bluuebunny/crossref_metadata_2025_split huggingface dataset:
The extracellular matrix of the developing cornea: diversity, deposition and function*
Authors: Bard J. B. L. , Bansal M. K. , Ross A. S. A. 
Published: April 2021

ABSTRACT
This paper examines the role of the extracellular matrix (ECM) in the development of the cornea. After a brief summary of the corneal structure and ECM, we describe evidence suggesting that the differen-tiation of neural crest (NC) cells into endothelium and fibroblasts is under the control of ocular ECM. We then examine the role of collagen I in stromal morpho-genesis by comparing normal corneas with those of homozygous Movl3 mice which do not make collagen We report that, in spite of this absence, the cellular morphology of the Movl3 eye is indistinguishable from that of the wild type. In the 16-day mutant stroma, however, the remaining collagens form small amounts of disorganized, thin fibrils rather than orthogonally organized 20 nm-diameter fibrils; a re-sult implying that collagen I plays only a structural role and that its absence is not compensated for. It also suggests that, because these remaining collagens will not form the normal fibrils that they will in vitro, fibrillogenesis in the corneal stroma differs from that elsewhere.
The latter part of the paper describes our current work on chick stromal deposition using corneal epi-thelia isolated with an intact basal lamina that lay down in vitro ∼3 /im-thick stromas of organized fibrils similar to that seen in vivo. This experimental system has yielded two unexpected results. First, the amount of collagen and proteoglycans produced by such epi-thelia is not dependent on whether its substratum is collagenous and we therefore conclude that stromal production by the intact epithelium is more auton-omous than hitherto thought. Second, chondroitin sulphate (CS), the predominant proteoglycan, appears to play no role in stromal morphogenesis: epithelia cultured in testicular hyaluronidase, which degrades CS, lay down stromas whose organization and fibril-diameter distribution are indistinguishable from con-trols. One possible role for CS, however, is as a lubricant which facilitates corneal growth: it could allow fibrils to move over one another without deform-ing their orthogonal organization. Finally, we have examined the processes of fibrillogenesis in the corneal stroma and conclude that they are different from those elsewhere in the embryo and in vitro, perhaps because there is in the primary stroma an unidentified, highly hydrated ECM macromolecule that embeds the fibrils and that may mediate their morphogenesis.
Strategies of head development: workshop report
Authors: Alberch P. , Kollar E. 
Published: April 2021

ABSTRACT
We organized this workshop as an open discussion around the theme of developmental organization and evolution of the vertebrate head. To sharpen the focus of the debate initially, we, the convenors, proposed a hypothetical ‘two-step’ model of head development. The first step would be conceptually analogous to insect development, in the sense that early in embryonic development, i.e. during, or shortly after, gastrulation, a segmental pattern is specified in the neuroectoderm and/or head mesoderm. By segmentation we mean the subdivision of an embryonic field into sharply defined populations of cells linearly arranged along the anterior-posterior axis. Each population of cells would correspond to a ‘segment’ and each segment would be characterized by divergent developmental fate and potency. This initial process of regionalization by segmentation would be followed by a series of morphogenetic events, involving deformation of epithelial sheets, cell migration and epithelial-mesenchymal inductive interactions.
Expression of intermediate filament proteins during development of Xenopus laevis III. Identification of mRNAs encoding cytokeratins typical of complex epithelia
Authors: Fouquet Bernadette , Herrmann Harald , Franz Jürgen K. , Franke Werner W. 
Published: April 2021

