Patterns of gene expression in the core of Spemann's organizer and activin-treated ectoderm in Cynops pyrrhogaster

The presumptive pharyngeal endoderm region of the Cynops early gastrula induces head or trunk-tail structures in sandwich culture. Activin-treated ectoderm can mimic this phenomenon at least at the histological level. The patterns of expression of organizer-specific genes were examined to compare these two inductive materials at the molecular level. A chordin cDNA clone from Cynops pyrrhogaster (Cychd) was isolated by reverse transcription-polymerase chain reaction (RT-PCR). Cychd mRNA was first detected in the presumptive pharyngeal endoderm and prechordal plate regions of stage 11 embryos, and was expressed continuously until stage 20. The spatiotemporal expression pattern of Cychd was similar to that of Xenopus chordin. The patterns of expression of organizer-related genes in the pharyngeal endoderm and activin-treated ectoderm were compared by RT-PCR analysis. Expression of Cychd in these two materials peaked at the time when they can induce head structures in sandwich culture. Expression of fork head and goosecoid did not change in the presumptive pharyngeal endoderm over this period. Cychd may play a key role in head formation in the Cynops embryo.

A role for SOX1 in neural determination

In vertebrates, the delineation of the neural plate from a region of the primitive ectoderm is accompanied by the onset of specific gene expression which in turn promotes the formation of the nervous system. Here we show that SOX1, an HMG-box protein related to SRY, is one of the earliest transcription factors to be expressed in ectodermal cells committed to the neural fate: the onset of expression of SOX1 appears to coincide with the induction of neural ectoderm. We demonstrate a role for SOX1 in neural determination and differentiation using an inducible expression P19 cell system as an in vitro model of neurogenesis. Misexpression of SOX1 can substitute for the requirement of retinoic acid to impart neural fate to competent ectodermal P19 cells. Using a series of antigenic markers which identify early neural cell types in combination with BrdU labeling, we demonstrate a temporal and spatial correlation between the differentiation of cell types along the dorsoventral axis of the neural tube and the downregulation of SOX1 expression. SOX1, therefore, defines the dividing neural precursors of the embryonic central nervous system (CNS).

prdl-a, a gene marker for hydra apical differentiation related to triploblastic paired-like head-specific genes

Two homeobox genes, prdl-a and prdl-b, which were isolated from a Hydra vulgaris cDNA library, encode paired-like class homeodomains highly related to that of the aristaless-related genes. In adult polyps, prdl-b is a marker for synchronously dividing nematoblasts while prdl-a displays an expression restricted to the the nerve cell lineage of the head region. During budding and apical regeneration, an early and transient prdl-a expression was observed in endodermal cells of the stump at a time when the head organizer is established. When apical regeneration was delayed upon concomittant budding, prdl-a expression was found to be altered in the stump. Furthermore, a specific anti-prdl-a protein immunoserum revealed that prdl-a was overexpressed in adult polyps of the Chlorohydra viridissima multiheaded mutant, with an expression domain extending below the tentacle ring towards the body column. Accordingly, prdl-a DNA-binding activity was enhanced in nuclear extracts from this mutant. These results suggest that prdl-a responds to apical forming signals and might thus be involved in apical specification. When a marine hydrozoan (Podocorynae carnea) was used, the anti-prdl-a antibody showed cross-reactivity with cells located around the oral region, indicating that prdl-a function is shared by other cnidaria. The ancestral role for prdl-a-related genes in the molecular definition of the head (or oral-surrounding region) is discussed.

