scRNA-seq analysis of cells comprising the amphioxus notochord

Cephalochordates occupy a key phylogenetic position for deciphering the origin and evolution of chordates, since they diverged earlier than urochordates and vertebrates. The notochord is the most prominent feature of chordates. The amphioxus notochord features coin-shaped cells bearing myofibrils. Notochord-derived hedgehog signaling contributes to patterning of the dorsal nerve cord, as in vertebrates. However, properties of constituent notochord cells remain unknown at the single-cell level. We examined these properties using Iso-seq analysis, single-cell RNA-seq analysis, and in situ hybridization (ISH). Gene expression profiles broadly categorize notochordal cells into myofibrillar cells and non-myofibrillar cells. Myofibrillar cells occupy most of the central portion of the notochord, and some cells extend the notochordal horn to both sides of the ventral nerve cord. Some notochord myofibrillar genes are not expressed in myotomes, suggesting an occurrence of myofibrillar genes that are preferentially expressed in notochord. On the other hand, non-myofibrillar cells contain dorsal, lateral, and ventral Müller cells, and all three express both hedgehog and Brachyury. This was confirmed by ISH, although expression of hedgehog in ventral Müller cells was minimal. In addition, dorsal Müller cells express neural transmission-related genes, suggesting an interaction with nerve cord. Lateral Müller cells express hedgehog and other signaling-related genes, suggesting an interaction with myotomes positioned lateral to the notochord. Ventral Müller cells also expressed genes for FGF- and EGF-related signaling, which may be associated with development of endoderm, ventral to the notochord. Lateral Müller cells were intermediate between dorsal/ventral Müller cells. Since vertebrate notochord contributes to patterning and differentiation of ectoderm (nerve cord), mesoderm (somite), and endoderm, this investigation provides evidence that an ancestral or original form of vertebrate notochord is present in extant cephalochordates.

Flexural rigidity of pressurized model notochords in regular packing patterns

The biomechanics of embryonic notochords are studied using an elastic membrane model. An initial study varying internal pressure and stiffness ratio determines tension and geometric ratios as a function of internal pressure, membrane stiffness ratio, and cell packing pattern. A subsequent three-point bending study determines flexural rigidity as a function of internal pressure, configuration, and orientation. Flexural rigidity is found to be independent of membrane stiffness ratio. Controlling for number and volume of cells and their internal pressure, the eccentric staircase pattern of cell packing has more than double the flexural rigidity of the radially symmetric bamboo pattern. Moreover, the eccentric staircase pattern is found to be more than twice as stiff in lateral bending than in dorsoventral bending. This suggests a mechanical advantage to the eccentric WT staircase pattern of the embryonic notochord, over patterns with round cross-section.

Cell contacts and pericellular matrix in the Xenopus gastrula chordamesoderm

Convergent extension of the chordamesoderm is the best-examined gastrulation movement in Xenopus. Here we study general features of cell-cell contacts in this tissue by combining depletion of adhesion factors C-cadherin, Syndecan-4, fibronectin, and hyaluronic acid, the analysis of respective contact width spectra and contact angles, and La3+ staining of the pericellular matrix. We provide evidence that like in other gastrula tissues, cell-cell adhesion in the chordamesoderm is largely mediated by different types of pericellular matrix. Specific glycocalyx structures previously identified in Xenopus gastrula tissues are absent in chordamesoderm but other contact types like 10-20 nm wide La3+ stained structures are present instead. Knockdown of any of the adhesion factors reduces the abundance of cell contacts but not the average relative adhesiveness of the remaining ones: a decrease of adhesiveness at low contact widths is compensated by an increase of contact widths and an increase of adhesiveness proportional to width. From the adhesiveness-width relationship, we derive a model of chordamesoderm cell adhesion that involves the interdigitation of distinct pericellular matrix units. Quantitative description of pericellular matrix deployment suggests that reduced contact abundance upon adhesion factor depletion is correlated with excessive accumulation of matrix material in non-adhesive gaps and the loss of some contact types.

A collagen-rich arch in the urochordate notochord coordinates cell shaping and multi-tissue elongation

Cell and tissue reshaping is crucial for coordinating three-dimensional pattern formation, in which the size and shape of the cells must be accurately regulated via signal transport and communication among tissues. However, the identity of signaling and transportation mechanisms in this process remains elusive. In our study, we identified an extracellular matrix (ECM) structure with a vertebra-like shape surrounding the central notochord tissue in the larval tail of the urochordate Ciona. Additionally, we verified that the ECM structure was formed de novo, mainly from collagens secreted by notochord cells. Fluorescence recovery after photobleaching and simulation results revealed that this structure was formed via diffusional collagen flow from a notochord that was restricted and molded in the spaces among tail tissues. We revealed that the collagen structure was essential for notochord cell arrangement and elongation. Furthermore, we observed that the central notochord connects with the epidermis through this ECM structure. The disruption of this structure by collagen knockdown and loss-of-collagen function caused the failure of notochord elongation. More importantly, the epidermis could not elongate proportionally with notochord, indicating that the collagen-rich structure serves as a scaffold to coordinate the concurrent elongation of the tail tissues. These findings provide insights into how the central tissue forms and molds its surrounding ECM structure, by not only regulating its own morphogenesis but also functioning as a scaffold for signal transmission to orchestrate the coordinated morphologic reshaping of the surrounding tissues.

Novel tissue thickening around the notochord sheath found in deformed Japanese eel Anguilla japonica leptocephali

Even though reared leptocephalus larvae of the Japanese eel Anguilla japonica have a high incidence of notochord deformities (>60%), the cause is unknown. We performed histological examinations of the notochord and associated organs in reared larvae to better understand the process causing notochord deformation in eel larvae. In deformed larvae, unknown tissue thickening was discovered near the notochord sheath. Azan staining revealed that these tissue thickenings are most likely collagen fibers within fibrous connective tissue. This was almost identical to the connective tissue found in the primordium of the vertebral body around the notochord sheath in properly metamorphosing larvae. Furthermore, the amount of the thyroid hormone triiodothyronine (T3) was significantly higher in deformed larvae than in normal larvae, indicating that notochord deformity is probably linked to metamorphosis despite the immature stage of growth. We suggest that the aberrant growth of connective tissue surrounding the notochord sheath induced by incomplete metamorphosis causes deformities in eel larvae. The reason why deformed larvae have greater thyroid hormone levels is still unknown. It is important to assess how environmental and dietary factors affect the thyroid hormone levels of eel larvae raised in captivity.

