bluuebunny/crossref_metadata_2025_split huggingface dataset:
Segmental repetition of neuronal phenotype sets in the chick embryo hindbrain
Authors: Clarke J. D. W. , Lumsden Andrew 
Published: April 2021

ABSTRACT
The neurons within the segmented hindbrain of the early chick embryo have been mapped with the neuronal tracers HRP and fluorescent lysinated dextran. We have categorised neurons according to their axonal pathways and have then compared rhombomeres with respect to the number and class of neurons present. The results indicate that most rhombomeres are similar in that they contain the same set of basic neuronal types but differ in that particular neuronal types are more abundant in some rhombomeres than others. The data support the concept that the hindbrain develops according to ‘variations on a segmental theme’ rather than ‘each segment is unique’. Many of the cell types occupy distinct mediolateral domains that are probably established by both the differential migration of some neuronal classes and the spatial segregation of distinct precursors. The caudal rhombomeres 7 and 8 are exceptional in that they do not have the full set of basic neuronal types and also contain two additional medial cell types that are not present rostrally. The mechanisms that may generate the regional diversity apparent in the more mature hindbrain are discussed.
Occurrence of dorsal axis-inducing activity around the vegetal pole of an uncleaved Xenopus egg and displacement to the equatorial region by cortical rotation
Authors: Fujisue M. , Kobayakawa Y. , Yamana K. 
Published: April 2021

ABSTRACT
Specification of the dorsoventral axis is a subject of great importance in amphibian embryogenesis. We have found that cytoplasm of the vegetal dorsal cells of a 16cell embryo of Xenopus laevis, when injected into the ventral vegetal cells of a recipient at the same stage, can induce formation of a second axis. In the present experiments, using the same assay procedure, we found that the cytoplasm around the vegetal pole of an egg before cortical rotation is also active in inducing a second axis, that the activity decreases throughout the second half of the cell cycle and appears in a presumptive dorsal equatorial region at the 2to 16-cell stages. This is the first demonstration of the localization of dorsal forming activity in any specific region of an egg. After UV irradiation, a treatment that is known to block cortical rotation and thereby inhibit axis specification, the activity remains near the vegetal pole beyond the first cell cycle and does not appear in an equatorial region, at least at the 16-cell stage. This suggests that cortical rotation or a related force is in some way involved in changes in distribution of the activity.
We also found that UV-irradiated 8-cell embryos can rescue dorsal development when they are cut into halves along the first cleavage plane. Histological examination revealed that the rescued embryos have a neural tube and notochord. In the half embryo, the animal and vegetal regions came into contact during wound healing, an event that enables the activity to localize in the new equator of an embryo. Therefore this rescue suggests that, if the activity is distributed only in the equatorial region, dorsal specification occurs. In fact, the dorsal side of the rescued embryos seems to correspond to the plane through which the embryos have been cut.
Based on our results, we propose (1) that a determinant that carries axis-inducing activity is first present around the vegetal pole, (2) that the determinant shifts from the vegetal pole to an equatorial region by or in close association with cortical rotation and (3) that occurrence of the determinant in the equatorial region is a prerequisite for axis specification.
Genomic organization and expression of the planarian homeobox genes Dth-1 and Dth-2
Authors: Garcia-Fernàndez Jordi , Baguñà Jaume , Saló Emili 
Published: April 2021