Regulation of bHLH-PAS protein subcellular localization during Drosophila embryogenesis

The Drosophila Single-minded and Tango basic-helix-loop-helix-PAS protein heterodimer controls transcription and embryonic development of the CNS midline cells, while the Trachealess and Tango heterodimer controls tracheal cell and salivary duct transcription and development. Expression of both single-minded and trachealess is highly restricted to their respective cell lineages, however tango is broadly expressed. The developmental control of subcellular localization of these proteins is investigated because of their similarity to the mammalian basic-helix-loop-helix-PAS Aromatic hydrocarbon receptor whose nuclear localization is dependent on ligand binding. Confocal imaging of Single-minded and Trachealess protein localization indicate that they accumulate in cell nuclei when initially synthesized in their respective cell lineages and remain nuclear throughout embryogenesis. Ectopic expression experiments show that Single-minded and Trachealess are localized to nuclei in cells throughout the ectoderm and mesoderm, indicating that nuclear accumulation is not regulated in a cell-specific fashion and unlikely to be ligand dependent. In contrast, nuclear localization of Tango is developmentally regulated; it is localized to the cytoplasm in most cells except the CNS midline, salivary duct, and tracheal cells where it accumulates in nuclei. Genetic and ectopic expression experiments indicate that Tango nuclear localization is dependent on the presence of a basic-helix-loop-helix-PAS protein such as Single-minded or Trachealess. Conversely, Drosophila cell culture experiments show that Single-minded and Trachealess nuclear localization is dependent on Tango since they are cytoplasmic in the absence of Tango. These results suggest a model in which Single-minded and Trachealess dimerize with Tango in the cytoplasm of the CNS midline cells and trachea, respectively, and the dimeric complex accumulates in nuclei in a ligand-independent mode and regulates lineage-specific transcription. The lineage-specific action of Single-minded and Trachealess derives from transcriptional activation of their genes in their respective lineages, not from extracellular signaling.

Domains of retinoid signalling and neurectodermal expression of zebrafish otx1 and goosecoid are mutually exclusive

Retinoid signalling plays an important role in embryonic pattern formation. Excess of retinoic acid during gastrulation results in axial defects in vertebrate embryos, suggesting that retinoids are involved in early anteroposterior patterning. To study retinoid signalling in zebrafish embryos, we developed a novel method to detect endogenous retinoids in situ in embryos, using a fusion protein of the ligand inducible transactivation domain of a retinoic acid receptor and a heterologous DNA binding domain. Using this method, we show that retinoid signalling is localized in zebrafish embryos in the region of the embryonic shield, and towards the end of gastrulation in a posterior dorsal domain. To investigate the relationships between the spatial distribution of retinoid signalling and the regulation of retinoid target genes, we studied the downregulation by retinoic acid of two genes expressed in anterior regions of the embryo, goosecoid and otx1. These experiments show that expression of both genes is strongly downregulated in the anterior neurectoderm of zebrafish embryos treated with retinoic acid, whereas mesendodermal expression is only mildly affected. Interestingly, a significant downregulation of goosecoid expression by retinoic acid was observed only during midgastrulation but not in earlier stages. In agreement with these results, spatial expression of goosecoid and otx1 does not overlap with the region of retinoid signalling in the late gastrula. Our data support the hypothesis that a localized retinoid signal is involved in axial patterning during early development, at least in part through the repression of anterior genes in posterior regions of the embryo. Furthermore, our data suggest that the action of retinoids is spatially as well as temporally regulated in the developing embryo.

The muscleblind gene participates in the organization of Z-bands and epidermal attachments of Drosophila muscles and is regulated by Dmef2

We report the embryonic phenotype of muscleblind (mbl), a recently described Drosophila gene involved in terminal differentiation of adult ommatidia. mbl is a nuclear protein expressed late in the embryo in pharyngeal, visceral, and somatic muscles, the ventral nerve cord, and the larval photoreceptor system. All three mbl alleles studied exhibit a lethal phenotype and die as stage 17 embryos or first instar larvae. These larvae are partially paralyzed, show a characteristically contracted abdomen, and lack striation of muscles. Our analysis of the somatic musculature shows that the pattern of muscles is established correctly, and they form morphologically normal synapses. Ultrastructural analysis, however, reveals two defects in the terminal differentiation of the muscles: inability to differentiate Z-bands in the sarcomeric apparatus and reduction of extracellular tendon matrix at attachment sites to the epidermis. Failure to differentiate both structures could explain the partial paralysis and contracted abdomen phenotype. Analysis of mbl expression in embryos that are either mutant for Dmef2 or ectopically express Dmef2 places mbl downstream of Dmef2 function in the myogenic differentiation program. mbl, therefore, may act as a critical element in the execution of two Dmef2-dependent processes in the terminal differentiation of muscles.