Genome-wide identification of notochord enhancers comprising the regulatory landscape of the brachyury locus in mouse

The node and notochord are important signaling centers organizing the dorso-ventral patterning of cells arising from neuro-mesodermal progenitors forming the embryonic body anlage. Owing to the scarcity of notochord progenitors and notochord cells, a comprehensive identification of regulatory elements driving notochord-specific gene expression has been lacking. Here, we have used ATAC-seq analysis of FACS-purified notochord cells from Theiler stage 12-13 mouse embryos to identify 8921 putative notochord enhancers. In addition, we established a new model for generating notochord-like cells in culture, and found 3728 of these enhancers occupied by the essential notochord control factors brachyury (T) and/or Foxa2. We describe the regulatory landscape of the T locus, comprising ten putative enhancers occupied by these factors, and confirmed the regulatory activity of three of these elements. Moreover, we characterized seven new elements by knockout analysis in embryos and identified one new notochord enhancer, termed TNE2. TNE2 cooperates with TNE in the trunk notochord, and is essential for notochord differentiation in the tail. Our data reveal an essential role of Foxa2 in directing T-expressing cells towards the notochord lineage.

Case of split notochord syndrome: a neonate with thoracic neuroenteric cyst, abdominal duodenal duplication cyst, malrotation and vertebral anomalies

The authors describe a case of a male neonate with split notochord syndrome presenting with cervico-thoracic deformity, thoracic neuroenteric cyst, separate abdominal duodenal duplication cyst and concurrent intestinal malrotation. This combination of abnormalities is very rare. When these lesions are suspected, patients must be investigated carefully.This case is presented not only to recount an infrequent combination of structural abnormalities but also to raise awareness of the signs that should point to clinical suspicion and prompt diagnosis.Following surgical excision of the thoracic neuroenteric cyst, the patient has made a good recovery.

Diverse logics and grammar encode notochord enhancers

The notochord is a defining feature of all chordates. The transcription factors Zic and ETS regulate enhancer activity within the notochord. We conduct high-throughput screens of genomic elements within developing Ciona embryos to understand how Zic and ETS sites encode notochord activity. Our screen discovers an enhancer located near Lama, a gene critical for notochord development. Reversing the orientation of an ETS site within this enhancer abolishes expression, indicating that enhancer grammar is critical for notochord activity. Similarly organized clusters of Zic and ETS sites occur within mouse and human Lama1 introns. Within a Brachyury (Bra) enhancer, FoxA and Bra, in combination with Zic and ETS binding sites, are necessary and sufficient for notochord expression. This binding site logic also occurs within other Ciona and vertebrate Bra enhancers. Collectively, this study uncovers the importance of grammar within notochord enhancers and discovers signatures of enhancer logic and grammar conserved across chordates.

Thiophanate-methyl induces notochord toxicity by activating the PI3K-mTOR pathway in zebrafish (Danio rerio) embryos

Thiophanate-methyl (TM), a typical pesticide widely used worldwide, was detected in rivers, soil, fruits, and vegetables. Thus, it is urgent to identify the potential harm of TM residual to non-target organisms and its molecular mechanisms. We used zebrafish (Danio rerio) in this study to evaluate TM toxicity. TM exposure induced developmental toxicity, including inhibited hatchability, reduced heart rates, restrained spontaneous locomotion, and decreased body length. Furthermore, we observed obvious toxicity in the notochord and detected increased expression levels of notochord-related genes (shha, col2a, and tbxta) by in situ hybridization in zebrafish larvae. In addition, calcein staining, alkaline phosphatase (ALP) activity analysis, and anatomic analysis indicated that TM induced notochord toxicity. We used rescue experiments to verify whether the PI3K-mTOR pathway involved in the notochord development was the cause of notochord abnormalities. Rapamycin and LY294002 (an inhibitor of PI3K) relieve notochord toxicity caused by TM, including morphological abnormalities. In summary, TM might induce notochord toxicity by activating the PI3K-mTOR pathway in zebrafish.

wnt16 regulates spine and muscle morphogenesis through parallel signals from notochord and dermomyotome

Bone and muscle are coupled through developmental, mechanical, paracrine, and autocrine signals. Genetic variants at the CPED1-WNT16 locus are dually associated with bone- and muscle-related traits. While Wnt16 is necessary for bone mass and strength, this fails to explain pleiotropy at this locus. Here, we show wnt16 is required for spine and muscle morphogenesis in zebrafish. In embryos, wnt16 is expressed in dermomyotome and developing notochord, and contributes to larval myotome morphology and notochord elongation. Later, wnt16 is expressed at the ventral midline of the notochord sheath, and contributes to spine mineralization and osteoblast recruitment. Morphological changes in wnt16 mutant larvae are mirrored in adults, indicating that wnt16 impacts bone and muscle morphology throughout the lifespan. Finally, we show that wnt16 is a gene of major effect on lean mass at the CPED1-WNT16 locus. Our findings indicate that Wnt16 is secreted in structures adjacent to developing bone (notochord) and muscle (dermomyotome) where it affects the morphogenesis of each tissue, thereby rendering wnt16 expression into dual effects on bone and muscle morphology. This work expands our understanding of wnt16 in musculoskeletal development and supports the potential for variants to act through WNT16 to influence bone and muscle via parallel morphogenetic processes.

In humans, genetic variants (DNA sequences that vary amongst individuals) have been identified that appear to influence two tissues, bone and skeletal muscle. However, how single genes and genetic variants exert dual influence on both tissues is not well understood. In this study, we found that the wnt16 gene is necessary for specifying the size and shape of both muscle and bone during development in zebrafish. We also disentangled how wnt16 affects both tissues: distinct cellular populations adjacent to muscle and bone secrete Wnt16, where it acts as a signal guiding the size and shape of each tissue. This is important because in humans, genetic variants near the WNT16 gene have effects on both bone- and muscle-related traits. This study expands our understanding of the role of WNT16 in bone and muscle development, and helps to explain how genetic variants near WNT16 affect traits for both tissues. Moreover, WNT16 is actively being explored as a target for osteoporosis therapies, thus our study could have implications with regard to the potential of targeting WNT16 to treat bone and muscle simultaneously.

Morphogenetic Roles of Hydrostatic Pressure in Animal Development

During organismal development, organs and systems are built following a genetic blueprint that produces structures capable of performing specific physiological functions. Interestingly, we have learned that the physiological activities of developing tissues also contribute to their own morphogenesis. Specifically, physiological activities such as fluid secretion and cell contractility generate hydrostatic pressure that can act as a morphogenetic force. Here, we first review the role of hydrostatic pressure in tube formation during animal development and discuss mathematical models of lumen formation. We then illustrate specific roles of the notochord as a hydrostatic scaffold in anterior-posterior axis development in chordates. Finally, we cover some examples of how fluid flows influence morphogenetic processes in other developmental contexts. Understanding how fluid forces act during development will be key for uncovering the self-organizing principles that control morphogenesis.

