bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffusible signal molecules controlling cell differentiation and patterning in Dictyostelium
Authors: Berks Mary , Traynor David , Carrin Ines , Insall Robert H. , Kay Robert R.
Published: April 2021

Slime moulds, such as Dictyostelium discoideum, have biochemical, physiological and probably developmental features in common with both plants and animals. During development separate Dictyostelium amoebae first aggregate into collecting centers to form small multicellular organisms which, in their slug form, can migrate over the substratum toward light. Eventually a slug culminates to form a fruiting body consisting of a cellular stalk supporting a mass of spores. Development is highly regulative, indicating that it is controlled by signalling between the cells. A number of diffusible signal molecules have been discovered, including cyclic AMP, the chemoattractant in aggregation, and DIF-1, a novel chlorinated phenyl alkanone, which acts as a specific inducer of stalk cell differentiation.

The migrating slug contains three types of precursor cell: prespore, prestalk A and prestalk B cells. Differentiation of these cells from uncommitted amoebae can be brought about in ceil cultures by cyclic AMP and DIF-1 acting in combination: cyclic AMP alone favours prespore, DIF-1 alone favours prestalk B, cyclic AMP and DIF-1 together favour prestalk A cell differentiation. There is evidence suggesting that these signals act in the same way in the intact aggregate.

A cytoplasmic DIF-1 binding protein has been discovered, whose level increases as cells become sensitive to DIF-1 and which binds DIF-1 with an affinity and specificity suggestive of a receptor. At the same time, cells are able to inactivate DIF-1 by a metabolic pathway involving at least 12 metabolites. Metabolism may also serve to produce gradients of DIF-1 in the aggregate or other signal molecules from DIF-1. Manipulation of the DIF-1 signalling system will be an important means of further elucidating its role in development.
Developmental expression and cellular localization of glucose transporter molecules during mouse preimplantation development
Authors: Aghayan Maryam , Rao L. V. , Smith Robert M. , Jarett Leonard , Charron Maureen J. , Thorens Bernard , Heyner Susan
Published: April 2021

Two general mechanisms mediate glucose transport, one is a sodium-coupled glucose transporter found in the apical border of intestinal and kidney epithelia, while the other is a sodium-independent transport system. Of the latter, several facilitated transporters have been identified, including GLUTl (erythrocyte/brain), GLUT2 (liver) and GLUT4 (adipose/muscle) isoforms. In this study, we used Western-blot analysis and high resolution immunoelectron microscopy (IEM) to investigate the stage-related expression and cellular localization of GLUTl, 2 and 4. The Western blot results demonstrate that GLUTl is detectable in the oocyte and throughout preimplantation development. GLUT2 isoforms were not detectable until the blastocyst stage, while the GLUT4 isoform was undetectable in the oocyte through blastocyst stages. The present findings confirm previous studies at the molecular level which demon-strated that mRNAs encoding the same GLUT isoforms are detectable at corresponding developmental stages. GLUTl and GLUT2 display different cellular distributions at the blastocyst stage as shown by IEM studies. GLUTl has a widespread distribution in both trophecto-derm and inner cell mass cells, while GLUT2 is located on trophectoderm membranes facing the blastocyst cavity. This observation suggests a different functional significance for these isoforms during mouse dereimplanttation development.
Isolation and expression of two novel Wnt/wingless gene homologues in Drosophila
Authors: Russell Jackie , Gennissen Annemiek , Nusse Roel
Published: April 2021

Wingless (wg), the Drosophila homologue of the mouse Wnt-1 proto-oncogene, is a segment polarity gene essential in each segment for normal Drosophila development. We here report the isolation of two novel Drosophila Wnt homologues, DWni-2 and DWnt-3, and thus the existence of a Wnt/wingless gene family in Drosophila. DWnt-2 and DWnt-3 map to chromosome 2 position 45E and chromosome X position 17A/B, respectively. DWnt-2 and DWni-3, like the other known Wnt genes, encode amino-terminal signal peptides suggesting that the gene products are secreted proteins. The putative translation product of DWnt-2 and the carboxy-terminal half of the deduced DiVzii-3 product are both rich in conserved cysteine residues. In comparison with other Wnt gene products, mostly about 40 × 103 relative molecular mass, the DWnt-3 protein has an extended amino terminus and a long internal insert, and its predicted relative molecular mass is 113 × 103. The expression patterns of these two Wnt/wg homologues are dynamic during Drosophila embryogenesis. The distribution of DWnt-2 transcripts is predominantly segmented, with the additional presence of transcripts in the presumptive gonads. Transcripts of both DWnt-2 and DWnt-3 appear to be associated with limb primordia in the embryo and may therefore specify limb development. DWnt-3 is also expressed in mesodermal and neurogenic regions. The distribution of D Wnt-3 transcripts in cells of the central nervous system (CNS) during Drosophila embryogenesis suggests that DWnt-3 could be involved in CNS development.
The somitogenetic potential of cells in the primitive streak and the tail bud of the organogenesis-stage mouse embryo
Authors: Tam Patrick P. L. , Tan Seong-Seng
Published: April 2021

