bluuebunny/crossref_metadata_2025 huggingface dataset:
Reformation of specific neuromuscular connections during axolotl limb regeneration: evidence that the first contacts are correct
Authors: Wilson Steve , Jesani Meena , Holder Nigel
Published: April 2021

Retrograde neuronal tracing with horseradish peroxidase was used to determine the position in the spinal cord of the motor neurone pools of a proximal (biceps) and a distal (extensor digitorum) limb muscle at various times during axolotl limb regeneration. It was found that from the earliest stages of muscle redifferentiation (as judged by light and electron microscopic analysis) the vast majority of axons innervating the regenerating muscles came from cells within the bounds of the normal motor neurone pool for each muscle. A few incorrect projections were noted in that the regenerating proximal muscle was sometimes innervated by some cells caudal to its normal motor neurone pool.

The results are discussed in terms of mechanisms that may be operating in the regenerating limb to ensure that specific neuromuscular connections are made.
Replication, integration and stable germ-line transmission of foreign sequences injected into early zebrafish embryos
Authors: Stuart Gary W. , Mcmurray James V. , Westerfield Monte
Published: April 2021

To generate stable lines of transgenic fish, early zebrafish embryos were injected with high concentrations of a linear bacterial plasmid. After injection, the foreign DNA was converted into a high molecular weight form and then amplified approximately tenfold during the initial rapid cleavages characteristic of the early embryo prior to gastrulation. While most of this DNA was subsequently degraded during gastrulation, some of the foreign sequences survived the gastrula stage and could be found in most of the injected fish at 3 weeks of age. Only about 5 % of fish analysed 4 months after the injection retained foreign DNA in their fins, usually at less than one copy per cell. One of these fish was also found to contain about 100 copies per cell of foreign DNA in a fraction of its germ cells. Approximately 20% of the F, offspring from this germ-line-positive parent inherited the foreign DNA, whereas 50 % of F2 progeny obtained from an identified F1 individual inherited these sequences. The 50 % transmission rate in F2 progeny was as expected for a single, heterozygous genomic insert. These observations indicate that injected DNA can be integrated into the fish genome, that the resulting transgenic fish are mosaic and that some of these mosaic individuals give rise to stable lines of transgenic fish.
The fate map of the cephalic neural primordium at the presomitic to the 3-somite stage in the avian embryo
Authors: Couly Gérard , Douarin Nicole M. Le
Published: April 2021

The fate map of the early neural plate and neural fold has been established at the cephalic level by using the quail–chick marker system (Le Douarin, 1969, 1973). The experimental design comprised the replacement of definite territories belonging to the neural plate and neural folds in the chick embryo by their counterparts from quail embryos at the same developmental stage. This technique is referred to as the isotopic and isochronic exchange of preneural tissues between these two species. The various types of experiments that were carried out are schematized in Fig. 2. The possibility of distinguishing quail from chick cells by the structure of their nuclei allowed the fate of the grafted territories to be recognized at later developmental stages ranging from 3 to 9 days of incubation (E3–E9). Fig. 1 illustrates the morphological changes in the anterior neural plate and neural ridges in the chick embryo at the early somitic stages.
Interactions and fates of avian craniofacial mesenchyme
Authors: Noden Drew M.
Published: April 2021

Craniofacial mesenchyme is composed of three mesodermal populations - prechordal plate, lateral mesoderm and paraxial mesoderm, which includes the segmented occipital somites and the incompletely seg- mented somitomeres - and the neural crest. This paper outlines the fates of each of these, as determined using quail-chick chimaeras, and presents similarities and differences between these cephalic populations and their counterparts in the trunk.

Prechordal and paraxial mesodermal populations are the sources of all voluntary muscles of the head. The latter also provides most of the connective precur- sors of the calvaria, occipital, otic-parietal and basisphenoid tissues. Lateral mesoderm is the source of peripharyngeal connective tissues; the most rostral skeletal tissues it forms are the laryngeal and tracheal cartilages.

When migrating neural crest cells encounter seg- mented paraxial mesoderm (occipital and trunk somites), most move into the region between the dermamyotome and sclerotome in the cranial half of each somite. In contrast, most cephalic crest cells migrate superficial to somitomeres. There is, however, a small subpopulation of the head crest that invades somitomeric mesoderm. These cells subsequently segregate presumptive myogenic precursors of vis- ceral arch voluntary muscles from underlying mesen- chyme.

In the neurula-stage avian embryo, all paraxial and lateral mesodermal populations contain precursors of vascular endothelial cells, which can be detected in chimaeric embryos using anti-quail endothelial anti- bodies. Some of these angioblasts differentiate in situ, contributing directly to pre-existing vessels or forming isolated, nonpatent, cords that subsequently vesiculate and fuse with nearby vessels. Many angioblasts mi- grate in all directions, invading embryonic mesenchy- mal and epithelial tissues and participating in new blood vessel formation in distant sites.

