bluuebunny/crossref_metadata_2025 huggingface dataset:
Rhythmicity in the early swimming of anuran larvae
Authors: Corner Michael
Published: April 2021

Observations were made upon two species of anurans, Xenopus laevis and Rana pipiens, of the early body movements and their modification after making sections through various levels of the central nervous system. Bursts of muscle contractions, effecting swimming, occur spontaneously and are identical to those evoked by tactile stimulation. Bursts are usually consistent in duration and occur at fairly regular intervals. In Rana they are generated by the spinal cord as well as by the hind-brain, but with a decreasing frequency at successively posterior levels. Early behavior in Xenopus is unchanged by removal of the forebrain and mid-brain, or by removal of the most anterior portion of the hind-brain. More extensive hind-brain removal alters the swimming pattern in most cases so that the bursts become either prolonged or abortive. Complete removal results in continual rather than periodic muscular activity. The early amphibian central nervous system is concluded to consist functionally of several excitatory and inhibitory rhythms acting upon an efferent mechanism which is capable of generating pulses at a constant mean frequency. Sensory input is held to alter briefly the net level of central excitation resulting from these endogenous rhythms.
Effects of Actinomycin D on the lens regenerating system
Authors: Yamada Tuneo , Roesel Marion E.
Published: April 2021

Effects of actinomycin D on transformation of iris into lens in Wolffian lens regeneration of adult T. viridescens were studied by injection into the body cavity of lentectomized animals. One microgram of the antibiotic per gram body weight was injected every day or every other day for a 3- to 12-day period at various phases of lens regeneration Administration of actinomycin during the early phase of lens regeneration for a period longer than 4 days suppressed the progress of regeneration. Suppression was complete or partial depending upon the experimental conditions. Four daily injections of the above indicated amount were given at various phases of lens regeneration. When the treatment began after appearance of primary lens fiber cells, differentiation of those cells proceeded normally, while developmental arrest and disintegration were observed in the lens epithelium. The actinomycin-treatment given after the regenerate has formed a well-differentiated lens caused a degenerative inhibition of the lens epithelium, but no appreciable change in the morphology of lens fibers. The non-regenerating iris and lens did not show any morphological sign of abnormalities after the same treatment. Data are discussed in connection with information on syntheses of RNA, proteins, and DNA occurring in the lens-regenerating system. A change in the pattern of transcription of genetic information is suggested as a part of the mechanism of tissue transformation.
True hermaphroditism in chimaeric mice
Authors: Tarkowski Andrzej K.
Published: April 2021

Histological examination of the reproductive systems of fourteen newborn chimaerae developed from fused eggs revealed eleven sexually normal animals and three true hermaphrodites. The genital apparatus of the hermaphrodites is described, the structure of the gonads being given special attention. Of the three hermaphrodites, one had one ovary with female sex ducts and an ovotestis with complete male ducts and incomplete female ones. The second had two ovotestes and a genital apparatus of nearly typically male type; the remnants of Müllerian ducts being retained only caudally. The third had one ovary with female sex ducts and one ovotestis with male sex ducts. The occurrence of true hermaphrodites strongly suggests that spontaneous true hermphroditism also develops from sex-chromosome mosaicism. The available literature on spontaneous hermaphroditism in rodents is critically reviewed. It is postulated that the frequencies of the four types of true hermaphroditism described (ovary-testis, ovary-ovotestis, testis-ovotestis, ovotestis-ovotestis) in mice and other rodents do not adequately reflect their actual incidence in nature in that the incidence of the lateral type of hermaphroditism is exaggerated. It is suggested that if the testicular tissue does, in fact, elaborate a morphogenetic stimulus influencing the differentiation of XX gonadal tissue, such a stimulus must be rather weak and act only near the site of its elaboration. The genetic sex of germ cells in chimaeric hermaphrodites remains unknown. It is postulated that the presence in the hermaphrodite gonads of oöcytes and male germ cells reflects only the secondary differentiation of germ cells, which need not reflect their genetic sex. While the ovarian tissue of the ovotestes examined contains only oöcytes, in the testicular tissue both oöcytes and spermatogonia are present. Initiation of meiosis of the female type in the testicular territory is attributed to the action of a meiotic stimulus elaborated by the ovarian tissue and diffusing through the testicular part.
The regenerative capacity of the tail in embryonic and post-natal lizards (Lacerta vivipara Jacquin)
Authors: Moffat (née Holder) Lynette A. , D’A. Bellairs A.
Published: April 2021