Hox group 3 paralogs regulate the development and migration of the thymus, thyroid, and parathyroid glands

The thymus, thyroid, and parathyroid glands in vertebrates develop from the pharyngeal region, with contributions both from pharyngeal endoderm and from neural crest cells in the pharyngeal arches. Hoxa3 mutant homozygotes have defects in the development of all three organs. Roles for the Hoxa3 paralogs, Hoxb3 and Hoxd3, were investigated by examining various mutant combinations. The thyroid defects seen in Hoxa3 single mutants are exacerbated in double mutants with either of its paralogs, although none of the double-mutant combinations resulted in thyroid agenesis. The results indicate that the primary role of these genes in thyroid development is their effect on the development and migration of the ultimobranchial bodies, which contribute the parafollicular or C-cells to the thyroid. Hoxb3, Hoxd3 double mutants show no obvious defects in the thymus or parathyroids. However, the removal of one functional copy of Hoxa3 from the Hoxb3, Hoxd3 double mutants (Hoxa3 +/-, Hoxb3-/-, Hoxd3-/-) results in the failure of the thymus and parathyroid glands to migrate to their normal positions in the throat. Very little is known about the molecular mechanisms used to mediate the movement of tissues during development. These results indicate that Hoxa3, Hoxb3, and Hoxd3 have highly overlapping functions in mediating the migration of pharyngeal organ primordia. In addition, Hoxa3 has a unique function with respect to its paralogs in thymus, parathyroid, and thyroid development. This unique function may be conferred by the expression of Hoxa3, but not Hoxb3 nor Hoxd3, in the pharyngeal pouch endoderm.

Regulation of ectodermal differentiation in Xenopus laevis animal caps treated with TPA and ammonium chloride

Animal caps isolated from Xenopus laevis embryos at the blastula stage were treated sequentially with NH4Cl, a known cement gland inducer, and with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a known neural inducer. The two artificial inducers were also used in reverse order to see if they can mimic the natural inducers acting during the progressive determination of the ectodermal organ. Immunofluorescence and whole-mount in situ hybridization were used to study the expression of tubulin, taken to indicate an early step on the pathway of cell elongation, and neural cell adhesion molecule (N-CAM) taken to indicate an early step in the determination of the nervous system. The expression of XCG-1, a marker of early specification of the cement gland, was also studied. The results showed that the two artificial inducers can mimic the effects of the natural inducers in animal cap explants. The TPA behaves like a neural inducer, reducing the number and the extension of the cement gland when added to the medium in addition to NH4Cl, before or after NH4Cl treatment. In the process of cement gland/neural induction, it is possible to redirect the ectoderm already specified as cement gland to neural tissue, but it does not seem possible to respecify the neural tissue as cement gland. Moreover, the animal caps were also cut into dorsal and ventral parts and the two halves were treated separately. The results were similar to those obtained with treatment of the entire animal cap, suggesting that a dorsal-ventral pattern is not yet established before the gastrula stage, and that in normal embryos there are boundaries between the effects of different inducers.

Multiple delta genes and lateral inhibition in zebrafish primary neurogenesis

In Drosophila, cells are thought to be singled out for a neural fate through a competitive mechanism based on lateral inhibition mediated by Delta-Notch signalling. In tetrapod vertebrates, nascent neurons express the Delta1 gene and thereby deliver lateral inhibition to their neighbours, but it is not clear how these cells are singled out within the neurectoderm in the first place. We have found four Delta homologues in the zebrafish--twice as many as reported in any tetrapod vertebrate. Three of these--deltaA, deltaB and deltaD--are involved in primary neurogenesis, while two--deltaC and deltaD--appear to be involved in somite development. In the neural plate, deltaA and deltaD, unlike Delta1 in tetrapods, are expressed in large patches of contiguous cells, within which scattered individuals expressing deltaB become singled out as primary neurons. By gene misexpression experiments, we show: (1) that the singling-out of primary neurons, including the unique Mauthner cell on each side of the hindbrain, depends on Delta-Notch-mediated lateral inhibition, (2) that deltaA, deltaB and deltaD all have products that can deliver lateral inhibition and (3) that all three of these genes are themselves subject to negative regulation by lateral inhibition. These properties imply that competitive lateral inhibition, mediated by coordinated activities of deltaA, deltaB and deltaD, is sufficient to explain how primary neurons emerge from proneural clusters of neuroepithelial cells in the zebrafish.