The developmental potency of cells isolated from the primitive streak and the tail bud of 8.5- to 13.5-day-old mouse embryos was examined by analyzing the pattern of tissue colonization after transplanting these cells to the primitive streak of 8.5-day embryos. Cells derived from these progenitor tissues contributed predominantly to tissues of the paraxial and lateral mesoderm. Cells isolated from older embryos could alter their segmental fate and participated in the formation of anterior somites after transplantation to the primitive streak of 8.5-day host embryo. There was, however, a developmental lag in the recruitment of the transplanted cells to the paraxial mesoderm and this lag increased with the extent of mismatch of developmental ages between donor and host embryos. It is postulated that certain forms of cell-cell or cell-matrix interaction are involved in the specification of segmental units and that there may be age-related variations in the interactive capability of the somitic progenitor cells during development. Tail bud mesenchyme isolated from 13.5-day embryos, in which somite formation will shortly cease, was still capable of somite formation after transplantation to 8.5-day embryos. The cessation of somite formation is therefore likely to result from a change in the tissue environment in the tail bud rather than a loss of cellular somitogenetic potency.
Mechanism of skin morphogenesis. II. Retinoic acid modulates axis orientation and phenotypes of skin appendages
Authors: Chuong Cheng-Ming , Ting Sheree A. , Widelitz Randall B. , Lee Yun-Shain
Published: April 2021

The factors that determine the axial orientation and phenotypes of skin appendages were analyzed by studying the effect of retinoic acid (RA) on embryonic chicken skin explant cultures. With RA uniformly distributed in the culture media, the feather buds became smaller, were disoriented or were transformed into scale-like structures in a concentration-dependent manner (from 0.05 –2.5 μM). With RA distributed as a gradient created by a RA-soaked anion exchange bead, a radial zone of inhibition with a rim of disoriented buds was observed. The new axis of the disoriented buds appeared to be determined by a combination of the original feather axis determining force and a new axial force pointing centrifugally away from the RA source. This observed result can be simulated with a computer model using a vectorial sum of different feather axial determination forces. The size of the inhibited zone is linearly correlated to the RA concentration and may be used to quantify the morphogenetic activity of retinoids. These effects are specific to developmental stages (Hamburg and Hamilton stage 31–34).

Both all-trans and 13-cis RA have morphogenetic activity. Retinol has no effect and retinal has a small inhibitory effect but neither phenotypic transformation nor axial disorientation were observed. The antero-posterior gradient of homeoprotein XlHbox 1 in feather buds became diffusive after RA treatment. RA dissolves dermal condensations and the distribution of N-CAM is altered from an anterior localized pattern to a diffusive presence in the bud cores. Endogenous retinoids in developing skins show developmental stage-dependent changes both quantitatively and qualitatively. The results suggest that RA either is or can modulate the endogenous morphogen(s) that determine the orientation and phenotype of skin appendages, and that this morphogenetic pathway involves Hox genes and adhesion molecules.
Developmentally regulated cell surface expression and function of c-kit receptor during lymphocyte ontogeny in the embryo and adult mice
Authors: Palacios Ronald , Nishikawa Shin-Ichi
Published: April 2021

We have used a c-kit-specific monoclonal antibody, immuno-fluorescence staining and flow fluorocytometry or microscopy analysis to assess the cell surface expression of the c-kit receptor on a panel of non-trans-formed clones representing different stages of T- and B-lymphocyte development, freshly isolated lymphoid cells from thymus, bone marrow and spleen of young adult C57BL/6 mice and cells from yolk sac, thymus and liver of developing C57BL/6 mouse embryos. Pro-T, Pro-B and Pre-B clones derived from thymus or liver of 14-day embryos are c-kit+. Starting at day 8 to 8.5 in yolk sac, day-10 in fetal liver, and day 11 to 12 in fetal thymus, there are many c-kit+ cells. The number of c-kit+ cells in liver and thymus increases up to day 15 and progressively decreases thereafter. Cell sorter purified c-kit+ day 14 fetal liver cells fully reconstitute the T and B cell compartments of immunodeficient Scid mice. Stromal cells or epithelial cells derived from fetal thymus or liver, which can support growth and differentiation of c-kit+ lymphocyte progenitor clones, synthesize mRNA for Steel Factor (SF), the ligand of c-kit. In the adult mouse, however, c-kit expression is restricted to very early stages of T- and B-lymphocyte development (multipotent progenitors, B-cell/myelocytic progenitors, Pro-T and Pro-B lymphocyte progenitors).

Most cells at the Pre-T, Pre-B and later stages of development do not bear detectable c-kit. Using Cos-1 cells tranfected with mouse SF-cDNA and an antagonistic c-kit receptor-specific antibody, we show that the c-kit/SF system contributes to the survival of lymphocyte progenitors and enhances the proliferative responses of these cells to other growth factors (i.e. IL2, IL3, IL4, IL7). However, the c-kit receptor/SF ligand pair is neither sufficient nor necessary for the differentiation of lymphocyte progenitors into mature T-or B-lymphocytes. Finally, in stromal cell lines from fetal liver and adult bone marrow and thymic epithelial cell lines the level of steady state SF-RNA transcripts is inversely correlated with that of IL-7-mRNA. Moreover, IL7 inhibits the synthesis of SF-mRNA in stromal cells and rIL6 abrogates this inhibitory effect of rIL7. Thus, the expression of SF in stromal cells is subjected to complex regulation by other cytokines produced by the same stromal cells or by neighboring cells in a given microenvironment. The results strongly suggest that the c-kit/SF system plays an important role in the very early stages of development of lymphocytes in the mouse.
Planar and vertical signals in the induction and patterning of the Xenopus nervous system
Authors: Altaba A. Ruiz i
Published: April 2021

