bluuebunny/crossref_metadata_2025 huggingface dataset:
Cytoskeletal elements link calcium channel activity and the cell cycle in early sea urchin embryos
Authors: Yazaki Ikuko , Tosti Elisabetta , Dale Brian
Published: April 2021

Using the whole-cell clamp technique, we show that L-type calcium channels are activated in early sea urchin blastomeres during M-phase and subsequently inactivated in S-phase. This cyclical channel behaviour occurs in the absence of the nucleus suggesting cytoplasmic regulation independent of the centrosome cycle. Puromycin at 100–400 μM does not prevent inactivation of the current showing that this phase, at least, does not require protein synthesis. Cytochalasin B at 2 μg/ml inhibits the cyclical activity in both M and S phases, while 100 μg/ml of colchicine inactivates the L-type current in M-phase and activates a large T-type calcium current in S-phase, suggesting that channel behaviour is regulated by cytoskeletal elements. Since, fragmentation experiments show the calcium channels to be clustered in the apical membrane, and some L-type calcium channel inhibitors induced a significant delay in the cell cycle, the channel may play a role in regulating cytokinesis possibly by contributing to local intracellular calcium gradients.
Axonal localisation of the CAM-like tyrosine phosphatase CRYPα: a signalling molecule of embryonic growth cones
Authors: Stoker Andrew W. , Gehrig Bernhard , Haj Fawaz , Bay Boon-Huat
Published: April 2021

Migrating embryonic growth cones require multiple, membrane-associated signalling molecules to monitor and respond to guidance cues. Here we present the first evidence that vertebrate cell adhesion molecule-like protein tyrosine phosphatases are likely to be components of this signalling system. CRYPα, the gene for an avian cell adhesion molecule-like phosphatase, is strongly expressed in the embryonic nervous system. In this study we have immunolocalised the protein in the early chick embryo and demonstrated its predominant localisation in axons of the central and peripheral nervous systems. This location suggests that the major, early role of the enzyme is in axonal development. In a study of sensory neurites in culture, we furthermore show that this phosphatase localises in migrating growth cones, within both the lamellipodia and filopodia. The dependence of growth cone migration on both cell adhesion and signalling through phosphotyrosine turnover, places the cell adhesion molecule-like CRYPα phosphatase in a position to be a regulator of these processes.
Drosophila nonmuscle myosin II is required for rapid cytoplasmic transport during oogenesis and for axial nuclear migration in early embryos
Authors: Wheatley Sally , Kulkarni Sanjay , Karess Roger
Published: April 2021

The X-linked Drosophila gene spaghetti squash (sqh) encodes the regulatory light chain of nonmuscle myosin II. To assess the requirement for myosin II in oogenesis and early embryogenesis, we induced homozygous germline clones of the hypomorphic mutation sqh1 in otherwise het-erozygous mothers. Developing oocytes in such sqh1 germline clones often failed to attain full size due to a defect in ‘dumping’, the rapid phase of cytoplasmic transport from nurse cells. In contrast to other dumpless mutants described to date, sqh1 egg chambers showed no evidence of ring canal obstruction, and no obvious alteration in the actin network. However the distribution of myosin II was abnormal. We conclude that the molecular motor responsible for cytoplasmic dumping is supplied largely, if not exclusively, by nurse cell myosin II and we suggest that regulation of myosin activity is one means by which cytoplasmic transport may be controlled during oocyte development. The eggs resulting from sqh1 clones, though smaller than normal, began development but exhibited an early defect in axial migration of cleavage nuclei towards the posterior pole of the embryo, in a similar manner to that seen in early cleavage eggs in which the actin cytoskeleton is disrupted. Thus both nurse cell dumping and axial migration require a maternally supplied myosin II.
Polydactylous limbs in Strong’s Luxoid mice result from ectopic polarizing activity
Authors: Chan David C. , Laufer Ed , Tabin Cliff , Leder Philip
Published: April 2021

Strong’s Luxoid (lstD) is a semidominant mouse mutation in which heterozygotes show preaxial hindlimb polydactyly, and homozygotes show fore- and hindlimb polydactyly. The digit patterns of these polydactylous limbs resemble those caused by polarizing grafts, since additional digits with posterior character are present at the anterior side of the limb. Such observations suggest that lstD limb buds might contain a genetically determined ectopic region of polarizing activity. Accordingly, we show that mutant embryos ectopically express the pattern-determining genes fibroblast growth factor 4 (fgf-4), sonic hedgehog (shh), and Hoxd-12 in the anterior region of the limb. Further, we show that anterior mesoderm from mutant limbs exhibits polarizing activity when grafted into host chicken limbs. In contrast to an experimentally derived polydactylous transgenic mouse, forelimbs of homozygotes show a normal pattern of Hoxb-8 expression, indicating that the duplication of polarizing tissue here occurs downstream or independently of Hoxb-8. We suggest that the lst gene product is involved in anteroposterior axis formation during normal limb development.
The role of Hoxa-3 in mouse thymus and thyroid development
Authors: Manley Nancy R. , Capecchi Mario R.
Published: April 2021