Embryonic morphogenesis signaling pathway mediated by JNK targets the transcription factor JUN and the TGF-beta homologue decapentaplegic

The dorsal surface of the Drosophila embryo is formed by the migration of the lateral epithelial cells to cover the amnioserosa. The Drosophila cJun-N-terminal kinase (DJNK) is essential for this process. Mutations in DJNK or the DJNK activator hemipterous (HEP) lead to incomplete dorsal closure, resulting in a hole in the dorsal cuticle. The molecules downstream of DJNK in this signaling pathway have not been established. Here we demonstrate that the basket1 (bsk1) mutation of DJNK causes decreased interaction with DJUN. Expression of decapentaplegic (DPP), a TGF-beta homologue, in the leading edge of the dorsal epithelium, is identified as a genetic target of the JNK pathway. A constitutive allele of JUN is able to rescue the dorsal closure defect of bsk1 and restores DPP expression. Furthermore, ectopic DPP rescues the defects in dorsal closure caused by bsk1. These data indicate that the interaction of DJNK with DJUN contributes to the dorsal closure signaling pathway and targets DPP expression.

Xenopus msx1 mediates epidermal induction and neural inhibition by BMP4

Epidermal fate in Xenopus ectoderm has been shown to be induced by a secreted growth factor, Bone Morphogenetic Protein 4 (BMP4). However, the molecular mechanism mediating this response is poorly understood. Here, we show that the expression of the homeobox gene, msx1, is an immediate early response to BMP4 in Xenopus embryos. The timing of expression and embryonic distribution of msx1 parallel those described for BMP4. Moreover, overexpression of msx1 in early Xenopus embryos leads to their ventralization as described for BMP4. Consistent with mediating a BMP type of signaling, overexpression of msx1 is sufficient to induce epidermis in dissociated ectoderm cells, which would otherwise form neural tissue. Finally, msx1 can also rescue neuralization imposed by a dominant negative BMP receptor (tBR) in ectodermal explants. We propose that Xenopus msx1 acts as a mediator of BMP signaling in epidermal induction and inhibition of neural differentiation.

Concentration-dependent patterning of the Xenopus ectoderm by BMP4 and its signal transducer Smad1

Morphogens are thought to establish pattern in early embryos by specifying several cell fates along a gradient of concentration; a well-studied example is the Drosophila protein decapentaplegic (DPP) acting in the wing disc. Recent work has established that bone morphogenetic protein 4 (BMP4), the vertebrate homologue of DPP, controls the fundamental choice between neural and epidermal fates in the vertebrate ectoderm, under the control of antagonists secreted by the organizer region of the mesoderm. We now show that BMP4 can act as a morphogen, evoking distinct responses in Xenopus ectodermal cells at high and low concentrations, in a pattern consistent with the positions of the corresponding cell types in the embryo. Moreover, this complex cellular response to extracellular BMP4 concentration does not require subsequent cell-cell communication and is thus direct, as required of a classical morphogen. We also show that the same series of cell types--epidermis, cement gland and neural tissue--can be produced by progressively inhibiting endogenous BMP signaling with specific antagonists, including the organizer factor noggin. Finally, expression of increasing doses of the signal transduction molecule Smad1 accurately reproduces the response to BMP4 protein. Since Smads have been shown to act in the nucleus, this finding implies a direct translation of extracellular morphogen concentration into transcription factor activity. We propose that a graded distribution of BMP activity controls the specification of several cell types in the gastrula ectoderm and that this extracellular gradient acts by establishing an intracellular and then nuclear gradient of Smad activity.

