bluuebunny/crossref_metadata_2025 huggingface dataset:
Genetic analysis of the floral initiation process (FLIP) in Arabidopsis
Authors: Schultz Elizabeth A. , Haughn George W.
Published: April 2021

Within the Arabidopsis inflorescence, two distinct developmental phases exist. The early inflorescence phase is characterized by nodes bearing coflorescences and leaves, and the late inflorescence phase by nodes bearing flowers. Four genes, TERMINAL FLOWER 1, LEAFY, APETALA1 and APETALA2 are necessary to initiate the switch from formation of early to formation of late inflorescence nodes at the appropriate time. We have investigated the relative roles of these genes in development by isolating and characterizing new alleles of TERMINAL FLOWER 1, LEAFY and APETALA1, and by constructing double mutants to test gene interactions. We suggest that the TERMINAL FLOWER 1 gene product is part of a mechanism that controls the timing of phase-switching in Arabidopsis. We propose that this mechanism involves factor(s) whose activity changes in response to shoot development and environmental variation. TERMINAL FLOWER 1 influences phase transitions in Arabidopsis, and appears to regulate the timing of expression of LEAFY, APETALA1 and APETALA2. LEAFY, APETALA1 and APETALA2 have partially redundant functions in initiating the floral program. In the absence of any one of the three genes, there is a gradual transition from coflorescence to flower-like lateral shoots. This suggests that (1) LEAFY, APETALA1 and APETALA2 are required in combination to ensure that the floral program is initiated rapidly and completely and (2) in the absence of one of the three genes, the others are activated slowly in response to the mechanism controlling timing of phase switching. Besides their role in establishing the floral program, phenotypes of flower-like lateral shoots in mutant inflorescences suggest that all three, LEAFY, APETALA1 and APETALA2, influence expression of whorl identity genes. Loss of LEAFY results in decreased Class B gene expression, as well as altered expression patterns of Class A and Class C genes. In the absence of either APETALA2 or APETALA1, reproductive organs develop in the perianth whorls, suggesting that both genes should be considered Class A organ identity genes, restricting Class C gene expression to inner whorls.
Sex-lethal, master and slave: a hierarchy of germ-line sex determination in Drosophila
Authors: Oliver Brian , Kim Young-Joon , Baker Bruce S.
Published: April 2021

Female sex determination in the germ line of Drosophila melanogaster is regulated by genes functioning in the soma as well as genes that function within the germ line. Genes known or suspected to be involved in germ-line sex determination in Drosophila melanogaster have been examined to determine if they are required upstream or downstream of Sex-lethal+, a known germ-line sex determination gene. Seven genes required for female-specific splicing of germ-line Sex-lethal+ pre-mRNA are identified. These results together with information about the tissues in which these genes function and whether they control sex determination and viability or just sex determination in the germ line have been used to deduce the genetic hierarchy regulating female germ-line sex determination. This hierarchy includes the somatic sex determination genes transformer+, transformer-2+ and doublesex+ (and by inference Sex-lethal+), which control a somatic signal required for female germ-line sex determination, and the germ-line ovarian tumor genes fused+, ovarian tumor+, ovo+, sans fille+, and Sex-lethal+, which are involved in either the reception or interpretation of this somatic sex determination signal. The fused+, ovarian tumor+, ovo+ and sans fille+ genes function upstream of Sex-lethal+ in the germ line.
Ultraviolet irradiation impairs epiboly in zebrafish embryos: evidence for a microtubule-dependent mechanism of epiboly
Authors: Strähle Uwe , Jesuthasan Suresh
Published: April 2021

Early morphogenesis of the teleost embryo is characterized by three orchestrated cell movements. Epiboly leads to spreading of the blastoderm over an uncleaved yolk cell while involution around the blastoderm margin and convergence movements towards the dorsal side generate the mesendodermal inner cell sheet and the axis rudiment, respectively. Irradiation of zebrafish zygotes with ultraviolet light selectively impairs epiboly resulting in embryos with open blastopores but well-formed anterior axes. Gastrulation movements are only marginally affected by ultraviolet irradiation. Involution of marginal cells in epiboly-retarded embryos takes place prior to 50% epiboly and thus appears independent of epiboly. Expression of dorsal and anterior marker genes is unaffected by ultraviolet irradiation.

The ultraviolet light effect is not restricted to the zygote stage as irradiation of later embryonic stages also impairs epiboly. The ultraviolet-sensitive targets may thus be maternally encoded components of the machinery driving epiboly. These targets appear to be microtubules: firstly, irradiated embryos show disorganized and less microtubules in the cytoplasmic layer of the yolk sphere; secondly, the ultraviolet light effect can be mimicked by the microtubule-depolymerising agent nocodazole. We suggest that epiboly is driven, at least partially, by motors that use microtubules radiating from the yolk syncytial layer into the yolk cytoplasmic layer. Together with an observed constrictive behaviour of the blastoderm margin, we propose a two-force model of epiboly: epiboly is initiated and driven by a pulling force dependent on microtubules in the yolk cytoplasmic layer; contraction at the margin operates in addition to aid closure of the blastopore.
Basic fibroblast growth factor induces differentiation of neural tube and neural crest lineages of cultured ectoderm cells from Xenopus gastrula
Authors: Kengaku Mineko , Okamoto Harumasa
Published: April 2021