Abstract
A Xenopus laevis mRNA encoding a cytokeratin of the basic (type II) subfamily that is expressed in postgastrulation embryos was cDNA-cloned and sequenced. Comparison of the deduced amino acid sequence of this polypeptide (513 residues, calculated mol. wt 55454; Mr ∼ 58 000 on SDS–PAGE) with those of other cytokeratins revealed its relationship to certain type II cytokeratins of the same and other species, but also remarkable differences. Using a subclone representing the 3′-untranslated portion of the 2·4kb mRNA encoding this cytokeratin, designated XenCK55(5development of n blot experiments, we found that it differs from the only other Xenopus type II cytokeratin known, i.e. the simple epithelium-type component XenCKl(8), in that it is absent in unfertilized eggs and pregastrulation embryos. XenCK55(5/6) mRNA was first detected at gastrulation (stage 11) and found to rapidly increase during neurulation and further development. It was also identified in Xenopus laevis cultured kidney epithelial cells of the line A6 and in the adult animal where it is a major polypeptide in the oesophageal mucosa but absent in most other tissues examined. The pattern of XenCK55(5/6) expression during embryonic development was similar to that reported for the type I polypeptides of the ‘XK81 subfamily’ previously reported to be embryo-specific and absent in adult tissues. Therefore, we used a XK81 mRNA probe representing the 3′-untranslated region in Northern blots, SI nuclease and hybrid-selection-translation assays and found the ∼ 1·6kb XK81 mRNA and the resulting protein of Mr∼ 48 000 not only in postgastrula embryos and tadpoles but also in the oesophagus of adult animals. Our results show that both these type II and type I cytokeratins are synthesized only on gastrulation and are very actively produced in early development. However, their synthesis is not restricted to developmental stages but is continued in at least one epithelium of the adult organism. These observations raise doubts on the occurrence of Xenopus cytokeratins that are strictly specific for certain embryonic or larval stages and absent in the adult. They rather suggest that embryonically expressed cytokeratins are also produced in some adult tissues, although in a restricted pattern of tissue and cell type distribution.
Organization of connective tissue patterns by dermal fibroblasts in the regenerating axolotl limb
Authors: Holder Nigel 
Published: April 2021

ABSTRACT
A set of tendons, aponeurotic sheets and retinaculae, which transduce muscle action from proximal limb levels to flexion and extension of the digits, is found in limbs of many vertebrates. This set of structures, here termed the digit tendon complex, is described for the axolotl forelimb. We show that the complex forms autonomously in muscleless axolotl limb regenerates produced from a cuff of unirradiated dermis surrounding an irradiated limb stump, and persists for up to a year after amputation. The pattern of other connective tissue structures, including the skeleton, is also normal.
Fibroblast condensations that may represent sets of these cells normally associated with muscles in the extensor and flexor compartments of the carpal region also form in muscleless limbs. The results are discussed in terms of the importance of the dermis in pattern regulation, selforganization of connective tissues in general and autonomous development of the digit tendon complex in particular.
Isolation of potential vertebrate limb-identity genes
Authors: Tabin Clifford J. 
Published: April 2021

ABSTRACT
Forelimbs and hindlimbs of tetrapods have different morphological patterns. One plausible explanation for the difference is that the cells that give rise to the limbs differentially express genes which control their pattern of development. Amphibian limb regeneration is an excellent system to test this hypothesis, since the same ultimate morphology is attained in regeneration as through embryogenesis. Using a combination of homeo-box probes and differential screening, I have isolated two newt genes which are differentially expressed in regenerating forelimbs and hindlimbs. One of these genes displays properties expected of a gene involved in controlling limb morphology, including expression in mesodermal tissue and constancy of expression upon transplantation. Based on sequence analysis, this gene appears to be homologous to a homeobox-containing gene previously isolated from frog and human libraries.
Systematic elimination of parthenogenetic cells in mouse chimeras
Authors: Fundele Reinald , Norris Michael L. , Barton Sheila C. , Reik Wolf , Surani M. Azim 
Published: April 2021

Abstract
The developmental potential of primitive ectoderm cells lacking paternal chromosomes was investigated by examining the distribution of parthenogenetic cells in chimeras. Using GPI-1 allozymes as marker, parthenogenetic cells were detected in most organs and tissues in adult chimeras. However, these cells were under severe selective pressure compared with cells from normal fertilized embryos. In the majority of chimeras, parthenogenetic cells in individual animals were observed in a limited number of tissues and organs and, even in these instances, their contribution was substantially reduced. Nevertheless, parthenogenetic cells were detected more consistently in some organs, especially the brain, heart, kidney and spleen. In contrast, there was apparently a systematic selection against parthenogenetic cells in some tissues, most notably in skeletal muscle, liver and pancreas. These results suggest that paternally derived genes are probably required not only for the development of extraembryonic structures but also for subsequent development of embryonic tissues derived from the primitive ectoderm lineage.
Expression of an engrailed-related protein is induced in the anterior neural ectoderm of early Xenopus embryos
Authors: Brivanlou Ali Hemmati , Harland Richard M. 
Published: April 2021