Targeted disruption of Hoxa-3 results in a number of regionally restricted defects in tissues and structures derived from or patterned by mesenchymal neural crest. However, analysis of mutant embryos with injections of a carbocyanine dye or with molecular markers that label these cells indicates that neither the amount nor the migration patterns of this neural crest population are grossly affected. Therefore, it appears that the loss of Hoxa-3 affects the intrinsic capacity of this neural crest cell population to differentiate and/or to induce proper differentiation of the surrounding pharyngeal arch and pouch tissues. Hoxa-3 mutant mice are athymic and show thyroid hypoplasia. Thymus development is first evident as an expansion of mesenchymal neural crest in the posterior part of the 3rd pharyngeal pouch. Prior to this expansion, a marked reduction in pax-1 expression is observed in these cells in the mutant embryos. As pax-1 mutant mice also show thymic hypoplasia, these results suggest that Hoxa-3 may be required to maintain pax-1 expression in these cells and that the reduction of pax-1 expression is part of the athymic teleology in Hoxa-3 mutant mice. The thyroid gland is formed from the fusion of two structures of separate embryonic origin, the thyroid diverticulum, which is formed from endodermal epithelium in the floor of the pharynx, and the ultimobranchial body, formed from mesenchymal neural crest in the 4th pharyngeal pouch. Both of these sites express Hoxa-3 and are defective in mutant mice. Often a vesicle is observed in mutant mice that is exclusively composed of calcitonin-producing cells, suggesting the persistence of an ultimobranchial body. Both aspects of the thyroid phenotype show variable expressivity among mutant animals, even on the two sides of the same mutant animal. This variability suggests the presence of a compensating gene or genes, whose utilization is stochastic. A reasonable candidate for providing this compensatory function is the paralogous gene Hoxb-3.
How do sea urchins invaginate? Using biomechanics to distinguish between mechanisms of primary invagination
Authors: Davidson Lance A. , Koehl M. A. R. , Keller Raymond , Oster George F.
Published: April 2021

The forces that drive sea urchin primary invagination remain mysterious. To solve this mystery we have developed a set of finite element simulations that test five hypothesized mechanisms. Our models show that each of these mechanisms can generate an invagination; however, the mechanical properties of an epithelial sheet required for proper invagination are different for each mechanism. For example, we find that the gel swelling hypothesis of Lane et al. (Lane, M. C., Koehl, M. A. R., Wilt, F. and Keller, R. (1993) Development 117, 1049-1060) requires the embryo to possess a mechanically stiff apical extracellular matrix and highly deformable cells, whereas a hypothesis based on apical constriction of the epithelial cells requires a more compliant extracellular matrix. For each mechanism, we have mapped out a range of embryo designs that work. Additionally, the simulations predict specific cell shape changes accompanying each mechanism. This allows us to design experiments that can distinguish between different mechanisms, all of which can, in principle, drive primary invagination.
Role of the proneural gene, atonal, in formation of Drosophila chordotonal organs and photoreceptors
Authors: Jarman Andrew P. , Sun Yan , Jan Lily Y. , Jan Yuh Nung
Published: April 2021

The Drosophila gene atonal encodes a basic helix-loop-helix protein similar to those encoded by the proneural genes of the achaete–scute complex (AS-C). The AS-C are required in the Drosophila PNS for the selection of neural precursors of external sense organs. We have isolated mutants of atonal, which reveal that this gene encodes the proneural gene for chordotonal organs and photoreceptors. In atonal mutants, all observable adult chordotonal organs, and almost all embryonic chordotonal organs fail to form; all adult photoreceptors are missing. For both types of sense organ, this defect is already apparent at the level of precursor formation. Therefore it is a failure in the epidermal-neural decision process i.e. a proneural defect. The failure to form photoreceptors results in atrophy of the atonal mutant imaginal disc, due to apoptosis and lack of stimulation of division. Lack of photoreceptors should also eliminate signalling that arises from differentiating photoreceptors and is required for morphogenetic furrow movement in the wild-type eye disc. Nevertheless, a remnant morphogenetic furrow is still observed in the atonal mutant disc. This presumably reflects the process of furrow initiation, which would not depend on signals from developing photoreceptors.
Transient and restricted expression during mouse embryogenesis of Dll1, a murine gene closely related to Drosophila Delta
Authors: Bettenhausen Berthold , Hrabê de Angelis Martin , Simon Dominique , Guénet Jean-Louis , Gossler Achim
Published: April 2021

The Drosophila Delta (Dl) gene is essential for cell-cell communication regulating the determination of various cell fates during development. Dl encodes a transmembrane protein, which contains tandem arrays of epidermal-growth-factor-like repeats in the extracellular domain and directly interacts with Notch, another transmembrane protein with similar structural features, in a ligand-receptor-like manner. Similarly, cell-cell interactions involving Delta-like and Notch-like proteins are required for cell fate determinations in C. elegans. Notch homologues were also isolated from several vertebrate species, suggesting that cell-to-cell signaling mediated by Delta- and Notch-like proteins could also underlie cell fate determination during vertebrate development. However, in vertebrates, no Delta homologues have yet been described.