The structure and development of the fracture planes in the tail of Lacerta vivipara are briefly described. The histological features of normal tail regeneration in baby and adult lizards are described. Regeneration occurs after autotomy or after amputation through either an intravertébral or an intervertebral plane, and becomes clearly visible after 2 to 3 weeks. The technique, based on the work of Panigel, of rearing embryos of lizards in a form of culture after removal from the mother, and of operating on them is described. In recent experiments over 40 per cent, of the embryos subjected to the operation of tail amputation at stages 30 – 32 in the normal table by Dufaure and Hubert survived to reach near hatching stages (39 – 40). A few hatched after periods of 17 to 24 days in culture at 28° C. Embryos operated on at very late stages generally succeeded in hatching. The effects of tail amputation during embryonic life were studied histologically in fifty-nine individuals. Almost all the embryos operated on at stages younger than 39 failed to regenerate after up to 36 days in culture, and showed appearances suggestive of rapid healing. Most of the embryos operated on at stages 39 – 40, very late in embryonic life, showed early regenerative changes. Others showed failure of regeneration, as in the younger embryos, and in a few specimens the results were doubtful. A possible reason for the lack of regenerative power in embryo lizards, and for its appearance at or shortly before hatching (or in nature, birth) is suggested. There is evidence that regeneration may depend upon the rate of healing and the extent of the wound surface. Substantial keratinization of the scales and stiffening of the sub-epidermal tissues occur towards the end of embryonic life. By preventing premature healing and helping to maintain a large wound surface such changes may provide the necessary conditions for regeneration. From the standpoint of natural selection the lack of regenerative power of the lizard embryo is not surprising, since regeneration would have no survival value until post-natal life.
Laminar specific attachment and neurite outgrowth of thalamic neurons on cultured slices of developing cerebral neocortex
Authors: Emerling Daniel E. , Lander Arthur D.
Published: April 2021

In nervous system development, the growth cones of advancing axons are thought to navigate to their targets by recognizing cell-surface and extracellular matrix molecules that act as specific guidance cues. To identify and map cues that guide the growth of a particular axonal system, the thalamocortical afferents, an assay was devised to examine short-term interactions of dissociated embryonic thalamic cells with living, ∼150 μm slices of developing mouse forebrain. Thalamic cells rapidly (<3 hours) and efficiently attached to and extended neurites on pre- and postnatal slices, but a broad zone throughout the neocortex was generally non-permissive for both thalamic cell attachment and the ingrowth of neurites. This zone coincided with the cortical plate at early stages (embryonic day 15), but later became restricted, in rostral-to-caudal fashion, to cortical laminae 2/3. Thus, at each stage, thalamic cells in vitro avoided just that area that thalamic axons confront, but generally do not enter, in vivo. In addition, neurites that extended on some layers were found to be significantly oriented in directions that coincide with the pathways that thalamic axons follow in vivo. These results imply that local adhesive cues and signals that affect process outgrowth are distributed among developing cortical laminae in a manner that could underlie much of the temporal and spatial patterning of thalamocortical innervation.
A Dictyostelium anterior-like cell mutant reveals sequential steps in the prespore prestalk differentiation pathway
Authors: Bichler Georg , Weijer Cornelis J.
Published: April 2021

We have isolated Dictyostelium cell type proportioning mutants in a screen for cAMP signal transduction mutants. All mutants showed reduced numbers of prespore cells as analyzed with prespore-specific antibodies and prespore- and prestalk-specific reporter gene constructs. Formation of prestalk and prespore cells in the mutant HP1 is initially almost normal; however, many prespore cells redifferentiate as anterior-like cells leading to altered cell type proportions. Synergy experiments of HP1 transformant cells with wild-type Ax2 slugs showed the existence of stable subpopulations of cells in the prestalk and prespore regions of the slug. HP1 cells sort exclusively to the back of the prestalk and prespore zone in slugs. These cells will form the upper and lower cup regions of the culminate. Ax2 transformant cells sort to the tip of the anterior prestalk and prespore zone in HP1 slugs and will form the stalk and spores in the culminate.

These experiments show that upper and lower cup derive from cells in different differentiation pathways. The lower cup cell derive from prespore cells, which redifferentiate as anterior-like cells in the back of the prespore zone. The upper cup cells derive from cells from the back of the prestalk zone. Synergy experiments showed that the mutation in HP1 results in an altered sensitivity to proportioning signals. The mutation in HP1 may result in a reduced sensitivity to cAMP, which facilitates the redifferentiation of prespore cells via anterior-like to lower cup cells and inhibits the transition from upper cup to stalk cells.
Developmental control of allelic methylation in the imprinted mouse Igf2 and H19 genes
Authors: Feil Robert , Walter Jörn , Allen Nicholas D. , Reik Wolf
Published: April 2021

The Insulin-like growth factor 2 (Igf2) and H19 genes are reciprocally imprinted and closely linked. Igf2 encodes a fetal growth-factor and is predominantly expressed from the paternal allele, while H19 is expressed from the maternal allele and encodes a transcript which may down-regulate cellular proliferation. One of the epigenetic modifications thought to be involved in parental imprinting is DNA methylation. Here we analyse methylation in two regions of the Igf2 gene, one approx. 3 kb upstream of the gene and one in the 3′ part of the gene. Both regions are more methylated on the expressed paternal chromosome. Genomic sequencing of individual chromosomes in the first region shows this parent-specific methylation to be highly mosaic; interestingly, individual sperm chromosomes carry different methylation patterns into the egg. In the more 3′ region, which is fully methylated in sperm, the level of methylation on the paternal allele is highly tissue-specific and is correlated with expression of the gene in fetal tissues. Hence, the paternal allele is highly methylated in fetal liver (high expression) but is undermethylated in fetal brain (virtually no expression). Adult choroid plexus, a brain tissue in which Igf2 is expressed from both alleles and H19 is not expressed, represents an apparent loss of imprinting. Here, both Igf2 and H19 adopt a paternal type methylation pattern on both parental chromosomes. Analysis of early-passage androgenetic and parthenogenetic embryonic stem (ES) cells shows that the methylation patterns of Igf2 and H19 on maternal and paternal chromosomes are very similar. Androgenetic and parthenogenetic teratomas derived from these ES cells show the appropriate paternal and maternal patterns, respectively, of allelic methylation in both genes. Our results suggest that allelic methylation patterns in Igf2 and H19 arise early in embryogenesis and change progressively during development. Some of these developmental changes are apparently under tissue-specific control.
Positioning adjacent pair-rule stripes in the posterior Drosophila embryo
Authors: Langeland James A. , Attai Sherwin F. , Vorwerk Kathy , Carroll Sean B.
Published: April 2021