The vertebrate nervous system is initially induced from a section of dorsal ectoderm by signal(s) from the underlying dorsal mesoderm during gastrulation. In an effort to identify the neural inducing factor(s) emanating from the dorsal mesoderm, we have examined the inductive action of various growth factors by applying them to ectoderm cells from Xenopus gastrulae (8to 12.5-hour age; embryonic stage 9+ to ) in a microculture system. Monoclonal antibodies that specifically recognize cellular differentiation antigens from three distinct ectoderm lineages (N1 for CNS neurons from neural tube, Me1 for melanophores from neural crest and E3 for skin epidermal cells from epidermal lineages, respectively) and a mesoderm lineage (Mu1 for muscle cells) were used as markers to monitor the differentiation of cultured ectoderm cells.

We found that basic fibroblast growth factor (bFGF) was capable of specifically and reproducibly inducing gastrula ectoderm cells to produce CNS neurons and melanophores at concentrations as low as 5 pM, a value about 50-fold lower than that required to induce the formation of muscle cells from blastula animal cap cells (6-hour age; stage 8+). The induction of neural lineages by bFGF was correlated with a suppression of epidermal differentiation in a dose-dependent manner. bFGF never induced the formation of muscle cells from gastrula ectoderm cells even at concentrations as high as 5 nM.

The response of ectoderm cells to bFGF changed dramatically during gastrulation. Ectoderm cells from early (8to 9-hour age; stage 9+ to 10) gastrula gave rise to CNS neurons, but yielded few melanophores. As ectoderm cells were prepared from gastrulae of increasing age, their competence to form neurons was gradually lost, whereas the production of melanophores was enhanced and peaked in 11-hour gastrula (stage ). The ability to form both neurons and melanophores was substantially reduced in 12.5-hour gastrula (stage ). By examining ectoderm cells from the ventral and dorsal sides independently, it was also shown that during gastrulation the change in response to bFGF of the ventral ectoderm preceded that of the dorsal ectoderm.

The state of competence of the ectoderm changed primarily due to intrinsic factors rather than by instruction from other parts of the gastrula embryo. This was shown by adding bFGF to cultures of ectoderm cells that were isolated at 9-hour (stage 10) and cultured for increasing periods to allow their autonomous development. The time course of both loss of neuronal competence and gain and loss of melanophore competence closely paralleled that observed in vivo during gastrulation. Further, we showed that the sensitivity of ectoderm cells to bFGF in the neuronal and melanophore induction processes decreased during later gastrula stages; i.e., higher doses of bFGF were required for older ectoderm cells to be fully induced. A possible role of bFGF in neural induction during normal development is discussed.
Expression of a lacZ transgene reveals floor plate cell morphology and macromolecular transfer to commissural axons
Authors: Campbell Robert M. , Peterson Alan C.
Published: April 2021

The floor plate is situated at the ventral midline of the neural tube and is an important intermediate target for commissural axons. During elongation, these axons converge bilaterally on the ventral midline neural tube and after crossing the floor plate make an abrupt rostral turn. Ample evidence indicates that the initial projection of commissural axons to the floor plate is guided by a chemotropic factor secreted by floor plate cells. However, the way in which the subsequent interaction of these axons with the floor plate leads them to make further trajectory changes remains undefined. In an effort to gain further understanding of the structure and function of floor plate cells, we have taken advantage of a line of transgenic mice in which these cells express galactosidase and thus can be stained by histochemical means. In this line, a genomic imprinting mechanism restricts the expression of the lacZ transgene to only a proportion of the floor plate cells, allowing their morphology to be appreciated with particular clarity. Our analysis revealed that the basal processes of floor plate cells are flattened in their rostrocaudal dimension and possess fine lateral branches which are aligned with commissural axons. Unexpectedly, -galactosidase activity was also detected within longer transverse linear profiles traversing the floor plate whose ultrastructural appearance was not that of floor plate cells but instead corresponded to that of commissural axons. Enzyme activity was not detected in more proximal axonal segments or in the neuronal cell bodies from which these axons originated. Therefore, we propose that the transgene product, and potentially other proteins synthesized by floor plate cells, can be transferred to decussating axons.
pannier, a negative regulator of achaete and scute in Drosophila, encodes a zinc finger protein with homology to the vertebrate transcription factor GATA-1
Authors: Ramain Philippe , Heitzler Pascal , Haenlin Marc , Simpson Pat
Published: April 2021

The gene pannier acts as a repressor of achaete and scute, two transcription factors expressed in discrete subsets of cells at the sites where neural precursors develop. Molecular analysis of mutant alleles revealed the presence of two functional domains within the pannier protein: a zinc finger domain showing homology to the GATA-1 family of vertebrate transcription factors and a domain comprising two putative amphipathic helices. Mutants associated with lesions in the zinc finger domain display an overexpression of achaete and scute and the development of extra neural precursors. Mutant proteins in which the domain including the putative helices is deleted act as hyperactive repressor molecules causing a loss of achaete/scute expression and a loss of neural precursors. Other results suggest that the activity of pannier may be modulated by association with position-specific factors.
Expression and function of the keratinocyte integrins
Authors: Watt Fiona M. , Jones Philip H.
Published: April 2021

