bluuebunny/crossref_metadata_2025 huggingface dataset:
The molecular basis of positional signalling: introduction
Authors: Kay R. R. , Smith J. C.
Published: April 2021

The importance of cell–cell interactions in embryonic development was first described by Driesch (1891), who showed that any of the blastomeres of the 2-cell or 4-cell sea-urchin embryo is capable of forming a complete embryo if cultured in isolation; this implied that in normal development each blastomere is aware of the other and will only form a half- or quarter-embryo, as appropriate. And it was only ten years later that Spemann (1901) discovered the phenomenon of embryonic induction, recently reviewed by Gurdon (1987) and defined as an interaction in which the differentiation of one group of cells is affected by a signal from an adjacent group. Thus the significance of cell signalling during development has been appreciated for almost a century, but, as has frequently been remarked, progress in the molecular analysis of the phenomenon has been slow compared with that in the younger disciplines of, for example, immunology and molecular biology.
Models for positional signalling with application to the dorsoventral patterning of insects and segregation into different cell types
Authors: Meinhardt Hans
Published: April 2021

Models of pattern formation and possible molecular realizations are discussed and compared with recent experimental observations. In application to the dorsoventral patterning of insects, it is shown that a superposition of two pattern-forming reactions is required. The first system generates the overall dorsoventral polarity of the oocyte, the second generates the positional information proper with a stripe-like region of high concentration along the ventral side of the embryo. A single reaction would be insufficient since the two reactions require different parameters. The model accounts for the orientation of the DV axes of the oocytes in the ovary of Musca domestica and Sarcophaga, independent of the DV axis of the mother, for the formation of several ventral furrows in the absence of the primary gurken/ torpedo system in Drosophila, as well as for the good size regulation of the dorsoventral axis as observed in some insect species.

Segregation of a homogeneous cell population into different cell types requires autocatalytic processes that saturate at relatively low concentrations and nondiffusible substances responsible for the autocatalytic feedback loops. Thus, these loops can be realized directly on the gene level via their gene products, for instance, by the mutual repression of two genes. A balance of the two cell types is achieved by a long-ranging substance interfering with the self-enhancing process. This substance is expected to have a more or less homogeneous distribution. This model accounts for the reestablishment of the correct proportion after an experimental interference and the change of determination after transplantation. Applications to the segregation of prestalk and prespore cells in Dictyostelium and of neuroblast cells from the ventral ectoderm in Drosophila are provided.
The process of localizing a maternal messenger RNA in Xenopus oocytes
Authors: Yisraeli Joel K. , Sokol Sergei , Melton D. A.
Published: April 2021

The maternal mRNA Vgl is localized to the vegetal pole during oogenesis in Xenopus. We have cultured oocytes in vitro to begin to understand how this localization occurs. Endogenous Vgl mRNA undergoes localization when oocytes are cultured in vitro, and synthetic Vgl mRNA injected into such oocytes is localized in the same fashion. Vgl mRNA is associated with a detergentinsoluble fraction from homogenized oocytes, suggesting a possible cytoskeletal association. The use of cytoskeletal inhibitors reveals a two-step process for localizing Vgl mRNA. Microtubule inhibitors such as nocodazole and colchicine inhibit the localization of Vgl mRNA in late stage in/early stage IV oocytes, but have no effect on Vgl mRNA once it is localized. The microfilament inhibitor cytochalasin B, however, has little effect on the translocation of Vgl mRNA in middle-stage oocytes but causes a release of the message in late-stage oocytes. We propose a model for the localization of Vgl mRNA in which translocation of the message to the vegetal cortex is achieved via cytoplasmic microtubules and the anchoring of the message at the cortex involves cortical microfilaments.
Cortical rotation of the Xenopus egg: consequences for the anteroposterior pattern of embryonic dorsal development
Authors: Gerhart J. , Danilchik M. , Doniach T. , Roberts S. , Rowning B. , Stewart R.
Published: April 2021

We first review cortical-cytoplasmic rotation, a microtubule-mediated process by which the Xenopus egg, like other amphibian eggs, transforms its polarized cylindrical symmetry into bilateral symmetry within the first cell cycle after fertilization. This transformation, the earliest of many steps leading to dorsal development, involves the displacement of the egg’s cortex relative to its cytoplasmic core by 30° in an animal-vegetal direction. As rotation is progressively reduced by microtubuledepolymerizing agents, embryos develop with body axes progressively deleted for dorsal structures at the anterior end. With no rotation, ventralized embryos are formed. In an effort to comprehend this progressive effect on embryonic organization, we go on to review subsequent developmental processes depending on rotation, and we propose, with evidence, that reduced rotation leads to a reduced number of vegetal dorsalizing cells, which induce during the blastula stage a Spemann organizer region of smaller than normal size. The reduced organizer then promotes a reduced amount of cell rearrangement (morphogenesis) at gastrulation. Reduced morphogenesis seems the proximate cause of the incompleteness of axial pattern, as shown further by the fact that embryos that are normal until the gastrula stage, if exposed to inhibitors of morphogenesis, develop body axes that are progressively less complete in their anterior dorsal organization the earlier their gastrulation had been blocked. We discuss why axial pattern might depend systematically on morphogenesis.
Short-range positional signals in the developing Drosophila eye
Authors: Tomlinson Andrew
Published: April 2021