The interactions leading to proper spatial pattern- ing of craniofacial skeletal, muscular, vascular and peripheral neural tissues has been studied by perform- ing heterotopic transplants of each of these mesoder- mal and neural crest populations. The results consist- ently indicate that connective tissue precursors, regardless of their origin, contain spatial information used by the precursors of muscles and blood vessels and by outgrowing peripheral nerves. Some of these connective tissue precursors (e.g. the neural crest, paraxial mesoderm) acquire their spatial pro- gramming while in association with the central ner- vous system or developing sensory epithelia (e.g. otic, optic, nasal epithelia).
Steroid-induced cell proliferation in vivo is associated with increased c-myc proto-oncogene transcript abundance
Authors: Rempel Sandra A. , Johnston Randal N.
Published: April 2021

Enhanced c-myc transcript abundance has been observed in a variety of human malignancies, in normal liver tissue induced to proliferate in vivo by partial hepatectomy and in cells in culture induced to proliferate with the addition of protein hormones and growth factors. Little is known, however, about the expression of cellular proto-oncogenes in cells induced to proliferate in vivo by steroid hormones. Experiments reported here indicate that when cells of the immature chicken oviduct are induced to undergo rapid in vivo proliferation by application of the estrogen hormone 17 β-estradiol, the onset of this proliferation is associated with a rapid, large, and transient increase in c-myc transcript abundance. When estrogen is administered to chickens in which the oviduct has already differentiated, neither massive cell proliferation nor large increases in c-myc transcript abundance are induced. We conclude that the abundance of c-myc transcripts in vivo correlates well with the degree of cell proliferation induced by steroid hormone.
Ectopic expression of homeotic genes caused by the elimination of the Polycomb gene in Drosophila imaginal epidermis
Authors: Busturia Ana , Morata Gines
Published: April 2021

The morphological patterns in the adult cuticle of Drosophila are determined principally by the homeotic genes of the bithorax and Antennapedia complexes. We find that many of these genes become indiscriminately active in the adult epidermis when the Pc gene is eliminated. By using the Pc3 mutation and various BX-C mutant combinations, we have generated clones of imaginal cells possessing different combinations of active homeotic genes.

We find that, in the absence of BX-C genes, Pc− clones develop prothoracic patterns; this is probably due to the activity of Sex combs reduced which overrules Antennapedia. Adding contributions of Ultrabithorax, abdominal-A and Abdominal-B results in thoracic or abdominal patterns. We have established a hierarchical order among these genes: Antp < Scr < Ubx < abd-A < Abd-B. In addition, we show that the engrailed gene is ectopically active in Pc− imaginal cells.
Somitomeres: mesodermal segments of vertebrate embryos
Authors: Jacobson Antone G.
Published: April 2021

Well before the somites form, the paraxial mesoderm of vertebrate embryos is segmented into somitomeres. When newly formed, somitomeres are patterned arrays of mesenchymal cells, arranged into squat, bilaminar discs. The dorsal and ventral faces of these discs are composed of concentric rings of cells. Somitomeres are formed along the length of the embryo during gastrulation, and in the segmental plate and tail bud at later stages. They form in strict cranial to caudal order. They appear in bilateral pairs, just lateral to Hensen’s node in the chick embryo. When the nervous system begins to form, the brain parts and neuromeres are in a consistent relationship to the somitomeres. Somitomeres first appear in the head, and the cranial somitomeres do not become somites, but disperse to contribute to the head the same cell types contributed by somites in the trunk region. In the trunk and tail, somitomeres gradually condense and epithelialize to become somites. Models of vertebrate segmentation must now take into account the early presence of these new morphological units, the somitomeres.

Somitomeres were discovered in the head of the chick embryo (Meier, 1979), with the use of stereo scanning electron microscopy. The old question of whether the heads of the craniates are segmented is now settled, at least for the paraxial mesoderm. Somitomeres have now been identified in the embryos of a chick, quail, mouse, snapping turtle, newt, anuran (Xenopus) and a teleost (the medaka). In all forms studied, the first pair of somitomeres abut the prosencephalon, but caudal to that, for each tandem pair of somitomeres in the amniote and teleost, there is but one somitomere in the amphibia. The mesodermal segments of the shark embryo are arranged like those of the amphibia.
The generation of periodic pattern during early Drosophila embryogenesis
Authors: Howard Ken
Published: April 2021

The first indication of the formation of segment primordia in Drosophila is expression of the segment-polarity genes in particular parts of each primordium. These patterns are controlled by another class of genes, the pair-rule genes, which show characteristic two-segment periodic expression. Each pair-rule gene has a unique domain of activity and in one view different combinations of pair-rule gene products directly control the expression of the segment-polarity genes.

There is a hierarchy within the pair-rule class revealed by pair-rule gene interactions. It is unlikely that these interactions generate the periodicity de novo. Instead, pair-rule genes respond to positional information generated by a system involving zygotic gap and maternal coordinate genes. In this paper, I will concentrate on the problem of the mechanism that generates these pair-rule patterns, the first periodic ones seen during segmentation. I will review and discuss some of the relevant literature, illustrating certain points with data from my recent work.
Functional differentiation and alveolar morphogenesis of primary mammary cultures on reconstituted basement membrane
Authors: Barcellos-Hoff M. H. , Aggeler J. , Ram T. G. , Bissell M. J.
Published: April 2021