Human keratinocytes express several adhesive receptors of the integrin family. Expression is normally confined to the basal (proliferative) layer of keratinocytes, both in mature epidermis and during development. Altered expression patterns are observed during wound healing, in psoriasis and in squamous cell carcinomas. Keratinocyte integrins are subject to both transcriptional and post-translational regulation and ligand binding ability can be modulated independently of expression. Studies with cultured keratinocytes suggest a variety of functions for the receptors: adhesion to extracellular matrix proteins, intercellular adhesion, stratification, lateral migration and the regulation of terminal differentiation. Three distinct subpopulations of basal keratinocytes, with characteristics of stem cells, transit amplifying cells and cells committed to differentiate, can be distinguished on the basis of differences in integrin expression and function.
Oligodendrocyte precursor (O-2A progenitor cell) migration; a model system for the study of cell migration in the developing central nervous system
Authors: Kiernan B. W. , ffrench-Constant Charles
Published: April 2021

Cell migration plays an important role in the development of complex multicellular organisms. The molecular mechanisms that regulate this migration are therefore of great interest. Unfortunately, however, analysis of cell migration in vertebrates is hampered by the inaccessability of the cells and the difficulty of manipulating their environment within the embryo. This review focusses on one particular migratory cell population, the oligodendrocyte precursor cell or O-2A progenitor cell, that gives rise to the myelin-forming oligodendrocytes within the CNS. These cells migrate extensively during normal development. They can be purified and grown in large numbers in cell culture, so allowing the use of reductionist approaches using cell and molecular biology techniques. Moreover, cultured cells will migrate within the CNS following transplantation. As a result, the migration of these cells in vivo can be analysed following manipulation in vitro. Taken together, we believe that the different properties of these cells makes them excellent candidates for studies addressing the control of cell migration in the developing nervous system.
puckered, a gene involved in position-specific cell differentiation in the dorsal epidermis of the Drosophila larva
Authors: Ring J. M. , Arias A. Martinez
Published: April 2021

The final pattern of the cuticle of the Drosophila larva depends on the position-specific behaviour of the epidermal cells during their differentiation. This behaviour is dictated, in part, by the relative position of the cells during embryogenesis which allows them to receive and integrate signals from their neighbours. The translation of this ‘positional information’ into pattern might depend on the activity of genes that are able to integrate the outcome of cell interactions and tranfer it to the genes responsible for cell differentiation. Mutations in the gene puckered cause spatially restricted defects during the differentiation of the larval epidermal cells. We present data that suggests puckered may be involved in linking positional information to cell differentiation.
An immunological study on the effect of brain extract on the developing nervous tissue in the chick embryo
Authors: McCallion D. J. , Langman J.
Published: April 2021

In previous experiments it has been shown that saline soluble extracts of adult chicken brain, when deposited on 24-to 32-hr. chick embryos, cause abnormalities of the brain and the eye. Since neither the soluble components of the brain extract, nor the chemical composition of the embryonic brain cells is known, the present study was undertaken to analyse by means of immunological techniques the soluble fractions of adult chicken brain; to study the appearance of these fractions in the course of embryonic development, and to determine the organ and species specificity of the various brain components. It was hoped that the information thus obtained might give a better understanding on the mode of action of adult tissue homogenates on the corresponding tissue in the embryo. The following results were obtained:

Adult brain extract, tested against chick brain antiserum by means of the agar diffusion technique causes the formation of five to eleven precipitin bands, indicating the presence of substantial amounts of five to eleven saline-soluble antigenic brain components. The total precipitin pattern consists of two distinct groups of bands : a group of four to eight rather vaguely defined bands, located close to the antiserum well and referred to as the ‘inner’ bands, and a group of one to three bands close to the antigen well and referred to as the ‘outer’ bands (Plate, 1,2). The antigenic brain components giving rise to the ‘inner’ bands arise during the first 5 days of development, that is the period during which neural plate formation, neural folding, and differentiation of the primitive brain vesicles occur. They have no organ specific characteristics, since they are present in identical form in all the other organs and tissues of the chick (Tables 1 and 2). The antigens giving rise to the ‘outer’ bands appear between the 5th and 12th day of development, that is the period during which the definitive cyto-architecture of the brain is established. They are also present in extracts of nervous retina, spinal cord and peripheral nerves, but are absent in all other tissues and organs of the chick. Hence, these antigens are highly specific for nervous tissue (Tables 1 and 2). The chick brain antigens causing the ‘inner’ and the ‘outer’ precipitin bands are present in antigenically identical form in brain tissue of the turkey and duck, to a lesser extent in that of some of the reptiles, but are absent in brain tissue of mammals, amphibians and fishes (Table 3). When the soluble components of the chick brain are separated by means of continuous flow electrophoresis and tested against brain antiserum, the most negatively charged fractions are found to form the ‘outer’ bands, while the positively charged molecules form the unspecific ‘inner’ precipitin bands.
The mechanism of vitamin A induced teratogenesis
Authors: Takekoshi Shoichi
Published: April 2021