Positional signals provided by immediate neighbours appear to direct developmental decisions in the eye of Drosophila. By a combined genetic and molecular approach the biochemical bases of the signal and reception mechanisms are being systematically dissected. Three key gene products have now been identified, sevenless is a transmembrane tyrosine kinase probably transducing positional signals that direct the R7 cell to its fate. The bride of sevenless gene product is on the signalling side of the mechanism and is required in R8 for R7 to develop. The type of protein bride of sevenless encodes is not yet known. The rough gene encodes a transcription factor on the signalling side required in R2 and RS for positional signals to be transmitted to neighbouring cells.
Formation and anatomy of the prestalk zone of Dictyostelium
Authors: Williams J. G. , Jermyn K. A. , Duffy K. T.
Published: April 2021

The pDd63 and pDd56 genes encode extracellular matrix proteins which, respectively, surround the migratory slug and mature stalk cells. Both genes are dependent for their expression upon, and rapidly induced by, DIF, the stalk cell inducer. Using these genes as cell-autonomous markers, we have defined three distinct kinds of ‘prestalk’ cells localized to different parts of the anterior region of the slug. At least one, and probably both, prestalk cell types initially differentiates at the base of the aggregate. The most abundant of the two prestalk cell types then migrates into the tip, the precursor of the prestalk zone which arises at the apex of the aggregate. Thus we believe that morphogenesis of the prestalk zone, the primary pattern-forming event in Dictyostelium development, involves a combination of positionally localized differentiation and directed cell migration. To account for the positionally localized différentiation of prestalk cells, we invoke the existence of gradients of the known antagonists of DIF - cAMP and NH3. We further suggest that differences in the motility of pstA and pstB cells might result from differences in their chemotactic responsiveness to cAMP signals propagated from the tip.
The biological effects of XTC-MIF: quantitative comparison with Xenopus bFGF
Authors: Green J. B. A. , Howes G. , Symes K. , Cooke K. J. , Smith J. C.
Published: April 2021

Mesoderm in Xenopus and other amphibian embryos is induced by signals from the vegetal hemisphere acting on equatorial or animal hemisphere cells. These signals are diffusible and two classes of candidate signal molecule have been identified: the fibroblast growth factor (FGF) and transforming growth factor β (TGF-β) types. In this paper, we compare the effects of cloned Xenopus basic FGF (XbFGF) and electophoretically homogeneous XTC-MIF (a TGF-β-like factor obtained from a Xenopus cell line) on animal pole expiants. We find that they have a similar minimum active concentration (0.1−0.2ng ml−1) but that, nonetheless, XTC-MIF is at least 40 times more active in inducing muscle. In general, we find that the two factors cause inductions of significantly different characters in terms of tissue type, morphology, gene expression and timing.

At low concentrations (0.1−1.0ngml-1) both factors induce the differentiation of ‘mesenchyme’ and ‘mesothelium’ as well as blood-like cells. These latter cells do not, however, react with an antibody to Xenopus globin. This raised the possibility that the identification of red blood cells in other studies on mesoderm induction might have been mistaken, but combinations of animal pole regions with ventral vegetal pole regions confirmed that genuine erythrocytes are formed. The identity of the blood-like cells formed in response to the inducing factors remains unknown. At higher concentrations XTC-MIF induces neural tissue, notochord, pronephros and substantial and often segmented muscle. By contrast, XbFGF only induces significant amounts of muscle above 24 ng ml”1 and even then this is much less than that induced by XTC-MIF.

For both factors an exposure of less than 30 min is effective. Competence of animal pole cells to respond to XbFGF is completely lost by the beginning of gastrulation (stage 10) while competence to XTC-MIF is detectable until somewhat later (stage 11). Since animal pole tissue is known to be able to respond to the natural inducer at least until stage 10, and perhaps until stage 10.5, this suggests that bFGF cannot be the sole inducer of mesoderm in vivo.

Taken together, these results are consistent with XTC-MIF being a dorsoanterior inducer and XbFGF a ventroposterior inducer, suggesting that body pattern is established by the interaction of two types of inducing signal. This model is discussed in view of the qualitative and quantitative differences between the factors.
The spatiotemporal distribution of N-CAM in the retinotectal pathway of adult goldfish detected by the monoclonal antibody D3
Authors: Bastmeyer Martin , Schlosshauer Burkhard , Stuermer Claudia A. O.
Published: April 2021

The spatiotemporal distribution of neural cell adhesion molecule (N-CAM) in the retinotectal system of adult goldfish was assessed by immunofluorescence using the monoclonal antibody (Mab) D3 against chick N-CAM. In immunoblots with extracts of cell surface membranes of fish brains, Mab D3 recognized a prominent band at 170K and a weak band at 130K (K=103Mr). N-CAM immunofluorescence on cells was restricted to the marginal growth zones of the retina and the tectum and, in normal fish, to the youngest axons from the new ganglion cells of the peripheral retinal margin. In fish with previously transected optic nerves (ONS), Mab D3 staining was found transiently on all axons from the site of the cut into the retinorecipient layers of the tectum, but disappeared from these axons 450 days after ONS.