The cellular mechanisms responsible for the formation of the Xenopus nervous system have been examined in total exogastrula embryos in which the axial mesoderm appears to remain segregated from prospective neural ectoderm and in recombinates of ectoderm and mesoderm. Posterior neural tissue displaying anteroposterior pattern develops in exogastrula ectoderm. This effect may be mediated by planar signals that occur in the absence of underlying mesoderm. The formation of a posterior neural tube may depend on the notoplate, a midline ectodermal cell group which extends along the anteroposterior axis. The induction of neural structures characteristic of the forebrain and of cell types normally found in the ventral region of the posterior neural tube requires additional vertical signals from underlying axial mesoderm. Thus, the formation of the embryonic Xenopus nervous system appears to involve the cooperation of distinct planar and vertical signals derived from midline cell groups.
Identifying targets of the rough homeobox gene of Drosophila: evidence that rhomboid functions in eye development
Authors: Freeman Matthew , Kimmel Bruce E. , Rubin Gerald M.
Published: April 2021

In order to identify potential target genes of the rough homeodomain protein, which is known to specify some aspects of the R2/R5 photoreceptor subtype in the Drosophila eye, we have carried out a search for enhancer trap lines whose expression is rough-dependent. We crossed 101 enhancer traps that are expressed in the developing eye into a rough mutant background, and have identified seven lines that have altered expression patterns. One of these putative rough target genes is rhomboid, a gene known to be required for dorsoventral patterning and development of some of the nervous system in the embryo. We have examined the role of rhomboid in eye development and find that, while mutant clones have only a subtle phenotype, ectopic expression of the gene causes the non-neuronal mystery cells to be transformed into photoreceptors. We propose that rhomboid is a part of a partially redundant network of genes that specify photoreceptor cell fate.
Primary culture of single ectodermal precursors of Drosophila reveals a dorsoventral prepattern of intrinsic neurogenic and epidermogenic capabilities at the early gastrula stage
Authors: Lüer Karin , Technau Gerhard M.
Published: April 2021

We have analyzed the development in vitro of individual precursor cells from the presumptive truncal segmental ectoderm of the Drosophila embryo to study the intrinsic component in the determination of cell fate. For each cultured cell, the original position within as well as the developmental stage of the donor embryo were known. Cells removed from the ventral neurogenic region develop neural clones. Cells from the dorsal ectoderm and from the dorsalmost part of the ventral neurogenic ectoderm develop epidermal clones. These two classes of clones differ with respect to their division pattern, adhesiveness, cell morphologies and the expression of cell-specific markers. Mixed neural/epidermal clones were obtained from a fraction of precursors at almost all dorsoventral sites.

We conclude that, at the onset of gastrulation, precursor cells of the truncal segmental ectoderm already have the capability to develop as either neuroblasts or epidermoblasts in the absence of further cell interactions. At the same time, positional cues distributed along the dorsoventral axis equip precursors with intrinsic preferences towards the neural or epidermal fate, thus defining a prepattern of high neurogenic preferences ventrally, and high epidermogenic preferences dorsally. It is likely that this prepattern is involved in defining the extent of the ventral neurogenic and dorsal epider-mogenic regions of the ectoderm. The roles of intrinsic capabilities versus extrinsic influences in the regulation of the characteristic pattern of segregation of the two lineages are discussed.
Characterisation of three shoot apical meristem mutants of Arabidopsis thaliana
Authors: Leyser H. M. Ottoline , Furner Ian J.
Published: April 2021

The shoot apical meristem of dicotyledonous plants is highly regulated both structurally and functionally, but little is known about the mechanisms involved in this regulation. Here we describe the genetic and phenotypic characterisation of recessive mutations at three loci of Arabidopsis thaliana in which meristem structure and function are disrupted. The loci are Clavata1 (Clv1), Fasciata1 (Fas1) and Fasciata2 (Fas2). Plants mutant at these loci are fasciated having broad, flat stems and disrupted phyllotaxy. In all cases, the fasciations are associated with shoot apical meristem enlargement and altered floral development. While all the mutants share some phenotypic features they can be divided into two classes. The pleiotropic fas1 and fas2 mutants are unable to initiate wild-type organs, show major alterations in meristem structure and have reduced root growth. In contrast, clv1 mutant plants show near wild-type organ phenotypes, more subtle changes in shoot apical meristem structure and wild-type root growth.
Induction of vasculogenesis and hematopoiesis in vitro
Authors: Flamme Ingo , Risau Werner
Published: April 2021