It Is generally recognized that some congenital malformations may be caused by an abnormal environment surrounding the mother in pregnancy. Many experiments have been reported concerning the teratogenesis due to various hypovitaminoses, but there are relatively few reports on teratogenesis resulting from the administration of a large quantity of vitamin. Among the latter, the best known are malformations caused by hypervitaminoses A and D.
Etude du mécanisme de la régulation dans ledéveloppement du bourgeon de membre de l’embryon de poulet
Authors: Kieny par Madeleine
Published: April 2021

On Distingue communément deux types de régulation proximo-distale des déficiences dans le développement du bourgeon de membre: la régulation des déficiences terminales et la régulation des déficiences intermédiaires.
Permeability alterations in amphibian embryos caused by salt solution and measured by tritiated thymidine uptake
Authors: Loeffler Clarence A. , Johnston Malcolm C.
Published: April 2021

In attempts to label the nuclei of intact amphibian embryos with tritiated thymidine we found, in agreement with other authors (Tencer, 1961; Quertier, 1962; Chibon, 1962), that when the nucleoside is dissolved in tap water or media of low salt concentration it fails to enter the embryo. However, when tritiated thymidine was either injected into the embryo or applied to isolated tissues immersed in salt solution, the marker diffused readily through the tissues. Evidently the outer surface of the amphibian embryo is furnished with a special permeability barrier which prevents or reduces the entrance of these molecules. This barrier corresponds most likely to what Holtfreter (1943a, 1943b) has described as the ‘surface coat’. In order that the results of our present experiments may be better understood some of the pertinent properties of the coat will be reviewed.
The transfer of coelomic eggs between frogs
Authors: Lavin Lora Harrer
Published: April 2021

In the analysis of events immediately preceding and following fertilization of the frog egg, it is frequently desirable to treat the eggs with various reagents. Any such treatment must circumvent the barrier represented by the fertilization membrane and the jelly layers. These membranes can be removed from fertilized eggs mechanically or by treatment with proteolytic enzymes (Spiegel, 1951) without impairing development. However, unfertilized eggs from which the jelly has been removed mechanically cannot be fertilized by sperm unless the jelly is restored, and chemical removal of the jelly also inhibits their ability to be parthenogenically activated (Barch & Shaver, 1959). Therefore an alternative method for obtaining jelly-free eggs for treatment prior to fertilization is necessary.
The relationship between size and vascularity in the spinal cord of developing Xenopus laevis
Authors: Sims R. T.
Published: April 2021

The ability of the blood vessels of the spinal cord to increase in number during growth without differentiation of the nerve cells was tested on six sexually immature female Xenopus and eight thyroidectomized tadpoles with suitable control animals. The mean number of blood vessels in the region of spinal cord between the origins of the second and third spinal nerves is greater in metamorphosed Xenopus which are 50 mm. long than in those which are 25 mm. long, and greater in thyroidectomized tadpoles which are 100 mm. long than in normal tadpoles at the same stage of development which are 30 mm. long. The mean number of vessels, which enter the cord from the ventral spinal artery, per unit length of spinal cord between the second and third spinal nerves is inversely proportional to the length of the cord in the metamorphosed animals, but is constant in the two groups of tadpoles. It is concluded that the ability to form new branches of the ventral spinal artery is lost during metamorphosis; the development of blood vessels in the central nervous system is not associated solely with differentiation of the nerve cells; and the histogenetic cell deaths in the developing spinal cord are not precipitated by an inadequate vascular system. The hypothesis is proposed that the growth of new blood vessels is stimulated by a humoral agent produced by the nerve tissue.
An inherited abnormality affecting the development of the yolk plasmodium and endoderm in Dermestes maculatus (Coleoptera)
Authors: Ede D. A.
Published: April 2021

A developmental abnormality characterized most obviously by longitudinal contraction of the embryo occurred in high frequency (up to 70 per cent.) among eggs from females of a selected line of the beetle Dermestes maculatus. In the majority of the abnormal embryos dorsal closure does not occur and the embryonic material forms a boat-shaped structure beneath the yolk. Stomo-daeal and proctodaeal invaginations are formed, but the endodermal wall of the mid-gut is absent. Development is abnormal from the time of blastoderm formation. The blastoderm is not completely cut off from the underlying yolk plasmodium, to which it remains connected by strands of cytoplasm. It is suggested that the failure of the yolk plasmodium to become clearly separated from the blastoderm accounts for the contraction of the embryo and also for the absence of the endodermal layer.
Sex-dependent expression of a transcription factor, Ad4BP, regulating steroidogenic P-450 genes in the gonads during prenatal and postnatal rat development
Authors: Hatano Osamu , Takayama Koichi , Imai Tsuneo , Waterman Michael R. , Takakusu Akira , Omura Tsuneo , Morohashi Ken-ichirou
Published: April 2021