We present a genetic and molecular analysis of two hairy (h) pair-rule stripes in order to determine how gradients of gap proteins position adjacent stripes of gene expression in the posterior of Drosophila embryos. We have delimited regulatory sequences critical for the expression of h stripes 5 and 6 to 302 bp and 526 bp fragments, respectively, and assayed the expression of stripe-specific reporter constructs in several gap mutant backgrounds. We demonstrate that posterior stripe boundaries are established by gap protein repressors unique to each stripe: h stripe 5 is repressed by the giant (gt) protein on its posterior border and h stripe 6 is repressed by the hunchback (hb) protein on its posterior border. Interestingly, Krüppel (Kr) limits the anterior expression limits of both stripes and is the only gap gene to do so, indicating that stripes 5 and 6 may be coordinately positioned by the Kr repressor. In contrast to these very similar cases of spatial repression, stripes 5 and 6 appear to be activated by different mechanisms. Stripe 6 is criti-cally dependent upon knirps (kni) for activation, while stripe 5 likely requires a combination of activating proteins (gap and non-gap). To begin a mechanistic understanding of stripe formation, we locate binding sites for the Kr protein in both stripe enhancers. The stripe 6 enhancer contains higher affinity Kr-binding sites than the stripe 5 enhancer, which may allow for the two stripes to be repressed at different Kr protein concentration thresholds. We also demonstrate that the kni activator binds to the stripe 6 enhancer and present evidence for a competitive mechanism of Kr repression of stripe 6.
Mutations in the FASS gene uncouple pattern formation and morphogenesis in Arabidopsis development
Authors: Torres-Ruiz Ramón A. , Jürgens Gerd
Published: April 2021

The pattern of cell division is very regular in Arabidopsis embryogenesis, enabling seedling structures to be traced back to groups of cells in the early embryo. Recessive mutations in the FASS gene alter the pattern of cell division from the zygote, without interfering with embryonic pattern formation: although no primordia of seedling structures can be recognised by morphological criteria at the early-heart stage, all elements of the body pattern are differentiated in the seedling. fass seedlings are strongly compressed in the apical-basal axis and enlarged circum-ferentially, notably in the hypocotyl. Depending on the width of the hypocotyl, fass seedlings may have up to three supernumerary cotyledons. fass mutants can develop into tiny adult plants with all parts, including floral organs, strongly compressed in their longitudinal axis. At the cellular level, fass mutations affect cell elongation and ori-entation of cell walls but do not interfere with cell polarity as evidenced by the unequal division of the zygote. The results suggest that the FASS gene is required for mor-phogenesis, i.e. oriented cell divisions and position-dependent cell shape changes generating body shape, but not for cell polarity which seems essential for pattern formation.
bcl-xLx is the major bcl-x mRNA form expressed during murine development and its product localizes to mitochondria
Authors: González-García Maribel , Pérez-Ballestero Rafael , Ding Liyun , Duan Linda , Boise Lawrence H. , Thompson Craig B. , Núñez Gabriel
Published: April 2021

Most examples of cell death in animals are controlled by a genetic program that is activated within the dying cell. The apoptotic process is further regulated by a set of genes that act as repressors of cell death. Of these, bcl-2 is expressed in a variety of embryonic and postnatal tissues which suggests a critical role for bcl-2 in organogenesis and tissue homeostasis. Surprisingly, mutant mice with targeted disruption of bcl-2 appear normal at birth and complete maturation of lymphoid tissues before succumbing to fulminant lymphopenia and polycystic renal disease by 2-5 weeks of age. This suggests that there may be genes other than bcl-2 that can regulate apoptosis during development. To begin to investigate this possibility, we have cloned and characterized the murine bcl-x gene, whose human counterpart displays striking homology to bcl-2. The predicted murine bcl-xL gene product exhibits a high level of amino acid identity (97%) to its human counterpart. Just like Bcl-2, the murine bcl-xL gene product can act as a dominant inhibitor of cell death upon growth factor withdrawal. In addition, the bulk of the bcl-xL product localizes to the periphery of mitochondria as assessed by a bcl-xL-tag expression system, suggesting that both Bcl-2 and Bcl-xL proteins prevent cell death by a similar mechanism. bcl-xL is the most abundant bcl-x mRNA species expressed in embryonic and adult tissues. The levels of bcl-xL mRNA appear higher than those of bcl-2 during embryonal development and in several adult organs including bone marrow, brain, kidney and thymus. In addition to bcl-xL, we have identified another form of bcl-x mRNA, bcl-xβ, that results from an unspliced bcl-x transcript. bcl-xβ mRNA is expressed in various embryonic and postnatal tissues. Surprisingly, the expression of bcl-xS (a negative regulator of programmed cell death) was undetectable by a sensitive S1-nuclease assay and polymerase chain reaction analysis of mouse tissues. Based on its tissue and developmental patterns of expression, it appears that bcl-x may play an important role in the regulation of cell death during development and tissue homeostasis.
Initial steps of myogenesis in somites are independent of influence from axial structures
Authors: Bober Eva , Brand-Saberi Beate , Ebensperger Cecilia , Wilting Jörg , Balling Rudi , Paterson Bruce M. , Arnold Hans-Henning , Christ Bodo
Published: April 2021

Formation of paraxial muscles in vertebrate embryos depends upon interactions between early somites and the neural tube and notochord. Removal of both axial struc-tures results in a complete loss of epaxial myotomal muscle, whereas hypaxial and limb muscles develop normally.