ABSTRACT
We have characterized the genomic organization of Dth1 and Dth-2, planarian homeobox-containing genes, previously described at the cDNA level (J. Garcia-Fernàndez, J. Baguñà and E. Saló (1991), Proc. Natl. Acad. Sci. USA, 88, 7338-7342). Genomic analysis shows that Dth-1 and Dth-2 genes encode proteins of 533 and 363 amino acids respectively. The open reading frame of Dth-1 is interrupted by two large introns of 8 kb and 12 kb Dth-2 also shows two introns, but these are short (42 bp and 44 bp) and the second interrupts helix III at position 44-45, as is the case with other homeobox genes from such divergent animals as Drosophila, honeybee, C. elegans, ascidians, and mouse, which suggests an ancient evolutionary relationship between these genes.
The spatial distribution of transcripts in adult tissues, determined by in situ hybridization, demonstrates that Dth-1 is expressed at a high level in the gastrodermal cells, while Dth-2 is expressed in the peripheral parenchyma, at higher levels in the dorsal than the ventral regions. Their specific spatial distribution suggests a possible role for these homeobox genes in determination and/or differentiation of specific cell types. The expression pattern of both genes is more or less continuous, but in Dth-1 clustered discontinuous labelling in areas surrounding the gastrodermis may indicate a specific expression of this gene in groups of undifferentiated cells (neoblasts) already committed or determined to gastrodermal cell fates.
In situ hybridization analysis during early regeneration shows expression only in the postblastema (stump) differentiated areas while no expression has been detected in the undifferentiated blastema, indicating that neither gene has a role in pattern formation mechanisms known to occur at the early stages of regeneration (0–3 days). Hence, Dth-1 and Dth-2 are planarian homeobox genes presumably involved in specific cell or tissue determination and/or differentiation.
Drosophila hairy pair-rule gene regulates embryonic patterning outside its apparent stripe domains
Authors: Lardelli Michael , Ish-Horowicz David 
Published: April 2021

ABSTRACT
The hairy (h) segmentation gene of Drosophila regulates segmental patterning of the early embryo, and is expressed in a set of anteroposterior stripes during the blastoderm stage. We have used a set of h gene deletions to study the h promoter and the developmental requirements for individual h stripes. The results confirm upstream regulation of h striping but indicate that expression in the anterodorsal head domain depends on sequences downstream of the two transcription initiation sites. Surprisingly, the two anterior-most h domains appear to be dispensable for head development and embryonic viability. One partial promoter deletion expresses ectopic h, leading to misexpression of other segmentation genes and embryonic pattern defects. We demonstrate that h affects patterning outside its apparent stripe domains, supporting a model in which primary pair-rule genes act as concentration-dependent transcriptional regulators, i.e. as local morphogens.
Proximal cis-acting elements cooperate to set Hoxb-7 (Hox-2.3) expression boundaries in transgenic mice
Authors: Vogels Ronald , Charité Jeroen , Graaff Wim de , Deschamps Jacqueline 
Published: April 2021

ABSTRACT
The Hox genes have been proved to be instrumental in establishing the positional identity of cells along the embryonic anteroposterior (A–P) axis. Studying the regulation of these genes is a first step toward elucidating the molecular basis of regionalization during embryogenesis. We report here on the identification of cis-acting elements controlling the expression of Hoxb-7 (Hox-2.3). We show that elements driving A–P restricted gene expression are located within the 3.5 kb proximal upstream sequences of the Hoxb-7 gene. A deletion analysis provides evidence for at least three cis-acting control elements upstream from Hoxb-7, and for cooperative interactions between some of these elements in generating the A–P restricted transgenic pattern. One element, conferring by itself Hox-like expression boundaries to the transgene, has been studied in more detail and found to act in an orientation- and promoter-dependent manner. Together the 3.5 kb sequences proximal to Hoxb-7 mediate A–P restricted Hoxb-7/lacZ gene expression in a domain showing rostral boundaries more posterior than those of Hoxb-7. The evolution throughout embryogenesis of the expression pattern of a transgene carrying these sequences has been analysed and shown to mimick that of the endogenous gene, except for a slight delay in the initial expression. We conclude that the transgenes that we tested, spanning a total of 27 kb genomic sequences, do not reproduce all the features of the Hoxb-7 expression pattern. The differences in expression between Hoxb-7 and the transgenes may reveal an aspect of the Hox regulation for which either remote cis-acting control elements and/or gene clustering is required. Additional features that may have favoured maintenance of clustered organisation during evolution are partial overlap of transcription units with the regulatory regions of the neighbouring genes, and cis-regulatory interactions between multiple Hox genes: not only do cis-acting control elements of the Hoxb-7 gene map in the 3′ untranslated sequences of the Hoxb-8 (Hox-2.4) gene, but our experiments suggest that Hoxb-7 control sequences modulate expression of the Hoxb-8 gene as well.
Retroviral expression of FGF-2 (bFGF) affects patterning in chick limb bud
Authors: Riley Bruce B. , Savage Mary P. , Simandl B. Kay , Olwin Bradley B. , Fallon John F. 
Published: April 2021