We investigated the expression of Ad4BP (also known as SF-1), a transcription factor regulating steroidogenic P-450 genes, in the steroidogenic tissues such as adrenal glands, testes and ovaries through the prenatal and postnatal life of rats. Ad4BP was detected in the primordial adrenal glands and gonads of the 13.5 day postcoitum (d.p.c.) fetus. After the appearance of Ad4BP, a steroidogenic P-450 (P-450(SCC)) was also detected in the adrenal glands and its amount increased gradually. In the fetal gonads of 14.5 d.p.c., a significant amount of Ad4BP was detected in the somatic cells of the testes, whereas only a trace amount was present in the ovaries. The sexually dimorphic expression of Ad4BP continued throughout the neonatal age. Drastic alterations occurred during the first to third week of postnatal age accompanied by functional and structural changes of the gonads. The expression of Ad4BP in the testes attained a maximal level one week after birth and decreased markedly thereafter. By contrast, increase of Ad4BP in the ovary was detected after the first postnatal week. Expression of P-450c17 showed a good correlation with the proliferation of Leydig cells in the testes and theca cells in the ovaries. Immunohistochemical studies revealed the presence of Ad4BP in Sertoli cells as well as Leydig cells up to the pubertal age. In the adult rat testis, however, staining of Sertoli cells decreased significantly. Ad4BP was detected in granulosa, theca, corpus luteum and interstitial gland cells in the ovary although the expression levels in granulosa cells varied among follicles. It is suggested that the Müllerian inhibitory substance gene may be a target of Ad4BP since this gene has a conserved Ad4-binding site within the promoter, which is recognized by Ad4BP expressed in the fetal testes.
Transcriptional regulation of tubulin gene expression in differentiating trochoblasts during early development of Patella vulgata
Authors: Damen Wim G. M. , van Grunsven Leo A. , van Loon André E.
Published: April 2021

The expression of α- and β-tubulin genes during the early development of the marine mollusk Patella vulgata has been investigated. From the 32-cell stage onwards, an enhanced expression of both α- and β-tubulin mRNAs was detected in the primary trochoblasts. After one additional cleavage, these cells become cleavage-arrested and then form cilia. They are the first cells to differentiate during Patella development. Later, α- and β-tubulin mRNA is also found in the accessory and secondary trochoblasts. Together these three cell-lines form the prototroch, the ciliated locomotory organ of the trochophore larva. The early and abundant expression of tubulin genes precede and accompany cilia formation in the trochoblasts and provides us with an excellent molecular differentiation marker for these cells.

Apart from the trochoblasts, tubulin gene expression was also found in other cells at some stages. At the 88-cell stage, elevated tubulin mRNA levels were found around the large nucleus of the mesodermal stem cell 4d. In later stages, tubulin gene expression was detected in the cells that form the flagella of the apical tuft and in the refractive bodies.

An α-tubulin gene was isolated and characterized. A lacZ fusion gene under control of the 5′ upstream region of this tubulin gene was microinjected into embryos at the two-cell stage. The reporter gene product was only detected in the three trochoblast cell-lines at the same time as tubulin genes were expressed in these cells.Reporter gene product was not detected in any other cells. Thus, this 5′ upstream region of this α-tubulin gene contains all the elements required for the correct spatiotemporal pattern of expression.
Progression of an inductive signal activates sporulation in Dictyostelium discoideum
Authors: Richardson Delwood L. , Loomis William F. , Kimmel Alan R.
Published: April 2021

spiA, a marker for sporulation, is expressed during the culmination stage of Dictyostelium development, when the mass of prespore cells has moved partly up the newly formed stalk. Strains containing a full-length spiA promoter/lacZ fusion were stained for β-galactosidase activity at intervals during development. The results indicate that expression of spiA initiates in prespore cells at the prestalk/prespore boundary (near the apex) and extends downward into the prespore mass as culmination continues. A spatial gradient of staining expands from the top of the prespore mass and intensifies until the front of activation reaches the bottom, whereupon the entire region stains darkly. The spiA promoter can be deleted to within 301 bp of the transcriptional start site with no effect on the relative strength, timing or spatial localization of expression. Further 5′ deletions from −301 to −175 reduce promoter strength incrementally, although timing and spatial expression are not affected. Deletions to −159 and beyond result in inactive promoters. Treatment of early developmental structures with 8-Br-cAMP in situ activates the intracellular cAMP-dependent protein kinase (PKA) and precociously induces spiA expression and sporulation. The absence of an apparent gradient of staining in these structures suggests that PKA is equivalently activatable throughout the prespore region and that all prespore cells are competent to express spiA. Thus, we postulate that the pattern of expression of spiA reveals the progression of an inductive signal for sporulation and suggest that this signal may originate from the prestalk cells at the apex.
lag-2 may encode a signaling ligand for the GLP-1 and LIN-12 receptors of C. elegans
Authors: Henderson Samuel T. , Gao Dali , Lambie Eric J. , Kimble Judith
Published: April 2021

The C. elegans lag-2 gene is required for several cell-cell interactions that rely on the receptors GLP-1 and LIN-12. In this paper, we report that lag-2 encodes a putative membrane protein with sequence similarity to Drosophila Delta, a proposed ligand for the Notch receptor. Furthermore, we show that the lag-2 promoter drives expression of a reporter protein in the signaling distal tip cell (DTC) of the DTC/germline interaction. By in situ hybridization, we have found that endogenous lag-2 mRNA is present in the DTC but not the germ line. One fusion protein, called LAG-2::β-gal(intra), rescues a lag-2 null mutant and can be detected in both DTC and germ line. Taking these results together, we propose that lag-2 may encode a signaling ligand for GLP-1/LIN-12 and that the entire LAG-2 protein may be taken up into the receiving cell during induction by GLP-1 and lateral signaling by LIN-12.
Positive and negative signals from mesoderm regulate the expression of mouse Otx2 in ectoderm explants
Authors: Ang Siew-Lan , Conlon Ronald A. , Jin Ou , Rossant Janet
Published: April 2021