Despite a large number of investigations of embryonic vascular development, in particular in avian embryos, the conditions under which the endothelial and hematopoietic cell lineages emerge remain unknown. As we demonstrate here, both endothelial and hematopoietic cells can be induced by treatment of dissociated quail epiblast with fibroblast growth factors in vitro. These cells aggregate in characteristic blood islands. In long-term culture, the induced endothelial cells gave rise to vascular structures in vitro, i.e. vasculogenesis. No induction was observed in the absence of fibroblast growth factors, and other growth factors like TGF-β, TGF-α and EGF were not capable of inducing blood island formation. Thus, the dissociated quail epiblast provides a remarkably simple test system to investigate cell lineage diversification in higher vertebrates.
Expression of the homeotic gene mab-5 during Caenorhabditis elegans embryogenesis
Authors: Cowing Deborah W. , Kenyon Cynthia
Published: April 2021

mab-5 is a member of a complex of homeobox-containing genes evolutionarily related to the Antennapedia and bithorax complexes of Drosophila melanogaster. Like the homeotic genes in Drosophila, mab-5 is required in a particular region along the anterior-posterior body axis, and acts during postembryonic development to give cells in this region their characteristic identities. We have used a mab-5-lacZ fusion integrated into the C. elegans genome to study the posterior-specific expression of mab-5 during embryogenesis. The mab-5-lacZ fusion was expressed in the posterior of the embryo by 180 minutes after the first cleavage, indicating that the mechanisms responsible for the position-specific expression of mab-5-lacZ act at a relatively early stage of embryogenesis. In embryos homozygous for mutaions in the par genes, which disrupt segregation of factors during early cleavages, expression of mab-5-lacZ was no longer localized to the posterior. This suggests that posterior-specific expression of mab-5 depends on the appropriate segregation of developmental factors during early embryogenesis. After extrusion of any blastomere of the four-cell embryo, descendants of the remaining three cells could still express the mab-5-lacZ fusion. In these partial embryos, however, the fusion was often expressed in cells scattered throughout the embryo, suggesting that cell-cell interactions and/or proper positioning of early blastomeres are required for mab-5 expression to be localized to the posterior.
Restoration of hair growth by surgical implantation of follicular dermal sheath
Authors: Horne Kenneth A. , Jahoda Colin A. B.
Published: April 2021

The capacity of lower follicle dermal sheath to restore hair growth was tested by removing the lower halves of follicles, and then immediately implanting material containing dermal sheath cells from these bases, into the remaining upper epidermal follicle cavity. Over 60% of recipient follicles produced stout emergent vibrissa fibres and some operations resulted in multiple hair production from a single follicle. Histological examination revealed new dermal papillae within large bulb structures which were sited below the level of amputation - a feature that indicated that the new dermal papilla was derived from implanted material. For many follicles, the failure to produce emergent fibres could be accounted for after histological examination. These results provide clear evidence that lower follicle dermal sheath cells are capable of replacing those of the dermal papilla and it shows that they can do so in the context of the upper follicle. However, because elements of lower follicle epidermis were present in the implant material, the interactive sequence of events cannot be established. Dermal sheath cells have immense potential for papilla cell replacement: questions remain as to whether the distinction between sheath and papilla cells is one of context, or whether the transition requires specific external influences.
Commitment along the dorsoventral axis of the sea urchin embryo is altered in response to NiCl2
Authors: Hardin Jeff , Coffman James A. , Black Steven D. , Mcclay David R.
Published: April 2021

Few treatments are known that perturb the dorsoventral axis of the sea urchin embryo. We report here that the dorsoventral polarity of the sea urchin embryo can be disrupted by treatment of embryos with NiCl2. Lytechinus variegatus embryos treated with 0.5 mM NiCl2 from fertilization until the early gastrula stage appear morphologically normal until the midgastrula stage, when they fail to acquire the overt dorsoventral polarity characteristic of untreated siblings. The ectoderm of normal embryos possesses two ventrolateral thickenings just above the vegetal plate region. In nickel-treated embryos, these become expanded as a circumferential belt around the vegetal plate. The ectoderm just ventral to the animal pole normally invaginates to form a stomodeum, which then fuses with the tip of the archenteron to produce the mouth. In nickeltreated embryos, the stomodeal invagination is expanded to become a circumferential constriction, and it eventually pinches off as the tip of the archenteron fuses with it to produce a mouth. Primary mesenchyme cells form a ring in the lateral ectoderm, but as many as a dozen spicule rudiments can form in a radial pattern. Dorsoventral differentiation of ectodermal tissues is profoundly perturbed: nickel-treated embryos underexpress transcripts of the dorsal (aboral) gene LvS1, they overexpress the ventral (oral) ectodermal gene product, EctoV, and the ciliated band is shifted to the vegetal margin of the embryo. Although some dorsoventral abnormalities are observed, animal-vegetal differentiation of the archenteron and associated structures seems largely normal, based on the localization of region-specific gene products. Gross differentiation of primary mesenchyme cells seems unaffected, since nickel-treated embryos possess the normal number of these cells. Furthermore, when all primary mesenchyme cells are removed from nickel-treated embryos, some secondary mesenchyme cells undergo the process of “conversion” (Ettensohn, C. A. and McClay, D. R. (1988) Dev. Biol. 125, 396-409), migrating to sites where the larval skeleton would ordinarily form and subsequently producing spicule rudiments. However, the skeletal pattern formed by the converted cells is completely radialized. Our data suggest that a major effect of NiCl2 is to alter commitment of ectodermal cells along the dorsoventral axis. Among the consequences appears to be a disruption of pattern formation by mesenchyme cells.
Characterization of a germ-line proliferation mutation in C. elegans
Authors: Beanan Maureen J. , Strome Susan
Published: April 2021