We report that chicken embryos, after surgical removal of the neural tube at the level of the unsegmented paraxial mesoderm, start to develop myotomal cells that express transcripts for the muscle-specific regulators MyoD and myogenin. These cells also make desmin, indicating that the initial steps of axial skeletal muscle formation can occur in the absence of the neural tube. However, a few days following the extirpation, the expression of MyoD and myogenin transcripts gradually disappears, and becomes almost undetectable after 4 days. From these observations we conclude that the neural tube is not required for the generation of the skeletal muscle cell lineage, but may support the survival or maitenance of further differen-tiation of the myotomal cell compartment.

Notochord transplanted medially or laterally to the unsegmented paraxial mesoderm leads to a ventralization of axial structures but does not entirely prevent the early appearance of myoblasts expressing MyoD transcripts. However, the additional notochord inhibits subsequent development and maturation of myotomes. Taken together, our data suggest that neural tube promotes, and notochord inhibits, the process of myogenesis in axial muscles at a developmental step following the initial expression of myogenic bHLH regulators.
Transcriptional regulation of string (cdc25): a link between developmental programming and the cell cycle
Authors: Edgar Bruce A. , Lehman Dara A. , O’Farrell Patrick H.
Published: April 2021

During postblastoderm embryogenesis in Drosophila, cell cycles progress in an invariant spatiotemporal pattern. Most of these cycles are differentially timed by bursts of transcription of string (cdc25), a gene encoding a phosphatase that triggers mitosis by activating the Cdc2 kinase. An analysis of string expression in 36 pattern-formation mutants shows that known patterning genes act locally to influence string transcription. Embryonic expression of string gene fragments shows that the complete pattern of string transcription requires extensive cis-acting regulatory sequences (>15.3 kb), but that smaller segments of this regulatory region can drive proper temporal expression in defined spatial domains. We infer that string upstream sequences integrate many local signals to direct string’s transcriptional program. Finally, we show that the spatiotemporal progression of string transcription is largely unaffected in mutant embryos specifically arrested in G2 of cycles 14, 15, or 16, or G1 of cycle 17. Thus, there is a regulatory hierarchy in which developmental inputs, not cell cycle inputs, control the timing of string transcription and hence cell cycle progression.
Immortalizing oncogenes subvert the establishment of granule cell identity in developing cerebellum
Authors: Gao Wei-Qiang , Hatten Mary E.
Published: April 2021

After implantation into the external germinal layer of early postnatal cerebellum, primary external germinal layer progenitor cells gave rise exclusively to granule neurons. In contrast, all major classes of cerebellar cells were observed following implantation of embryonic day 13 cerebellar precursor cells into the external germinal layer. These results suggest that granule cells arise from precursors with a restricted potential. In contrast to results with the primary external germinal layer population, cell lines established from external germinal layer cells, by infection with a retrovirus containing the SV40 large T-antigen oncogene, gave rise to several cerebellar cell types upon implantation. These included granule neurons, one subclass of stellate interneurons, Golgi cells, Bergmann glia and astrocytes. From these results, we conclude that early postnatal external germinal layer progenitors are normally fated to a granule cell identity and that expression of the SV40 large T-antigen oncogene subverts mechanisms that control granule neuron fate.
Heparin-binding EGF-like growth factor gene is induced in the mouse uterus temporally by the blastocyst solely at the site of its apposition: a possible ligand for interaction with blastocyst EGF-receptor in implantation
Authors: Das Sanjoy K. , Wang Xiao-Ning , Paria Bibhash C. , Damm Deborah , Abraham Judith A. , Klagsbrun Michael , Andrews Glen K. , Dey Sudhansu K.
Published: April 2021

Heparin-binding EGF-like growth factor (HB-EGF) is a newly discovered member of the EGF family of growth factors. HB-EGF can bind to two loci on cell surfaces, heparan sulphate proteoglycans and EGF-receptor (EGFR), and either one or both of these interactions could play a role in cell-cell interactions. In the rodent, increased endometrial vascular permeability at the site of blastocyst apposition is considered to be an earliest discernible prerequisite event in the process of implantation and this event coincides with the initial attachment reaction between the blastocyst trophectoderm and uterine luminal epithelium. This investigation demonstrates that the HB-EGF gene is expressed in the mouse uterine luminal epithelium surrounding the blastocyst 6-7 hours before the attachment reaction that occurs at 2200-2300 hours on day 4 of pregnancy. It was further demonstrated that this gene is not expressed in the luminal epithelium at the site of the blastocyst apposition during the progesterone-maintained delayed implantation, but is readily induced in the luminal epithelium surrounding an activated blastocyst after termination of the delay by an estrogen injection. In vitro studies showed that HB-EGF induced blastocyst EGF-R autophosphorylation, and promoted blastocyst growth, zona-hatching and trophoblast outgrowth. These results suggest possible interactions between the uterine HB-EGF and blastocyst EGF-R very early in the process of implantation, earlier than any other embryo-uterine interactions defined to date at the molecular level.
Neurogenic and proneural genes control cell fate specification in the Drosophila endoderm
Authors: Tepass Ulrich , Hartenstein Volker
Published: April 2021