ABSTRACT
To investigate the role of fibroblast growth factor-2 (basic fibroblast growth factor) in chick limb development, we constructed a replication-defective spleen necrosis virus to ectopically express fibroblast growth factor-2 in stage 2022 chick limb bud. Because infecting cells in vivo proved to be inefficient, limb bud cells were dissociated, infected in vitro, and then grafted back into host limbs. This procedure caused duplications of anterior skeletal elements, including proximal humerus, distal radius, and digits 2 and 3. Eighty-nine percent of host wings receiving infected grafts at their anterior borders had duplications of one or more of these elements. The frequency of duplication declined dramatically when infected cells were grafted to progressively more posterior sites of host limb buds, and grafting to the posterior border had no effect at all. Several techniques were used to determine the role of infected tissue in forming skeletal duplications. First, staining with an fibroblast growth factor-2 specific monoclonal antibody showed higher than endogenous levels of fibroblast growth factor-2 expression associated with extra elements. Second, the host/donor composition of duplicated elements was determined by simultaneously infecting donor cells with viruses encoding fibroblast growth factor-2 or β-galactosidase; donor tissue was then visualized by X-gal staining. Patterns of ectopic fibroblast growth factor-2 expression and X-gal staining confirmed the presence of infected donor tissue near duplicated structures, but the duplicated skeletal elements themselves showed very little staining. Similar results were obtained in duplications caused by infected quail wing bud cells grafted to the chick wing bud. These observations suggest that fibroblast growth factor-2expressing donor tissue induced host tissue to form normally patterned extra elements. In support of this conclusion, implanting beads containing fibroblast growth factor-2 caused partial duplications of digit 2. These data provide the first direct evidence that fibroblast growth factor-2 plays a role in patterning in the limb bud.
Cell proliferation in the gastrulating chick embryo: a study using BrdU incorporation and PCNA localization
Authors: Sanders E. J. , Varedi M. , French A. S. 
Published: April 2021

ABSTRACT
Cell proliferation in the gastrulating chick embryo was assessed using two independent techniques which mark cells in S phase of the mitotic cycle: nuclear incorporation of bromodeoxyuridine (BrdU) detected immunocytochemically and immunolocalization of proliferating cell nuclear antigen (PCNA). Computer-reconstructed maps were produced showing the distribution of labelled nuclei in the primitive streak and the cell layers. These distributions were also normalized to take into account regional differences in cell density across the embryo. Results from a 2 hour pulse of BrdU indicated that although cells at caudal levels of the primitive streak showed the highest incorporation, this region showed a similar proportion of labelled cells to the surrounding caudal regions of the epiblast and mesoderm when normalized for cell density. The entire caudal third of the embryo showed the highest proportion of cells in S phase. Cells of Hensen’s node showed a relatively low rate of incorporation and, although the chordamesoderm cells showed many labelled nuclei, this appeared to be a reflection of a high cell density in this region. Combining this result with results from a 4 hour pulse of BrdU permitted mapping of cell generation time across the entire embryo. Generation times ranged from a low value of approximately 2 hours at caudal levels of both the epiblast and mesoderm, to an upper value of approximately 10 hours in the rostral regions of the primitive streak, in the mid-lateral levels of the epiblast and in the chordamesoderm rostral to Hensen’s node.
Cells at caudal regions of the primitive streak showed a generation time of approximately 5 hours. Taking into account that cells are generally considered to be continuously moving through the primitive streak, we conclude that cell division, as judged by generation time, is greatly reduced during transit through this region, despite the presence there of cells in S phase and M phase. Immunocytochemical localization of PCNA-positive nuclei gave generally similar distributions to those obtained with BrdU incorporation, confirming that this endogenous molecule is a useful S-phase marker during early embryogenesis. Mid-levels and caudal levels of the primitive streak showed the highest numbers of positive nuclei, and the highest proportion of labelling after cell density was accounted for. As with BrdU incorporation, the highest proportions of PCNA-positive nuclei were found towards the caudal regions of the epiblast and mesoderm. These results suggest that the differential growth of the caudal region of the embryo at this time is a direct consequence of elevated levels of cell proliferation in this region. However, no evidence was obtained to indicate that the primitive streak cells, or the cells of the emerging mesoderm, are highly proliferative, and we therefore conclude that the morphogenesis of the early mesoderm does not rely on elevated rates of cell division.
trkC, a receptor for neurotrophin-3, is widely expressed in the developing nervous system and in non-neuronal tissues
Authors: Tessarollo Lino , Tsoulfas Pantelis , Martin-Zanca Dionisio , Gilbert Debra J. , Jenkins Nancy A. , Copeland Neal G. , Parada Luis F. 
Published: April 2021