A niche for axial stem cells - A cellular perspective in amniotes

The head-tail axis in birds and mammals develops from a growth zone in the tail-end, which contains the node. This growth zone then forms the tailbud. Labelling experiments have shown that while many cells leave the node and tailbud to contribute to axial (notochord, floorplate) and paraxial (somite) structures, some cells remain resident in the node and tailbud. Could these cells be resident axial stem cells? If so, do the node and tailbud represent an instructive stem cell niche that specifies and maintains these stem cells? Serial transplantation and single cell labelling studies support the existence of self-renewing stem cells and heterotopic transplantations suggest that the node can instruct such self-renewing behaviour. However, only single cell manipulations can reveal whether self-renewing behaviour occurs at the level of a cell population (asymmetric or symmetric cell divisions) or at the level of single cells (asymmetric divisions only). We combine data on resident cells in the node and tailbud and review it in the context of axial development in chick and mouse, summarising our current understanding of axial stem cells and their niche and highlighting future directions of interest.

Hyaluronan receptor CD44: developmentally regulated expression and role in the early chick embryo

CD44 is a membrane glycoprotein and is the main receptor for hyaluronan. We studied CD44 expression and spatio-temporal distribution by RT-PCR and immunofluorescence, and used an anti-CD44 blocking antibody to perturb CD44-depended signalling programs in the early chick embryo. The intense CD44 levels we detected in the morula embryo (XI) were of particular interest, suggestive of a maternally stored transcript. Intriguingly, the early presence of CD44 seemed to be essential for the rapid synthesis of hyaluronan. At stage XIII (blastula), CD44 expression was intense in the epiblast and hypoblast. During gastrulation (HH3-4), the cells ingressing into the primitive groove and migrating, and the blood islands, expressed CD44 intensely. At HH8, the folding neural plate showed polarity regulation of CD44 expression, and expression was also intense in neural crest, notochord, and blood islands. During early organogenesis, CD44 was expressed intensely in the developing cranial and caudal neural tube that showed polarity regulation, in optic stalks, otic vesicles, pre-and migratory neural crest cells, ganglia, notochord, pharynx, gut, liver, aortae, heart, somites, vascular area, amnion and chorion, and was distinct in extracellular matrix of cranial neural tube and otic vesicle lumens. Antibody-mediated perturbation of CD44 function resulted in unorganized extracellular matrix, loss of tissue spaces, grossly abnormal notochord, intermingling of clumped neuroectoderm and mesenchyme, absence of somites and blood vessels and inhibition of neural crest cell emigration. CD44 has various pivotal roles in matrix integrity and tissue patterning, consistent with its known biochemical features and interactions with hyaluronan, growth factors, receptors and other signaling molecules.

The Degenerate Tale of Ascidian Tails

Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the "notoball." We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

Single-cell analysis of cell fate bifurcation in the chordate Ciona

BACKGROUND

Inductive signaling interactions between different cell types are a major mechanism for the further diversification of embryonic cell fates. Most blastomeres in the model chordate Ciona robusta become restricted to a single predominant fate between the 64-cell and mid-gastrula stages. The deeply stereotyped and well-characterized Ciona embryonic cell lineages allow the transcriptomic analysis of newly established cell types very early in their divergence from sibling cell states without the pseudotime inference needed in the analysis of less synchronized cell populations. This is the first ascidian study to use droplet scRNAseq with large numbers of analyzed cells as early as the 64-cell stage when major lineages such as primary notochord first become fate restricted.

RESULTS AND CONCLUSIONS

We identify 59 distinct cell states, including new subregions of the b-line neural lineage and the early induction of the tail tip epidermis. We find that 34 of these cell states are directly or indirectly dependent on MAPK-mediated signaling critical to early Ciona patterning. Most of the MAPK-dependent bifurcations are canalized with the signal-induced cell fate lost upon MAPK inhibition, but the posterior endoderm is unique in being transformed into a novel state expressing some but not all markers of both endoderm and muscle. Divergent gene expression between newly bifurcated sibling cell types is dominated by upregulation in the induced cell type. The Ets family transcription factor Elk1/3/4 is uniquely upregulated in nearly all the putatively direct inductions. Elk1/3/4 upregulation together with Ets transcription factor binding site enrichment analysis enables inferences about which bifurcations are directly versus indirectly controlled by MAPK signaling. We examine notochord induction in detail and find that the transition between a Zic/Ets-mediated regulatory state and a Brachyury/FoxA-mediated regulatory state is unexpectedly late. This supports a "broad-hourglass" model of cell fate specification in which many early tissue-specific genes are induced in parallel to key tissue-specific transcriptional regulators via the same set of transcriptional inputs.

A cis-regulatory change underlying the motor neuron-specific loss of Ebf expression in immotile tunicate larvae

Many species in the tunicate family Molgulidae have independently lost their swimming larval form and instead develop as tailless, immotile larvae. These larvae do not develop structures that are essential for swimming such as the notochord, otolith, and tail muscles. However, little is known about neural development in these nonswimming larvae. Here, we studied the patterning of the Motor Ganglion (MG) of Molgula occulta, a nonswimming species. We found that spatial patterns of MG neuron regulators in this species are conserved, compared with species with swimming larvae, suggesting that the gene networks regulating their expression are intact despite the loss of swimming. However, expression of the key motor neuron regulatory gene Ebf (Collier/Olf/EBF) was reduced in the developing MG of M. occulta when compared with molgulid species with swimming larvae. This was corroborated by measuring allele-specific expression of Ebf in hybrid embryos from crosses of M. occulta with the swimming species M. oculata. Heterologous reporter construct assays in the model tunicate species Ciona robusta revealed a specific cis-regulatory sequence change that reduces expression of Ebf in the MG, but not in other cells. Taken together, these data suggest that MG neurons are still specified in M. occulta larvae, but their differentiation might be impaired due to reduction of Ebf expression levels.

Down-Regulation of hspb9 and hspb11 Contributes to Wavy Notochord in Zebrafish Embryos Following Exposure to Polychlorinated Diphenylsulfides

It is hypothesized that key genes, other than ahr2, are present and associated with the development of a unique type of notochord malformation known as wavy notochord in early life stages of zebrafish following exposure to polychlorinated diphenylsulfides (PCDPSs). To investigate the potential mechanism(s), time-dependent developmental morphologies of zebrafish embryos following exposure to 2500 nM 2,4,4',5-tetra-CDPS, 2,2',4-tri-CDPS or 4,4'-di-CDPS were observed to determine the developmental time point when notochord twists began to occur (i.e., 21 h-postfertilization (hpf)). Simultaneously, morphometric measurements suggested that PCDPS exposure did not affect notochord growth at 21 or 120 hpf; however, elongation of the body axis was significantly inhibited at 120 hpf. Transcriptome analysis revealed that the retardation of body growth was potentially related with dysregulation of transcripts predominantly associated with the insulin-associated Irs-Akt-FoxO cascade. Moreover, knockdown and gain-of-function experiments in vivo on codifferentially expressed genes demonstrated that reduced expression of hspb9 and hspb11 contributed to the occurrence of wavy notochord. The results of this study strongly support the hypothesis that the notochord kinks and twists are triggered by the down-regulation of hspb9 and hspb11, and intensified by body growth retardation along with normal notochord length in PCDPS-exposed zebrafish embryos.