Growing retinal axons in vitro exhibited N-CAM immunofluorescence throughout their entire extent, including their growth cones. Glial cells cultured from regenerating optic nerves were, however, unlabeled.

These data are consistent with the idea that N-CAM is involved in adhesive interactions of growing axons. The temporally regulated expression of N-CAM on the new retinal axons may contribute to the creation of the age-related organization of the axons in the retinotectal pathway of fish.
Nuclear transfer from teratocarcinoma cells into mouse oocytes and eggs
Authors: Modlinski J. A. , Gerhäuser D. , Lioi B. , Winking H. , Illmensee K.
Published: April 2021

A spontaneous ovarian teratocarcinoma was isolated from a LT/Sv mouse female and converted into an ascites tumor from which embryonal carcinoma (EC) cells were dissociated. Non-enucleated and enucleated, activated oocytes were fused with EC cells and either cultured in vitro or transferred into ligated oviducts of Swiss/A females. The nucleocytoplasmic hybrids cultured in vitro up to 22 h were examined cytologically at various time intervals. EC nuclei showed morphological remodelling in the foreign cytoplasm. EC chromosomes and female pronuclear chromosomes together formed a common metaphase. The nucleocytoplasmic hybrids developed in vivo were analyzed cytologically between the first and third day after oviduct transfer. The majority of embryos developed abnormally and, in a few instances, they had passed several cleavage divisions and reached, at best, a developmental stage resembling a premature morula. Fertilized, enucleated eggs were fused with EC cells or microinjected with EC nuclei. The resulting nucleocytoplasmic hybrids were either cultured in vitro or in vivo up to the fourth day. Enzyme tests were carried out on the nuclear transplant embryos, using electrophoretic variants of glucose phosphate isomerase (GPI) in order to distinguish between EC nuclei (GPI-A) and recipient eggs (GPI-B). The EC-specific GPI could be detected in about one third of the embryos analyzed and, in several instances, also together with the egg-specific GPI. Most of them were arrested during early cleavage divisions. Some embryos cleaved abnormally or mimicked normal embryogenesis. In a few instances, development resulted in embryos that resembled late preimplantation embryos.
Mechanisms of pigment pattern formation in the quail embryo
Authors: Richardson Michael K. , Hornbruch Amata , Wolpert Lewis
Published: April 2021

One hypothesis to account for pigment patterning in birds is that neural crest cells migrate into all feather papillae. Local cues then act upon the differentiation of crest cells into melanocytes. This hypothesis is derived from a study of the quail-chick chimaera (Richardson et al., Development 107, 805–818, 1989). Another idea, derived from work on larval fish and amphibia, is that pigment patterns arise from the differential migration of crest cells. We want to know which of these mechanisms can best account for pigment pattern formation in the embryonic plumage of the quail wing. Most of the feather papillae on the dorsal surface of the wing are pigmented, while many on the ventral surface are white.

When ectoderm from unpigmented feather papillae is grown in culture, it gives rise to melanocytes. This indicates that neural crest cells are present in white feathers but that they fail to differentiate. If the wing tip is inverted experimentally then the pigment pattern is inverted also. This is difficult to explain in terms of a model based on migratory pathways, unless one assumes that the pathways became re-routed.

When an extra polarizing region is grafted to the anterior margin of the wing bud, a duplication develops in: (1) the pattern of skeletal elements; (2) the pattern of feather papillae; (3) the feather pigment pattern. The pigment pattern was not a precise mirror image although some groups of papillae showed a high degree of symmetry in their pigmentation. Both the tip inversions and the duplications produce discontinuities in the feather and pigment patterns. No evidence of intercalation was found in these cases. We conclude that pigment patterning in birds is determined by local cues acting on melanocyte differentiation, rather than by the differential migration of crest cells. Positional values along the anteroposterior axis of the pigment pattern are determined by a gradient of positional information. Thus the pigment patterns, feather patterns and cartilage patterns of the wing may all be specified by a similar mechanism.
Three markers of adult non-myelin-forming Schwann cells, 217c(Ran-1), A5E3 and GFAP: development and regulation by neuron–Schwann cell interactions
Authors: Jessen K. R. , Morgan L. , Stewart H. J. S. , Mirsky R.
Published: April 2021

Immunohistochemical methods are used to investigate in detail the development and regulation of three proteins (217c(Ran-1),A5E3 and GFAP) specifically associated with adult non-myelin-forming Schwann cells in the rat sciatic nerve, from embryo day 15 to maturity. 217c(Ran-1),which is probably the NGF-receptor, and A5E3 are expressed by the majority of cells in the nerve at embryo day 15 and by essentially all cells at embryo day 18. GFAP first appears at embryo day 18; this is an intrinsically programmed developmental event which occurs in cultured Schwann cells even in the absence of serum. Postnatally, the expression of 217c(Ran-1),A5E3 and GFAP is suppressed in cells that form myelin but retained in non-myelin-forming Schwann cells. Mature myelin-forming cells nevertheless maintain the potential to express all three proteins but will only do so if removed from contact with myelinated axons. In neuron-free cultures Schwann cells express all three proteins.