ABSTRACT
We have used a monoclonal antibody directed against the C-terminus of the Drosophila inverted homeodomain to detect a nuclear protein in brain cells of Xenopus laevis embryos. We refer to this antigen as the Xenopus EN protein. The EN protein is localized at midneunda stage to a band of cells in the anterior portion of the neural plate, on each side of the neural groove. Later in development, the expression coincides with the boundary of the midbrain and hindbrain, and persists at least to the swimming tadpole stage. These properties make the EN protein an excellent molecular marker for anterior neural structures. In embryos where inductive interactions between mesodermal and ectodermal tissues have been perturbed, the expression of the EN protein is altered; in embryos that have been anterodorsalized by LiCl treatment, the region that expresses the EN protein is expanded, but still well organized. In ventralized UV-irradiated embryos, the absence of the protein is correlated with the absence of anterior neural structures. In extreme exogastrulae, where the contacts between head mesoderm and prospective neurectoderm are lost, the EN protein is not expressed.
engrailed gene expression in the abdominal segment of Oncopeltus: gradients and cell states in the insect segment
Authors: Campbell Gerard L. , Caveney Stanley 
Published: April 2021

ABSTRACT
A monoclonal antibody that recognizes the product of the segmental gene, engrailed (en), of Drosophila has been used to analyse expression of the homologous gene of Oncopeltus. engrailed expression in the abdominal segment of larval Oncopeltus is confined to a narrow band of epidermal cells localized immediately anterior to the segment border. Expression varies in intensity dur-ing postembryonic development: no gene product is detectable in newly moulted larvae, but reappears soon after initiation of intermoult activities. One possible function of en in this system is revealed by a series of operations confronting cells from different anteropos-terior levels in the segment. New segment borders are generated only when en-expressing cells confront cells from the anteriormost region of the segment. All other combinations result in intercalation of intermediate intrasegmental levels. It is therefore suggested that the most important function of en is the establishment of new, and presumably the maintenance of existing, segment borders.
Collagen fibril bundles: a branching assembly unit in tendon morphogenesis
Authors: Birk David E. , Southern James F. , Zycband Emanuel I. , Fallon John T. , Trelstad Robert L. 
Published: April 2021

ABSTRACT
The assembly, deposition and organization of collagen fibril bundles and their composite fibrils were studied during morphogenesis of the chick embryo tendon using electron microscopy, serial sections and computer-assisted three-dimensional reconstruction techniques. The 14-day chick embryo is a stage when tendon architecture is being established and rapid changes in the mechanical properties occur between days 14 and 17 of development. Tendon matrix structure develops from discrete sub-units, bundles of collagen fibrils. The bundles branch; undergo a gradual rotation over several micrometers; are intimately associated with the cellular elements of the developing tendon; and form arborizing networks within and among fascicles. The organization of discrete fibril segments into bundles, during the establishment of tendon architecture and function, where the segmental fibrillar components could interact with the interfibrillar matrix as well as with adjacent fibrils would contribute to the stabilization of this structure. The observed gradual rotation of the bundles would serve to stabilize the immature bundle through the physical twining of the composite fibrils while the extensive branching of the bundles observed at 14-days of development and their intimate association with the cellular elements would provide a higher order of structure stabilization.
The development of asymmetry: the sidedness of drug-induced limb abnormalities is reversed in situs inversus mice
Authors: Brown Nigel A. , Hoyle Christine I. , McCarthy Afshan , Wolpert Lewis 
Published: April 2021