An essential feature of mammary gland differentiation during pregnancy is the formation of alveoli composed of polarized epithelial cells, which, under the influence of lactogenic hormones, secrete vectorially and sequester milk proteins. Previous culture studies have described either organization of cells polarized towards lumina containing little or no demonstrable tissue-specific protein, or establishment of functional secretory cells exhibiting little or no glandular architecture. In this paper, we report that tissue-specific vectorial secretion coincides with the formation of functional alveoli-like structures by primary mammary epithelial cells cultured on a reconstituted basement membrane matrix (derived from Engelbreth-Holm-Swarm murine tumour). Morphogenesis of these unique three-dimensional structures was initiated by cell-directed remodelling of the exogenous matrix leading to reorganization of cells into matrix-ensheathed aggregates by 24 h after plating. The aggregates subsequently cavitated, so that by day 6 the cells were organized into hollow spheres in which apical cell surfaces faced lumina sealed by tight junctions and basal surfaces were surrounded by a distinct basal lamina.

The profiles of proteins secreted into the apical (luminal) and basal (medium) compartments indicated that these alveoli-like structures were capable of an appreciable amount of vectorial secretion. Immunoprecipitation with a broad spectrum milk antiserum showed that more than 80% of caseins were secreted into the lumina, whereas iron-binding proteins (both lactoferrin and transferrin) were present in comparable amounts in each compartment. Thus, these mammary cells established protein targeting pathways directing milk-specific proteins to the luminal compartment. A time course monitoring secretory activity demonstrated that establishment of tissue-specific vectorial secretion and increased total and milk protein secretion coincided with functional alveolar-like multicellular architecture.

This culture system is unique among models of epithelial cell polarity in that it demonstrates several aspects of epithelial cell polarization: vectorial secretion, apical junctions, a sequestered compartment and formation of a basal lamina. These lumina-containing structures therefore reproduce the dual role of mammary epithelia to secrete vectorially and to sequester milk proteins. Thus, in addition to maintaining tissue-specific cytodifferentiation and function, a basement membrane promotes the expression of tissue-like morphogenesis.
Expression of intermediate filament proteins during development of Xenopus laevis: I.cDNA clones encoding different forms of vimentin
Authors: Herrmann Harald , Fouquet Bernadette , Franke Werner W.
Published: April 2021

To provide a basis for studies of the expression of genes encoding the diverse kinds of intermediate-filament (IF) proteins during embryogenesis of Xenopus laevis we have isolated and characterized IF protein cDNA clones. Here we report the identification of two types of Xenopus vimentin, Viml and Vim4, with their complete amino acid sequences as deduced from the cloned cDNAs, both of which are expressed during early embryogenesis. In addition, we have obtained two further vimentin cDNAs (Vim2 and 3) which are sequence variants of closely related Viml. The high evolutionary conservation of the amino acid sequences (Viml: 458 residues; Mr ∼ 52800; Vim4: 463 residues; Mr∼ 53500) to avian and mam malian vimentin and, to a lesser degree, to desmin from the same and higher vertebrate species, is emphasized, including conserved oligopeptide motifs in their head domains. Using these cDNAs in RNA blot and ribonu clease protection assays of various embryonic stages, we observed a dramatic increase of vimentin RNA at stage 14, in agreement with immunocytochemical results ob tained with antibody VIM-3B4. The significance of very weak mRNA signals detected in earlier stages is dis cussed in relation to negative immunocytochemical re sults obtained in these stages. The first appearance of vimentin has been localized to a distinct mesenchymal cell layer underlying the neural plate or tube, respect ively. The results are discussed in relation to programs of de novo synthesis of other cytoskeletal proteins in amphibian and mammalian development.
Spots and stripes: the patterning spectrum in the cellular slime mould Polysphondylium pallidum
Authors: Mcnally James G. , Cox Edward C.
Published: April 2021

Whorls of the cellular slime mould Polysphondylium pallidum originate as spherical cell masses that during normal morphogenesis produce tips only at equidistant positions around their equator. We have observed a series of new patterns in whorls that differ from normal whorls only in that they are larger or more elongated. Among the novel patterns found were arrays of tips distributed fairly regularly over the whole whorl surface, as well as striped patterns detected at earlier stages with a tip-specific antigen. These altered patterns demonstrate that a whorl’s size and shape are by themselves important factors in pattern determination. We have compared the range of observed patterns to those predicted by a variety of different theories. We find that while no one theory can account in detail for all of our observations, predictions based on Turing’s scheme of pattern formation come the closest.
Identification of an adult-specific glial progenitor cell
Authors: Wolswijk Guus , Noble Mark
Published: April 2021

We have found that glial progenitor cells isolated from the optic nerves of adult rats are fundamentally different from their counterparts in perinatal animals. In our studies on bipotential oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, we have seen that O-2Aadutt progenitor cells can be distinguished from O-2Aperinatal progenitors by their morphology and antigenic pheno type, their much longer cell cycle time (65 h versus 18 h), slower rate of migration rate (4μmh-1versus 21μm h–1), and their time course of differentiation into oligodendrocytes or type-2 astrocytes in vitro (⩽3 days versus >5 days). At least some of the differences between 0–2Aadult and 0–2Aperinatal progenitor cells appear to be clearly related to the differing cellular requirements of the adult and perinatal central nervous system (CNS).