The C. elegans germ line is generated by extensive proliferation of the two germ-line progenitor cells present in newly hatched larvae. We describe genetic and phenotypic characterization of glp-4, a locus whose product is required for normal proliferation of the germ line. glp-4(bn2ts) mutant worms raised at the restrictive temperature contain approximately 12 germ nuclei, in contrast to the 700-1000 present in wild-type adults. The few germ cells present in sterile glp-4 adults appear to be arrested at prophase of the mitotic cell cycle. This cell-cycle disruption prevents the germ cells from entering meiosis and differentiating into gametes. Shifting sterile glp-4 worms to the permissive temperature enables their germ cells to undergo extensive proliferation and form gametes, demonstrating that the bn2-induced cell-cycle arrest is reversible and that proliferation and differentiation of germ cells can be uncoupled from development of the somatic gonad. The glp-4(bn2ts) mutation can be used to generate large populations of worms that are severely depleted in germ cells, facilitating determination of whether any gene of interest is expressed in the germ line or soma or both.
Regulation of expression of fibronectin and its receptor, α5β1, during development and regeneration of peripheral nerve
Authors: Lefcort Frances , Venstrom Kristine , McDonald John A. , Reichardt Louis F.
Published: April 2021

The extracellular matrix glycoprotein, fibronectin, is a potent promoter of peripheral neurite outgrowth. Interactions of peripheral neurons with fibronectin have been shown to be primarily mediated by the β1 class of integrin heterodimers. In the present study, we have examined the expression and regulation of fibronectin and its integrin receptor, α5 β1, in developing and regenerating chick peripheral nerve. We show that fibronectin and α5β1 α5β1 are expressed at comparatively high levels in developing nerve with α5β1 expression on axons and non-neuronal cells. With nerve maturation, both proteins are less prominently expressed and the cellular pattern of α5β1,expression becomes more restricted. Following lesion of mature nerve, both fibronectin and α5β1, are strongly induced with prominent expression of α5β1, on regenerating neurites and Schwann cells. The elevation in fibronectin levels in the regenerating nerve is highest in the vicinity of the lesion, an area undergoing extensive cellular remodeling including Schwann cell migration and growth cone extension. Our results suggest that fibronectin and its receptor, α5β1, may mediate functionally important interactions in the development and regeneration of peripheral nerve.
Local and transient expression of E-cadherin involved in mouse embryonic brain morphogenesis
Authors: Shimamura Kenji , Takeichi Masatoshi
Published: April 2021

We found that E-cadherin (uvomorulin) is transiently expressed in restricted regions of the metencephalon, mesencephalon and diencephalon of mouse embryonic brain. This expression first occurred in parts of the mes-encephalon and diencephalon at around E9.5, and subsequently extended to the primordia of cerebellum, the dorsal midline of mesencephalon and some other regions of the embryonic brain. These E-cadherin expressions ceased by E15 except at the dorsal midline. Immuno-histological analyses showed that E-cadherin-positive cells are radially arranged in the neural tube and the E-cadherin-positive regions are sharply demarcated from E-cadherin-negative regions. Axons extending from some of the E-cadherin-positive regions also expressed this molecule. When embryonic brains were dissociated into single cells and cultured as monolayers, E-cadherin-positive cells formed clusters that were seg-regated from E-cadherin-negative cells. E9.5 brain fragments containing metencephalon and mesencephalon were isolated, explanted on Nuclepore filters and cul-tured in the absence or presence of antibodies to E-cad-herin. This antibody treatment removed most of the E-cadherin molecules from the explants and consequently affected their growth pattern. To analyze cellular events induced by the antibody treatment, we stained these explants with an antiserum to En whose distribution was found to overlap in part with that of E-cadherin and found that the pattern of En staining was altered by the anti-E-cadherin antibody treatment. These results suggest that the local and transient expression of E-cad-herin in embryonic brain is involved in regional pattern formation in this organ.
The role of initial cells in maize anther morphogenesis
Authors: Dawe R. Kelly , Freeling Michael
Published: April 2021

The near absence of cell movement in plants makes clonal analysis a particularly informative method for reconstructing the early events of organ formation. We traced the patterns of cell division during maize anther development by inducing sector boundaries that preceded the earliest events of anther initiation. In doing this, we were able to estimate the smallest number of cells that are fated to form an anther, characteristic cell division patterns that occur during anther morphogenesis, and the relationship between the pre-existing symmetry of the initial cells and the final symmetry of the mature anther. Four general conclusions are made: (1) anthers are initiated from small groups of 12 or fewer cells in each of two floral meristematic layers; (2) the early growth of the anther is more like a shoot than a glume or leaf; (3) cell ancestry does not dictate basic structure and (4) the orientation of initial cells predicts the orientation of the four pollen-containing microsporangia, which define the axes of symmetry on the mature anther. The final point is discussed with other data, and an explanation involving a ‘structural template’ is invoked. The idea is that the orientation of initial cells within the floral meristem establishes an architectural pattern into which anther cells are recruited without regard to their cellular lineages. The structural template hypothesis may prove to be generally applicable to problems of pattern formation in plants.
Tissue-specific ecdysone responses: regulation of the Drosophila genes Eip28/29 and Eip40 during larval development
Authors: Andres Andrew J. , Cherbas Peter
Published: April 2021