We have isolated a novel mouse gene, Dll1 (delta-like gene 1), which maps to the mouse t-complex and whose deduced amino acid sequence strongly suggests that Dll1 represents a mammalian gene closely related to Drosophila Delta. Dll1 is transiently expressed during gastrulation and early organogenesis, and in a tissue-restricted manner in adult animals. Between day 7 and 12.5 of development, expression was detected in the paraxial mesoderm, closely correlated with somitogenesis, and in subsets of cells in the nervous system. In adult animals, transcripts were detected in lung and heart. Dll1 expression in the paraxial mesoderm and nervous system is strikingly similar to the expression of mouse Notch1 during gastrulation and early organogenesis. The overlapping expression patterns of the Dll1 and Notch1 genes suggest that cells in these tissues can communicate by interaction of the Dll1 and Notch1 proteins. Our results support the idea that Delta- and Notch-like proteins are involved in cell-to-cell communication in mammalian embryos and suggest a role for these proteins in cellular interactions underlying somitogenesis and development of the nervous system.
Epithelial planar polarity in the developing Drosophila eye
Authors: Wehrli Marcel , Tomlinson Andrew
Published: April 2021

Experiments with the insect ectoderm have suggested that planar polarity in epithelia results from the local orientation of cells to the slope of a gradient of positional information. Here we show that planar polarity in the Drosophila eye is inverted when the morphogenetic wave that sweeps through the presumptive retinal epithelium is induced to move in the reverse direction. We suggest that the movement of the morphogenetic wave may be causal in establishing the planar polarity of this epithelium.
Dynamics of thin filopodia during sea urchin gastrulation
Authors: Miller Jeffrey , Fraser Scott E. , McClay David
Published: April 2021

At gastrulation in the sea urchin embryo, a dramatic rearrangement of cells establishes the three germ layers of the organism. Experiments have revealed a number of cell interactions at this stage that transfer patterning information from cell to cell. Of particular significance, primary mesenchyme cells, which are responsible for production of the embryonic skeleton, have been shown to obtain extensive positional information from the embryonic ectoderm. In the present study, high resolution Nomarski imaging reveals the presence of very thin filopodia (0.2-0.4 μm in diameter) extending from primary mesenchyme cells as well as from ectodermal and secondary mesenchyme cells. These thin filopodia sometimes extend to more than 80 μm in length and show average growth and retraction rates of nearly 10 μm/minute. The filopodia are highly dynamic, rapidly changing from extension to resorption; frequently, the resorption changes to resumption of assembly. The behavior, location and timing of active thin filopodial movements does not correlate with cell locomotion; instead, there is a strong correlation suggesting their involvement in cell-cell interactions associated with signaling and patterning at gastrulation. Nickel-treatment, which is known to create a patterning defect in skeletogenesis due to alterations in the ectoderm, alters the normal position-dependent differences in the thin filopodia. The effect is present in recombinant embryos in which the ectoderm alone was treated with nickel, and is absent in recombinant embryos in which only the primary mesenchyme cells were treated, suggesting that the filopodial length is substratum dependent rather than being primary mesenchyme cell autonomous. The thin filopodia provide a means by which cells can contact others several cell diameters away, suggesting that some of the signaling previously thought to be mediated by diffusible signals may instead be the result of direct receptor-ligand interactions between cell membranes.
Three genes of the MAP kinase cascade, mek-2, mpk-1/sur-1 and let-60 ras, are required for meiotic cell cycle progression in Caenorhabditis elegans
Authors: Church Diane L. , Guan Kun-Liang , Lambie Eric J.
Published: April 2021

In the germline of Caenorhabditis elegans hermaphrodites, meiotic cell cycle progression occurs in spatially restricted regions. Immediately after leaving the distal mitotic region, germ cells enter meiosis and thereafter remain in the pachytene stage of first meiotic prophase for an extended period. At the dorsoventral gonadal flexure, germ cells exit pachytene and subsequently become arrested in diakinesis. We have found that exit from pachytene is dependent on the function of three members of the MAP kinase signaling cascade. One of these genes, mek-2, is a newly identified C. elegans MEK (MAP kinase kinase). The other two genes, mpk-1/sur-1 (MAP kinase) and let-60 ras, were previously identified based on their roles in vulval induction and are shown here to act in combination with mek-2 to permit exit from pachytene. Through genetic mosaic analysis, we demonstrate that the expression of mpk-1/sur-1 is required within the germline to permit exit from pachytene.
The Drosophila 63F early puff contains E63-1, an ecdysone-inducible gene that encodes a novel Ca2+-binding protein
Authors: Andres Andrew J. , Thummel Carl S.
Published: April 2021