Otx2, a mouse homolog of the Drosophila orthodenticle gene, is first widely expressed in the epiblast but becomes progressively restricted to the anterior third of the embryo by the headfold stage. This progressive restriction corre-lates with the anterior migration of mesoderm in the embryo, suggesting that interactions with mesoderm may be involved in setting up the anterior domain of Otx2 expression in vivo. Using explant-recombination assays, we have obtained evidence that a positive signal from anterior mesendoderm is required to stabilize expression of Otx2 in vivo, whereas a negative signal from the later-forming posterior mesendoderm represses Otx2 expression in the posterior part of the embryo. We have also found that exogenous retinoic acid can mimic the effect of this negative signal and reduces the anterior domain of Otx2 expression.
The pha-4 gene is required to generate the pharyngeal primordium of Caenorhabditis elegans
Authors: Mango Susan E. , Lambie Eric J. , Kimble Judith
Published: April 2021

In the 4-cell Caenorhabditis elegans embryo, two blastomeres are destined to generate pharyngeal cells, each by a distinct developmental strategy: one pathway is inductive, while the other is autonomous. Here, we identify the pha-4 locus. In animals lacking pha-4 activity, an early step in pharyngeal organogenesis is blocked: no pharyngeal primordium is formed and differentiated pharyngeal cells are absent. Most other tissues are generated normally in pha-4 mutants, including cells related to pharyngeal cells by cell lineage and position. Thus, pha-4 activity is required to form the pharyngeal primordium. We propose that pha-4 marks a convergence of the inductive and autonomous pathways of pharyngeal development and suggest that establishment of pharyngeal organ identity is a crucial step for pharyngeal organogenesis.
Localization of determinants for formation of the anterior-posterior axis in eggs of the ascidian Halocynthia roretzi
Authors: Nishida Hiroki
Published: April 2021

Unfertilized eggs of the ascidian Halocynthia roretzi are radially symmetrical along the animal-vegetal axis. After fertilization, ooplasmic segregation results in formation of an anterior-posterior axis horizontally, and eggs become bilaterally symmetrical. When 8-15% of the cytoplasm of the posterior-vegetal region of the egg was removed after the second phase of ooplasmic segregation, most of the embryos completed gastrulation but developed into radialized larvae along the animal-vegetal axis with no apparent anterior-posterior axis. Removal of cytoplasm from other regions did not affect formation of this latter axis. The cleavage pattern of the embryos that were deficient in posterior-vegetal cytoplasm (PVC) exhibited radial symmetry instead of the complicated bilateral symmetry of normal embryos. Detailed comparisons of cleavage patterns revealed the duplication of the anterior cleavage pattern in the originally posterior halves of the PVC-deficient embryos. The PVC-deficent larvae lacked muscle cells, which are normally derived from the posterior blas-tomeres. Examination of the developmental fates of the early blastomeres of the PVC-deficient embryos revealed that all of the vegetal blastomeres had assumed anterior fates. These results suggest that the PVC-deficient embryos are totally anteriorized.

When posterior-vegetal cytoplasm was transplanted to the anterior-vegetal position of PVC-deficient eggs, the axial deficiency was overcome, and reversal of the anterior-posterior axis was observed. The results of transplantation of posterior-vegetal cytoplasm to the anterior-vegetal position in normal eggs demonstrated that formation of the anterior structure is suppressed by posterior-vegetal cytoplasm. These results suggest that posterior fate is specified by the presence of posterior-vegetal cytoplasm, while anterior fate is specified by the absence of posterior-vegetal cytoplasm. Thus, posterior-vegetal cytoplasm determines the anterior-posterior axis by generating the posterior cleavage pattern and conferring posterior fates on cells, as well as by inhibiting anterior fates that would otherwise occur by default.
Multiple requirements for the receptor serine/threonine kinase thick veins reveal novel functions of TGFβ homologs during Drosophila embryogenesis
Authors: Affolter Markus , Nellen Denise , Nussbaumer Ute , Basler Konrad
Published: April 2021