ABSTRACT
The Trk family of tyrosine kinases encodes receptors for nerve growth factor-related neurotrophins. Here we present a developmental expression study of trkC, which encodes a receptor for neurotrophin-3 (NT-3). Like the related genes, trk and trkB, trkC is expressed primarily in neural lineages although the pattern is complex and includes non-neuronal cells. Direct comparison with trk and trkB developmental expression patterns permits the following observations. (1) trkC is expressed in novel neural tissues where other Trk genes are silent. (2) Some tissues appear to coexpress trkB and trkC receptors in the embryo and in the adult. (3) trkC expression can be detected in the gastrulating embryo. These data provide insights into the role of Trk-family receptors and nerve growth factor-related neurotrophins during development and suggest that, in addition to regulating neuronal survival and differentiation, the neurotrophin/Trk receptor system may have broader physiological effects. Finally, interspecific mouse backcrosses have been used to map the location of each of the Trk genes on mouse chromosomes. Alignment with available chromosomal maps identify possible linkage between the Trk genes and known neurological mutations.
PDGF and its receptors in the developing rodent retina and optic nerve
Authors: Mudhar Hardeep S. , Pollock, Richard A. , Wang Chiayeng , Stiles Charles D. , Richardson William D. 
Published: April 2021

ABSTRACT
We have used in situ hybridization to visualize cells in the developing rat retina and optic nerve that express mRNAs encoding the A and B chains of platelet-derived growth factor (PDGF-A and PDGF-B), and the alpha and beta subunits of the PDGF receptor (PDGFR αR and PDGF βR). We have also visualized PDGF-A protein in these tissues by immunohistochemistry. In the retina, PDGF-A mRNA is present in pigment epithelial cells, ganglion neurons and a subset of amacrine neurons. PDGF-A transcripts accumulate in ganglion neurons during target innervation and in amacrine neurons around the time of eye opening, suggesting that PDGFA expression in these cells may be regulated by targetderived signals or by electrical activity. In the mouse retina, PDGF-A immunoreactivity is present in the cell bodies, dendrites and proximal axons of ganglion neurons, and throughout the inner nuclear layer. PDGFR αR mRNA is expressed in the retina by astrocytes in the optic fibre layer and by a subset of cells in the inner nuclear layer that might be Müller glia or bipolar neurons. Taken together, our data suggest short-range paracrine interactions between PDGF-A and PDGFR, the ligand and its receptor being expressed in neighbouring layers of cells in the retina. In the optic nerve, PDGF-A immunoreactivity is present in astrocytes but apparently not in the retinal ganglion cell axons. PDGF αR+ cells in the optic nerve first appear near the optic chiasm and subsequently spread to the retinal end of the nerve; these PDGFαR+ cells are probably oligodendrocyte precursors (Pringle et al., 1992). RNA transcripts encoding PDGF-B and PDGFβR are expressed by cells of the hyaloid and mature vascular systems in the eye and optic nerve.
Oligodendrocyte precursors originate at the ventral ventricular zone dorsal to the ventral midline region in the embryonic rat spinal cord
Authors: Noll Elizabeth , Miller Robert H. 
Published: April 2021