Pulses of ecdysone at the end of Drosophila larval development dramatically reprogram gene expression as they signal the onset of metamorphosis. Ecdysone directly induces several early puffs in the salivary gland polytene chromosomes that, in turn, activate many late puffs. Three early puffs, at 2B5, 74EF, and 75B, have been studied at the molecular level. Each contains a single ecdysone primary-response gene that encodes a family of widely expressed transcription factors. We report here a molecular characterization of the 63F early puff. Unexpectedly, we have found this locus to be significantly different from the previously characterized early puff loci. First, the 63F puff contains a pair of ecdysone-inducible genes that are transcribed in the larval salivary glands: E63-1 and E63-2. Second, E63-1 induction in late third instar larvae appears to be highly tissue-specific, restricted to the salivary gland. Third, E63-1 encodes a novel Ca2+-binding protein related to calmodulin. The discovery of an ecdysone-inducible Ca2+-binding protein provides a foundation for integrating steroid hormone and calcium second messenger signaling pathways and generates an additional level for potential regulation of the ecdysone response.
A sea urchin homologue of the chordate Brachyury (T) gene is expressed in the secondary mesenchyme founder cells
Authors: Harada Yoshito , Yasuo Hitoyoshi , Satoh Noriyuki
Published: April 2021

Chordates are thought to have emerged from some common ancestor of deuterostomes by organizing shared anatomical and embryological features including a notochord, a dorsal nerve cord and pharyngeal gill slits. Because the notochord is the most prominent feature of chordates and because the Brachyury (T) gene is essential for notochord formation, the T gene is a key molecular probe with which to explore the origin and evolution of chordates. We investigated whether the sea urchin (echin- oderm) conserves the T gene and, if so, where the sea urchin T gene is expressed.

A cDNA clone for the sea urchin T (HpTa) gene contained a long open reading frame that encodes a polypeptide of 434 amino acids. Although the overall degree of amino acid identity was not very high (52%, sea urchin/mouse), in the T domain of the N terminus the amino acid identity was 73% (sea urchin/mouse). The HpTa gene is present as a single copy per haploid genome. As with the chordate T gene, the expression of HpTa is transient, being first detected in the swimming blastula, maximally transcribed in the gastrula, decreasing at the prism larval stage and barely detectable at the pluteus larval stage. HpTa tran- scripts were found in the secondary mesenchyme founder cells, vegetal plate of the mesenchyme blastula, extending tip of the invaginating archenteron and, finally, the secondary mesenchyme cells at the late-gastrula stage. The results suggest that the T gene was organized prior to the branching of a lineage leading to chordates from that leading to echinoderms and that, during sea urchin development, the T gene is utilized to specify the developmental fate of embryonic cells to the secondary mesenchyme.
Expression of the unc-4 homeoprotein in Caenorhabditis elegans motor neurons specifies presynaptic input
Authors: Miller David M. , Niemeyer Charles J.
Published: April 2021

In the nematode, Caenorhabditis elegans, VA and VB motor neurons arise from a common precursor cell but adopt different morphologies and synapse with separate sets of interneurons in the ventral nerve cord. A mutation that inactivates the unc-4 homeodomain gene causes VA motor neurons to assume the VB pattern of synaptic input while retaining normal axonal polarity and output; the disconnection of VA motor neurons from their usual presynaptic partners blocks backward locomotion. We show that expression of a functional unc-4-β-galactosidase chimeric protein in VA motor neurons restores wild-type movement to an unc-4 mutant. We propose that unc-4 controls a differentiated characteristic of the VA motor neurons that distinguishes them from their VB sisters, thus dictating recognition by the appropriate interneurons. Our results show that synaptic choice can be controlled at the level of transcription in the post-synaptic neuron and identify a homeoprotein that defines a subset of cell-specific traits required for this choice.
Targeted mutation of the murine goosecoid gene results in craniofacial defects and neonatal death
Authors: Yamada Gen , Mansouri Ahmed , Torres Miguel , Stuart Edward T. , Blum Martin , Schultz M. , Robertis Eddy M. De , Gruss Peter
Published: April 2021

The goosecoid gene encodes a homeodomain-containing protein that has been identified in a number of species and has been implicated in a variety of key developmental processes. Initially suggested to be involved in organizing the embryo during early development, goosecoid has since been demonstrated to be expressed during organogenesis – most notably in the head, the limbs and the ventrolateral body wall. To investigate the role of goosecoid in embryonic development, we have inactivated the gene by gene targeting to generate mice mutant for the goosecoid gene. Mice that are homozygous for the goosecoid mutation do not display a gastrulation phenotype and are born; however, they do not survive more than 24 hours. Analysis of the homozygotes revealed numerous developmental defects affecting those structures in which goosecoid is expressed during its second (late) phase of embryonic expression. Predominantly, these defects involve the lower mandible and its associated musculature including the tongue, the nasal cavity and the nasal pits, as well as the components of the inner ear (malleus, tympanic ring) and the external auditory meatus. Although the observed phenotype is in accordance with the late expression domains of goosecoid in wild-type embryos, we suggest that the lack of an earlier phenotype is the result of functional compensation by other genes.
Origin and specification of type II sensory neurons in Drosophila
Authors: Brewster Rachel , Bodmer Rolf
Published: April 2021