The properties of the 0–2Aadult progenitor cells may make these cells ideally suited for the needs of the adult CNS, where rapid exponential increases in the number of oligodendrocytes and 0–2A progenitor cells would be inappropriate. However, the properties of the 0–2Aadult progenitor cells are such that they may not be able to replace oligodendrocytes in sufficient numbers to repair extensive or recurrent damage in the adult brain, such as in patients suffering from the human demyelinating disease multiple sclerosis. Moreover, available information about other tissues suggests that the transition from perinatal to adult progenitor cell types may rep resent a developmental mechanism of general import ance.
Ammonia depletion and DIF trigger stalk cell differentiation in intact Dictyostelium discoideum slugs
Authors: Wang Mei , Schaap Pauline
Published: April 2021

The differentiation-inducing factor, DIF, was previously shown to induce stalk cell differentiation in Dictyostelium discoideum cells incubated as submerged monolayers. We investigated the mechanism that regulates the differentiation of stalk cells in the intact organism. It was found that in migrating or submerged slugs DIF cannot induce stalk cell differentiation, which is most likely due to the presence of a DIF antagonist. Cyclic AMP and ammonia were earlier reported to act as DIF antagonists in vitro. We show here that ammonia, but not cAMP, acts as an antagonist for DIF-induced stalk cell differentiation in vivo. DIF can induce stalk cell differentiation when ammonia levels in the slug are enzymically depleted. However, depletion of cAMP levels does not increase the efficacy of DIF. We propose that the induction of stalk cell differentiation during early culmination may be triggered by a drop in ammonia levels inside the organism.
Locomotory competence and laminin-specific cell surface binding sites are lost during myoblast differentiation
Authors: Goodman Simon L. , Deutzmann Rainer , Nurcombe Victor
Published: April 2021

The specific interaction of embryonal cells with the extracellular matrix (ECM) is one of the principal forces influencing embryonal development (Hay, 1984; Trinkaus, 1984). We used a muscle satellite cell line (MM14dy) to determine the relationship between locomotory response to laminin and the expression of specific cell surface binding sites for it.

Time lapse videomicroscopic analysis was used to study the locomotory response and radioligand binding assays and cell attachment assays were used to follow the expression levels of binding sites for laminin and its subfragments E8 and El –4. We report here the novel finding that the ability of MM14dy to locomote over laminin diminishes and finally vanishes as the cells differentiate. The simultaneous drop in expression of binding sites for laminin is interpreted as being of potential significance during development and repair.
Consequences of somite manipulation on the pattern of dorsal root ganglion development
Authors: Kalcheim Chaya , Teillet Marie-Aimée
Published: April 2021

We have investigated dorsal root ganglion formation, in the avian embryo, as a function of the composition of the paraxial somitic mesoderm. Three or four contiguous young somites were unilaterally removed from chick embryos and replaced by multiple cranial or caudal halfsomites from quail embryos. Migration of neural crest cells and formation of DRG were subsequently visualized both by the HNK-1 antibody and the Feulgen nuclear stain.

At advanced migratory stages (as defined by Teillet et al. Devi Biol. 120, 329 – 347 1987), neural crest cells apposed to the dorsolateral faces of the neural tube were distributed in a continuous, nonsegmented pattern that was indistinguishable on unoperated sides and on sides into which either half of the somites had been grafted. In contrast, ventrolaterally, neural crest cells were distributed segmentally close to the neural tube and within the cranial part of each normal sclerotome, whereas they displayed a nonsegmental distribution when the graft involved multiple cranial half-somites or were virtually absent when multiple caudal half-somites had been implanted.

In spite of the identical dorsal distribution of neural crest cells in all embryos, profound differences in the size and segmentation of DRG were observed during gangliogenesis (E4 – 9) according to the type of graft that had been performed. Thus when the implant consisted of compound cranial half-somites, giant, coalesced ganglia developed, encompassing the entire length of the graft. On the other hand, very small, dorsally located ganglia with irregular segmentation were seen at the level corresponding to the graft of multiple caudal halfsomites. We conclude that normal morphogenesis of dorsal root ganglia depends upon the craniocaudal integrity of the somites.
Alternative splicing of fibronectin is temporally and spatially regulated in the chicken embryo
Authors: Ffrench-Constant Charles , Hynes Richard O.
Published: April 2021

The primary gene transcript for the adhesive extracellular matrix glycoprotein fibronectin (FN) is alternatively spliced in three regions (EIIIA, EIIIB and V). At least one of these regions (V) has been shown to encode cell-binding sites, suggesting that splicing represents a mechanism to create functionally different forms of FN at different times and places. In order to test this hypothesis, we have examined the extent of alternative splicing of fibronectin during embryonic development. The distribution of the different spliced forms of FN mRNA in developing chicken embryos was determined using probes specific for the spliced regions in ribonuclease protection and in situ hybridization experiments. At embryonic day 2–4 (E2–4), all three spliced regions were included wherever FN mRNA was detected. At E16, however, we found spatially distinct splicing differences within the embryo, with cell-type-specific splicing excluding EIIIA and/or EIIIB in some tissues. In contrast, we did not detect exclusion of the V region. In a more detailed developmental study of the simplest of these tissues, the chorioallantoic membrane, we found that EIIIB was preferentially excluded after the completion of growth. These results suggest that FN splicing is used during development as a mechanism to create different forms of FN within the extracellular matrix by the inclusion or exclusion of specific segments. The data are consistent with an essential role for one of these segments, E111B, in the migration and/or proliferation of embryonic cells prior to their terminal differentiation and also suggest possible roles for the EIIIA segment.
Analysis of lens cell fate and eye morphogenesis in transgenic mice ablated for cells of the lens lineage
Authors: Breitman Martin L. , Bryce Dawn Marie , Giddens Elizabeth , Clapoff Susan , Goring Daphne , Tsui Lap-Chee , Klintworth Gordon K. , Bernstein Alan
Published: April 2021