Expression of gamma-adducin is associated with regions of morphogenetic cell movement in the chick embryo

The adducin family of cytoskeletal proteins are known to mediate actin driven cell movements. Here we describe the cloning and expression pattern of a gene encoding gamma-adducin in the chick embryo. Expression of this gene is strikingly restricted to the epithelial cell layer (with a few exceptions including emerging notochord and lateral mesoderm). Gamma-adducin is detected at particularly high levels in cell populations undergoing important morphogenetic movements, such as epiblast approaching the primitive streak, regressing spinal cord primordia and closing neural tube.

Phenotypic analysis of a novelchordinmutant in medaka

We have isolated and characterized a ventralized mutant in medaka (the Japanese killifish; Oryzias latipes), which turned out to have a mutation in the chordin gene. The mutant exhibits ventralization of the body axis, malformation of axial bones, over-bifurcation of yolk sac blood vessels, and laterality defects in internal organs. The mutant exhibits variability of phenotypes, depending on the culture temperature, from embryos with a slightly ventralized phenotype to those without any head and trunk structures. Taking advantages of these variable and severe phenotypes, we analyzed the role of Chordin-dependent tissues such as the notochord and Kupffer's vesicle (KV) in the establishment of left-right axis in fish. The results demonstrate that, in the absence of the notochord and KV, the medaka lateral plate mesoderm autonomously and bilaterally expresses spaw gene in a default state.

A rare symptomatic presentation of ecchordosis physaliphora: neuroradiological and surgical management

We report a case of ecchordosis physaliphora, an uncommon benign lesion originating from embryonic notochordal remnants, intradurally located in the prepontine cistern, that unusually presented associated with symptoms. MRI detected and precisely located the small mass. At surgery, a cystic gelatinous nodule was found ventral to the pons, contiguous with the dorsal wall of the clivus via a small pedicle. Histological examination diagnosed the lesion as an ecchordosis physaliphora. Here we focus on the analysis of the neuroradiological aspects that play a crucial role from both a diagnostic and a therapeutic standpoint.

First single movement of turkey embryos

The purpose of the study was to characterize the first contraction of an isolated muscle in turkey embryo. The space of time of the contraction since beginning of incubation, topography, morphogenesis and histology of the concerned muscle and its mechanical counterpart are described.

MATERIAL AND METHODS

From the 3rd day of incubation on until the 6th day the embryos were continuously watched through a cellophane window in the eggshell. The installing of the window followed a certain time schedule to reveal the influence of the experimental conditions. For histology the embryos were fixed in Bouin's fluid, then completely cut in serial sections of 5 microm thickness and stained according to Masson-Goldner's trichrome procedure plus resorcin-fuchsin. Wire frame 3D reconstructions were performed to reveal the topography of the region.

RESULTS

A paired muscle 1 mm long and 0.1 mm broad, derived by fusion of the four occipital myomeres, is responsible for the first individual contraction. The contraction produces a stretching in the neck region. The muscle named M. occipitalis primordialis consists of four end-to-end connected groups of mononucleated muscle cells; insofar it looks like early muscle in fishes and amphibians. The muscle contains two types of cells according to the cell nuclei. The elastic rod-shaped notochord represents an endoskeleton. Immediately after contraction it brings the body of the embryo back into its former shape. In the neck region the diameter of the notochord is less and, therefore, the elasticity of notochord is higher than further caudal. The floor plate may prevent damage of the neural tube during excursion of the notochord. The floor plate is flanked by two floor plate posts, which have a filamentous content like the floor plate. Their function may be fastening of floor plate and protection of nerve tissue. The passive pulse movements in the occipital region for 2 days before first contraction are considered to be of importance in orientation and extent of the consecutive active reactions.

CONCLUSION

Vital observation accompanying serial section examination showed to be a suitable approach to biomechanical investigations in embryology. It allowed even after nearly 180 years of intensive morphological studies of avian embryos to find a new, up till now not described muscle.

Retroclival ecchordosis physaliphora: MR imaging and review of the literature

BACKGROUND AND PURPOSE

Ecchordosis physaliphora (EP), found in about 2% of autopsies, is a clinically inconspicuous notochordal remnant appearing at the dorsal wall of the clivus. To our knowledge, a systematic review of its MR features does not exist. The aim of this study was to describe the MR imaging findings of incidentally found retroclival EP with special respect to its differentiation from intradural chordomas.

METHODS

We reviewed 300 consecutive 1.5-T MR imaging studies that included thin-section transverse T2-weighted images of the skull base for the presence of a retroclival EP. In cases in which an EP was identified, two neuroradiologists observed MR signal intensity characteristics, contrast enhancement, size, form, stalk of EP, and signal intensity changes of the adjacent clivus.

RESULTS

Five cases with retroclival EP were found (incidence, 1.7%). In all cases, the ecchordoses was hyperintense on T2-weighted images and hypointense on T1-weighted images. Contrary to the reported findings in chordomas, none of the lesions showed contrast enhancement. In four cases, there were signal intensity changes in the adjacent clivus. A stalklike connection between clivus and EP was seen in three patients.

CONCLUSION

Because of the benign character of EP and the difficulties in its histopathologic differentiation from chordomas, precise knowledge of the radiologic characteristics of EP is important. On the basis of these five cases and a review of literature, contrast enhancement and the presence of clinical symptoms seem to be highly reliable parameters in the differential diagnosis of intradural chordoma and EP.

Benign Notochordal Cell Tumors: A Comparative Histological Study of Benign Notochordal Cell Tumors, Classic Chordomas, and Notochordal Vestiges of Fetal Intervertebral Discs

Intraosseous benign notochordal cell tumors are recently recognized conditions that may undergo malignant transformation to classic chordomas. This study attempts to define the morphologic and immunohistochemical characteristics of 34 benign notochordal cell tumors by contrasting them with classic chordomas and the notochordal vestiges in fetal intervertebral discs. Benign notochordal cell tumors were characterized by well-demarcated though unencapsulated sheets of adipocyte-like vacuolated and less vacuolated eosinophilic cells within axial bones. The round nuclei were mildly polymorphic but bland. The tumor cells often contained cytoplasmic eosinophilic hyaline globules and lack any intercellular myxoid matrix or necrosis. The involved bone trabeculae were often sclerotic without evidence of bone destruction. The histologic features were different from those of both notochordal vestiges in fetal intervertebral discs and classic chordomas. There was overlap in immunohistochemical reactivity of benign notochordal cell tumors and chordomas, but notochordal vestiges failed to demonstrate cytokeratin 18 positivity. A more appropriate term for the lesions is "benign notochordal cell tumor" rather than "notochordal rest" or "notochordal hamartoma" as they are not rests and do not fulfill the definition of hamartoma. Benign notochordal cell tumors do not need any surgical procedure and must be adequately recognized to prevent unnecessary operations.