The peripheral nervous system (PNS) of Drosophila is a preferred model for studying the genetic basis of neuro-genesis because its simple and stereotyped pattern makes it ideal for mutant analysis. Type I sensory organs, the external (bristle-type) sensory organs (es) and the internal (stretch-receptive) chordotonal organs (ch), have been postulated to derive from individual ectodermal precursor cells that undergo a stereotyped pattern of cell division. Little is known about the origin and specification of type II sensory neurons, the multiple dendritic (md) neurons. Using the flp/FRT recombinase system from yeast, we have determined that a subset of md neurons derives from es organ lineages, another subset derives from ch organ lineages and a third subset is unrelated to sensory organs. We also provide evidence that the genes, numb and cut, are both required for the proper differentiation of md neurons.
Posterior stripe expression of hunchback is driven from two promoters by a common enhancer element
Authors: Margolis Jonathan S. , Borowsky Mark L. , SteingrÍmsson EirÍkur , Shim Chung Wha , Lengyel Judith A. , Posakony James W.
Published: April 2021

The gap gene hunchback (hb) is required for the formation and segmentation of two regions of the Drosophila embryo, a broad anterior domain and a narrow posterior domain. Accumulation of hb transcript in the posterior of the embryo occurs in two phases, an initial cap covering the terminal 15% of the embryo followed by a stripe at the anterior edge of this region. By in situ hybridization with transcript-specific probes, we show that the cap is composed only of mRNA from the distal transcription initiation site (P1), while the later posterior stripe is composed of mRNA from both the distal and proximal (P2) transcription initiation sites. Using a series of genomic rescue constructs and promoter-lacZ fusion genes, we define a 1.4 kb fragment of the hb upstream region that is both necessary and sufficient for posterior expression. Sequences within this fragment mediate regulation by the terminal gap genes tailless (tll) and huckebein, which direct the formation of the posterior hb stripe. We show that the tll protein binds in vitro to specific sites within the 1.4 kb posterior enhancer region, providing the first direct evidence for activation of gene expression by tll. We propose a model in which the anterior border of the posterior hb stripe is determined by tll concentration in a manner analogous to the activation of anterior hb expression by bicoid.
Defective bone formation in Krox-20 mutant mice
Authors: Levi Giovanni , Topilko Piotr , Schneider-Maunoury Sylvie , Lasagna Marco , Mantero Stefano , Cancedda Ranieri , Charnay Patrick
Published: April 2021

Endochondral ossification is the prevalent mode of vertebrate skeleton formation; it starts during embryogenesis when cartilage models of long bones develop central regions of hypertrophy which are replaced by bony trabeculae and bone marrow. Although several transcription factors have been implicated in pattern formation in the limbs and axial skeleton, little is known about the transcriptional regulations involved in bone formation. We have created a null allele in the mouse Krox-20 gene, which encodes a zinc finger transcription factor, by in frame insertion of the E. coli lacZ gene and shown that hindbrain segmentation and peripheral nerve myelination are affected in Krox-20−/− embryos. We report here that Krox-20 is also activated in a subpopulation of growth plate hypertrophic chondrocytes and in differentiating osteoblasts and that its disruption severely affects endochondral ossification. Krox-20−/− mice develop skeletal abnormalities including a reduced length and thickness of newly formed bones, a drastic reduction of calcified trabeculae and severe porosity. The periosteal component to bone formation and calcification does not appear to be affected in the homozygous mutant suggesting that the major role for Krox-20 is to be found in the control of the hypertrophic chondrocyte-osteoblast interactions leading to endosteal bone formation.
Retinoic acid establishes ventral retinal characteristics
Authors: Hyatt G. A. , Schmitt E. A. , Marsh-Armstrong N. , McCaffery P. , Dräger U. C. , Dowling J. E.
Published: April 2021

The developing eye is known to be rich in retinoic acid (RA), and perturbations in RA levels during formation of the optic primordia, as well as RA receptor mutations, cause retinal malformations, especially in ventral eye regions. To test the hypothesis that RA plays a role in the establishment of ventral retinal characteristics, we examined several dorsal and ventral ocular markers in RA-treated zebrafish. The optic stalk represents the ventral-most region of the early eye field. During normal development, the optic stalks constrict, decreasing in width and are gradually replaced by the optic nerve. Systemic high RA levels cause an expansion in the optic stalk with an increased cell content and a patent lumen. In addition, the stalks do not constrict and persist into later stages of development indicating an enhancement of early ventral eye characteristics. Expression of the transcription factor pax[b], normally confined to the ventral retina, expands into the dorsal retina following RA treatment, whereas msh[c], normally expressed in the dorsal retinal pole, disappears. Activity of an aldehyde dehydrogenase that normally occupies the dorsal third of the retina is reduced or abolished following high systemic RA. When a localized RA source, an RA-soaked bead, is placed next to the developing eye, a fissure resembling the choroid fissure appears in the eye facing the bead. Taken together, these observations suggest that RA is involved in the determination of the ventral retina.
A targeted mouse Otx2 mutation leads to severe defects in gastrulation and formation of axial mesoderm and to deletion of rostral brain
Authors: Ang Siew-Lan , Jin Ou , Rhinn Muriel , Daigle Nathalie , Stevenson Lois , Rossant Janet
Published: April 2021