Mouse Dll3: a novel divergent Delta gene which may complement the function of other Delta homologues during early pattern formation in the mouse embryo

Mouse delta-like 3 (Dll3), a novel vertebrate homologue of the Drosophila gene Delta was isolated by a subtracted library screen. In Drosphila, the Delta/Notch signalling pathway functions in many situations in both embryonic and adult life where cell fate specification occurs. In addition, a patterning role has been described in the establishment of the dorsoventral compartment boundary in the wing imaginal disc. Dll3 is the most divergent Delta homologue identified to date. We confirm that Dll3 can inhibit primary neurogenesis when ectopically expressed in Xenopus, suggesting that it can activate the Notch receptor and therefore is a functional Delta homologue. An extensive expression study during gastrulation and early organogenesis in the mouse reveals a diverse and dynamic pattern of expression. The three major sites of expression implicate Dll3 in somitogenesis and neurogenesis and in the production of tissue from the primitive streak and tailbud. A careful comparison of Dll3 and Dll1 expression by double RNA in situ hybridisation demonstrates that these genes have distinct patterns of expression, but implies that together they operate in many of the same processes. We postulate that during somitogenesis Dll3 and Dll1 coordinate in establishing the intersomitic boundaries. We confirm that, during neurogenesis in the spinal cord, Dll1 and Dll3 are expressed by postmitotic cells and suggest that expression is sequential such that cells express Dll1 first followed by Dll3. We hypothesise that Dll1 is involved in the release of cells from the precursor population and that Dll3 is required later to divert neurons along a specific differentiation pathway.

The role of the msh homeobox gene during Drosophila neurogenesis: implication for the dorsoventral specification of the neuroectoderm

Development of the Drosophila central nervous system begins with the delamination of neural and glial precursors, called neuroblasts, from the neuroectoderm. An early and important step in the generation of neural diversity is the specification of individual neuroblasts according to their position. In this study, we describe the genetic analysis of the msh gene which is likely to play a role in this process. The msh/Msx genes are one of the most highly conserved families of homeobox genes. During vertebrate spinal cord development, Msx genes (Msx1-3) are regionally expressed in the dorsal portion of the developing neuroectoderm. Similarly in Drosophila, msh is expressed in two longitudinal bands that correspond to the dorsal half of the neuroectoderm, and subsequently in many dorsal neuroblasts and their progeny. We showed that Drosophila msh loss-of-function mutations led to cell fate alterations of neuroblasts formed in the dorsal aspect of the neuroectoderm, including a possible dorsal-to-ventral fate switch. Conversely, ectopic expression of msh in the entire neuroectoderm severely disrupted the proper development of the midline and ventral neuroblasts. The results provide the first in vivo evidence for the role of the msh/Msx genes in neural development, and support the notion that they may perform phylogenetically conserved functions in the dorsoventral patterning of the neuroectoderm.

The mesodermal expression of rolling stone (rost) is essential for myoblast fusion in Drosophila and encodes a potential transmembrane protein

In homozygous rolling stone embryos, the fusion of myoblasts to syncytial myotubes is diminished. Nevertheless, the visceral mesoderm, the heart mesoderm, and few somatic muscles are properly formed. Thus, we postulate a central role of rolling stone for the fusion process within the somatic mesoderm. We have cloned the rolling stone gene, and the deduced protein sequence is in accordance with a transmembrane protein, which agrees with the enrichment of Rost in the membrane fraction of Drosophila embryos. No homologous genes have been described so far. rolling stone is expressed in the embryonic nervous system and cells of the somatic mesoderm, most notable in muscle founder cells. To elucidate the function of rolling stone for myoblast fusion, we applied a knock-out strategy. The expression of an antisense rolling stone transcript specifically within the mesoderm of wild-type embryos results in fusion defects of myoblasts, proving that the rolling stone expression in the mesoderm is responsible for the rolling stone phenotype. We suggest that rolling stone is a member of a group of genes that are necessary for the fusion process during myogenesis.

Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation

Vertebrate organogenesis is initiated at sites that are often morphologically indistinguishable from the surrounding region. Here we have identified Pax9 as a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. We provide evidence that the sites of Pax9 expression in the mandibular arch are positioned by the combined activity of two signals, one (FGF8) that induces Pax9 expression and the other (BMP2 and BMP4) that prevents this induction. Thus it appears that the position of the teeth is determined by a combination of two different types of signaling molecules produced in wide but overlapping domains rather than by a single localized inducer. We suggest that a similar mechanism may be used for specifying the sites of development of other organs.

Cooperation of BMP7 and SHH in the induction of forebrain ventral midline cells by prechordal mesoderm

Ventral midline cells at different rostrocaudal levels of the central nervous system exhibit distinct properties but share the ability to pattern the dorsoventral axis of the neural tube. We show here that ventral midline cells acquire distinct identities in response to the different signaling activities of underlying mesoderm. Signals from prechordal mesoderm control the differentiation of rostral diencephalic ventral midline cells, whereas notochord induces floor plate cells caudally. Sonic hedgehog (SHH) is expressed throughout axial mesoderm and is required for the induction of both rostral diencephalic ventral midline cells and floor plate. However, prechordal mesoderm also expresses BMP7 whose function is required coordinately with SHH to induce rostral diencephalic ventral midline cells. BMP7 acts directly on neural cells, modifying their response to SHH so that they differentiate into rostral diencephalic ventral midline cells rather than floor plate cells. Our results suggest a model whereby axial mesoderm both induces the differentiation of overlying neural cells and controls the rostrocaudal character of the ventral midline of the neural tube.

Dynamic expression of chicken Sox2 and Sox3 genes in ectoderm induced to form neural tissue

The chick genes, cSox2 and cSox3, are members of a large family of genes that encode transcription factors. Previous studies have shown that these genes are predominantly expressed in the central nervous system during embryonic development. We show that cSox3 is expressed throughout the ectoderm that is competent to form nervous tissue before neural induction. The expression of cSox3 is lost from cells as they undergo gastrulation to form nonectodermal tissues; the transcription factor, Brachyury, appears in cells about to undergo gastrulation a short time before cSox3 transcripts are lost. Therefore, Brachyury expression may act functionally upstream of cSox3 downregulation. cSox3 expression is also lost from non-neuronal ectoderm shortly after the neural plate becomes morphologically apparent. cSox2 expression increases dramatically in the central nervous system as neural ectoderm is established. The appearance of cSox2 in neural ectoderm represents one of the earliest molecular responses to neural induction documented thus far.

Bone morphogenetic proteins (BMPs) as regulators of dorsal forebrain development

Bone Morphogenetic Proteins (BMPs) play crucial roles in a variety of developmental processes, but their functions during early vertebrate brain development are largely unknown. To investigate this problem, we have compared by in situ hybridization the expression of five Bmp genes belonging to the Drosophila Decapentaplegic (Bmp2 and Bmp4) and 60A subgroups (Bmp5, Bmp6 and Bmp7). Striking co-expression of these Bmps is observed within the dorsomedial telencephalon, coincident with a future site of choroid plexus development. Bmp co-expression overlaps that of Msx1 and Hfh4, and is complementary to that of Bf1. The domain of Bmp co-expression is also associated with limited growth of the neuroectoderm, as revealed by morphological observation, reduced cell proliferation, and increased local programmed cell death. In vitro experiments using explants from the embryonic lateral telencephalic neuroectoderm reveal that exogenous BMP proteins (BMP4 and BMP2) induce expression of Msx1 and inhibit Bf1 expression, a finding consistent with their specific expression patterns in vivo. Moreover, BMP proteins locally inhibit cell proliferation and increase apoptosis in the explants. These results provide evidence that BMPs function during regional morphogenesis of the dorsal telencephalon by regulating specific gene expression, cell proliferation and local cell death.