The Drosophila genes Eip28/29 and Eip40 are expressed in Kc cells and are rapidly induced by the steroid hor-mone ecdysone. The molecular basis for Eip28/29’s regulation in those cells has been studied in some detail. To determine how this regulation relates to normal development, we have examined the expression of both genes throughout Drosophila development, with special attention to Eip28/29 and the final larval instar. Eip28/29 expression is complex; there are tissues in which it is never expressed, others in which it is continuously expressed at a low level and tissues in which its expression is regulated without obvious relationship to endocrine events. However high-level Eip28/29 expression always correlates with the presence of ecdysone and there is good evidence that Eip28/29 is directly regulated by the hormone in some tissues and at some stages. Most striking are the induction of Eip28/29 transcripts in numerous tissues at the last larval molt, their induction in the epidermis at the time of the ‘late 3rd transition’, their extinction in the same tissue by the premetamorphic ecdysone peak, and their induction by that peak in the lymph gland, hemocytes and proventriculus. These contrasting regulatory behaviors provide a well-defined model for studying the developmental specificity of steroid responses.

Eip40 appears to be ecdysone-inducible only in the lymph gland and there only at the premetamorphic peak. The similarities been Eip28/29 and Eip40 regulation in the lymph gland and Kc cells support the idea that Kc cells are derived from a hematopoietic ancestor.
The epithelium of the dorsal marginal zone of Xenopus has organizer properties
Authors: Shih John , Keller Ray
Published: April 2021

We have investigated the properties of the epithelial layer of the dorsal marginal zone (DMZ) of the Xeno - pus laevis early gastrula and found that it has inductive properties similar to those of the entire Spemann orga- nizer. When grafts of the epithelial layer of the DMZ of early gastrulae labelled with fluorescein dextran were transplanted to the ventral sides of unlabelled host embryos, they induced secondary axes composed of notochord, somites and posterior neural tube. The orga- nizer epithelium rescued embryos ventralized by UV irradiation, inducing notochord, somites and posterior neural tube in these embryos, while over 90% of ven- tralized controls showed no such structures. Combina- tions of organizer epithelium and ventral marginal zone (VMZ) in explants of the early gastrula resulted in con- vergence, extension and differentiation of dorsal meso- dermal tissues, whereas similar recombinants of non- organizer epithelium and the VMZ did none of these things. In all cases, the axial structures forming in response to epithelial grafts were composed of labelled graft and unlabelled host cells, indicating an induction by the organizer epithelium of dorsal, axial morpho- genesis and tissue differentiation among mesodermal cells that otherwise showed non-axial development.

Serial sectioning and scanning electron microscopy of control grafts shows that the epithelial organizer effect occurs in the absence of contaminating deep cells adher- ing to the epithelial grafts. However, labelled organizer epithelium grafted to the superficial cell layer con- tributed cells to deep mesodermal tissues, and organizer epithelium developed into mesodermal tissues when deliberately grafted into the deep region. This shows that these prospective endodermal epithelial cells are able to contribute to mesodermal, mesenchymal tissues when they move or are moved into the deep environ- ment. These results suggest that in normal development, the endodermal epithelium may influence some aspects of the cell motility underlying the mediolateral interca- lation (see Shih, J. and Keller, R. (1992) Development 116, 901-914), as well as the tissue differentiation of mesodermal cells. These results have implications for the analysis of mesoderm induction and for analysis of variations in the differentiation and morphogenetic function of the marginal zone in different species of amphibians.
Induction of a RARβ2-lacZ transgene by retinoic acid reflects the neuromeric organization of the central nervous system
Authors: Zimmer Andreas , Zimmer Anne
Published: April 2021

The hormone retinoic acid (RA) has been implicated in the organization of the anteroposterior (AP) body axis. In this paper, we describe the effects of RA on the activity of the RA-inducible retinoic acid receptor-2 (RAR 2) promoter. When transgenic embryos carrying a RAR 2-lacZ reporter gene were exposed to a single dose of RA between gestational days 8.5 to 10.5, lacZ expression was induced in the anterior central nervous system (CNS). Strikingly, the transgene was expressed in a segmented pattern reminiscent of that of Drosophila ‘pair-rule’ genes. RA treatment of midgastrulation embryos at day 7.5 disturbed the segmentation and pro-duced severe craniofacial defects. We discuss the possi-bility that the entire anterior CNS is segmented and that this segmentation is reflected by the RAR 2-lacZ induc-tion pattern.
Relationships between mesoderm induction and the embryonic axes in chick and frog embryos
Authors: Stern Claudio D. , Hatada Yohko , Selleck Mark A. J. , Storey Kate G.
Published: April 2021

The hypoblast is generally thought to be responsible for inducing the mesoderm in the chick embryo because the primitive streak, and subsequently the embryonic axis, form according to the orientation of the hypoblast. However, some cells become specified as embryonic mesoderm very late in development, towards the end of the gastrulation period and long after the hypoblast has left the embryonic region. We argue that induction of embryonic mesoderm and of the embryonic axis are different and separable events, both in amniotes and in amphibians. We also consider the relationships between the dorsoventral and anteroposterior axes in both groups of vertebrates.
Fate mapping the neural plate and the intraembryonic mesoblast in the upper layer of the chicken blastoderm with xenografting and time-lapse videography
Authors: Bortier Hilde , Vakaet L. C. A.
Published: April 2021