Mouse Otx2 is a bicoid-class homeobox gene, related to the Drosophila orthodenticle (otd) gene. Expression of this gene is initially widespread in the epiblast at embryonic day 5.5 but becomes progressively restricted to the anterior end of the embryo at the headfold stage. In flies, loss of function mutations in otd result in deletion of pre-antennal and antennal segments; which leads to the absence of head structures derived from these segments. To study the function of Otx2 in mice, we have generated a homeobox deletion mutation in this gene. Mice homozygous for this mutation show severe defects in gastrulation and in formation of axial mesoderm and loss of anterior neural tissues. These results demonstrate that Otx2 is required for proper development of the epiblast and patterning of the anterior brain in mice, and supports the idea of evolutionary conservation of the function of Otd/Otx genes in head development in flies and mice.
The establishment of the hepatic architecture is a prerequisite for the development of a lobular pattern of gene expression
Authors: Notenboom Robbert G. E. , de Boer Piet A. J. , Moorman Antoon F. M. , Lamers Wouter H.
Published: April 2021

We have studied the expression patterns of ammonia-metabolising enzymes and serum proteins in intraspleni-cally transplanted embryonic rat hepatocytes by in situ hybridisation and immunohistochemical analysis. The enzymic phenotype of individually settled hepatocytes was compared with that of hepatocytes being organised into a three-dimensional hepatic structure. Our results demon-strate that development towards the terminally differenti-ated state with zonal differences in enzyme content requires the incorporation of hepatocytes into lobular structures. Outside such an architectural context, phenotypic matura-tion becomes arrested and hepatocytes linger in the pro-todifferentiated state. These features identify the foetal period as a crucial time for normal liver development and show that the establishment of the terminally differentiated hepatocellular phenotype, beginning with the differen-tiation of hepatocytes from the embryonic foregut, is realised via a multistep process.
The alignment of the axis of asymmetry in regenerating protoplasts of the moss, Ceratodon purpureus, is determined independently of axis polarity
Authors: Cove D. J. , Quatrano R. S. , Hartmann E.
Published: April 2021

Ceratodon protoplasts regenerate by polar outgrowth to form cell filaments. The kinetics of regeneration show that some cellular event has to be completed before regenera-tion can be initiated. The development of the regeneration axis is strongly influenced by light, with axis alignment and axis polarity being fixed independently. We define axis alignment as the relationship of the regeneration axis to the incident light, independent of polarity. Thus protoplasts regenerating directly towards, or directly away from the light source are defined as being similarly aligned but with opposite polarity. Protoplasts that regenerate in unidirec-tional red light form axes that are aligned parallel to the light direction, with about 70% being polarised towards the light and about 30% away. In unidirectional blue or white light, almost all protoplasts regenerate towards the light but axis alignment is determined less stringently. Re- ori-entation of protoplasts regenerating in unidirectional light shows that axis alignment is fixed between 8 and 9 hours before protoplasts regenerate and that axis polarity is fixed later. When protoplasts are removed from directional light to either non-directional light or to darkness, regeneration axes continue to be aligned by the earlier directional stimulus for at least 24 hours. Thus although axis alignment is fixed only about 8 hours before regeneration, in the absence of contradictory information about direc-tionality in the light environment, protoplasts retain a memory of light direction for much longer. However, both reorientation and removal from a directional light field have profound effects on axis polarity; the pattern observed in undisturbed protoplasts being lost. To account for these observations, we propose that separate gradients are established independently to determine the alignment and polarity of the regeneration axis respectively. The alignment gradient is established rapidly and is steeper in red than in blue or white light, the polarity gradient is established slowly and is steeper in white or blue light than in red. These studies will now allow a genetic dissection of these processes in moss.
Rpx: a novel anterior-restricted homeobox gene progressively activated in the prechordal plate, anterior neural plate and Rathke’s pouch of the mouse embryo
Authors: Hermesz Edit , Mackem Susan , Mahon Kathleen A.
Published: April 2021

We have isolated a new murine homeobox gene, Rpx (for Rathke’s pouch homeobox), that is dynamically expressed in the prospective cephalic region of the embryo during gastrulation. Early expression is seen in the anterior midline endoderm and prechordal plate precursor. Expression is subsequently activated in the overlying ectoderm of the cephalic neural plate, suggesting that inductive contact with Rpx-expressing mesendoderm is required for this expression. Subsequently, Rpx expression is extinguished in the mesendoderm while remaining in the prospective prosencephalic region of the neural plate ectoderm. Ultimately, transcripts become restricted to Rathke’s pouch, the primordium of the pituitary, which is known to be derived from the most anterior ectoderm of the early embryo. Down regulation of Rpx in the pouch coincides with the differentiation of pituitary-specific cell types. Rpx is the earliest known marker for the pituitary primordium, suggestive of a role in the early determination or differentiation of the pituitary. Since Rpx is expressed so dynamically and so early in the anterior region of the embryo, and since its early expression domain is much more extensive than the region fated to form the pituitary, it is likely that Rpx is involved in the initial determination of the anterior (prechordal) region of the embryo.
Clones in the chick diencephalon contain multiple cell types and siblings are widely dispersed
Authors: Golden Jeffrey A. , Cepko Constance L.
Published: April 2021

