bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oriented cell divisions and cellular morphogenesis in the zebrafish gastrula and neurula: a time-lapse analysis
Authors: Concha Miguel L. , Adams Richard J. 
Published: April 2021

ABSTRACT
We have taken advantage of the optical transparency of zebrafish embryos to investigate the patterns of cell division, movement and shape during early stages of development of the central nervous system. The surface-most epiblast cells of gastrula and neurula stage embryos were imaged and analysed using a computer-based, time-lapse acquisition system attached to a differential interference contrast (DIC) microscope. We find that the onset of gastrulation is accompanied by major changes in cell behaviour. Cells collect into a cohesive sheet, apparently losing independent motility and integrating their behaviour to move coherently over the yolk in a direction that is the result of two influences: towards the vegetal pole in the movements of epiboly and towards the dorsal midline in convergent movements that strengthen throughout gastrulation. Coincidentally, the plane of cell division becomes aligned to the surface plane of the embryo and oriented in the anterior-posterior (AP) direction. These behaviours begin at the blastoderm margin and propagate in a gradient towards the animal pole. Later in gastrulation, cells undergo increasingly mediolateral-directed elongation and autonomous convergence movements towards the dorsal midline leading to an enormous extension of the neural axis. Around the equator and along the dorsal midline of the gastrula, persistent AP orientation of divisions suggests that a common mechanism may be involved but that neither oriented cell movements nor shape can account for this alignment.
When the neural plate begins to differentiate, there is a gradual transition in the direction of cell division from AP to the mediolateral circumference (ML). ML divisions occur in both the ventral epidermis and dorsal neural plate. In the neural plate, ML becomes the predominant orientation of division during neural keel and nerve rod stages and, from late neural keel stage, divisions are concentrated at the dorsal midline and generate bilateral progeny (C. Papan and J. A. Campos-Ortega (1994) Roux’s Arch. Dev. Biol. 203, 178-186). Coincidentally, cells on the ventral surface also orient their divisions in the ML direction, cleaving perpendicular to the direction in which they are elongated. The ML alignment of epidermal divisions is well correlated with cell shape but ML divisions within the neuroepithelium appear to be better correlated with changes in tissue morphology associated with neurulation.
The spatial relationships among cutaneous, muscle sensory and motoneuron axons during development of the chick hindlimb
Authors: Honig Marcia G. , Frase Priscilla A. , Camilli Suzanne J. 
Published: April 2021

ABSTRACT
Previous studies have suggested that interactions with other axons are important in sensory axon pathfinding in the developing chick hindlimb. Yet the nature of these interactions remains unknown, in part because information about the spatial relationships among the different kinds of axons is lacking. To obtain this information, we combined retrograde axonal tracing with an immunofluorescent labelling approach that distinguishes between sensory and motoneuron axons. This allowed us to follow the trajectories of sensory axons having a known destination, while also identifying their neighbors.
We found that as sensory and motoneuron axons meet in the spinal nerves and travel into the limb, sensory axons remain bundled together. The large bundles that are present proximally gradually split into smaller bundles as the axons course distally in the spinal nerves; more distally, some bundles join to again form large bundles. Younger, later-growing sensory axons appear to grow primarily along bundles of older sensory axons that grew out earlier.
Starting from very proximal levels, axons projecting along an individual cutaneous nerve are found together in bundles that are situated in characteristic regions of each spinal nerve. Some of these bundles are initially interspersed with bundles of axons projecting along other nerves, thereby indicating that the initial position of a cutaneous axon in the spinal nerves does not strictly determine its subsequent trajectory. As they travel distally, bundles of axons projecting along one cutaneous nerve gradually join one another, becoming increasingly separated from axons having different destinations.
In contrast, muscle sensory axons are situated adjacent to motoneuron axons innervating the same muscle for much of their course. This suggests that muscle sensory axons may be guided to the appropriate muscles by fasciculating along motoneuron axons.
Taken together, the results show that sensory axons projecting along different nerves are different from one another and respond to cues in their environment to navigate through the spinal nerves and plexus. Thus, sensory neurons must be intrinsically specified with respect to their peripheral targets. Sensory axons appear to respond differentially to the axons they encounter, segregating from axons that project along different nerves and often growing with axons destined for the same nerve, suggesting that fasciculation may aid pathfinding.
Gli3 (Xt) and formin (ld) participate in the positioning of the polarising region and control of posterior limb-bud identity
Authors: Zúñiga Aimée , Zeller Rolf 
Published: April 2021