The split notochord syndrome with dorsal enteric fistula

The authors describe a new case of the split notochord syndrome (SNS) with dorsal enteric fistula in an eight-month-old-boy. The child had also central nervous system anomalies including dysgenetic corpus callosum, absence of septum pellucidum, triventricular hydrocephalus, small posterior fossa, and lipomyelomeningocele, gastrointestinal system anomalies such as malrotation, wandering spleen, and right inguinal hernia. The clinical features, embryogenesis, and literature were reviewed briefly.

First histologically confirmed case of a classic chordoma arising in a precursor benign notochordal lesion: differential diagnosis of benign and malignant notochordal lesions

The first histologically confirmed case of a classic chordoma arising in a precursor benign notochordal lesion is presented and the differential diagnosis between benign and malignant notochordal lesions is discussed. A 57-year-old man presented with a classic chordoma in the coccyx. The resected specimen demonstrated a small intraosseous benign notochordal lesion in the coccyx, which was adjacent to the classic chordoma. Also seen were two separate, similar benign lesions in the sacrum. The classic chordoma consisted of multiple lobules that were separated by thin fibrous septa and that showed cords or strands of atypical physaliphorous cells set within an abundant myxoid matrix. In contrast, the benign lesions consisted of intraosseous sheets of bland physaliphorous cells without any extracellular matrix. The affected bone trabeculae showed sclerotic reactions. It was concluded that benign and malignant notochordal lesions can be distinguished microscopically.

Ascidian embryos as a model system to analyze expression and function of developmental genes

Ascidians have served as an appropriate experimental system in developmental biology for more than a century. The fertilized egg develops quickly into a tadpole larva, which consists of a small number of organs including epidermis, central nervous system with two sensory organs, endoderm and mesenchyme in the trunk, and notochord and muscle in the tail. This configuration of the ascidian tadpole is thought to represent the most simplified and primitive chordate body plan. Their embryogenesis is simple, and lineage of embryonic cells is well documented. The ascidian genome contains a basic set of genes with less redundancy compared to the vertebrate genome. Cloning and characterization of developmental genes indicate that each gene is expressed under discrete spatio-temporal pattern within their lineage. In addition, the use of various molecular techniques in the ascidian embryo system highlights its advantages as a future experimental system to explore the molecular mechanisms underlying the expression and function of developmental genes as well as genetic circuitry responsible for the establishment of the basic chordate body plan. This review is aimed to highlight the recent advances in ascidian embryology.

Chick CFC controls Lefty1 expression in the embryonic midline and nodal expression in the lateral plate

Members of the EGF-CFC family of proteins have recently been implicated as essential cofactors for Nodal signaling. Here we report the isolation of chick CFC and describe its expression pattern, which appears to be similar to Cfc1 in mouse. During early gastrulation, chick CFC was asymmetrically expressed on the left side of Hensen's node as well as in the emerging notochord, prechordal plate, and lateral plate mesoderm. Subsequently, its expression became confined to the heart fields, notochord, and posterior mesoderm. Implantation experiments suggest that chick CFC expression in the lateral plate mesoderm is dependent on BMP signaling, while in the midline its expression depends on an Activin-like signal. The asymmetric expression domain within Hensen's node was not affected by application of FGF8, Noggin, or Shh antibody. Implantation of cells expressing human or mouse CFC2, or chick CFC on the right side of Hensen's node randomized heart looping without affecting expression of genes involved in left-right axis formation, including SnR, Nodal, Car, or Pitx2. Application of antisense oligodeoxynucleotides to the midline of Hamburger-Hamilton stage 4-5 embryos also randomized heart looping, but in contrast to the overexpression experiments, antisense oligodeoxynucleotide treatment resulted in bilateral expression of Nodal, Car, Pitx2, and NKX3.2, whereas Lefty1 expression in the midline was transiently lost. Application of the antisense oligodeoxynucleotides to the lateral plate mesoderm abolished Nodal expression. Thus, chick CFC seems to have a dual function in left-right axis formation by maintaining Nodal expression in the lateral plate mesoderm and controlling expression of Lefty1 expression in the midline territory.

Shh and Wnt signaling pathways converge to control Gli gene activation in avian somites

The regulation of the Gli genes during somite formation has been investigated in quail embryos. The Gli genes are a family encoding three related zinc finger transcription factors, Gli1, Gli2 and Gli3, which are effectors of Shh signaling in responding cells. A quail Gli3 cDNA has been cloned and its expression compared with Gli1 and Gli2. These studies show that Gli1, Gli2 and Gli3 are co-activated at the time of somite formation, thus providing a mechanism for regulating the initiation of Shh signaling in somites. Embryo surgery and paraxial mesoderm explant experiments show that each of the Gli genes is regulated by distinct signaling mechanisms. Gli1 is activated in response to Shh produced by the notochord, which also controls the dorsalization of Gli2 and Gli3 following their activation by Wnt signaling from the surface ectoderm and neural tube. This surface ectoderm/neural tube Wnt signaling has both negative and positive functions in Gli2 and Gli3 regulation: these signals repress Gli3 in segmental plate mesoderm prior to somite formation and then promote somite formation and the somite-specific activation of Gli2 and Gli3. These studies, therefore, establish a role for Wnt signaling in the control of Shh signal transduction through the regulation of Gli2 and Gli3, and provide a mechanistic basis for the known synergistic actions of surface ectoderm/neural tube and notochord signaling in somite cell specification.

SHH-N upregulates Sfrp2 to mediate its competitive interaction with WNT1 and WNT4 in the somitic mesoderm

Dorsoventral polarity of the somitic mesoderm is established by competitive signals originating from adjacent tissues. The ventrally located notochord provides the ventralizing signals to specify the sclerotome, while the dorsally located surface ectoderm and dorsal neural tube provide the dorsalizing signals to specify the dermomyotome. Noggin and SHH-N have been implicated as the ventralizing signals produced by the notochord. Members of the WNT family of proteins, on the other hand, have been implicated as the dorsalizing signals derived from the ectoderm and dorsal neural tube. When presomitic explants are confronted with cells secreting SHH-N and WNT1 simultaneously, competition to specify the sclerotome and dermomyotome domains within the naive mesoderm can be observed. Here, using these explant cultures, we provide evidence that SHH-N competes with WNT1, not only by upregulating its own receptor Ptc1, but also by upregulating Sfrp2 (Secreted frizzled-related protein 2), which encodes a potential WNT antagonist. Among the four known Sfrps, Sfrp2 is the only member expressed in the sclerotome and upregulated by SHH-N recombinant protein. We further show that SFRP2-expressing cells can reduce the dermomyotome-inducing activity of WNT1 and WNT4, but not that of WNT3a. Together, our results support the model that SHH-N at least in part employs SFRP2 to reduce WNT1/4 activity in the somitic mesoderm.