The thalamus, hypothalamus and epithalamus of the vertebrate central nervous system are derived from the embryonic diencephalon. These regions of the nervous system function as major relays between the telencephalon and more caudal regions of the brain. Early in development, the diencephalon morphologically comprises distinct units known as neuromeres or prosomeres. As development proceeds, multiple nuclei, the functional and anatomical units of the diencephalon, derive from the neuromeres. It was of interest to determine whether progenitors in the diencephalon give rise to daughters that cross nuclear or neuromeric boundaries. To this end, a highly complex retroviral library was used to infect diencephalic progenitors. Retrovirally marked clones were found to contain neurons, glia and occasionally radial glia. The majority of clones dispersed in all directions, resulting in sibling cells populating multiple nuclei within the diencephalon. In addition, several distinctive patterns of dispersion were observed. These included clones with siblings distributed bilaterally across the third ventricle, clones that originated in the lateral ventricle, clones that crossed neuromeric boundaries, and clones that crossed major boundaries of the developing nervous system, such as the diencephalon and mesencephalon. These findings demonstrate that progenitor cells in the diencephalon are multipotent and that their daughters can become widely dispersed.
Mutations in the 8 kDa dynein light chain gene disrupt sensory axon projections in the Drosophila imaginal CNS
Authors: Phillis R. , Statton D. , Caruccio P. , Murphey R. K.
Published: April 2021

Mutations in an 8 kDa (8 ×103Mr) cytoplasmic dynein light chain disrupt sensory axon trajectories in the imaginal nervous system of Drosophila. Weak alleles are behaviorally mutant, female-sterile and exhibit bristle thinning and bristle loss. Null alleles are lethal in late pupal stages and alter neuronal anatomy within the imaginal CNS. We utilized P[Gal4] inserts to examine the axon projections of stretch receptor neurons and an engrailed-lacZ construct to characterize the anatomy of tactile neurons. In mutant animals both types of sensory neurons exhibited altered axon trajectories within the CNS, suggesting a defect in axon pathfinding. However, the alterations in axon trajec-tory did not prevent these axons from reaching their normal termination regions. In the alleles producing these neuronal phenotypes, expression of the cytoplasmic dynein 8 kDa light chain gene is completely absent. These results demonstrate a new function for the cytoplasmic dynein light chain in the regulation of axonogenesis and may provide a point of entry for studies of the role of cellular motors in growth cone guidance.
Discrete developmental stages during teliospore formation in the corn smut fungus, Ustilago maydis
Authors: Banuett Flora , Herskowitz Ira
Published: April 2021

Ustilago maydis is a dimorphic fungus with a yeast-like non-pathogenic form and a filamentous (hyphal) pathogenic form that induces tumor formation in maize. Within mature tumors’ hyphae give rise to teliospores’ which are round’ diploid cells surrounded by a specialized cell wall. Here we describe the time course of fungal development in the plant with a focus on the morphological changes in the hyphae and the pathway of teliospore formation. We confirm and extend earlier observations that U. maydis hyphae branch extensively on the leaf surface and intra-cellularly before induction of tumors. We observe that at later stages the filaments undergo a series of discrete morphogenetic changes leading to teliospore formation. In particular’ we show that the hyphae become embedded in a mucilaginous matrix within the tumor cells and the hyphal tips become modified. The hyphae then undergo fragmentation to release individual cells that exhibit a variety of shapes on their way to becoming rounded. Finally’ a specialized cell wall is deposited. Support for the existence of such a pathway comes from analysis of a mutant defective in the fuz1 gene: inactivation of fuz1 blocks production of the mucilaginous matrix and fragmentation of the hyphae’ leading to a defect in teliospore formation. The different morphological changes that occur while in the plant but not in culture suggest that plant inputs play a key role in fungal development.
Cell autonomous functions of the receptor tyrosine kinase TIE in a late phase of angiogenic capillary growth and endothelial cell survival during murine development
Authors: Partanen Juha , Puri Mira C. , Schwartz Lois , Fischer Klaus-Dieter , Bernstein Alan , Rossant Janet
Published: April 2021

TIE is a receptor tyrosine kinase expressed in both mature endothelial cells and their precursors, as well as in some hematopoietic cells. Mouse embryos homozygous for a disrupted Tie allele die at midgestation due to impaired endothelial cell integrity and resulting hemorrhage. Here we have performed chimeric analysis to study further the function of the murine TIE in the development of embryonic vasculature and in the hematopoietic system. Cells lacking a functional Tie gene (tielcztielczn– cells) contributed to the embryonic vasculature at E10.5 as efficiently as cells heterozygous for a targeted Tie allele (tielcz+ cells). Thus, TIE does not play a significant role in vasculogenesis or in early angiogenic processes, such as formation of the intersomitic arteries and limb bud vascularization. At E15.5 tielcztielczn– cells still readily contributed to major blood vessels and to endothelial cells of organs such as lung and heart, which have been suggested to be vascularized by angioblast differentiation. In contrast, the tielcztielczn– cells were selected against in the capillary plexuses of several angiogenically vascularized tissues, such as brain and kidney. Our results thus support,a role for TIE in late phases of angiogenesis but not vasculogenesis. Furthermore, the results suggest that different mechanisms regulate early and late angiogenesis and provide support for a model of differential organ vascularization by vasculogenic or angiogenic processes. Analysis of adult chimeras suggested that TIE is required to support the survival or proliferation of certain types of endothelial cells demonstrating heterogeneity in the growth/survival factor requirements in various endothelial cell populations.