Differentiation of distinct cell types at specific locations within a developing organism depends largely on the ability of cells to communicate. A major class of signalling proteins implicated in cell to cell communication is represented by members of the TGFβ superfamily. A corresponding class of transmembrane serine/threonine kinases has recently been discovered that act as cell surface receptors for ligands of the TGFβ superfamily. The product of the Drosophila gene decapentaplegic (dpp) encodes a TGFβ homolog that plays multiple roles during embryogenesis and the development of imaginal discs. Here we describe the complex expression pattern of thick veins (tkv), which encodes a receptor for dpp. We make use of tkv loss-of-function mutations to examine the consequences of the failure of embryonic cells to respond to dpp and/or other TGFβ homologs. We find that while maternal tkv product allows largely normal dorsoventral pattering of the embryo, zygotic tkv activity is indispensable for dorsal closure of the embryo after germ band retraction. Fur-thermore, tkv activity is crucial for patterning the visceral mesoderm; in the absence of functional tkv gene product, visceral mesoderm parasegment 7 cells fail to express Ultra-bithorax, but instead accumulate Antennapedia protein. The tkv receptor is therefore involved in delimiting the expression domains of homeotic genes in the visceral mesoderm. Interestingly, tkv mutants fail to establish a proper tracheal network. Tracheal braches formed by cells migrating in dorsal or ventral directions are absent in tkv mutants. The requirements for tkv in dorsal closure, visceral mesoderm and trachea development assign novel functions to dpp or a closely related member of the TGFβ superfamily.
Segment and cell type lineage restrictions during pharyngeal arch development in the zebrafish embryo
Authors: Schilling Thomas F. , Kimmel Charles B.
Published: April 2021

In zebrafish, the segmental series of pharyngeal arches is formed predominantly by two migratory cell types, neural crest and paraxial mesoderm, which arise in the early embryo. Neural crest cells migrate ventrally out of the neuroepithelium and into the arches to form cartilage, neurons, glia and pigment cells. Surrounding mesoderm generates muscles and endothelia. We labeled individual pharyngeal precursor cells with fluorescent dyes and found that their clonal progeny were confined to single segments and generated single cell types. When a neural crest or mesodermal cell was marked before migration into the pharynx, its progeny dispersed but generally remained confined to a single arch primordium. Such segmental restrictions arose first in the most rostral arches, mandibular and hyoid, and progressed caudally. The phenotypes of progeny generated by single cells were examined in the mandibular arch. Clones derived from premigratory neural crest cells generally did not contribute to more than one cell type. Further, the progenitors of some cell types were spatially separated in the premigratory crest. In particular, neurogenic crest cells were situated further laterally than cells that generate cartilage and connective tissues, while pigment and glial cell progenitors were more evenly distributed. Based on these results we suggest that arch precursors may be specified as to their eventual fates before the major morphogenetic movements that form the arch primordia. Further, cell movements are restricted during segmentation establishing a group of arch precursors as a unit of developmental patterning, as in the fashion of vertebrate rhombomeres or segmental lineage compartments in Drosophila.
Pax-3 is required for the development of limb muscles: a possible role for the migration of dermomyotomal muscle progenitor cells
Authors: Bober Eva , Franz Thomas , Arnold Hans-Henning , Gruss Peter , Tremblay Patrick
Published: April 2021

Limb muscles in vertebrates originate from dermomyotomal cells, which during early development migrate from the ventrolateral region of somites into the limb buds. These progenitor cells do not express any muscle-specific marker genes or myogenic transcription factors until they reach their destination in the limbs. Here, we demonstrate by in situ hybridization that myogenic cells in somites and a population of presumably migratory muscle precursor cells in somatopleural tissue as well as myoblasts in the developing limbs express Pax-3. Significantly, in homozygous splotch mutant mice, which synthesize altered Pax-3 mRNA but make no normal protein, no cells positive for Pax-3 transcripts can be detected in the region of migrating limb muscle precursors or in the limb itself. In contrast, myotomal precursor cells and axial skeletal muscles contain Pax-3 transcripts also in the mutant. Interestingly, these animals fail to develop limb musculature as demon-strated by the lack of hybridization with various probes for myogenic transcription factors (Myf-5, myogenin, MyoD) but make apparently normal axial muscles. These observations suggest that Pax-3 is necessary for the formation of limb muscles, affecting either the generation of myogenic precursors in the somitic dermomyotome or the migration of these cells into the limb field.
The fate of the CNS midline progenitors in Drosophila as revealed by a new method for single cell labelling
Authors: Bossing Torsten , Technau Gerhard M.
Published: April 2021

We present a new method for marking single cells and tracing their development through embryogenesis. Cells are labelled with a lipophilic fluorescent tracer (DiI) in their normal positions without impaling their membranes. The dye does not diffuse between cells but is transferred to the progeny, disclosing their morphology in all detail. Behaviour of labelled cells can be observed in vivo (cell divisions, morphogenetic movements and differentiation). Following photoconversion of the dye, fully differentiated clones can be analyzed in permanent preparations.

We apply this method for cell lineage analysis of the embryonic Drosophila CNS. Here we describe the fate of the CNS midline cells. We present the complete lineages of these cells in the fully differentiated embryo and show that variability exists in segmental numbers of the midline progenitors as well as in the composition of their lineages.
A primary requirement for nodal in the formation and maintenance of the primitive streak in the mouse
Authors: Conlon Frank L , Lyons Karen M , Takaesu Norma , Barth Katrin S , Kispert Andreas , Herrmann Bernhard , Robertson Elizabeth J
Published: April 2021