The Drosophila endoderm segregates into three non-neural cell types, the principle midgut epithelial cells, the adult midgut precursors, and the interstitial cell precursors, early in development. We show that this process occurs in the absence of mesoderm and requires proneural and neurogenic genes. In neurogenic mutants the principle midgut epithelial cells are missing and the other two cell types develop in great excess. Consequently, the midgut epithelium does not form. In achaete-scute complex and daughterless mutants the interstitial cell precursors do not develop and the number of adult midgut precursors is strongly reduced. Development of the principle midgut epithelial cells and formation of the midgut epithelium is restored in neurogenic proneural double mutants. The neu rogenic/proneural genes are, in contrast to the neuroectoderm, not expressed in small clusters of cells but initially homogeneously in the endoderm suggesting that no prepattern exists which determines the position of the segregating cells. Hence, the segregation pattern solely depends on neurogenic/proneural gene interaction. Proneural genes are required but not sufficient to determine specific cell fates because they are required for cell type specification in both ectoderm and endoderm. Our data also suggest that the neurogenic/proneural genes are involved in the choice between epithelial versus mesenchymal cell morphologies.
Extracellular FGF-1 acts as a lens differentiation factor in transgenic mice
Authors: Robinson Michael L. , Overbeek Paul A. , Verran Deborah J. , Grizzle William E. , Stockard Cecil R. , Friesel Robert , Maciag Thomas , Thompson John A.
Published: April 2021

The vertebrate ocular lens undergoes a spatially defined pattern of differentiation which may be regulated by the ocular distribution of proteins from the fibroblast growth factor (FGF) family. The ability of altered FGF-1 (acidic FGF) distribution to disrupt the normal pattern of lens differentiation was evaluated by the production of transgenic mice which express FGF-1 under the control of the lens-specific αA-crystallin promoter. Since FGF-1 lacks a classical signal peptide consensus sequence, transgenic mice were also produced with a chimeric construct containing the signal peptide sequence of the FGF-4 gene fused in frame to the coding sequences of the FGF-1 cDNA in order to obtain extracellular expression of the transgene. The presence of transgenic mRNA and protein was confirmed by in situ hybridization, Western analysis and immunohistochemistry. The ocular histology of newborn and young adult transgenic mice expressing FGF-1 without a signal peptide appeared normal. In contrast, mice expressing secreted FGF-1 exhibited lens abnormalities including the elongation of anterior epithelial cells. Epithelial cell elongation was accompanied by expression of the fiber cell differentiation marker, β-crystallin. These observations provide an in vivo demonstration that FGF-1 can induce anterior lens epithelial cells to express characteristics consistent with the onset of fiber cell differentiation. The transgenic induction of differentiation confirms that normal lens morphology reflects an asymmetric distribution of inductive factors within the eye.
Mutations affecting somite formation and patterning in the zebrafish, Danio rerio
Authors: van Eeden Fredericus J. M. , Granato Michael , Schach Ursula , Brand Michael , Furutani-Seiki Makoto , Haffter Pascal , Hammerschmidt Matthias , Heisenberg Carl-Philipp , Jiang Yun-Jin , Robert Donald A. Kane , Kelsh N. , Mullins Mary C. , Odenthal Jörg , Warga Rachel M. , Allende Miguel L. , Weinberg Eric S. , Nüsslein-Volhard Christiane
Published: April 2021

Somitogenesis is the basis of segmentation of the mesoderm in the trunk and tail of vertebrate embryos. Two groups of mutants with defects in this patterning process have been isolated in our screen for zygotic mutations affecting the embryonic development of the zebrafish (Danio rerio). In mutants of the first group, boundaries between individual somites are invisible early on, although the paraxial mesoderm is present. Later, irregular boundaries between somites are present. Mutations in fused somites (fss) and beamter (bea) affect all somites, whereas mutations in deadly seven (des), after eight (aei) and white tail (wit) only affect the more posterior somites. Mutants of all genes but wit are homozygous viable and fertile. Skeletal stainings and the expression pattern of myoD and snail1 suggest that anteroposterior patterning within individual somites is abnormal. In the second group of mutants, formation of the horizontal myoseptum, which separates the dorsal and ventral part of the myotome, is reduced. Six genes have been defined in this group (you-type genes). you-too mutants show the most severe phenotype; in these the adaxial cells, muscle pioneers and the primary motoneurons are affected, in addition to the horizontal myoseptum.