This work, together with our previous observations on N-CAM, shows that removal of a diverse set of surface proteins and a change in intermediate filament expression is one of the major consequences of axon to Schwann cell signalling during myelination in the rat sciatic nerve. Unlike myelin-forming cells, adult nonmyelin-forming Schwann cells remain very similar to embryonic and newborn cells with respect to expression of surface proteins, in contrast to the previously established developmental changes that occur in their surface lipids.
Fetal and adult localization of C/EBP: evidence for combinatorial action of transcription factors in cell-specific gene expression
Authors: Kuo C. Frank , Xanthopoulos Kleanthis G. , Darnell James E.
Published: April 2021

C/EBP is a positive-acting transcription factor important for hepatocyte-specific expression present not only in hepatocytes but also a limited number of other cell types in adult mice. By Northern blot analysis and in situ hybridization experiments with mouse embryos and adult tissues, we first detected C/EBP mRNA in hepatocytes at the 13th gestational day, when no other cell types give detectable signals and thus by this test C/EBP is, at least in the embryo, a ‘liver-specific’ factor. Only trace amounts of C/EBP were seen in the yolk sac and no mRNA was detectable in choroid plexus in either embryos or adult animals. Both these cell types produce some proteins (e.g. albumin, transthyretin, a-l antitrypsin and others) that are also made in the liver where C/EBP is important for their production; thus either fewer factors or different factors govern yolk sac and choroid plexus production of these proteins. C/EBP mRNA was not detected in fetal brain but was present in several regions of the adult mouse brain again emphasizing that this factor does not appear to have a very early embryologic role. In the adult brain, it was most concentrated in CAI to CA4 regions of the hippocampus, in cerebellar Purkinje cells, and in layer II and III of the cortex.
Structural and compositional divergencies in the extracellular matrix encountered by neural crest cells in the white mutant axolotl embryo
Authors: Perris R. , Löfberg J. , FällströM C. , Boxberg Y. Von , Olsson L. , Newgreen D. F.
Published: April 2021

The skin of the white mutant axolotl larva is pigmented differently from that of the normal dark due to a local inability of the extracellular matrix (ECM) to support subepidermal migration of neural crest-derived pigment cell precursors. In the present study, we have compared the ECM of neural crest migratory pathways of normal dark and white mutant embryos ultrastructurally, immunohistochemically and biochemically to disclose differences in their structure/composition that could be responsible for the restriction of subepidermal neural crest cell migration in the white mutant axolotl. When examined by electron microscopy, in conjunction with computerized image analysis, the structural assembly of interstitial and basement membrane ECMs of the two embryos was found to be largely comparable. At stages of initial neural crest cell migration, however, fixation of the subepidermal ECM in situ with either Karnovsky-ruthenium red or with periodate-lysine-paraform-aldehyde followed by ruthenium red-containing fixatives, revealed that fibrils of the dark matrix were significantly more abundant in associated electron-dense granules. This ultrastructural discrepancy of the white axolotl ECM was specific for the subepidermal region and suggested an abnormal proteoglycan distribution. Dark and white matrices of the medioventral migratory route of neural crest cells had a comparable appearance but differed from the corresponding subepidermal ECMs. Immunohistochemistry revealed only minor differences in the distribution of fibronectin, laminin, collagen types I, and IV, whereas collagen type III appeared differentially distributed in the two embryos. Chondroitin- and chondroitin-6-sulfate-rich proteogly cans were more prevalent in the white mutant embryo than in the dark, especially in the subepidermal space. Membrane microcarriers were utilized to explant site-specifically native ECM for biochemical analysis. Two dimensional gel electrophoresis of these regional matrices revealed a number of differences in their protein content, principally in constituents of apparent molecular masses of 30– 90 000. Taken together our observations suggest that local divergences in the concentration/assembly of low and high molecular mass proteins and proteoglycans of the ECM encountered by the moving neural crest cells account for their disparate migratory behavior in the white mutant axolotl.
Different patterns of transcription from the two Antennapedia promoters during Drosophila embryogenesis
Authors: Bermingham John R. , Martinez-Arias Alfonso , Petitt Matthew G. , Scott Matthew P.
Published: April 2021

The homeotic genes of Drosophila control the differentiation of segments during development. Mutations in these genes cause one or more segments to develop structures normally found elsewhere in the organism. Several studies have shown that the spatial patterns of homeotic gene transcription are highly complex, and that these precise patterns of transcription are critical to normal development. The homeotic gene Antennapedia (Antp), a member of the Antennapedia Complex, is required for the correct differentiation of thoracic segments in both embryos and adults. The patterns of total Antp transcript and protein accumulation have been described in detail, but the contribution of each promoter to the overall pattern in embryos has not been reported. We have examined in detail the spatial distribution of transcripts from each of the Antp promoters in both embryo sections and whole embryos by in situ hybridization using promoter-specific probes. We show that the transcripts from each of the two promoters accumulate in distinct, but overlapping patterns during embryogenesis. The results demonstrate that the two Antp promoters are differentially regulated in embryos and provide a basis for examining the regulation of the two promoters and characterizing more fully the func tion of Antp during embryogenesis. In addition, we have examined the regulation of each of the Antp promoters by genes of the bithorax complex (BX-C). We show that in BX-C− embryos both promoters are derepressed in the abdomen.
Effects of the lethal yellow (AY) mutation in mouse aggregation chimeras
Authors: Barsh Gregory S. , Lovett Michael , Epstein Charles J.
Published: April 2021