Chimeric analysis of adult hematopoietic cell populations, including peripheral platelets and bone marrow progenitor cells, revealed that tielcztielczn– cells were able to contribute to these cell types in a way indistinguishable from tielcz+ or wild-type cells. Thus, the primary function of TIE appears to be restricted to the endothelial cell lineage.
The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controling dorsoventral patterning of Xenopus mesoderm
Authors: Onichtchouk Darya , Gawantka Volker , Dosch Roland , Delius Hajo , Hirschfeld Kirsten , Blumenstock Claudia , Niehrs Christof
Published: April 2021

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstück pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.
Early gene expression along the animal-vegetal axis in sea urchin embryoids and grafted embryos
Authors: Ghiglione Christian , Emily-Fenouil Francoise , Chang Patrick , Gache Christian
Published: April 2021

The HE gene is the earliest strictly zygotic gene activated during sea urchin embryogenesis. It is transiently expressed in a radially symmetrical domain covering the animal-most two-thirds of the blastula. The border of this domain, which is orthogonal to the primordial animal-vegetal axis, is shifted towards the animal pole in Li+-treated embryos. Exogenous micromeres implanted at the animal pole of whole embryos, animal or vegetal halves do not modify the extent and localization of the HE expression domain. In grafted embryos or animal halves, the Li+ effect is not affected by the presence of ectopic micromeres at the animal pole. A Li+-induced shift of the border, similar to that seen in whole embryos, occurs in embryoids develop-ing from animal halves isolated from 8-cell stage embryos or dissected from unfertilised eggs. Therefore, the spatial restriction of the HE gene is not controlled by the inductive cascade emanating from the micromeres and the pattern-ing along the AV-axis revealed by Li+ does not require interactions between cells from the animal and vegetal halves. This suggests that maternal primary patterning in the sea urchin embryo is not limited to a small vegetal center but extends along the entire AV axis.
PAR-2 is asymmetrically distributed and promotes association of P granules and PAR-1 with the cortex in C. elegans embryos
Authors: Boyd Lynn , Guo Su , Levitan Diane , Stinchcomb Dan T. , Kemphues Kenneth J.
Published: April 2021

The par genes participate in the process of establishing cellular asymmetries during the first cell cycle of Caenorhabditis elegans development. The par-2 gene is required for the unequal first cleavage and for asymmetries in cell cycle length and spindle orientation in the two resulting daughter cells. We have found that the PAR-2 protein is present in adult gonads and early embryos. In gonads, the protein is uniformly distributed at the cell cortex, and this subcellular localization depends on micro-filaments. In the one-cell embryo, PAR-2 is localized to the posterior cortex and is partitioned into the posterior daughter, P1, at the first cleavage. PAR-2 exhibits a similar asymmetric cortical localization in P1, P2, and P3, the asym-metrically dividing blastomeres of germ line lineage. This distribution in embryos is very similar to that of PAR-1 protein.

By analyzing the distribution of the PAR-2 protein in various par mutant backgrounds we found that proper asymmetric distribution of PAR-2 depends upon par-3 activity but not upon par-1 or par-4. par-2 activity is required for proper cortical localization of PAR-1 and this effect requires wild-type par-3 gene activity. We also find that, although par-2 activity is not required for posterior localization of P granules at the one-cell stage, it is required for proper cortical association of P granules in P1.
Appropriate pattern formation following regulative regeneration in the hindbrain neural tube
Authors: Díaz Carmen , Glover Joel C.
Published: April 2021

Patterns of neuronal differentiation and axon outgrowth were examined in regenerated neural tissue of the hindbrain of the chicken embryo. Specific regions of the hindbrain neural tube corresponding to identified rhom-bomeres were selectively removed unilaterally by micro-surgery. Of the embryos that survived, about 35% exhibited regulative regeneration, wherein the missing hemi-rhombomere was reconstituted to normal size and morphology through compensatory proliferation and migration from adjacent tissue. The capacity for regener-ation depended on the embryonic stage at which the ablation was performed and on whether the midline floor plate was included in the ablation. Ablations performed prior to the formation of morphologically overt rhom-bomere boundaries led to significantly lower frequencies of regulative regeneration than ablations performed after the formation of boundaries. Ablations that included the floor plate led to significantly lower frequencies of regulative regeneration than ablations sparing the floor plate. Regu-lative regeneration was also observed at low frequency following excision of an entire rhombomere.

Within the regenerated hemi-rhombomere, identified neuron groups appeared in their normal positions and dif-ferentiated normal patterns of migration and axon outgrowth. The fidelity of this patterning, however, depended on the mediolateral position of the neuron group, being more reliable for neuron groups in the basal plate than in the alar plate. These results confirm the extensive regulative potential of the neural tube and demonstrate the capacity for appropriately patterned cellular differen-tiation within the regenerated tissue.