ABSTRACT
We are studying the development of handedness, in particular the relationships between handed structures with bilateral symmetry, for example the limbs, and those with lateral asymmetry, such as the heart, lungs and gut. Asymmetric (unilateral) developmental limb abnormalities can be induced by chemical treatment of mouse embryos, either in utero by acetazolamide, or in culture by misonidazole. We have examined these effects in mice homozygous for the iv gene. The development of bilateral symmetry in iv/iv mice is normal, but the control of asymmetry appears to be random, that is 50 % develop normally (situs solitus), 50 % with laterally inverted viscera (situs inversus). We find that the handedness of induced asymmetric limb defects is highly correlated with embryonic visceral situs. Right limb defects are induced in situs solitus embryos, left-sided defects in situs inversus. This suggests that the mechanism of induction of asymmetric defects is not related to any intrinsic difference between the development of left and right limbs, but is connected to visceral asymmetry. In addition, the high correlation of limb defects with situs was observed in culture as well as in utero suggesting that the maternal environment plays no role in the development of asymmetry.
The directed growth of retinal axons towards surgically transposed tecta in Xenopus; an examination of homing behaviour by retinal ganglion cell axons
Authors: Taylor J. S. H. 
Published: April 2021

ABSTRACT
The growth of optic axons towards experimentally rotated tecta has been studied. In stage 24/25 embryos, a piece of the dorsal neural tube, containing the dorsal midbrain rudiment, was rotated through 180°. At later stages of development, the pathways of growing optic axons were investigated by labelling with either horseradish peroxidase or fluorescent dye. It is shown that retinal ganglion cell axons followed well-defined pathways, in spite of the abnormal structure of the brain, and were able to locate displaced tecta. This directed outgrowth of retinal axons in the optic tracts appears to be related either to the tectum or to some other component included in the graft operations. In tadpoles in which the midbrain rudiment was removed, optic axons still followed the normal course of the optic tract. This observation argues against long-range target attraction as being essential in guiding growing retinal axons towards the tectum. An alternative axon guidance mechanism, selective fasciculation, is discussed as a possible alternative to exaplain the directed axon outgrowth which occurs in both the normal and in these experimentally manipulated tadpoles.
Range and stability of cell fate determination in isolated sea urchin blastomeres
Authors: Livingston Brian T. , Wilt Fred H. 
Published: April 2021

Abstract
We have examined the developmental potential of blastomeres isolated from either the animal (mesomeres) or vegetal (macromeres-micromeres) half of 16-cell embryos of the sea urchin Lytechinus pictus. We have also examined the effects of two known vegetalizing agents on the development of isolated mesomeres; LiCl treatment and combination with micromeres, the small blastomeres found at the vegetal pole of the 16-cell embryo. The markers for differentiation used were both morphological (invaginations, spicules and pigment cells) and molecular (gut-specific alkaline phosphatase activity, and monoclonal antibodies against antigens specific for gut and oral ectoderm). Embryoids derived from isolated mesomeres expressed markers characteristic of vegetal differentiation only at very low levels. They did express an antigen characteristic of animal development, the oral ectoderm antigen, but with an altered pattern. Isolated macromere-micromere pairs expressed all markers characteristic of vegetal development, but did not express the marker characteristic of animal development. Increasing concentrations of LiCl caused isolated mesomeres to give rise to embryoids with an increasing tendency to express vegetal markers of differentiation, and it was found that expression of different vegetal markers begin to appear at different concentrations of LiCl. LiCl also caused the marker for oral ectoderm to be expressed in a more normal pattern. Combining micromeres with mesomeres also induced mesomere derivatives to differentiate in a vegetal manner. Micromeres were not completely effective in inducing a more normal pattern of expression of the marker for oral ectoderm. The treatment of isolated mesomeres with both LiCl and micromeres produces a synergistic effect resulting in embryoids expressing markers not induced by either treatment alone.
Differential expression of Hox 3.1 protein in subregions of the embryonic and adult spinal cord
Authors: Awgulewitsch Alexander , Jacobs Donna 
Published: April 2021