Transgenic mice carrying the diphtheria toxin A gene driven by mouse γ2-crystallin promoter sequences manifest microphthalmia due to ablation of fiber cells in the ocular lens. Here we map ablation events in the lens by crossing animals hemizygous for the ablation construct with transgenic mice homozygous for the in situ lacT reporter gene driven by identical γ2 crystallin promoter sequences. By comparing the spatial distribution of tacZ-expressing cells and the profile of γ-crystallin gene expression in the lenses of normal and microphthalmic offspring, the contributions of specific cell types to lens development were examined. The results suggest that phenotypically and developmentally distinct populations of lens fiber cells are able to contribute to the lens nucleus during organogenesis. We also show that dosage of the transgene and its site of integration influence the extent of ablation. In those mice homozygous for the transgene and completely lacking cells of the lens lineage, we show that the sclera, cornea, and ciliary epithelium are reduced in size but, otherwise, reasonably well formed. In contrast, the anterior chamber, iris, and vitreous body are not discernible while the sensory retina is highly convoluted and extensively fills the vitreous chamber.
The oral-aboral axis of a sea urchin embryo is specified by first cleavage
Authors: Cameron R. Andrew , Fraser Scott E. , Britten Roy J. , Davidson Eric H.
Published: April 2021

Several lines of evidence suggest that the oral–aboral axis in Strongylocentrotus purpuratus embryos is specified at or before the 8-cell stage. Were the oral-aboral axis specified independently of the first cleavage plane, then a random association of this plane with the blastomeres of the four embryo quadrants in the oral-aboral plane (viz. oral, aboral, right and left) would be expected. Lineage tracer dye injection into one blastomere at the 2-cell stage and observation of the resultant labeling patterns demonstrates instead a strongly non-random association. In at least ninety percent of cases, the progeny of the aboral blastomeres are associated with those of the left lateral blastomeres and the progeny of the oral blastomeres with the right lateral ones, respectively. Thus, ninety percent of the time the oral pole of the future oral-aboral axis lies 45° clockwise from the first cleavage plane as viewed from the animal pole. The nonrandom association of blastomeres after labeling of the 2-cell stage implies that there is a mechanistic relation between axis specification and the positioning of the first cleavage plane.
Mouse neural crest cells secrete both urokinase-type and tissue-type plasminogen activators in vitro
Authors: Menoud P. A. , Debrot S. , Schowing J.
Published: April 2021

Neural tubes of E8·5 day mouse embryos were dissected and cultured in serum substitute-supplemented medium to allow the emigration of neural crest cells. After 48 h of culture the neural tubes were removed. The neural crest cells were then cultured for 12 h in serum-free medium, and their culture supernatant was studied by electro-phoresis and zymography. The cultured cells were shown to secrete both urokinase-type and tissue-type plasminogen activators. When the truncal neural tube was divided in four equal segments, the secretion pattern of the two types of plasminogen activators was similar for the cells from the three most anterior segments; cells having migrated from the most caudal one, i.e. consisting of the neural plate, secreted a higher level of urokinase-type plasminogen activator. The secretion in vitro of plasminogen activators by neural crest cells is in accord with the postulated importance of these proteases in cellular migration.
Cellular contacts required for neural induction in Xenopus embryos: evidence for two signals
Authors: Dixon Jane E. , Kintner Chris R.
Published: April 2021

Neurogenesis begins in amphibian embryos around the time of gastrulation when a portion of the ectoderm receives an inducing signal from dorsal mesoderm. Two different proposals have been made for how ectoderm must come into contact with dorsal mesoderm in order for the inducing signal to pass between the two tissues. Induction in one proposal would require normal gastru-lation movements to bring dorsal mesoderm under-neath, and into apposition with, the overlying ectoderm. The inducing signal in this case would pass between dorsal mesoderm and ectoderm as apposed tissue layers. The other proposal is that induction requires only a small contact between ectoderm and dorsal mesoderm at the boundary they share before gastrulation. The in-ducing signal by this proposal would pass laterally across this small area of contact between mesoderm and ectoderm, perhaps before gastrulation, and spread within the ectodermal cell layer. Since it is not known to what extent neurogenesis depends on each of these proposed contacts between ectoderm and dorsal meso-derm, we have generated explants of embryonic tissue in which one or the other type of contact between meso-derm and ectoderm is favored. The amount of neural tissue formed under these various conditions was then assessed using a quantitative RNase protection assay to measure the levels of two neural-specific RNA tran-scripts. The results show that neural tissue forms efficiently when ectoderm and dorsal mesoderm only interact laterally within a plane of tissue. In contrast, neural tissue, forms extremely poorly when ectoderm is placed experimentally in apposition with involuting, anterior-dorsal mesoderm. Finally, a synergistic effect is obtained when both types of contacts can occur between ectoderm and mesoderm indicating that two signals may be required for neural induction in Xenopus embryos.
A vital dye analysis of the timing and pathways of avian trunk neural crest cell migration
Authors: Serbedzija George N. , Bronner-Fraser Marianne , Fraser Scott E.
Published: April 2021