ABSTRACT
The precursors for oligodendrocytes, the myelinating cells of the vertebrate CNS, appear to be initially restricted to ventral regions of the embryonic rat spinal cord. These cells subsequently populate dorsal spinal cord regions where they acquire the mature characteristics of oligodendrocytes. To determine the location and timing of proliferation of oligodendrocyte precursors in the ventral spinal cord, and to map their pathways of migration in vivo, an assay that identifies mitotic cells was used in conjunction with antibodies that distinguish astrocytes, oligodendrocytes and their precursors.
Between E16.5 and E18.5, two hours after a maternal injection of BrdU, the majority of proliferating cells were located in a discrete cluster at the ventral ventricular zone dorsal to the ventral midline region of the developing spinal cord. By contrast, 12 – 24 hours following a BrdU injection at E16.5, increasing numbers of labeled cells were seen in the dorsal and more lateral locations of the spinal cord. These observations suggest that BrdU-labeled ventral ventricular cells, or their progeny migrate dorsally and laterally during subsequent spinal cord development. To determine the nature of these proliferating cells, cultures of dorsal and ventral spinal cord from BrdU-labeled animals were doublelabeled with antibodies that identify oligodendrocytes or astrocytes and anti-BrdU. In dorsal spinal cord cultures derived from animals that had received a single injection of BrdU at E16.5, the majority of proliferating cells differentiated into astrocytes while, in ventrally derived cultures from the same animals, the majority of proliferating cells differentiated into oligodendrocytes. In dorsal cultures prepared from animals that received multiple injections of BrdU between E16.5 and E18.5, many more cells were labeled with BrdU and approximately half of these differentiated into oligodendrocytes. These observations suggest that during embryonic development proliferating oligodendrocyte precursors are initially located at the ventral ventricular zone dorsal to the ventral midline region of the spinal cord and during subsequent maturation these cells or their progeny migrated dorsally in the ventricular region of the spinal cord, and laterally to reside in the developing white matter.
Inactivation of an X-linked transgene in murine extraembryonic and adult tissues
Authors: Dandolo Luisa , Stewart Colin L. , Mattei Marie-Genevieve , Avner Philip R. 
Published: April 2021

ABSTRACT
Transgenes located on the X chromosome have been used to study the mechanisms involved in X-chromosome inactivation. Analysis of the transgenic mouse strain M-TKneo1 carrying a neomycin resistance gene inserted in the X chromosome showed that, in adult somatic tissues, this transgene is subject to X-inactivation and to de novo methylation as other endogenous Xlinked genes.
During mouse embryogenesis, X-linked genes show a preferential paternal inactivation in extraembryonic tissues, whereas these genes are subject to random inactivation in embryonic tissues. It has been suggested that, in the mouse, the extraembryonic tissues carry a parental imprint at the time of inactivation.
The study of the neo transgene expression in extraembryonic endoderm has shown not only that neo is inactivated but also that, at the RNA level, paternal inactivation of the transgene seems essentially complete.
The differences between our results and previously obtained results with a mouse α-fetoprotein transgene, which was only inactivated in neonatal tissues but not in extraembryonic tissues, are discussed.
Mechanical evaluation of theories of neurulation using computer simulations
Authors: Clausi David A. , Brodland G. Wayne 
Published: April 2021

ABSTRACT
Current theories about the forces that drive neurulation shape changes are evaluated using computer simulations. Custom, three-dimensional, finite element-based computer software is used. The software draws on current engineering concepts and makes it possible to construct a ‘virtual’ embryo with any user-specified mechanical properties. To test a specific hypothesis about the forces that drive neurulation, the whole virtual embryo or any selected part of it is ascribed with the force generators specified in the hypothesis. The shape changes that are produced by these forces are then observed and compared with experimental data.
The simulations demonstrate that, when uniform, isotropic circumferential microfilament bundle (CMB) constriction and cephalocaudal (axial) elongation act together on a circular virtual neural plate, it becomes keyhole shaped. When these forces act on a spherical (amphibian) embryo, dorsal surface flattening occurs. Simulations of transverse sections further show that CMB constriction, acting with or without axial elongation, can produce numerous salient transverse features of neurulation. These features include the sequential formation of distinct neural ridges, narrowing and thickening of the neural plate, skewing just medial to the ridges, ‘hinge’ formation and neural tube closure. No region-specific ‘programs’ or non-mechanical cell-cell communications are used. The increase in complexity results entirely from mechanical interactions. The transverse simulations show how changes to the driving forces would affect the patterns of shape change produced.
Hypotheses regarding force generation by microtubules, intercellular adhesions and forces extrinsic to the neural plate are also evaluated. The simulations show that these force-generating mechanisms do not, by themselves, produce shape changes that are consistent with normal development. The simulations support the concept of cooperation of forces and suggest that neurulation is robust because redundant force generating mechanisms exist.