Abstract
Synthetic oligopeptides derived from the predicted Hox 3.1 protein coding sequence were used for the production of antibodies (anti-aa2) that specifically recognize Hox 3.1 protein in tissue sections. These antibodies were applied in immunohistochemical studies to monitor the expression of Hox 3.1 protein within the central nervous system (CNS) of embryonic and adult mice. We demonstrate congruency between the distinct Hox 3.1 RNA and protein expression patterns in the developing spinal cord by direct comparison of in situ hybridization and immunohistochemical staining in frozen sagittal sections from embryos of 12.5 days of gestation. A distinct pattern of spatially restricted expression of Hox 3.1 protein within the spinal cord was first detected at around 10.5 days of embryonic development. Within certain anteroposterior limits the geometries of this expression pattern change drastically during subsequent embryonic stages, concomitant with important cytoarchitectural changes in the developing spinal cord. Analyses on subcellular levels indicate predominant accumulation of Hox 3.1 protein within nuclei of neuronal cells. In addition to the nuclear localization in subsets of embryonic cells, persistent accumulation of Hox 3.1 protein was shown in nuclei of fully differentiated and mature neuronal cells of the adult CNS.
Genome imprinting and development in the mouse
Authors: Surani M. Azim , Kothary Rashmi , Allen Nicholas D. , Singh Prim B. , Fundele Reinald , Ferguson-Smith Anne C. , Barton Sheila C. 
Published: April 2021

ABSTRACT
Development in mammals is influenced by genome imprinting which results in differences in the expression of some homologous maternal and paternal alleles. This process, initiated in the germline, can continue following fertilization with interactions between oocyte cytoplasmic factors and the parental genomes involving modifier genes. Further epigenetic modifications may follow to render the ‘imprints’ heritable through subsequent cell divisions during development. Imprinting of genes can be critical for their dosage affecting embryonic growth, cell proliferation and differentiation.
The cumulative effects of all the imprinted genes are observed in androgenones (AG) and parthenogenones (PG), which reveal complementary phenotypes with respect to embryonic and extraembryonic tissues. The presence of PG cells in chimeras causes growth retardation, while that of AG cells enhanced growth. AG cells apparently have a higher cell proliferation rate and, unlike PG cells, are less prone to selective elimination. However, the PG germ cells are exempt from cell selection. In chimeras, PG cells are more likely to be found in ectodermal derivatives such as epidermis and brain in contrast to AG cells which make pronounced contributions to many mesodermal derivatives such as muscle, kidney, dermis and skeleton. The presence of androgenetic cells in chimeras also results in the disproportionate elongation of the anterior-posterior axis and sometimes in the abnormal development of skeletal elements along the axis. Genetic studies highlight the influence of subsets of imprinted genes, and identify those that are critical for development.
Imprinting by DNA methylation: from transgenes to endogenous gene sequences
Authors: Reik Wolf , Howlett Sarah K. , Surani M. Azim 
Published: April 2021

ABSTRACT
A number of transgenes in the mouse show variation in methylation and expression phenotypes dependent on parental transmission. It appears that there exist at least two types of transgene imprinting; one is retained on an essentially homozygous background, while the other requires heterozygosity at some modifying loci in the genome and is observed as differences in phenotype in reciprocal crosses. For this type of imprinting to occur, the parental origin of the modifier locus itself is important, and parental asymmetry may involve specific interactions between egg cytoplasm and the chromosomes. Based on the identification of ‘methylation polymorphism’ in the mouse genome, we also show that endogenous gene sequences can undergo imprinting by DNA methylation.
Expression of the IGF-ll/mannose-6-phosphate receptor mRNA and protein in the developing rat
Authors: Senior Paul V. , Byrne Simon , Brammar William J. , Beck Felix 
Published: April 2021

ABSTRACT
The insulin-like growth factor II (IGF-II) receptor is identical to the mannose-6-phosphate receptor (M-6-P), but its role as a somatomedin transducer is uncertain. IGF-II/M-6-P receptor expression was studied by in situ hybridization (ISH) in the developing rat. Expression occurs in extra-embryonic membranes at the time of IGF-n mRNA induction and later at paracrine/ autocrine sites of IGF-II action (skeletal muscle and perichondrium) in the embryo. Highest levels of receptor mRNA occur in heart and major vessels. Postnatally transcription is strongly down-regulated. This suggests a role for the IGF-II/M-6-P receptor in IGF-II action or turnover during development distinct from its role in lysosomal transport.