The horizontal myoseptum is also missing in mutants that lack a notochord. The similarity of the somite phenotype in mutants lacking the notochord and in the you- type mutants suggests that the genes mutated in these two groups are involved in a signaling pathway from the notochord, important for patterning of the somites.
Neural degeneration mutants in the zebrafish, Danio rerio
Authors: Furutani-Seiki Makoto , Jiang Yun-Jin , Brand Michael , Heisenberg Carl-Philipp , Houart Corinne , Beuchle Dirk , Eeden Fredericus J. M. van , Granato Michael , Haffter Pascal , Hammerschmidt Matthias , Kane Donald A. , Kelsh Robert N. , Mullins Mary C. , Odenthal Jörg , Nüsslein-Volhard Christiane
Published: April 2021

Forty zebrafish mutants with localized or general neural degeneration are described. The onset and duration of degeneration and the distribution of ectopically dying cells are specific characteristics of each mutant. Mutants are classified into four groups by these parameters. Class I: late focal neural degeneration mutants. These 18 mutants have restricted cell death mainly in the tectum and the dorsal hindbrain after 36 hours. The degeneration does not spread and disappears at later stages of development. Class II: early focal neural degeneration mutants. Ten mutants in this class exhibit transient restricted degeneration affecting mainly the diencephalon, the hindbrain and the spinal cord at 20 hours. The midbrain is less affected. The degeneration shifts to the dorsal diencephalon and the tectum at 36 hours. Class III: late spreading neural degeneration mutants. The 8 mutants in this class display a degeneration that is first seen in the tectum and subsequently spreads throughout the nervous system from 36 hours on. Class IV: early general neural degeneration mutants. This class of four mutants already shows overall cell degeneration in the nervous system at the 15-somite stage.

Three of the class I mutants show a change in the pattern of gene expression in the anlage of a brain structure prior to the onset of degeneration. These results suggest that focal cell death may be a useful clue for the detection of early patterning defects of the vertebrate nervous system in regions devoid of visible landmarks.
Spatially restricted expression of PlOtp, a Paracentrotus lividus Orthopedia- related homeobox gene, is correlated with oral ectodermal patterning and skeletal morphogenesis in late-cleavage sea urchin embryos
Authors: Di Bernardo Maria , Castagnetti Stefania , Bellomonte Daniela , Oliveri Paola , Melfi Raffaella , Palla Franco , Spinelli Giovanni
Published: April 2021

Several homeobox genes are expressed in the sea urchin embryo but their roles in development have yet to be elucidated. Of particular interest are homologues of homeobox genes that in mouse and Drosophila are involved in patterning the developing central nervous system (CNS). Here, we report the cloning of an orthopedia (Otp)-related gene from Paracentrotus lividus, PlOtp. Otp is a single copy zygotic gene that presents a unique and highly restricted expression pattern. Transcripts were first detected at the mid-gastrula stage in two pairs of oral ectoderm cells located in a ventrolateral position, overlying primary mesenchyme cell (PMC) clusters. Increases in both transcript abundance and the number of Otp-expressing cells were observed at prism and pluteus stages. Otp transcripts are symmetrically distributed in a few ectodermal cells of the oral field. Labelled cells were observed close to sites of active skeletal rod growth (tips of the budding oral and anal arms), and at the juxtaposition of stomodeum and foregut. Chemicals known to perturb PMC patterning along animal-vegetal and oral-aboral axes altered the pattern of Otp expression. Vegetalization by LiCl caused a shift in Otp-expressing cells toward the animal pole, adjacent to shifted PMC aggregates. Nickel treatment induced expression of the Otp gene in an increased number of ectodermal cells, which adopted a radialized pattern. Finally, ectopic expression of Otp mRNA affected patterning along the oral-aboral axis and caused skeletal abnormalities that resembled those exhibited by nickel-treated embryos. From these results, we conclude that the Otp homeodomain gene is involved in short-range cell signalling within the oral ectoderm for patterning the endoskeleton of the larva through epithelial- mesenchymal interactions.
Targeted disruption of the homeobox transcription factor Nkx2-3 in mice results in postnatal lethality and abnormal development of small intestine and spleen
Authors: Pabst Oliver , Zweigerdt Robert , Arnold Hans-Henning
Published: April 2021

The homeodomain transcription factor Nkx2-3 is expressed in gut mesenchyme and spleen of embryonic and adult mice. Targeted inactivation of the Nkx2-3 gene results in severe morphological alterations of both organs and early postnatal lethality in the majority of homozygous mutants. Villus formation in the small intestine appears considerably delayed in Nkx2-3−/− foetuses due to reduced proliferation of the epithelium, while massively increased growth of crypt cells ensues in surviving adult mutants. Interestingly, differentiated cell types of the intestinal epithelium are present in homozygous mutants, suggesting that Nkx2-3 is not required for their cell lineage allocation or migration-dependent differentiation. Hyperproliferation of the gut epithelium in adult mutants is associated with markedly reduced expression of BMP-2 and BMP-4, suggesting that these signalling molecules may be involved in mediating non-cell-autonomous control of intestinal cell growth.