The 413.d insertional mutation arrests mouse development shortly after gastrulation. nodal, a novel TGFb-related gene, is closely associated with the locus. The present study provides direct evidence that the proviral insertion causes a loss of function mutation. nodal RNA is initially detected at day 5.5 in the primitive ectoderm. Concomitant with gastrulation, expression becomes restricted to the proximal posterior regions of the embryonic ectoderm. nodal RNA is also expressed in the primitive endoderm overlying the primitive streak. A few hours later, expression is strictly confined to the periphery of the mature node. Interestingly 413.d mutant embryos show no morphological evidence for the formation of a primitive streak. Nonetheless, about 25% of mutant embryos do form randomly positioned patches of cells of a posterior mesodermal character. Data presented in this report demonstrate the involvement of a TGFb-related molecule in axis formation in mammals.
Multiple developmental defects in Engrailed-1 mutant mice: an early mid-hindbrain deletion and patterning defects in forelimbs and sternum
Authors: Wurst Wolfgang , Auerbach Anna B. , Joyner Alexandra L.
Published: April 2021

During mouse development, the homeobox-containing gene En-1 is specifically expressed across the mid-hindbrain junction, the ventral ectoderm of the limb buds, and in regions of the hindbrain, spinal cord, somites and somite-derived tissues. To address the function of En-1 during embryogenesis, we have generated mice homozygous for a targeted deletion of the En-1 homeobox. En-1 mutant mice died shortly after birth and exhibited multiple developmental defects. In the brains of newborn mutants, most of the colliculi and cerebellum were missing and the third and fourth cranial nerves were absent. A deletion of mid-hindbrain tissue was observed as early as 9.5 days of embryonic development and the phenotype resembles that previously reported for Wnt-1 mutant mice. In addition, patterning of the forelimb paws and sternum was disrupted, and the 13th ribs were truncated. The results of these studies suggest a cell autonomous role for En-1 in generation and/or survival of mid-hindbrain precursor cells and also a non-cell autonomous role in signaling normal development of the limbs and possibly sternum.
The expression pattern of a novel gene encoding brain-fatty acid binding protein correlates with neuronal and glial cell development
Authors: Kurtz Andreas , Zimmer Andreas , Schnütgen Frank , Brüning Gerold , Spener Friedrich , Müller Thomas
Published: April 2021

Fatty acid binding proteins (FABPs) are a multigene family of small intracellular proteins that bind hydrophobic ligands. In this report we describe the cloning and expression pattern of a novel member of this gene family that is specifically expressed in the developing and adult nervous system and thus was designated brain (B)-FABP. B-FABP is closely related to heart (H)-FABP with 67% amino acid identity. B-FABP expression was first detected at mouse embryonic day 10 in neuroepithelial cells and its pattern correlates with early neuronal differentiation. Upon further development, B-FABP was confined to radial glial cells and immature astrocytes. B-FABP mRNA and protein were found in glial cells of the peripheral nervous system such as satellite cells of spinal and cranial ganglia and ensheathing cells of the olfactory nerve layer from as early as embryonic day 11 until adulthood. In the adult mouse brain, B-FABP was found in the glia limitans, in radial glial cells of the hippocampal dentate gyrus and Bergman glial cells. These findings suggest a function of B-FABP during neurogenesis or neuronal migration in the developing nervous system. The partially overlapping expression pattern with that of cellular retinoid binding proteins suggests that B-FABP is involved in the metabo-lism of a so far unknown hydrophobic ligand with potential morphogenic activity during CNS development.
blistered: a gene required for vein/intervein formation in wings of Drosophila
Authors: Fristrom Dianne , Gotwals Philip , Eaton Suzanne , Kornberg Thomas B. , Sturtevant Mark , Bier Ethan , Fristrom James W.
Published: April 2021

We have characterized the blistered (bs) locus phenotypically, genetically and developmentally using a set of new bs alleles. Mutant defects range from wings with ectopic veins and intervein blisters to completely ballooned wings where the distinction between vein and intervein is lost. Mosaic analyses show that severe bs alleles behave largely autonomously; homozygous patches having vein-like properties. Developmental analyses were undertaken using light and electron microscopy of wild-type and bs wings as well as confocal microscopy of phalloidin- and laminin-stained preparations. bs defects were first seen early in the prepupal period with the failure of apposition of dorsal and ventral wing epithelia. Correspondingly, during definitive vein/intervein differentiation in the pupal period (18–36 hours after puparium formation), the extent of dorsal/ventral reapposition is reduced in bs wings. Regions of the wing that fail to become apposed differentiate properties of vein cells; i.e. become constricted apically and acquire a laminin-containing matrix basally.

To further understand bs function, we examined genetic interactions between various bs alleles and mutants of two genes whose products have known functions in wing development. (i) rhomboid, a component of the EGF-R signalling pathway, is expressed in vein cells and is required for specification of vein cell fate. rhove mutations (lacking rhomboid in wings) suppress the excess vein formation and associated with bs. Conversely, rho expression in prepupal and pupal bs wings is expanded in the regions of increased vein formation. (ii) The integrin genes, inflated and myospheroid, are expressed in intervein cells and are required for adhesion between the dorsal and ventral wing surfaces. Loss of integrin function results in intervein blisters. Integrin mutants interact with bs mutants to increase the frequency of intervein blisters but do not typically enhance vein defects. Both developmental and genetic analyses suggest that the bs product is required during metamorphosis for the initiation of intervein development and the concomitant inhibition of vein development.