Abstract
During initiation of limb-bud outgrowth in vertebrate embryos, the polarising region (limb-bud organizer) is established upon activation of the Sonic Hedgehog (SHH) signaling molecule at the posterior limb-bud margin. Another hallmark of establishing anteroposterior limb-bud identities is the colinear activation of HoxD genes located at the 5′ end of the cluster (5′HoxD genes). The unique and shared functions of Gli3 and formin in these determinative events were genetically analyzed using single and double homozygous Extra-toes (Xt; disrupting Gli3) and limb deformity(ld; disrupting formin) mouse embryos. Analysis of the limb skeletal phenotypes reveals genetic interaction of the two genes. In addition to loss of digit identity and varying degrees of polydactyly, proximal skeletal elements are severely shortened in Xt;ld double homozygous limbs. The underlying molecular defects affect both establishment of the polarising region and posterior limb-bud identity. In particular, the synergism between Gli3- and formin-mediated mesenchyme-AER interactions positions the SHH signaling center at the posterior limb-bud margin. The present study shows that establishment and positioning of the polarising region is regulated both by restriction of Shh through Gli3 and its positive feedback regulation through formin. Concurrently, Gli3 functions independently of formin during initial posterior nesting of 5′HoxD domains, whereas their subsequent distal restriction and anterior expansion depends on genetic interaction of Gli3 and formin.
Multipotential progenitors of the mammalian enteric nervous system capable of colonising aganglionic bowel in organ culture
Authors: Natarajan Dipa , Grigoriou Maria , Marcos-Gutierrez Camelia V. , Atkins Chris , Pachnis Vassilis 
Published: April 2021

ABSTRACT
The enteric nervous system of vertebrates is derived from neural crest cells that invade the gut wall and generate a highly organised network of enteric ganglia. Among the genes that play an important role in ENS development is c- Ret, mutations of which result in failure of formation of enteric ganglia (intestinal aganglionosis). To further understand the development of the mammalian ENS in general and the mechanism of action of the RET RTK in particular, we have developed and used an organotypic culture system of mouse fetal gut. At the stage of culture initiation, the gut is partially populated by undifferentiated ENS progenitors, but culture for several days results in extensive neuronal and glial differentiation. Using this organ culture system, we have compared the development of the ENS in wild-type and RET-deficient gut and showed that the aganglionic phenotype observed in vivo is consistently reproduced under the in vitro culture conditions. Microinjection of RET+ cells isolated from E11.5 mouse bowel into wild-type or RET-deficient aganglionic gut in organ culture, results in extensive repopulation of their wall by exogenously derived neurons and glia. Finally, using a similar approach, we demonstrate that single RET+ cells introduced into the wall of wild-type gut generate both cell lineages of the ENS, i.e. neurons and glia. Our data show the NC-derived RET+ population of fetal gut in mammalian embryos consists of multipotential progenitors capable of colonising efficiently both wild-type and RET-deficient aganglionic bowel in organ culture.
NeuroD regulates multiple functions in the developing neural retina in rodent
Authors: Morrow Eric M. , Furukawa Takahisa , Lee Jacqueline E. , Cepko Constance L. 
Published: April 2021