Regulation of Ci-tropomyosin-like, a Brachyury target gene in the ascidian, Ciona intestinalis

Brachyury is a sequence-specific transcriptional activator that is essential for notochord differentiation in a variety of chordates. In vertebrates, Brachyury is expressed throughout the presumptive mesoderm, but becomes restricted to the notochord at later stages of development. In ascidians, such as Ciona intestinalis, Brachyury is expressed exclusively in the notochord and does not exhibit an early pan-mesodermal pattern. Subtractive hybridization screens were recently used to identify potential Ciona Brachyury (Ci-Bra) target genes (Takahashi, H., Hotta, K., Erives, A., Di Gregorio, A., Zeller, R. W., Levine, M. and Satoh, N. (1999). Genes Dev. 13, 1519–1523). Of the genes that were identified in this screen, one corresponds to a new member of the tropomyosin superfamily, Ciona tropomyosin (Ci-trop). Here we show that Ci-trop is specifically expressed in the developing notochord beginning at gastrulation, and expression persists in the notochord during tailbud and tadpole stages. A 3 kb region of the Ci-trop 5′-flanking sequence was characterized via electroporation of lacZ fusion genes into fertilized Ciona eggs. A minimal, 114 bp enhancer was identified that is sufficient to direct the expression of a heterologous promoter in the notochord. DNA binding assays indicate that this enhancer contains two sets of low-affinity Brachyury half-sites, which are bound in vitro by a GST/Ci-Bra fusion protein. Deletion of the distal sites inactivates the notochord-specific staining pattern mediated by an otherwise normal Ci-trop/lacZ transgene. These results suggest that Ci-trop is a direct target gene of Ci-Bra and that Brachyury plays an immediate role in the cellular morphogenesis of the notochord.

Pancreas development and diabetes

The past few years have seen an increase in interest about the molecular and genetic events regulating pancreas development. Transcription factors such as Pdx1, p48 and Nkx2.2 have been shown to be essential for the proper differentiation of exocrine and endocrine tissue; however, pancreas development also involves intricate interactions between the pancreatic epithelium and its surrounding mesenchyme. Signalling factors emanating from the notochord have been shown to repress Sonic hedgehog expression in the endoderm whereas signals originating from the pancreatic mesenchyme determine the proportion of exocrine to endocrine tissue. Understanding the molecular and genetic events underlying pancreas development also opens the door for devising new therapeutic strategies against pancreatic diseases such as diabetes and cancer.

Delta-mediated specification of midline cell fates in zebrafish embryos

BACKGROUND

Fate mapping studies have shown that progenitor cells of three vertebrate embryonic midline structures - the floorplate in the ventral neural tube, the notochord and the dorsal endoderm - occupy a common region prior to gastrulation. This common region of origin raises the possibility that interactions between midline progenitor cells are important for their specification prior to germ layer formation.

RESULTS

One of four known zebrafish homologues of the Drosophila melanogaster cell-cell signaling gene Delta, deltaA (dlA), is expressed in the developing midline, where progenitor cells of the ectodermal floorplate, mesodermal notochord and dorsal endoderm lie close together before they occupy different germ layers. We used a reverse genetic strategy to isolate a missense mutation of dlA, dlAdx2, which coordinately disrupts the development of floorplate, notochord and dorsal endoderm. The dlAdx2 mutant embryos had reduced numbers of floorplate and hypochord cells; these cells lie above and beneath the notochord, respectively. In addition, mutant embryos had excess notochord cells. Expression of a dominant-negative form of Delta protein driven by mRNA microinjection produced a similar effect. In contrast, overexpression of dlA had the opposite effect: fewer trunk notochord cells and excess floorplate and hypochord cells.

CONCLUSION

Our results indicate that Delta signaling is important for the specification of midline cells. The results are most consistent with the hypothesis that developmentally equivalent midline progenitor cells require Delta-mediated signaling prior to germ layer formation in order to be specified as floorplate, notochord or hypochord.

Sequential roles for Fgf4, En1 and Fgf8 in specification and regionalisation of the midbrain

Experiments involving tissue recombinations have implicated both early vertical and later planar signals in the specification and polarisation of the midbrain. Here we investigate the role of fibroblast growth factors in regulating these processes in the avian embryo. We show that Fgf4 is expressed in the notochord anterior to Hensen's node before transcripts for the earliest molecular marker of midbrain tissue in the avian embryo, En1, are detected. The presence of notochord is required for the expression of En1 in neural plate explants in vitro and FGF4 mimics this effect of notochord tissue. Subsequently, a second member of the fibroblast growth factor family, Fgf8, is expressed in the isthmus in a manner consistent with it providing a polarising signal for the developing midbrain. Using a retroviral vector to express En1 ectopically, we show that En1 can induce Fgf8 expression in midbrain and posterior diencephalon. Results of the introduction of FGF8 protein into the anterior midbrain or posterior diencephalon are consistent with it being at least part of the isthmic activity which can repolarise the former tissue and respecify the latter to a midbrain fate. However, the ability of FGF8 to induce expression of genes which have earlier onsets of expression than Fgf8 itself, namely En1 and Pax2, strongly suggests that the normal function of FGF8 is in maintaining patterns of gene expression in posterior midbrain. Finally, we provide evidence that FGF8 also provides mitogenic stimulation during avian midbrain development.

Hox gene expression reveals regionalization along the anteroposterior axis of the zebrafish notochord

The vertebrate Hox genes have been shown to confer regional identity along the anteroposterior axis of the developing embryo, especially within the central nervous system (CNS) and the paraxial mesoderm. The notochord has been shown to play vital roles in patterning adjacent tissues along both the dorsoventral and mediolateral axes. However, the notochord's role in imparting anteroposterior information to adjacent structures is less well understood, especially as the notochord shows no morphological distinctions along the anteroposterior axis and is not generally described as a segmental or compartmentalized structure. Here we report that four zebrafish hox genes: hoxb1, hoxb5, hoxc6 and hoxc8 are regionally expressed along the anteroposterior extent of the developing notochord. Notochord expression for each gene is transient, but maintains a definite, gene-specific anterior limit throughout its duration. The hox gene expression in the zebrafish notochord is spatially colinear with those genes lying most 3' in the hox clusters having the most anterior limits. The expression patterns of these hox cluster genes in the zebrafish are the most direct molecular evidence for a system of anteroposterior regionalization of the notochord in any vertebrate studied to date.

Neuroradiological characteristics of ecchordosis physaliphora. Case report and review of the literature

An extremely rare case of ecchordosis physaliphora is presented in which the authors focus especially on its radiological characteristics. The patient complained of a headache with no other neurological abnormalities. A thorough radiological examination revealed a small intradural prepontine mass with no bone destruction of the clivus. Magnetic resonance imaging was very useful in visualizing this mass as a low signal intensity lesion on T1-weighted images and as a high signal intensity lesion on T2-weighted images without any contrast enhancing effects. At surgery, a cystic gelatinous nodule was found ventral to the pons; the nodule was connected to the dorsal wall of the clivus via a delicate stalk. Histological studies proved that this was an ecchordosis physaliphora. Review of the literature demonstrates that the reported cases of ecchordoses have many common radiological features that would suggest the diagnosis of this rare disease.