Transcriptional regulation of Notch and Delta: requirement for neuroblast segregation in Drosophila

Segregation of a single neural precursor from each proneural cluster in Drosophila relies on Notch-mediated lateral signalling. Studies concerning the spacing of precursors for the microchaetes of the peripheral nervous system suggested the existence of a regulatory loop between Notch and its ligand Delta within each cell that is under transcriptional control. Activation of Notch leads to repression of the achaete-scute genes which themselves regulate transcription of Delta, perhaps directly. Here we have tested a requirement for transcriptional regulation of Notch and/or Delta during neuroblast segregation in embryos, by providing Notch and Delta ubiquitously at uniform levels. Neuroblast segregation occurs normally under conditions of uniform Notch expression. Under conditions of uniform Delta expression, a single neuroblast segregates from each proneural group in 80% of the cases, more than one in the remaining 20%. Thus transcriptional regulation of Delta is largely dispensable. We discuss the possibility that segregation of single precursors in the central nervous system may rely on a heterogeneous distribution of neural potential between different cells of the proneural group. Notch signalling would enable all cells to mutually repress each other and only a cell with an elevated neural potential could overcome this repression.

Expression of heparan sulfate proteoglycan (perlecan) in the mouse blastocyst is regulated during normal and delayed implantation

Previous studies have shown that expression of the heparan sulfate proteoglycan, perlecan, on the external trophectodermal cell surfaces of mouse blastocysts increases during acquisition of attachment competence. However, it is not clear if this change in perlecan protein expression also is reflected at the level of perlecan mRNA expression. In the present investigation, the spatial and temporal patterns of perlecan mRNA expression in the mouse embryo during the periimplantation period were examined by in situ hybridization and reverse transcriptase-polymerase chain reaction. In addition, a delayed implantation model was used to determine the expression of perlecan mRNA and protein in dormant and estrogen-activated hatched blastocysts. The results demonstrate that perlecan mRNA expression is low in morulae, but increases in Day 4 blastocysts, attaining maximal expression in Day 4.5 attachment-competent blastocysts. In contrast, perlecan mRNA is detected in both the dormant and estrogen-activated delayed blastocysts; however, within 12 hr of blastocyst activation by estrogen, both perlecan protein and heparan sulfate chain expression markedly increase. Taken together, these results suggest that during normal development perlecan mRNA expression increases with the acquisition of attachment competence. Moreover, perlecan protein expression also is attenuated during delayed implantation and appears to increase in response to nidatory estrogen, perhaps via the increased translation of preexisting perlecan mRNA.

Evidence that absence of Wnt-3a signaling promotes neuralization instead of paraxial mesoderm development in the mouse

Wnt-3a mutant embryos show defects caudal to the forelimb level; somites are absent, the notochord is disrupted, and the central nervous system has a pronounced dysmorphology. Previous studies revealed that the primary defects of the mutant embryos are likely to be in the process of paraxial mesoderm formation. In this study, we analyzed the phenotype of Wnt-3a mutant embryos at early somite stages (8.0 days post coitum), when somite formation is initiated. In Wnt-3a mutants, cells which have ingressed through the primitive streak do not migrate laterally but remain under the streak and form an ectopic tubular structure. Several neural-specific molecular markers, but no paraxial mesoderm markers, are expressed in this structure, suggesting that the ectopic tube is an additional neural tube. In normal embryos, Wnt-3a is expressed in the primitive ectoderm, including the cells which are fated to give rise to the paraxial mesoderm and neurectoderm, but expression is absent in migrating mesoderm cells. These results suggest that Wnt-3a signaling may play a role in regulating paraxial mesodermal fates, at the expense of neurectodermal fates, within the primitive ectoderm of the gastrulating mouse embryo.

tbx6, a Brachyury-related gene expressed by ventral mesendodermal precursors in the zebrafish embryo