The disposition of the Anlage fields of the neural plate and the intraembryonic mesoblast in the upper layer of the chicken blastoderm was studied at the primitive streak stage prior to the regression of Hensen’s node (stages 5V to 6V, L. Vakaet (1970)Arch. Biol. 81, 387426). Chicken blastoderms were cultured by New’s technique on a mixture of thin egg white and agar. The anterior half of the deep layer was reflected with a tungsten needle. A circular fragment of the upper layer was punched out with a pulled out Pasteur pipette and discarded. It was replaced with an isotopic and isopolar piece of quail upper layer that was punched out with the same pipette. The deep layer was replaced and the chimeras were reincubated for 24 hours. The xenografts were followed with time-lapse videography. After fixation, the quail cells were located using Le Douarin’s quail nucleolar marker technique. Integrating the observations with time-lapse videography and the results of Feulgen stained sections, we have drawn a new fate map of the disposition of the Anlage fields in the upper layer of the chicken blastoderm at stages prior to the regression of Hensen’s node (stages 5V to 6V). The disposition of the neural plate and of the notochord, somites, nephrotome and lateral plates was therefore determined before the Anlage fields are morphologically discernible. The pathway of the fields in the upper layer towards their disposition was documented with time-lapse videography in chimeric chicken blastoderms that developed normally.
Homeostatic balance between dorsal and cactus proteins in the Drosophila embryo
Authors: Govind Shubha , Brennan Lynn , Steward Ruth
Published: April 2021

The maternal-effect gene dorsal encodes the ventral morphogen that is essential for elaboration of ventral and ventrolateral fates in the Drosophila embryo. Dorsal belongs to the rel family of transcription factors and controls asymmetric expression of zygotic genes along the dorsoventral axis. The dorsal protein is cytoplasmic in early embryos, possibly because of a direct interaction with cactus. In response to a ventral signal, dorsal protein becomes partitioned into nuclei of cleavagestage syncytial blastoderms such that the ventral nuclei have the maximum amount of dorsal protein, and the lateral and dorsal nuclei have progressively less protein. Here we show that transgenic flies containing the dorsal cDNA, which is driven by the constitutively active hsp83 promoter, exhibits rescue of the dorsal phenotype. Transformed lines were used to increase the level of dorsal protein. Females with dorsal levels roughly twice that of wild-type produced normal embryos, while a higher level of dorsal protein resulted in phenotypes similar to those observed for loss-of-function cactus mutations. By manipulating the cactus gene dose, we found that in contrast to a dorsal/cactus ratio of 2.5 which resulted in fully penetrant weak ventralization, a cactus/dorsal ratio of 3.0 was acceptable by the system. By manipulating dorsal levels in different cactus and dorsal group mutant backgrounds, we found that the relative amounts of ventral signal to that of the dorsalcactus complex is important for the elaboration of the normal dorsoventral pattern. We propose that in a wildtype embryo, the activities of dorsal and cactus are not independently regulated; excess cactus activity is deployed only if a higher level of dorsal protein is available. Based on these results we discuss how the ventral signal interacts with the dorsal-cactus complex, thus forming a gradient of nuclear dorsal protein.
Apical-basal pattern formation in the Arabidopsis embryo: studies on the role of the gnom gene
Authors: Mayer Ulrike , Büttner Gabriele , Jürgens Gerd
Published: April 2021

gnom is one of several genes that make substantial contributions to pattern formation along the apical-basal axis of polarity in the Arabidopsis embryo as indicated by the mutant seedling phenotype. The apical and basal end regions of the body pattern, which include the meristems of the shoot and the root, fail to form, and a minority of mutant embryos lack morphological features of apical-basal polarity. We have investigated the developmental basis of the gnom mutant phenotype, taking advantage of a large number of EMS-induced mutant alleles. The seedling phenotype has been traced back to the early embryo in which the asymmetric division of the zygote is altered, now producing two nearly equal-sized cells. The apical daughter cell then undergoes abnormal divisions, resulting in an octant embryo with about twice the normal number of cells while the uppermost derivative of the basal cell fails to become the hypophysis, which normally contributes to root development. Consistent with this early effect, gnom appears to be epistatic to monopteros in doubly mutant embryos, suggesting that, without prior gnom activity, the monopteros gene cannot promote root and hypocotyl development. On the other hand, when root formation was induced in bisected seedlings, wild-type responded whereas gnom mutants failed to produce a root but formed callus instead. These results suggest that gnom activity promotes asymmetric cell division which we believe is necessary both for apical-basal pattern formation in the early embryo and for root formation in tissue culture.
The Drosophila gene pointed encodes two ETS-like proteins which are involved in the development of the midline glial cells
Authors: Klämbt Christian
Published: April 2021