The Ay allele is a recessive lethal mutation at the mouse agouti locus, which results in embryonic death around the time of implantation. In the heterozygous state, Ay produces several dominant pleiotropic effects, including an increase in weight gain and body length, a susceptibi lity to hepatic, pulmonary and mammary tumors, and a suppression of the agouti phenotype, which results in a yellow coat color. To investigate the cellular action of Ay with regard to its effects upon embryonic viability and adult-onset obesity, we generated a series of aggregation chimeras using embryos that differ in their agouti locus genotype. Embryos derived from Ay/a×X.Ay/a matings were aggregated with those derived from A/A × A/A matings, and genotypic identification of the resultant chimeras was accomplished using a molecular probe at the Emv-15 locus that distinguishes among the three different alleles, Ay, A, and a. Among 50 chimeras, 25 analyzed as liveborns and 25 as 9.5 day embryos, 29 were a/a ↔ A/A and 21 were Ay/a ↔ A/A. The absence of AyIAy↔ A/A chimeras demonstrates that Ay/Ay cells cannot be rescued in a chimeric environment, and the relative deficiency of Ay/a ↔ A/A chimeras suggests that, under certain conditions, Ay heterozygosity may par tially affect cell viability or proliferation. In the 25 liveborn chimeras, Ay/a ↔ A/A animals became obese as adults and a/a ↔ A/A animals did not. There was no correlation between genotypic proportions and rate of weight gain, which shows that, with regard to its effects on weight gain, Ay heterozygosity is cell non-auton-omous.
Coexistence of perinatal and adult forms of a glial progenitor cell during development of the rat optic nerve
Authors: Wolswijk Guus , Riddle Peter N. , Noble Mark
Published: April 2021

We have studied the developmental appearance of the O-2Aadult progenitor cell, a specific type of oligodendro cyte-type-2 astrocyte (O-2A) progenitor cell that we have identified previously in cultures prepared from the optic nerves of adult rats. O-2Aadult progenitors differ from their counterparts in perinatal animals (O-2Aperina,al progenitor cells) in antigenic phenotype, morphology, cell cycle time, rate of migration, time course of differen tiation into oligodendrocytes or type-2 astrocytes and sensitivity to the lytic effects of complement in vitro. In the present study, we have found that O-2Aadult progeni tor-like cells first appear in the developing optic nerve approximately 7 days after birth and that by 1 month after birth these cells appear to be the dominant progeni tor population in the nerve. Howevér, the perinatal-to-adult transition in progenitor populations is a gradual one and O-2Aadult and O-2Aptrinata progenitors coexist in the optic nerve for 3 weeks or more. In addition, cells derived from optic nerves of P21 rats express character istic features of O-2Aadultand O-2Aptrinata progenitors for extended periods of growth in the same tissue culture dish. Our results thus indicate that the properties that distinguish these two types of O-2A progenitors from each other are expressed in apparently identical environ ments. Thus, these cells must either respond to different signals present in the environment, or must respond with markedly different behaviours to the binding of identical signalling molecules.
Lateral inhibition and cell fate during neurogenesis in Drosophila: the interactions between scute, Notch and Delta
Authors: Cabrera Carlos V.
Published: April 2021

A comparison of the patterns of expression of AS-C (T3) RNA and protein suggests that an important level of regulation occurs post-transcriptionally. First, when the RNA is abundant in the early embryo the protein is barely detectable. Later, the protein starts to accumulate in only a subset of the nuclei of those cells expressing the RNA. Only the cells in the subsets become the neuroblasts. This post-transcriptional regulation is suppressed in embryos mutant for the genes Notch and Delta; where all cells expressing RNA accumulate protein. These findings suggest that deployment of T3 protein expression is one of the causal factors that assigns specific fates to the neuroblasts and, in consequence, a basis for the mechanism of lateral inhibition is proposed.
The Effect of Adenosine Triphosphate and its Decomposition Products on Cortisone Induced Teratology
Authors: Gordon Harry W. , Tkaczyk Walter , Peer Lyndon A. , Bernhard William G.
Published: April 2021

Cortisone has been demonstrated by several investigators to be a teratogenic agent when administered to pregnant mice (Baxter & Fraser, 1950; Fraser & Feinstat, 1951; Ingalls & Curley, 1957; Walker & Fraser, 1957; Gordon et al., 1961). The degree of occurrence was found to vary with strain (Walker & Fraser, 1957; Fraser et al., 1954), maternal age, weight and litter size (Kalter, 1956; 1957).
Malformation of the Extremity in the Mouse Foetus Caused by X-radiation of the Mother During Pregnancy
Authors: Murakami Ujihiro , Kameyama Yoshiro , Nogami Hiroshi
Published: April 2021