Classical embryology experiments have indicated the existence of dorsal-type and ventral-type mesoderms that arise as a consequence of mesoderm induction during vertebrate development. Here we report that the zebrafish tbx6 gene, a member of the Brachyury-related T-box family of genes, is exclusively expressed by ventral mesendoderm. Three observations link the expression of tbx6 to ventral mesoderm specification. First, the gene is initially expressed at the onset of gastrulation within a ventrolateral subpopulation of cells that express the pan-mesodermal gene, no tail (Brachyury). Second, the mesoderm-inducing factors activin and bFGF activate tbx6 expression in animal caps. Third, dorsalization of the mesendodermal precursor population following exposure of embryos to lithium ions causes down-regulation of tbx6 transcription. tbx6 is expressed transiently in the involuting derivatives of the ventral mesendoderm, which give rise to nonaxial mesodermal tissues; its expression is extinguished as tissue differentiation progresses. Transcription of tbx6 commences about an hour after initiation of expression of the pan-mesendodermal gene no tail and the organizer gene goosecoid. The dependence of tbx6 expression on no tail activity was examined in no tail mutant embryos. The activation of tbx6 transcription in ventral mesoderm does not depend on no tail gene activity. However, no tail appears to contribute to the maintenance of normal levels of tbx6 transcription and may be required for tbx6 transcription in the developing tail.

Each member of the Id gene family exhibits a unique expression pattern in mouse gastrulation and neurogenesis

We have performed a detailed comparative in situ hybridization analysis to examine the patterns of expression of all the members of the Id gene family (Id1-4) during murine gastrulation and neurogenesis. During gastrulation, both Id1 and Id3 are expressed in the tissues derived from the inner cell mass from 5.5 dpc onward, whereas Id2 is expressed in tissues derived from trophoblasts. Id4 expression is absent during this period of development. Embryonic Id1 messages are detected during gastrulation on the proximal side of the embryonic ectoderm, which is the border between the embryo proper and the extraembryonic tissues, and the expression of Id3 is found throughout the entire embryo proper. This unique pattern of expression of the different members of the Id family suggests a nonredundant role for these genes in antagonizing the activity of bHLH transcription factors during very early mouse development. During neurogenesis, the expression of each member of the Id family is present in an unique pattern along the dorsal-ventral axis of the neural tube: In the early stages of spinal cord development, both Id1 and Id2 are expressed in the roof plate, whereas Id3 is expressed both in the roof and the floor plates. As development progresses, the expression of both Id1 and Id3 is detected in the dividing neuroblasts, whereas Id2 and 4 are expressed in presumptive neurons which are undergoing maturation. The expression patterns of all the members of the Id gene family persist throughout the entire CNS, both in the spinal cord and in the brain. In addition, the characteristic expression of Id2 and Id4 in more mature neurons is reiterated both in the PNS and in the neurons of some of the sensory organs. These data suggest that the expression of different subgroups of the Id gene family may have different physiological consequences and thereby contributes in unique ways to specify the differentiation state of neuronal cells during development.

Isolation, characterization, and expression of Le-msx, a maternally expressed member of the msx gene family from the glossiphoniid leech, Helobdella

The msx gene family is one of the most highly conserved of the nonclustered homeobox-containing genes. We have isolated an msx homolog (Le-msx) from the glossiphoniid leech, Helobdella robusta, and characterized its pattern of expression by whole mount in situ hybridization. In situ expression and reverse transcription polymerase chain reaction (RT-PCR) data results show that Le-msx is a maternal transcript initially uniformly distributed in the cortex of immature oocytes that becomes asymmetrically localized to the polar regions of the uncleaved zygote. This is the earliest reported expression for the msx gene family and the first maternally expressed homeodomain-containing transcription factor reported in annelids. During embryonic development, Le-msx is expressed in all 10 embryonic stem cells and their segmental founder cell descendants. At midembryonic stages, Le-msx is expressed in the expanding germinal plate. Le-msx is confined to the central nervous system and nephridia at late (stage 9) stages and subsequently disappears from nephridia. In addition, we present a phylogenetic hypothesis for the evolution of the msx gene family, including the identification of a putative C. elegans msx homolog and the realignment of the sponge msx homolog to the NK class of homeodomain genes.