To permit a more detailed analysis of neural crest cell migratory pathways in the chick embryo, neural crest cells were labelled with a nondeleterious membrane intercalating vital dye, Dil. All neural tube cells with endfeet in contact with the lumen, including premigratory neural crest cells, were labelled by pressure injecting a solution of Dil into the lumen of the neural tube. When assayed one to three days later, migrating neural crest cells, motor axons, and ventral root cells were the only cells types external to the neural tube labelled with Dil. During the neural crest cell migratory phase, distinctly labelled cells were found along: (1) a dorsolateral pathway, under the epidermis, as well adjacent to and intercalating through the dermamyotome; and (2) a ventral pathway, through the rostral portion of each sclerotome and around the dorsal aorta as described previously. In contrast to those cells migrating through the sclerotome, labelled cells on the dorsolateral pathway were not segmentally arranged along the rostrocaudal axis. Dil-labelled cells were observed in all truncal neural crest derivatives, including subepidermal presumptive pigment cells, dorsal root ganglia, and sympathetic ganglia. By varying the stage at which the injection was performed, neural crest cell emigration at the level of the wing bud was shown to occur from stage 13 through stage 22. In addition, neural crest cells were found to populate their derivatives in a ventral-to-dorsal order, with the latest emigrating cells migrating exclusively along the dorsolateral pathway.
Regeneration and pattern formation in planarians: II. Local origin and role of cell movements in blastema formation
Authors: Saló Emili , Baguñá Jaume
Published: April 2021

In planarians, blastema cells do not divide, and growth of blastema is thought to result from the steady accumulation, beneath the wound epithelium, of undifferentiated cells produced by cell division in the stump. However, whether these cells come only from local stump sources or whether cells placed far from the wound can also participate, after long-range migrations, in the growth of blastema, is still uncertain.

To study this problem, we have analysed three basic parameters of the process of regeneration: cell kinetics of blastema growth; number of cells produced by mitosis in the stump areas near the wound (postblastema); and rates of movement (‘migration’) of undifferentiated cells using grafting procedures with nuclear and chromosomal markers.

The results show that: (1) cells near the wound area spread (move) ar higher rates than cells placed far from it (90–140 μm day− 1versus 40–50/ μm day− 1); (2) cells originally placed farther than 500 fan from the wound boundary are hardly represented within 3- and 5-day-old blastemata; and (3) the number of cells produced by mitosis within a 200-300 fun postblastema area around the wound seem sufficient to explain, provided their rates of movement are taken into account, the increasing number of blastema cells. From this, it is concluded that blastema cells in planarians originate from local stump areas, and that mitotic activity jointly with local cell movement within a 200–300 μm postblastema area around the wound match the increasing number of blastema cells during regeneration. The implications of these results for blastema growth and pattern formation mechanisms are discussed.
Modifications of oogenesis and development in marsupial frogs
Authors: del Pino Eugenia M.
Published: April 2021

Early development in animals is guided by the RNA reserves of the egg (see Davidson, 1986 for a review), and therefore variations in eggs might correlate with changes in their patterns of early development. Frogs are organisms especially suitable for analyzing the relationship of oogenesis to development, because many frog species have evolved distinctive changes to avoid or diminish their period of aquatic dependence (Lamotte and Lescure, 1977; Duellman and Trueb, 1986). The analysis of the developmental adaptations that accompany the modes of frog reproduction, however, is hindered by the fact that the best investigated frogs, such as Xenopus laevis or Rana pipiens, have aquatic reproduction and similar modes of oogenesis and early development, although the former is in a family considered to be primitive (Pipidae), with adult specializations for an extreme aquatic life-style, whereas the latter is in an advanced family (Ranidae), with more pronounced terrestrial adaptations in the adult (Table 1). The modes of reproduction found in the 21 living families of frogs is shown in Table 1. The traditional order of anuran classification has been maintained in Table 1, but the higher taxa are not given. In an ancient and diverse group, such as the anurans, there might be many cases of similarity due to derived features, and the current knowledge of many characters and their evolutionary change does not allow the reconstruction of the anuran phylogeny (Duellman and Trueb, 1986).
Mesoderm-inducing factors and Spemann’s organiser phenomenon in amphibian development
Authors: Cooke Jonathan
Published: April 2021

Certain proteins from ‘growth factor’ families can initiate mesodermal development in animal cap cells of the amphibian blastula. Cells that are in early stages of their response to one such factor, XTC-MIF (Smith et al. 1988), initiate the formation of a new axial body plan when grafted to the ventral marginal zone of a similarly aged host embryo (Cooke et al. 1987). This replicates the natural control of this phase of development by the dorsal blastoporal lip when similarly grafted; the classical ‘organiser’ phenomenon. I have explored systematically the effect, upon the outcome of this pattern formation using defined inducing factors, of varying graft size, XTC-MIF concentration to which graft cells were exposed, length of exposure before grafting, and host age. The ‘mesodermal organiser’ status, evoked by the factor, appears to be stable, and the variables most influencing the degree of completeness and orderliness of second patterns are graft size and factor concentration.