Glial-neuronal cell interactions at the ventral midline are necessary for the proper elaboration of commissures in the embryonic CNS of Drosophila. In particular, migrating midline glial cells are required for the separation of segmental commissures. During this process the glial cells recognize specific neuronal cells at the midline, they migrate posteriorly along their cell processes and thereby separate the segmental commissures. The gene pointed (pnt) is required for this glialneuronal cell interaction, as loss of function mutations lead to a change in the migration behavior of the midline glial cells. As a consequence, anterior and posterior commissures do not become separated and appear fused. Molecular analysis of pointed has revealed two differently spliced types of transcripts, which are encoded in a region extending over 55 kb of genomic sequence. In the CNS both transcript classes are expressed in cells of the midline, including the midline glial cells. Sequence analysis of cDNA clones corresponding to both transcript types reveals two different pointed proteins which share an ETS domain common to a number of transcription factors related to the vertebrate ets oncogene. Furthermore, one pointed protein form contains an additional domain of homology of approx. 80 amino acids in length, which is shared by only a subset of the ETS protein family.
Type VI collagen expression is upregulated in the early events of chondrocyte differentiation
Authors: Quarto Rodolfo , Dozin Beatrice , Bonaldo Paolo , Cancedda Ranieri , Colombatti Alfonso
Published: April 2021

Dedifferentiated chondrocytes cultured adherent to the substratum proliferate and synthesize large amounts of type I collagen but when transferred to suspension culture they decrease proliferation, resume the chondrogenic phenotype and the synthesis of type II collagen, and continue their maturation to hypertrophic chondrocyte (Castagnola et al., 1986, J.Cell Biol. 102, 2310-2317). In this report, we describe the developmentally regulated expression of type VI collagen in vitro in differentiating avian chondrocytes. Type VI collagen mRNA is barely detectable in dedifferentiated chondrocytes as long as the attachment to the substratum is maintained, but increases very rapidly upon passage of the cells into suspension culture reaching a peak after 48 hours and declining after 5-6 days of suspension culture. The first evidence of a rise in the mRNA steadystate levels is obtained already at 6 hours for the 3(VI) chain. Immunoprecipitation of metabolically labeled cells with type VI collagen antibodies reveals that the early mRNA rise is paralleled by an increased secretion of type VI collagen in cell media. Induction of type VI collagen is not the consequence of trypsin treatment of dedifferentiated cells since exposure to the actin-dis-rupting drug cytochalasin or detachment of the cells by mechanical procedures has similar effects. In 13-day-old chicken embryo tibiae, where the full spectrum of the chondrogenic differentiation process is represented, expression of type VI collagen is restricted to the articular cartilage where chondrocytes developmental stage is comparable to stage I (high levels of type II collagen expression). The steep rise in type VI collagen synthesis suggests that the transition of chondrocytes from a stage characterized by high levels of type I collagen to a later stage with a predominance of type II collagen (stage I chondrocytes) might be subdivided further in an early phase (stage Ia) characterized by a high and transient type VI collagen expression and a later phase (stage Ib) where type II collagen is predominant. These data might suggest a functional relationship between type VI collagen expression and the chondrogenic maturation process.
A Y-chromosomal effect on blastocyst cell number in mice
Authors: Burgoyne Paul S.
Published: April 2021

Karyotypic and cell number analysis of 3.5 day post coitum preimplantation mouse embryos was used to determine whether XY embryos had more cells than XX embryos at the late morula/early blastocyst stage. This proved to be the case for the CD1 strain (for which it had previously been shown that XY embryos form a blastocoel earlier than XX embryos) and for the MF1 strain. However, this increased cell number was not seen in MF1 embryos carrying an RIII strain Y in place of the MF1 Y. Furthermore, interstrain crosses between CD1 and the MF1,YRIII strain showed that the cell number increase segregated with the CD1 Y but not with the RIII Y. It is concluded that the CD1 and MF1 Y chromosomes carry a factor that accelerates the rate of preimplantation development.
The triple origin of skull in higher vertebrates: a study in quail-chick chimeras
Authors: Couly Gérard F. , Coltey Pierre M. , Douarin Nicole M. Le
Published: April 2021

We have used the quail-chick chimera technique to study the origin of the bones of the skull in the avian embryo. Although the contribution of the neural crest to the facial and visceral skeleton had been established previously, the origin of the vault of the skull (i.e. frontal and parietal bones) remained uncertain. Moreover formation of the occipito-otic region from either the somitic or the cephalic paraxial mesoderm had not been experimentally investigated. The data obtained in the present and previous works now allow us to assign a precise embryonic origin from either the mesectoderm, the paraxial cephalic mesoderm or the five first somites, to all the bones forming the avian skull. We distinguish a skull located in front of the extreme tip of the notochord which reaches the sella turcica and a skull located caudally to this boundary. The former (‘prechordal skull’) is derived entirely from the neural crest, the latter from the mesoderm (cephalic or somitic) in its ventromedial part (‘chordal skull’) and from the crest for the parietal bone and for part of the otic region. An important point enlighten in this work concerns the double origin of the corpus of the sphenoid in which basipresphenoid is of neural crest origin and the basipostsphenoid is formed by the cephalic mesoderm. Formation of the occipito-otic region of the skeleton is particularly complex and involves the cooperation of the five first somites and the paraxial mesoderm at the hindbrain level. The morphogenetic movements leading to the initial puzzle assembly could be visualized in a reproducible way by means of small grafts of quail mesodermal areas into chick embryos.

The data reported here are discussed in the evolutionary context of the ‘New Head’ hypothesis of Gans and Northcutt (1983, Science, 220, 268-274).