Spleens of Nkx2-3 mutants are generally smaller and contain drastically reduced numbers of lymphatic cells. The white pulp appears anatomically disorganized, possibly owing to a homing defect in the spleen parenchyme. Moreover, some of the Nkx2-3 mutants exhibit asplenia. Taken together these observations indicate that Nkx2-3 is essential for normal development and functions of the small intestine and spleen.
Xenopus GDF6, a new antagonist of noggin and a partner of BMPs
Authors: Chang Chenbei , Hemmati-Brivanlou Ali
Published: April 2021

In Xenopus, ectodermal cell fates are determined by antagonistic interaction between the BMP subfamily of TGF-β ligands and the organizer-specific secreted factors (e.g. noggin, chordin and follistatin). Inhibition of BMP function by these factors can convert cells from an epidermal to a neural cell fate. In this study, we report that GDF6, a new member of the Xenopus TGF-β family, can function in antagonistic interaction with neural inducers. GDF6 induces epidermis and inhibits neural tissue in dissociated cells, and this activity is blocked by the presence of noggin. We demonstrate that GDF6 binds directly to the neural inducer noggin. Furthermore, we find that GDF6 and BMP2 can form heterodimers and the process seems to require cotranslation of the proteins in the same cells. In normal embryos, GDF6 and BMP2 are coexpressed in several places, including the edge of the neural plate at early neurula stages, suggesting that GDF6 may synergize with BMPs to regulate patterning of the ectoderm. Our data show for the first time that noggin can bind directly to and inhibit another TGF-β family member: GDF6. In addition, BMP and GDF6 heterodimers may play an important role in vivo to regulate cell fate determination and patterning.
The zebrafish diwanka gene controls an early step of motor growth cone migration
Authors: Zeller Jörg , Granato Michael
Published: April 2021

During vertebrate embryogenesis different classes of motor axons exit the spinal cord and migrate on common axonal paths into the periphery. Surprisingly little is known about how this initial migration of spinal motor axons is controlled by external cues. Here, we show that the diwanka gene is required for growth cone migration of three identified subtypes of zebrafish primary motoneurons. In diwanka mutant embryos, motor growth cone migration within the spinal cord is unaffected but it is strongly impaired as motor axons enter their common path to the somites. Chimera analysis shows that diwanka gene activity is required in a small set of myotomal cells, called adaxial cells. We identified a subset of the adaxial cells to be sufficient to rescue the diwanka motor axon defect. Moreover, we show that this subset of adaxial cells delineates the common axonal path prior to axonogenesis, and we show that interactions between these adaxial cells and motor growth cones are likely to be transient. The studies demonstrate that a distinct population of myotomal cells plays a pivotal role in the early migration of zebrafish motor axons and identify the diwanka gene as a somite-derived cue required to establish an axonal path from the spinal cord to the somites.
Heparan sulfate proteoglycans are essential for FGF receptor signaling during Drosophila embryonic development
Authors: Lin Xinhua , Buff Eugene M. , Perrimon Norbert , Michelson Alan M.
Published: April 2021

The Drosophila sugarless and sulfateless genes encode enzymes required for the biosynthesis of heparan sulfate glycosaminoglycans. Biochemical studies have shown that heparan sulfate glycosaminoglycans are involved in signaling by fibroblast growth factor receptors, but evidence for such a requirement in an intact organism has not been available. We now demonstrate that sugarless and sulfateless mutant embryos have phenotypes similar to those lacking the functions of two Drosophila fibroblast growth factor receptors, Heartless and Breathless. Moreover, both Heartless- and Breathless-dependent MAPK activation is significantly reduced in embryos which fail to synthesize heparan sulfate glycosaminoglycans. Consistent with an involvement of Sulfateless and Sugarless in fibroblast growth factor receptor signaling, a constitutively activated form of Heartless partially rescues sugarless and sulfateless mutants, and dosage-sensitive interactions occur between heartless and the heparan sulfate glycosaminoglycan biosynthetic enzyme genes. We also find that overexpression of Branchless, the Breathless ligand, can partially overcome the requirement of Sugarless and Sulfateless for Breathless activity. These results provide the first genetic evidence that heparan sulfate glycosaminoglycans are essential for fibroblast growth factor receptor signaling in a well defined developmental context, and support a model in which heparan sulfate glycosaminoglycans facilitate fibroblast growth factor ligand and/or ligand-receptor oligomerization.
The midbrain-hindbrain boundary genetic cascade is activated ectopically in the diencephalon in response to the widespread expression of one of its components, the medaka gene Ol-eng2
Authors: Ristoratore Filomena , Carl Matthias , Deschet Karine , Richard-Parpaillon Laurent , Boujard Daniel , Wittbrodt Joachim , Chourrout Daniel , Bourrat Franck , Joly Jean-Stéphane
Published: April 2021

In vertebrates, the engrailed genes are expressed at early neurula stage in a narrow stripe encompassing the midbrain-hindbrain boundary (MHB), a region from which a peculiar structure, the isthmus, is formed. Knock-out experiments in mice demonstrated that these genes are essential for the development of this structure and of its derivatives. In contrast, little is known about the effect of an overexpression of engrailed genes in vertebrate development. Here we report the isolation of Ol-eng2, a medaka fish (Oryzias latipes) engrailed gene. We have monitored the effects of its widespread expression following mRNA injections in 1- and 2-cell medaka and Xenopus embryos. We found that the ectopic expression of Ol-eng2 predominantly results in an altered development of the anterior brain, including an inhibition of optic vesicle formation. No change in the patterns of mesencephalic and telencephalic markers were observed. In contrast, expressions of markers of the diencephalon were strongly repressed in injected embryos. Furthermore, the endogenous Ol-eng2, Pax2, Wnt1 and Fgf8, which are essential components of the MHB genetic cascade, were ectopically expressed in this region. Therefore, we propose that Ol-eng2 induces de novo formation of an isthmus-like structure, which correlates with the development of ectopic midbrain structures, including optic tectum. A competence of the diencephalon to change to a midbrain fate has been demonstrated in isthmic graft experiments. Our data demonstrate that this change can be mimicked by ectopic engrailed expression alone.
Lineage-tracing cells born in different domains along the PD axis of the developing Drosophila leg
Authors: Weigmann Katrin , Cohen Stephen M.
Published: April 2021