Ascidian embryogenesis and the origins of the chordate body plan

For more than a century, ascidians have been a widely used system for classic embryological studies. Ascidians possess simple, well-defined cell-lineages, compact genomes, rapid development and world-wide distribution. Transgenic DNA can be introduced into developing embryos using simple electroporation methods. The ascidian larva represents the most simplified chordate body plan and provides a useful model for studying the molecular pathways underlying the morphogenesis and differentiation of the notochord and neural tube.

Rostral truncation of a cyclostome, Lampetra japonica, induced by all-trans retinoic acid defines the head/trunk interface of the vertebrate body

The effect of all-trans retinoic acid on embryogenesis was studied in a cyclostome, Lampetra japonica. Treatment with 0.05-0.5 microM retinoic acid on early gastrula and early neurula resulted in loss of the pharynx and in the rostral truncation of the neural tube. The mouth, pharynx, esophagus, heart, endostyle, and rostral brain were missing with graded severity. In the severest case, the embryo consisted only of trunk segments, especially myotomes that extended to the rostral end of the axis. The effect appeared to be dose- and stage-dependent: Rostral pharyngeal arches were more vulnerable to a lower amount of retinoic acid, and earlier treatment resulted in severer defects. The initial protrusion of the anterior axis started equally in control and retinoic acid-treated embryos, implying that the head morphogenesis is omitted in treated embryos. By identifying the number of myotomes based on the differentiation of hypobranchial muscles, there seemed to be no myotomes lost by retinoic acid-induced truncation. The rostral truncation, therefore, was not simply a limitation of the anterior axis but was restricted to the ventral portion; only the branchial arches disappeared with normally developing myotomes dorsally. The absent region can be defined as the vertebrate head in a morphological sense, including the branchiomeric and preotic paraxial regions as well as the heart. The results suggest the presence of distinct programs between somitomeric and branchiomeric portions of the body, providing a developmental basis for the dual-metamerical body plan of vertebrates.

The one bone spine: a failure of notochord/sclerotome signalling?

Three cases of extensive vertebral fusions with absent clivo-axial angle are presented. The 'bone-within-bone' appearance in two patients with almost complete fusion of the spine suggested ossification of the notochord and perinotochordal sheath. On the basis of the radiological appearances and the results of recent molecular genetic studies on vertebrate embryos, the suggested time of segmentation failure along the axis of the craniovertebral junction and between vertebrae is the third to fifth week of gestation. The possible roles of the Pax-1 gene and of signalling between notochord and sclerotome are discussed, concluding that an early defect of the notochord may be responsible for this type of failure of segmentation. Indications for surgery in these cases included cord compression with brachialgia and 'chin-on-chest' deformity causing severely restricted visual fields. A critical review of clinical lessons learned in the operative treatment is presented.

lim6, a novel LIM homeobox gene in the zebrafish: comparison of its expression pattern with lim1

A novel LIM class homeobox gene, lim6, was isolated from a zebrafish embryonic cDNA library. The encoded protein shares a high degree of sequence similarity with the previously described Lim1 and Lim5 proteins. This study compares the spatial and temporal expression pattern of the closely related lim6 and lim1 genes during early embryogenesis. Generally, lim6 mRNA was found at rather low amounts compared to lim1 mRNA. At the shield stage, lim6 mRNA, similar to lim1 mRNA, was predominantly expressed in the shield. Lim6 was transiently expressed in a restricted region of the anterior neural plate at the bud stage, distinct from the expression of lim1 in the notochord and the pronephros and pronephric ducts. During the segmentation period, the lim6 gene started to be expressed in single cells in the spinal cord, followed by a gradually increasing wide-spread expression throughout the CNS. During this stage, lim1 mRNA disappeared in the notochord and pronephric ducts and was found in the pronephroi and single cells in the CNS. In 24 hr embryos, lim6 and lim1 were expressed in the fore-, mid-, and hindbrain and the spinal cord, except that lim1 mRNA was limited to two small domains in the telencephalon, whereas lim6 mRNA was widely expressed in this region. A comparison of expression of lim1 and lim6 and of the previously characterized lim5 show that, in spite of close sequence similarity, distinct expression patterns imply nonredundant functions for each member of this group of genes.

Characterization of the INT6 mammary tumor gene product

INT6 is a unique gene, highly conserved throughout evolution and associated with mammary tumorigenesis in the mouse. Although it is expressed in all adult tissues of the mouse and early in embryonic development, its function is unknown. To study the normal distribution and the potential function of the Int6 gene products, we produced antibodies against synthetic peptides specific for the Int6 protein. Western blot and immunoprecipitation analysis demonstrated a 43-kD major gene product that is localized in the cytosolic fraction of mammary cell homogenates. This latter observation is supported by immunoperoxidase analysis, which shows a strong staining anti-Int6 peptide in the perinuclear region of the HC11 mammary epithelial cell line, suggesting a possible localization in the Golgi apparatus. Further immunocytochemical studies in the mouse embryo show that Int6 expression is prevalent in migrating neural crest cells, in the notochord, and in condensing cartilage between 9.5 and 14.5 days of development. In these embryonic tissues, Int6 staining co-localizes with the staining of ricinus lectin, and giantin, proteins that are specifically associated with the Golgi apparatus. The restricted expression of the protein within the Golgi apparatus and its strong conservation throughout evolution suggest that Int6 may perform an essential cellular function.

Regulation of paraxis expression and somite formation by ectoderm- and neural tube-derived signals

During vertebrate embryogenesis, the paraxial mesoderm becomes segmented into somites, which form as paired epithelial spheres with a periodicity that reflects the segmental organization of the embryo. As a somite matures, the ventral region gives rise to a mesenchymal cell population, the sclerotome, that forms the axial skeleton. The dorsal region of the somite remains epithelial and is called dermomyotome. The dermomyotome gives rise to the trunk and limb muscle and to the dermis of the back. Epaxial and hypaxial muscle precursors can be attributed to distinct somitic compartments which are laid down prior to overt somite differentiation. Inductive signals from the neural tube, notochord, and overlying ectoderm have been shown to be required for patterning of the somites into these different compartments. Paraxis is a basic helix-loop-helix transcription factor expressed in the unsegmented paraxial mesoderm and throughout epithelial somites before becoming restricted to epithelial cells of the dermomyotome. To determine whether paraxis might be a target for inductive signals that influence somite patterning, we examined the influence of axial structures and surface ectoderm on paraxis expression by performing microsurgical operations on chick embryos. These studies revealed two distinct phases of paraxis expression, an early phase in the paraxial mesoderm that is dependent on signals from the ectoderm and independent of the neural tube, and a later phase that is supported by redundant signals from the ectoderm and neural tube. Under experimental conditions in which paraxis failed to be expressed, cells from the paraxial mesoderm failed to epithelialize and somites were not formed. We also performed an RT-PCR analysis of combined tissue explants in vitro and confirmed that surface ectoderm is sufficient to induce paraxis expression in segmental plate mesoderm. These results demonstrate that somite formation requires signals from adjacent cell types and that the paraxis gene is a target for the signal transduction pathways that regulate somitogenesis.