Different groups of the ddN strain mice were exposed to X-radiation of 300 r., 200 r. and 150 r. from the 8th to 13th day of pregnancy. In offspring from mothers treated on the 11th day, malformations of the extremities were observed in groups treated with over 200 r. Scapular hypoplasia and shortening of the fore-limb, as a result of absence, synostosis, shortening, flexure or curvature of the limb bone, etc., were typical malformations. However, in the group treated with 150 r. of X-radiations the incidence of such malformations was very low. In offspring from mothers treated on the 12th day, digital reductions, on account of ectrodactylias, were the typical malformation in the fore-limb. Ectrodactylia was dominant in the second digit of the fore-limbs, but involvement of the fifth digit was also seen. In the hind-limbs, the incidence was a little lower and the predominant site of involvement was the third digit. Both in the fore- and hind-limbs, the fourth digit was the most resistant to malformation. Other digital malformations occurred more often in the hind-limb than in the fore-limbs, though not frequently. The formation of ectrodactylias after X-radiation was studied in a follow-up experiment. The process began with cellular necrosis in the hand and foot plate corresponding to the predominant site of ectrodactylias, i.e. chiefly in the second digit of the fore-limb and in the third digit of the hindlimb. The process may damage the precursor of the pre-cartilage. In contrast to the regenerative neural tissue, compensation does not take place in the mesenchymal tissue once the destruction is severe, and the reduction of digits will therefore persist. The effect of X-radiation upon a developing digital primordium may be a directly destructive process. The critical stage for extremity malformations due to thalidomide in human beings corresponds with the critical stage for X-ray malformations of the extremities in mouse.
‘Generic’ physical mechanisms of morphogenesis and pattern formation
Authors: Newman Stuart A. , Comper Wayne D.
Published: April 2021

The role of ‘generic’ physical mechanisms in morphogenesis and pattern formation of tissues is considered. Generic mechanisms are defined as those physical processes that are broadly applicable to living and nonliving systems, such as adhesion, surface tension and gravitational effects, viscosity, phase separation, convection and reaction–diffusion coupling. They are contrasted with ‘genetic’ mechanisms, a term reserved for highly evolved, machine-like, biomolecular processes. Generic mechanisms acting upon living tissues are capable of giving rise to morphogenetic rearrangements of cytoplasmic, tissue and extracellular matrix components, sometimes leading to ‘microfingers’, and to chemical waves or stripes. We suggest that many mor phogenetic and patterning effects are the inevitable outcome of recognized physical properties of tissues, and that generic physical mechanisms that act on these properties are complementary to, and interdependent with genetic mechanisms. We also suggest that major morphological reorganizations in phylogenetic lineages may arise by the action of generic physical mechanisms on developing embryos. Subsequent evolution of genetic mechanisms could stabilize and refine developmental outcomes originally guided by generic effects.
Homologous and heterologous enhancers modulate spatial expression but not cell-type specificity of the murine γF-crystallin promoter
Authors: Yu Calvin C.-K. , Tsui Lap-Chee , Breitman Martin L.
Published: April 2021

Previous studies have shown that mouse γF-crystallin sequences –759 to +45, which include the core promoter and two upstream enhancer elements, contain sufficient information for directing gene expression to terminally differentiated fiber cells of the ocular lens. To investigate the role that proximal sequences of the mouse γF-crystallin promoter play in the developmental regulation of gene expression, we generated transgenic mice containing the lacZ gene driven by either mouse γF-crystallin sequences –171 to +45, which lack functional enhancers, or a hybrid hamster αA-/mouse γF-crystallin promoter, which contains the hamster αA-crystallin enhancer instead of operational γF-crystallin enhancers. In situ analysis of lacZ expression in these mice revealed that the mouse γF-crystallin promoter segment –171 to +45, which shows low activity in vitro, is able to direct gene expression to the fiber cells in the nucleus of the lens. However, animals expressing γ171–lacZ show both a lower level of expression of the lacZ gene and a narrower pattern of staining in the lens nucleus than mice expressing γ759–lacZ, which contains the two enhancer elements located between –392 and –278 and –226 to –123. Moreover, animals expressing the lacZ gene driven by the hybrid αA-/γF-crystallin promoter show a pattern of staining in the lens nucleus similar to that of mice expressing γ759–lacZ, indicating that although the αA-crystallin enhancer is unable to alter the cell-type specificity conferred by the mouse γF-crystallin proximal promoter segment –171 to +45, it is able to expand the spatial domain of gene expression in a manner similar to the native γF-crystallin enhancers. The significance of these results, in terms of the mechanisms governing spatial regulation of gene expression, is discussed.
Expression of the nerve growth factor-regulated NGFI-A and NGFI-B genes in the developing rat
Authors: Watson Mark A. , Milbrandt Jeffrey
Published: April 2021