Patterning of the developing limbs by the secreted signaling proteins Wingless, Hedgehog and Dpp takes place while the imaginal discs are growing rapidly. Cells born in regions of high ligand concentration may be displaced through growth to regions of lower ligand concentration. We have used a novel lineage-tagging method to address the reversibility of cell fate specification by morphogen gradients. We find that responses to Hedgehog and Dpp in the wing disc are readily reversible. In the leg, we find that cells readily adopt more distal fates, but do not normally shift from distal to proximal fate. However, they can do so if given a growth advantage. These results indicate that cell fate specification by morphogen gradients remains largely reversible while the imaginal discs grow. In other systems, where growth and patterning are uncoupled, nonreversible specification events or ‘ratchet’ effects may be of functional significance.
Regeneration of isthmic tissue is the result of a specific and direct interaction between rhombomere 1 and midbrain
Authors: Irving Carol , Mason Ivor
Published: April 2021

The midbrain-hindbrain boundary, or isthmus, is the source of signals that are responsible for regional specification of both the midbrain and anterior hindbrain. Fibroblast growth factor 8 (Fgf8) is expressed specifically at the isthmus and there is now good evidence that it forms at least part of the patterning signal. In this study, we use Fgf8 as a marker for isthmic cells to examine how interactions between midbrain and hindbrain can regenerate isthmic tissue and, thereby, gain insight into the normal formation and/or maintenance of the isthmus. We show that Fgf8-expressing tissue with properties of the isthmic organiser is generated when midbrain and rhombomere 1 tissue are juxtaposed but not when midbrain contacts any other rhombomere. The use of chick/quail chimeras shows that the isthmic tissue is largely derived from rhombomere 1. In a few cases a small proportion of the Fgf8-positive cells were of midbrain origin but this appears to be the result of a local respecification to a hindbrain phenotype, a process mimicked by ectopic FGF8. Studies in vitro show that the induction of Fgf8 is the result of a direct planar interaction between the two tissues and involves a diffusible signal.
The Phox2 homeodomain proteins are sufficient to promote the development of sympathetic neurons
Authors: Stanke Matthias , Junghans Dirk , Geissen Markus , Goridis Christo , Ernsberger Uwe , Rohrer Hermann
Published: April 2021

The development of sympathetic neurons is controlled by a network of transcriptional regulators, including the paired homeodomain proteins Phox2a and Phox2b. To understand the role of Phox2 proteins in more detail, the effect of Phox2 overexpression was analysed in the avian peripheral nervous system. Phox2a expression in neural crest cultures elicited a strong increase in the number of sympathoadrenergic cells. Expression of Phox2a in the chick embryo promoted the generation of additional neurons expressing the noradrenergic marker genes DBH and TH, pan-neuronal genes SCG10 and NF160 and cholinergic genes ChAT and VAChT. Phox2a-induced neurons were found in ectopic locations such as dorsal root ganglia and peripheral nerve. Sympathoadrenergic development could be elicited in cultures of E5 dorsal root ganglia, demonstrating the presence of Phox2a-responsive cells in non-autonomic peripheral ganglia. Phox2b induced ectopic neurons in the chick embryo in the same way as Phox2a. These results show that Phox2 proteins are sufficient to promote sympathetic neuron generation and control, directly or indirectly, the expression of a large number of genes characteristic for sympathetic neurons.
Regulation of neuronal K+ currents by target-derived factors: opposing actions of two different isoforms of TGFβ
Authors: Cameron Jill S. , Dryer Laurence , Dryer Stuart E.
Published: April 2021

The developmental expression of macroscopic Ca2+-activated K+ currents in chick ciliary ganglion neurons is dependent on an avian ortholog of TGFβ1, known as TGFβ4, secreted from target tissues in the eye. Here we report that a different isoform, TGFβ3, is also expressed in a target tissue of ciliary ganglion neurons. Application of TGFβ3 inhibits the functional expression of whole-cell Ca2+-activated K+ currents evoked by 12 hour treatment with either TGFβ1 or β-neuregulin-1 in ciliary ganglion neurons developing in vitro. TGFβ3 had no effect on voltage-activated Ca2+ currents. A neutralizing antiserum specific for TGFβ3 potentiates stimulation of Ca2+-activated K+ currents evoked by a target tissue (iris) extract in cultured ciliary ganglion neurons, indicating that TGFβ3 is an inhibitory component of these extracts. Intraocular injection of TGFβ3 causes a modest but significant inhibition of the expression of Ca2+-activated K+ currents in ciliary ganglion neurons developing in vivo. Further, intraocular injection of a TGFβ3-neutralizing antiserum stimulates expression of Ca2+-activated K+ currents in ciliary ganglion neurons developing in vivo, indicating that endogenous TGFβ3 regulates the functional expression of this current. The normal developmental expression of functional Ca2+-activated K+ currents in ciliary ganglion neurons developing in vivo is therefore regulated by two different target-derived isoforms of TGFβ, which produce opposing effects on the electrophysiological differentiation of these neurons.