The function of silberblick in the positioning of the eye anlage in the zebrafish embryo

In zebrafish, as in other vertebrates, an initially singular eye field within the neural plate has to split during morphogenesis to allow the development of two separated eyes. It has been suggested that anterior progression of midline tissue within the neural plate is involved in the bilateralization of the eye field. Mutations in the recently identified silberblick (slb) gene cause an incomplete separation of the eyes. During gastrulation and early somitogenesis, the ventral midline of the central nervous system (CNS) together with the underlying axial mesendoderm is shortened and broadened in slb embryos. While in wild-type embryos the ventral CNS midline extends to the anterior limit of the neural plate at the end of gastrulation, there is a gap between the anterior tip of the ventral CNS midline and the anterior edge of the neural plate in slb. To investigate the cause for the shortening of the ventral CNS midline in slb we determined the fate of labeled ventral CNS midline cells in wild-type and slb embryos at different stages of development. In slb, anterior migration of ventral CNS midline cells is impaired, which indicates that migration of these cells is needed for elongation of the ventral CNS midline. The anterior shortening of the ventral CNS midline in slb leads to medial instead of bilateral induction of optic stalks followed by a partial fusion of the eyes at later developmental stages. The analysis of the slb phenotype indicates that anterior migration of midline cells within the neural plate is required for proper induction and subsequent bilateralization of an initially singular eye field. These findings may therefore provide a starting point in elucidating the role of neural plate morphogenesis in positioning of the eyes.

FINE STRUCTURE AND MORPHOGENIC MOVEMENTS IN THE GASTRULA OF THE TREEFROG, HYLA REGILLA

The blastoporal groove of the early gastrula of the treefrog, Hyla regilla, was examined with the electron microscope. The innermost extension of the groove is lined with invaginating flask- and wedge-shaped cells of entoderm and mesoderm. The distal surfaces of these cells bear microvilli which are underlain with an electron-opaque layer composed of fine granular material and fibrils. The dense layer and masses of vesicles proximal to it fill the necks of the cells. In flask cells bordering the forming archenteron the vesicles are replaced by large vacuoles surrounded by layers of membranes. The cells lining the groove are tightly joined at their distal ends in the region of the dense layer. Proximally, the cell bodies are separated by wide intercellular spaces. The cell body, which is migrating toward the interior of the gastrula, contains the nucleus plus other organalles and inclusions common to amphibian gastrular cells. A dense layer of granular material, vesicles, and membranes lies beneath the surface of the cell body and extends into pseudopodium-like processes and surface undulations which cross the intercellular spaces. A special mesodermal cell observed in the dorsal lining of the groove is smaller and denser than the surrounding presumptive chordamesodermal cells. A long finger of cytoplasm, filled with a dense layer, vesicles and membranes, extends from its distal surface along the edge of the groove, ending in a tight interlocking with another mesodermal cell. Some correlations between fine structure and the mechanics of gastrulation are discussed, and a theory of invagination is proposed, based on contraction and expansion of the dense layer and the tight junctions at distal cell surfaces.

AN ELECTRON MICROSCOPE STUDY OF THE ENDOPLASMIC RETICULUM IN NEWT NOTOCHORD CELLS AFTER DISTURBANCE WITH ULTRASONIC TREATMENT AND SUBSEQUENT REGENERATION

Ultrasonic treatment of the tails of Triturus alpestris tadpoles, at intensities of 8 to 15 watts/cm², at 1 megacycle/sec., for 5 minutes, disrupted the epidermis and caused pycnosis in individual cells of the muscle and neural tube, but caused no damage to the notochord that could be detected by light microscopy. Electron microscopy showed that this ultrasonic treatment disordered nearly all the endoplasmic reticulum (ER) of the notochord cells into irregularly rounded vesicles, but within 3 hours after treatment some parallel arrays of normal endoplasmic reticulum were seen near, and continuous with, the outer nuclear membrane. In addition, a re-ordering of the previously disordered ER took place throughout the cytoplasm, in some cases. A classification was made of the state of the ER as shown in electron micrographs of material fixed immediately, 3, and 24 hours after treatment. This showed that more than half the total endoplasmic reticulum in notochord cells was normal again by 24 hours after treatment.

Formin isoforms are differentially expressed in the mouse embryo and are required for normal expression of fgf-4 and shh in the limb bud

Mice homozygous for the recessive limb deformity (ld) mutation display both limb and renal defects. The limb defects, oligodactyly and syndactyly, have been traced to improper differentiation of the apical ectodermal ridge (AER) and shortening of the anteroposterior limb axis. The renal defects, usually aplasia, are thought to result from failure of ureteric bud outgrowth. Since the ld locus gives rise to multiple RNA isoforms encoding several different proteins (termed formins), we wished to understand their role in the formation of these organs. Therefore, we first examined the embryonic expression patterns of the four major ld mRNA isoforms. Isoforms I, II and III (all containing a basic amino terminus) are expressed in dorsal root ganglia, cranial ganglia and the developing kidney including the ureteric bud. Isoform IV (containing an acidic amino terminus) is expressed in the notochord, the somites, the apical ectodermal ridge (AER) of the limb bud and the developing kidney including the ureteric bud. Using a lacZ reporter assay in transgenic mice, we show that this differential expression of isoform IV results from distinct regulatory sequences upstream of its first exon. These expression patterns suggest that all four isoforms may be involved in ureteric bud outgrowth, while isoform IV may be involved in AER differentiation. To define further the developmental consequences of the ld limb defect, we analyzed the expression of a number of genes thought to play a role in limb development. Most significantly, we find that although the AERs of ld limb buds express several AER markers, they do not express detectable levels of fibroblast growth factor 4 (fgf-4), which has been proposed to be the AER signal to the mesoderm. Thus we conclude that one or more formins are necessary to initiate and/or maintain fgf-4 production in the distal limb. Since ld limbs form distal structures such as digits, we further conclude that while fgf-4 is capable of supporting distal limb outgrowth in manipulated limbs, it is not essential for distal outgrowth in normal limb development. In addition, ld limbs show a severe decrease in the expression of several mesodermal markers, including sonic hedgehog (shh), a marker for the polarizing region and Hoxd-12, a marker for posterior mesoderm. We propose that incomplete differentiation of the AER in ld limb buds leads to reduction of polarizing activity and defects along the anteroposterior axis.