ABSTRACT
The expression and function of the basic helix-loop-helix (bHLH) transcription factor NeuroD were studied in the developing neural retina in rodent. neuroD was expressed in areas of undetermined retinal cells as well as developing photoreceptors and amacrine interneurons. Expression was maintained in a subset of mature photoreceptors in the adult retina. Using both loss-of-function and gain-of-function approaches, NeuroD was found to play multiple roles in retinal development. (1) NeuroD was found to be a critical regulator of the neuron versus glial cell fate decision. Retinal explants derived from NeuroD-null mice demonstrated a three-to fourfold increase in Müller glia. Forced expression of neuroD in progenitors in rat using retroviruses hastened cell cycle withdrawal and blocked gliogenesis in vivo. (2) NeuroD appeared to regulate interneuron development, favouring amacrine over bipolar differentiation. Forced NeuroD expression resulted in an increase in amacrine interneurons and a decrease in bipolar interneurons. In the complementary experiment, retinae derived from NeuroD-null mice demonstrated a twofold increase in bipolar interneurons and a delay in amacrine differentiation. (3) NeuroD appeared to be essential for the survival of a subset of rod photoreceptors. In conclusion, these results implicate NeuroD in a variety of developmental functions including cell fate determination, differentiation and neuron survival.
A Wnt signaling pathway controls Hox gene expression and neuroblast migration in C. elegans
Authors: Maloof Julin N. , Whangbo Jennifer , Harris Jeanne M. , Jongeward Gregg D. , Kenyon Cynthia 
Published: April 2021

ABSTRACT
The specification of body pattern along the anteroposterior (A/P) body axis is achieved largely by the actions of conserved clusters of Hox genes. Limiting expression of these genes to localized regional domains and controlling the precise patterns of expression within those domains is critically important for normal patterning. Here we report that egl-20, a C. elegans gene required to activate expression of the Hox gene mab-5 in the migratory neuroblast QL, encodes a member of the Wnt family of secreted glycoproteins. We have found that a second Wnt pathway gene, bar-1, which encodes a β-catenin/Armadillo-like protein, is also required for activation of mab-5 expression in QL. In addition, we describe the gene pry-1, which is required to limit expression of the Hox genes lin-39, mab-5 and egl-5 to their correct local domains. We find that egl-20, pry-1 and bar-1 all function in a linear genetic pathway with conserved Wnt signaling components, suggesting that a conserved Wnt pathway activates expression of mab-5 in the migratory neuroblast QL. Moreover, we find that members of this Wnt signaling system play a major role in both the general and fine-scale control of Hox gene expression in other cell types along the A/P axis.
The Notch pathway helps to pattern the tips of the Drosophila tracheal branches by selecting cell fates
Authors: Llimargas Marta 
Published: April 2021

ABSTRACT
The Drosophila tracheal system consists of a stereotyped network of epithelial tubes formed by several tracheal cell types. By the end of embryogenesis, when the general branching pattern is established, some specialised tracheal cells then mediate branch fusion while others extend fine terminal branches. Here evidence is presented that the Notch signalling pathway acts directly in the tracheal cells to distinguish individual fates within groups of equivalent cells. Notch helps to single out those tracheal cells that mediate branch fusion by blocking their neighbours from adopting the same fate. This function of Notch would require the restricted activation of the pathway in specific cells. In addition, and probably later, Notch also acts in the selection of those tracheal cells that extend the terminal branches. Both the localised expression and the mutant phenotypes of Delta, a known ligand for Notch, suggest that Delta may activate Notch to specify cell fates at the tips of the developing tracheal branches.
The Hedgehog morphogen and gradients of cell affinity in the abdomen of Drosophila
Authors: Lawrence1 Peter A. , Casal José , Struhl Gary 
Published: April 2021