NGFI-A and NGFI-B are two genes that are induced in PC 12 cells treated with nerve growth factor and encode zinc-finger proteins that may act as transcription factors. To study the function of these genes in vivo, their expression in rat embryos and postnatally developing tissues was examined. Both genes are expressed during embryogenesis from E12 to E18. In postnatally developing animals, these genes are induced in cortex, midbrain and cerebellum, suggesting that they may function in one or many of the changes that occur during postnatal maturation of the brain. Both genes are expressed at a low, constitutive level in a subset of other non-neuronal tissues, but also demonstrate developmental changes in expression in many other organs. To examine expression of these genes in ganglia of the peripheral nervous system, a sensitive and quantitative assay employing reverse transcription and polymerase chain reaction was developed. Results obtained with this assay demonstrated that expression of both NGFI-A and NGFI-B is modulated in the peripheral nervous system during postnatal maturation, but not in a pattern concordant with previously characterized NGF receptor expression. These studies suggest that NGFI-A and NGFI-B serve multiple, tissue-specific functions and are under complex regulation in vivo.
Cell patterning in the Drosophila segment: spatial regulation of the segment polarity gene patched
Authors: Hidalgo Alicia , Ingham Philip
Published: April 2021

Intrasegmental patterning in the Drosophila embryo requires the activity of the segment polarity genes. The acquisition of positional information by cells during embryogenesis is reflected in the dynamic patterns of expression of several of these genes. In the case of patched, early ubiquitous expression is followed by its repression in the anterior portion of each parasegment; subsequently each broad band of expression splits into two narrow stripes. In this study we analyse the contribution of other segment polarity gene functions to the evolution of this pattern; we find that the first step in patched regulation is under the control of engrailed whereas the second requires the activity of both cubitus interruptusD and patched itself. Furthermore, the products of engrailed, wingless and hedgehog are essential for maintaining the normal pattern of expression of patched.
Competitive titration in living sea urchin embryos of regulatory factors required for expression of the Cyllla actin gene
Authors: Franks Roberta R. , Anderson Roger , Moore James G. , Hough-Evans Barbara R. , Britten Roy J. , Davidson Eric H.
Published: April 2021

Previous studies have located some twenty distinct sites within the 2.3 kb 5′ regulatory domain of the sea urchin Cyllla cytoskeletal actin gene, where there occur in vitro high-specificity interactions with nuclear DNA-binding proteins of the embryo. This gene is activated in late cleavage, exclusively in cells of the aboral ectoderm cell lineages. In this study, we investigate the functional importance in vivo of these sites of DNA-protein interaction. Sea urchin eggs were coinjected with a fusion gene construct in which the bacterial chloramphenicol acetyltransferase (CAT) reporter gene is under the control of the entire CyIIIa regulatory domain, together with molar excesses of one of ten nonoverlapping competitor subfragments of this domain, each of which contains one or a few specific site(s) of interaction. The exogenous excess binding sites competitively titrate the available regulatory factors away from the respective sites associated with the CyIIIa · CAT reporter gene. This provides a method for detecting in vivo sites within the regulatory domain that are required for normal levels of expression, without disturbing the structure of the regulatory domain. We thus identify five nonoverlapping regions of the regulatory DNA that apparently function as binding sites for positively acting transcriptional regulatory factors. Competition with a subfragment bearing an octamer site results in embryonic lethality. We find that three other sites display no quantitative competitive interference with CyIIIa · CAT expression, though as shown in the accompanying paper, two of these sites are required for control of spatial expression. We conclude that the complex CyIIIa regulatory domain must assess the state of many distinct and individually necessary interactions in order to properly regulate CyIIIa transcriptional activity in development.
Negative spatial regulation of the lineage specific CyIIIa actin gene in the sea urchin embryo
Authors: Hough-Evans Barbara R. , Franks Roberta R. , Zeller Robert W. , Britten Roy J. , Davidson Eric H.
Published: April 2021

The CyHIa · CAT fusion gene was injected into Stron-gylocentrotus purpuratus eggs, together with excess ligated competitor sequences representing subregions of the CyIIIa regulatory domain. In this construct, the chloramphenicol acetyltransferase (CAT) reporter gene is placed under the control of the 2300 nucleotide upstream regulatory domain of the lineage-specific CyHIa cytoskeletal actin gene. CAT mRNA was detected by in situ hybridization in serial sections of pluteus stage embryos derived from the injected eggs. When carrier DNA lacking competitor CyHIa fragments was coinjected with CyHIa · CAT, CAT mRNA was observed exclusively in aboral ectoderm cells, i.e. the territory in which the CyHIa gene itself is normally expressed (as also reported by us previously). The same result was obtained when five of seven different competitor subfragments bearing sites of DNA-protein interaction were coinjected. However, coinjection of excess quantities of either of two widely separated, nonhomologous fragments of the CyHIa regulatory domain produced a dramatic ectopic expression of CAT mRNA in the recipient embryos. CAT mRNA was observed in gut, mesenchyme cells and oral ectoderm in these embryos. We conclude that these fragments contain regulatory sites that negatively control spatial expression of the CyHIa gene.
Vimentin downregulation is an inherent feature of murine erythropoiesis and occurs independently of lineage
Authors: Sangiorgi Frank , Woods Catherine M. , Lazarides Elias
Published: April 2021