Inappropriately large grafts are not effective. A Xenopus basic fibroblast growth factor homologue, present in the embryo and known to be a strong inducer but of mesoderm with a different character from that induced by XTC-MIF, produced no episode of pattern formation at all when tested in the procedure described in this paper. Organiser status of grafts that have been exposed to mixtures of the two factors is set entirely by the supplied XTC-MIF concentration. Lineage labelling of these grafts, and of classical dorsal lip grafts, reveals closely similar though not identical patterns of contribution to the new structure within the host. Implications of the results for the normal mechanism of body pattern formation are discussed.
The mouse Hox-1.4 gene: primary structure, evidence for promoter activity and expression during development
Authors: Galliot Brigitte , Dollé Pascal , Vigneron Marc , Featherstone Mark S. , Ron Agnes Ba , Duboule Denis
Published: April 2021

This study reports the structure of the mouse homeobox-containing gene Hox-1.4 of the HOX-1 cluster, as well as its expression pattern during embryonic and fetal development. The overall structure of this gene includes two major exons, the second of which encodes the homeo-domain. The putative Hox-1.4 protein displays similarities with products of homologous genes located at the same relative positions in other HOX clusters. A fragment extending 360 base pairs (bp) upstream of a transcriptional start site was shown to be able to promote transcription in transfected cells. This fragment is GC-rich and contains binding sites for the Spl transcription factor. In situ hybridization studies revealed the Hox-1.4 expression pattern during development. As already reported for several other murine Hox genes, Hox-1.4 is expressed in the fetal central nervous system (CNS), in structures derived from somitic mesodermal condensations (sclerotomes, prevertebrae) as well as in several mesodermal components of various organs and structures such as lungs, gut, stomach, intestine and meso- and metanephros. This expression pattern is in good agreement with recent proposals concerning the involvement of such genes in the establishment of the vertebrate body plan as well as the relationship between the positions of these genes within their clusters and the anteroposterior restriction of their expression domains.
Nuclear transplantation of male primordial germ cells in the mouse
Authors: Tsunoda Y. , Tokunaga T. , Imai H. , Uchida T.
Published: April 2021

We examined the developmental ability of enucleated eggs receiving embryonic nuclei and male primordial germ cells (PGCs) in the mouse. Reconstituted eggs developed into the blastocyst stage only when an earlier 2-cell nucleus was transplanted (36 %) but very rarely if the donor nucleus was derived from a later 2-cell, 8-cell, or inner cell mass of a blastocyst (0–3%). 54–100%, 11–67 %, 6–43 % and 6–20 % of enucleated eggs receiving male PGCs developed to 2-cell, 4-cell, 8-cell and blastocyst stage, respectively, in culture. The overall success rate when taking into account the total number of attempts at introducing germ cells was actually 0-6 %. Live fetuses were not obtained after transfer of reconstituted eggs to recipients, although implantation sites were observed.

The developmental ability of reconstituted eggs in relation to embryonic genome activation and genomic imprinting is discussed.
The human blastocyst: cell number, death and allocation during late preimplantation development in vitro
Authors: Hardy Kate , Handyside Alan H. , Winston Robert M. L.
Published: April 2021

The development of 181 surplus human embryos, including both normally and abnormally fertilized, was observed from day 2 to day 5, 6 or 7 in vitro. 63/149 (42 %) normally fertilized embryos reached the blastocyst stage on day 5 or 6. Total, trophectoderm (TE) and inner cell mass (ICM) cell numbers were analyzed by differential labelling of the nuclei with polynucleotide specific fluorochromes. The TE nuclei were labelled with one fluorochrome during immunosurgical lysis, before fixing the embryo and labelling both sets of nuclei with a second fluorochrome (Handyside and Hunter, 1984, 1986). Newly expanded normally fertilized blastocysts on day 5 had a total of 58.3±8.1 cells, which increased to 84.4±5.7 and 125.5±19 on days 6 and 7, respectively. The numbers of TE cells were similar on days 5 and 6 (37.9±6.0 and 40.3±5.0, respectively) and then doubled on day 7 (80.6±15.2). In contrast, ICM cell numbers doubled between days 5 and 6 (20.4±4.0 and 41.9±5.0, respectively) and remained virtually unchanged on day 7 (45.6±10.2). There was widespread cell death in both the TE and ICM as evidenced by fragmenting nuclei, which increased substantially by day 7. These results are compared with the numbers of cells in morphologically abnormal blastocysts and blastocysts derived from abnormally fertilized embryos. The nuclei of arrested embryos were also examined. The number of TE and ICM cells allocated in normally fertilized blastocysts appears to be similar to the numbers allocated in the mouse. Unlike the mouse, however, the proportion of ICM cells remains higher, despite cell death in both lineages.
The zygotic control of Drosophila pair-rule gene expression: II. Spatial repression by gap and pair-rule gene products
Authors: Carroll Sean B. , Vavra Stephanie H.
Published: April 2021