ABSTRACT
The adult abdomen of Drosophila is a chain of anterior (A) and posterior (P) compartments. The engrailed gene is active in all P compartments and selects the P state. Hedgehog enters each A compartment across both its anterior and posterior edges; within A its concentration confers positional information. The A compartments are subdivided into an anterior and a posterior domain that each make different cell types in response to Hedgehog. We have studied the relationship between Hedgehog, engrailed and cell affinity. We made twin clones and measured the shape, size and displacement of the experimental clone, relative to its control twin. We varied the perceived level of Hedgehog in the experimental clone and find that, if this level is different from the surround, the clone fails to grow normally, rounds up and sometimes sorts out completely, becoming separated from the epithelium. Also, clones are displaced towards cells that are more like themselves: for example groups of cells in the middle of the A compartment that are persuaded to differentiate as if they were at the posterior limit of A, move posteriorly. Similarly, clones in the anterior domain of the A compartment that are forced to differentiate as if they were at the anterior limit of A, move anteriorly. Quantitation of these measures and the direction of displacement indicate that there is a U-shaped gradient of affinity in the A compartment that correlates with the U-shaped landscape of Hedgehog concentration. Since affinity changes are autonomous to the clone we believe that, normally, each cell’s affinity is a direct response to Hedgehog. By removing engrailed in clones we show that A and P cells also differ in affinity from each other, in a manner that appears independent of Hedgehog. Within the P compartment we found some evidence for a U-shaped gradient of affinity, but this cannot be due to Hedgehog which does not act in the P compartment.
Axon repulsion from the midline of the Drosophila CNS requires slit function
Authors: Battye Robin , Stevens Adrienne , Jacobs J. Roger 
Published: April 2021

ABSTRACT
Guidance of axons towards or away from the midline of the central nervous system during Drosophila embryogenesis reflects a balance of attractive and repulsive cues originating from the midline. Here we demonstrate that Slit, a protein secreted by the midline glial cells provides a repulsive cue for the growth cones of axons and muscle cells. Embryos lacking slit function show a medial collapse of lateral axon tracts and ectopic midline crossing of ventral muscles. Transgene expression of slitin the midline restores axon patterning. Ectopic expression of slit inhibits formation of axon tracts at locations of high Slit production and misdirects axon tracts towards the midline. slit interacts genetically with roundabout, which encodes a putative receptor for growth cone repulsion.
Timing and cell interactions underlying neural induction in the chick embryo
Authors: Darnell Diana K. , Stark Michael R. , Schoenwolf Gary C. 
Published: April 2021

ABSTRACT
Previous studies on neural induction have identified regionally localized inducing activities, signaling molecules, potential competence factors and various other features of this important, early differentiation event. In this paper, we have developed an improved model system for analyzing neural induction and patterning using transverse blastoderm isolates obtained from gastrulating chick embryos. We use this model to establish the timing of neural specification and the spatial distribution of perinodal cells having organizer activity. We show that a tissue that acts either as an organizer or as an inducer of an organizer is spatially co-localized with the prospective neuroectoderm immediately rostral to the primitive streak in the early gastrula. As the primitive streak elongates, this tissue with organizing activity and the prospective neuroectoderm rostral to the streak separate. Furthermore, we show that up to and through the mid-primitive streak stage (i.e., stage 3c/3+), the prospective neuroectoderm cannot self-differentiate (i.e., express neural markers and acquire neural plate morphology) in isolation from tissue with organizer activity. Signals from the organizer and from other more caudal regions of the primitive streak act on the rostral prospective neuroectoderm and the latter gains potency (i.e., is specified) by the fully elongated primitive streak stage (i.e., stage 3d). Transverse blastoderm isolates containing non-specified, prospective neuroectoderm provide an improved model system for analyzing early signaling events involved in neuraxis initiation and patterning.
Wnt-7a in feather morphogenesis: involvement of anterior-posterior asymmetry and proximal-distal elongation demonstrated with an in vitro reconstitution model
Authors: Widelitz Randall B. , Jiang Ting-Xin , Janet Chen Chia-Wei , Stott N. Susan , Jung Han-Sung , Chuong Cheng-Ming 
Published: April 2021