In mammalian erythropoiesis, the mature cells of the primitive lineage remain nucleated while those of the definitive lineage are anuclear. One of the molecular and structural changes that precedes enucleation in cells of the definitive lineage is the cessation in the expression of the gene for the intermediate filament (IF) protein vimentin and the removal of all vimentin filaments from the cytoplasm. We show here that in immature primitive cells vimentin is synthesized and forms a cytoplasmic network of IFs. As differentiation proceeds in vivo, vimentin gene expression is downregulated in these cells; this is accompanied by the loss of vimentin filaments from the cytoplasm. This loss temporally coincides with the nucleus becoming freely mobile within the cytoplasm, suggesting that, while IF removal is not directly linked to the physical process of enucleation, it may be a prerequisite for the initiation of nuclear mobility in both lineages. These changes are also observed in early primitive cells cultured in vitro, suggesting that they constitute an intrinsic part of the murine erythroid differentiation program independent of lineage and hematopoietic microenvironment.
Ectopic expression of N-cadherin perturbs histogenesis in Xenopus embryos
Authors: Fujimori Toshihiko , Miyatani Seiji , Takeichi Masatoshi
Published: April 2021

Xenopus embryos express N-cadherin in a pattern similar to that observed in other species, and cells expressing Xenopus N-cadherin can bind to cells expressing chicken N-cadherin in vitro. To investigate the developmental role of this molecule, we injected mRNA encoding chicken N-cadherin into one blastomere of 2-cell-stage Xenopus embryos and examined the effect of its expression on their development. The ectopic expression of N-cadherin occurred in various regions of the injected embryos and induced abnormal histogenesis, such as thickening, clumping or fusion of cell layers. These results suggest that the precise quantitative and qualitative regulation of the expression of cadherins is essential to embryonic morphogenesis.
Isolation of the mouse Hox-2.9 gene; analysis of embryonic expression suggests that positional information along the anterior-posterior axis is specified by mesoderm
Authors: Frohman Michael A. , Boyle Monica , Martin Gail R.
Published: April 2021

It is rapidly becoming accepted that the vertebrate neural tube, in particular the hindbrain, develops into a segmented structure. After segment formation, cells in the neural tube do not cross segmental boundaries, and segment-specific gene expression is observed. However, it is not known what positional cues instruct the neural tube to express genes in this restricted manner. We have cloned a murine homeobox-containing gene, Hox-2.9, whose expression in the neural tube at E9.5 is restricted to a segment of the hindbrain known as rhombomere 4. A study of its expression pattern earlier in development revealed that prior to the start of neurulation (E7.5) Hox-2.9 is expressed within and posterior to the embryonic mesoderm that will participate in hindbrain formation. With the onset of neurulation, expression then becomes detectable in the neural plate as well, but only in the part that overlies the Hox-2.9-expressing mesoderm; it is not detected in the more anterior neuroectoderm that will form the future midbrain and forebrain. On the basis of these findings, we propose that the mesoderm is providing cues that serve to instruct the overlying neuroectoderm with respect to its position along the anteroposterior axis and that Hox-2.9 participates in or reflects this process. As neurulation continues and individual segments form, a second phase of expression is detected in the neural tube in which high levels of Hox-2.9 transcripts become restricted to rhombomere 4. Hox-2.9 expression is also detected in the developing branchial arch units of the hindbrain region, in a pattern that suggests to us that here, too, mesoderm is providing a localized signal that induces Hox-2.9 expression, in this case in endoderm of the pharynx and in superficial ectoderm. In general, we interpret the expression patterns of Hox-2.9 in the hindbrain region as suggesting that the specific mechanisms of pattern formation in mammals are fundamentally similar to those of amphibians and avians - i.e. anteroposterior positional information is acquired by mesoderm, mesoderm induces positional values within (neuro-) ectoderm and endoderm, and both events occur within a restricted window of time.
Structure and expression pattern of the murine Hox-3.2 gene
Authors: Erselius Jörn R , Goulding Martyn D. , Gruss Peter
Published: April 2021

The murine homeobox-containing gene Hox-3.2 is the most 5’ member of the Hox-3 complex on chromosome 15 isolated to date. Conceptual translation of the longest ORF gives a protein of 260 amino acids lacking the conserved hexapeptide found in most homeobox genes. Northern analysis detects three transcripts of 1.5, 1.9 and 3.2 kb in day 9 to 15 p.c. embryos. As early as day 8.5 p.c., transcripts can be detected in the posterior part of the embryo by in situ hybridization. At this developmental stage no or only very weak expression is visible in the neural plate. At day 10.5 Hox-3.2 is detected in the ventral part of the neural tube with a sharp anterior boundary at the level of the third thoracic pre vertebra. This anterior boundary remains at day 12.5 and day 14.5. In contrast to Hox-3.1, Hox-3.2 is not expressed in the dorsal horns containing the sensory neurons at day 14.5 p.c. Hox-3.2 transcripts are also detected in the posterior pre vertebrae, the hindlimb buds and the cortex of the developing kidney. Unlike Hox-1.4 and Hox-1.3 and their paralogs, Hox-3.2, -2.5 and -4.4 (5.2) show strikingly different anterior boundaries of expression in the CNS and prevertebrae.