We examined gene expression patterns in certain single and double pair-rule mutant embryos to determine which of the largely repressive pair-rule gene interactions are most likely to be direct and which interactions are probably indirect. From these studies we conclude that: (i) hairy+ and even-skipped (eve+) regulate the fushi tarazu (ftz) gene; (ii) eve+ and runt+ regulate the hairy gene; (iii) runt+ regulates the eve gene; but, (iv) runt does not regulate the ftz gene pattern, and hairy does not regulate the eve gene pattern. These pair-rule interactions are not sufficient, however, to explain the periodicity of the hairy and eve patterns, so we examined specific gap gene mutant combinations to uncover their regulatory effects on these two genes. Our surprising observation is that the hairy and eve genes are expressed in embryos where the three key gap genes hunchback (hb), Krüppel (Kr), and knirps (kni) have been removed, indicating that these gap genes are not essential to activate the pair-rule genes. In fact, we show that in the absence of either hb+ or kni+, or both gap genes, the Kr+ product represses hairy expression. These results suggest that gap genes repress hairy expression in the interstripe regions, rather than activate hairy expression in the stripes. The molecular basis of pair-rule gene regulation by gap genes must involve some dual control mechanisms such that combinations of gap genes affect pair-rule transcription in a different manner than a single gap gene.
Dynamics of Drosophila eye development and temporal requirements of sevenless expression
Authors: Basler Konrad , Hafen Ernst
Published: April 2021

The development of the compound eye of Drosophila consists of a linear, stereotyped program starting at the posterior end of the eye imaginal disc and progressing towards the anterior border. The determination of the R7 photoreceptor cells is part of this process and is dependent on the sevenless gene. In this study, we used a heat-shock-inducible sevenless gene as a conditional allele to determine the exact temporal requirements of sevenless gene expression and to reveal the stages of ommatidial development during which the presumptive R7 cell can respond to the presence of sevenless protein.

Our results indicate that sevenless gene function is only required during a brief, defined period for the initiation of R7 development; subsequently sevenless is dispensable for both differentiation and function of the R7 photoreceptors. Furthermore, using rescue of R7 cells as an internal marker to monitor the progression of eye development we could examine when and at what rate ommatidial columns form.
Myonuclear birthdates distinguish the origins of primary and secondary myotubes in embryonic mammalian skeletal muscles
Authors: Harris A. J. , Duxson M. J. , Fitzsimons R. B. , Rieger F.
Published: April 2021

Myotubes were isolated from enzymically disaggregated embryonic muscles and examined with light microscopy. Primary myotubes were seen as classic myotubes with chains of central nuclei within a tube of myofilaments, whereas secondary myotubes had a smaller diameter and more widely spaced nuclei. Primary myotubes could also be distinguished from secondary myotubes by their specific reaction with two monoclonal antibodies (MAbs) against adult slow myosin heavy chain (MHC). Myonuclei were birthdated with [3H]thymidine autoradiography or with 2-bromo-5 ′-deoxyuridine (BrdU) detected with a commercial monoclonal antibody. After a single pulse of label during the 1 –2 day period when primary myotubes were forming, some primary myotubes had many myonuclei labelled, usually in adjacent groups, while in others no nuclei were labelled. If a pulse of label was administered after this time labelled myonuclei appeared in most secondary myotubes, while primary myotubes received few new nuclei. Labelled and unlabelled myonuclei were not grouped in the secondary myotubes, but were randomly interspersed. We conclude that primary myotubes form by a nearly synchronous fusion of myoblasts with similar birthdates. In contrast, secondary myotubes form in a progressive fashion, myoblasts with asynchronous birthdates fusing laterally with secondary myotubes at random positions along their length. These later-differentiating myoblasts do not fuse with primary myotubes, despite being closely apposed to their surface. Furthermore, they do not generally fuse with each other, as secondary myotube formation is initiated only in the region of the primary myotube endplate.
The function of PS integrins in Drosophila wing morphogenesis
Authors: Wilcox Michael , Diantonio Aaron , Leptin Maria
Published: April 2021

Integrins are found on many cell types during the development of most organisms. In Drosophila their functions can be analysed genetically. An analysis of lethal mutations in a PS integrin gene showed that the integrins were required for muscle attachment and for certain cell sheet migrations during embryogenesis. In this paper we use viable mutations in integrin component genes to look at integrin function in the later stages of development of one adult structure, the wing. We show that two known viable mutations, one which has its primary effect on the fly’s escape response, the other on wing morphogenesis, are mutations in the ß and PS2α subunits, respectively, of the PS integrins. The mutation non-jumper (mysnj42) in the ß subunit leads to wasting of the thoracic jump muscles. Flies in which the dosage of this allele is reduced (and no wildtype copy is present) show defects also in wing morphogenesis. The two surfaces of the wing fail to connect properly, resulting in ‘blistering’ of the wing and the formation of extra crossveins. The mutation in the gene for the PS2α integrin subunit, inflated, also leads to a failure in wing surface apposition and consequent wing blistering. When the two mutations are combined, the mutant phenotype is greatly enhanced. Thus, one of the roles of the PS integrins in late Drosophila development is to ensure the correct apposition and patterning of the wing epithelia.