ABSTRACT
How do vertebrate epithelial appendages form from the flat epithelia? Following the formation of feather placodes, the previously radially symmetrical primordia become anterior-posterior (A-P) asymmetrical and develop a proximo-distal (P-D) axis. Analysis of the molecular heterogeneity revealed a surprising parallel of molecular profiles in the A-P feather buds and the ventral-dorsal (V-D) Drosophila appendage imaginal discs. The functional significance was tested with an in vitro feather reconstitution model. Wnt-7a expression initiated all over the feather tract epithelium, intensifying as it became restricted first to the primordia domain, then to an accentuated ring pattern within the primordia border, and finally to the posterior bud. In contrast, sonic hedgehog expression was induced later as a dot within the primordia. RCAS was used to overexpress Wnt-7a in reconstituted feather explants derived from stage 29 dorsal skin to further test its function in feather formation. Control skin formed normal elongated, slender buds with A-P orientation, but Wnt-7a overexpression led to plateau-like skin appendages lacking an A-P axis. Feathers in the Wnt-7a overexpressing skin also had inhibited elongation of the P-D axes. This was not due to a lack of cell proliferation, which actually was increased although randomly distributed. While morphogenesis was perturbed, differentiation proceeded as indicated by the formation of barb ridges. Wnt-7a buds have reduced expression of anterior (Tenascin) bud markers. Middle (Notch-1) and posterior bud markers including Delta-1 and Serrate-1 were diffusely expressed. The results showed that ectopic Wnt-7a expression enhanced properties characteristic of the middle and posterior feather buds and suggest that P-D elongation of vertebrate skin appendages requires balanced interactions between the anterior and posterior buds.
Axial mesendoderm refines rostrocaudal pattern in the chick nervous system
Authors: Rowan Autumn M. , Stern Claudio D. , Storey Kate G. 
Published: April 2021

ABSTRACT
There has long been controversy concerning the role of the axial mesoderm in the induction and rostrocaudal patterning of the vertebrate nervous system. Here we investigate the neural inducing and regionalising properties of defined rostrocaudal regions of head process/prospective notochord in the chick embryo by juxtaposing these tissues with extraembryonic epiblast or neural plate explants. We localise neural inducing signals to the emerging head process and using a large panel of region-specific neural markers, show that different rostrocaudal levels of the head process derived from headfold stage embryos can induce discrete regions of the central nervous system. However, we also find that rostral and caudal head process do not induce expression of any of these molecular markers in explants of the neural plate. During normal development the head process emerges beneath previously induced neural plate, which we show has already acquired some rostrocaudal character. Our findings therefore indicate that discrete regions of axial mesendoderm at headfold stages are not normally responsible for the establishment of rostrocaudal pattern in the neural plate. Strikingly however, we do find that caudal head process inhibits expression of rostral genes in neural plate explants. These findings indicate that despite the ability to induce specific rostrocaudal regions of the CNS de novo, signals provided by the discrete regions of axial mesendoderm do not appear to establish regional differences, but rather refine the rostrocaudal character of overlying neuroepithelium.
her1, a zebrafish pair-rule like gene, acts downstream of notch signalling to control somite development
Authors: Takke Christina , Campos-Ortega José A. 
Published: April 2021

ABSTRACT
During vertebrate embryonic development, the paraxial mesoderm becomes subdivided into metameric units known as somites. In the zebrafish embryo, genes encoding homologues of the proteins of the Drosophila NOTCH signalling pathway are expressed in the presomitic mesoderm and expression is maintained in a segmental pattern during somitogenesis. This expression pattern suggests a role for these genes during somite development. We misexpressed various zebrafish genes of this group by injecting mRNA into early embryos. RNA encoding a constitutively active form of NOTCH1a (notch1a-intra) and a truncated variant of deltaD [deltaD(Pst)], as well as transcripts of deltaC and deltaD, the hairy-E(spl) homologues her1 and her4, and groucho2 were tested for their effects on somite formation, myogenesis and on the pattern of transcription of putative downstream genes. In embryos injected with any of these RNAs, with the exception of groucho2 RNA, the paraxial mesoderm differentiated normally into somitic tissue, but failed to segment correctly. Activation of NOTCH results in ectopic activation of her1 and her4. This misregulation of the expression of her genes might be causally related to the observed mesodermal defects, as her1 and her4 mRNA injections led to effects similar to those seen with notch1a-intra. deltaC and deltaD seem to function after subdivision of the presomitic mesoderm, since the her gene transcription pattern in the presomitic mesoderm remains essentially normal after misexpression of delta genes. Whereas NOTCH signalling alone apparently does not affect myogenesis, zebrafish groucho2 is involved in differentiation of mesodermal derivatives.