cells having distinct potentialities.

Cross- and triple-ratios of human body parts during development.

Recently developed landmark-based geometric morphometry has been used to depict the morphological development of organisms. In geometry, four landmarks can be mapped to any other four by Möbius transformations, if the cross-ratio of the landmarks is invariant and vice versa. To geometrically analyze the morphological development of the human body, we examined the cross-ratio of three consecutive body parts that are segmented by four landmarks in their configuration. Moreover, we introduced the triple-ratio of five landmarks that segments four consecutive parts (e.g., the shoulder, upper arm, forearm, and hand) and examined their growth patterns. The cross- and triple-ratios of the upper limb and shoulder girdle in fetuses were constant when biomechanical landmarks were used, although the cross-ratio of the upper limb varied when anatomical landmarks were used. The cross-ratios of the lower limbs, trunk, and pelvic girdles in fetuses differed from their corresponding cross-ratios in adults. These results suggest Möbius growth in the fetal upper limb and shoulder girdle but not in the other body parts examined. However, the growth balance of the three contiguous body parts was represented by the developmental change in the cross-ratio. Therefore, the cross- and triple-ratios may be applicable for simple but significant assessments of growth balance or proportion of the body parts.