Distinct roles for Fgf, Wnt and retinoic acid in posteriorizing the neural ectoderm. (1/172)

Early neural patterning in vertebrates involves signals that inhibit anterior (A) and promote posterior (P) positional values within the nascent neural plate. In this study, we have investigated the contributions of, and interactions between, retinoic acid (RA), Fgf and Wnt signals in the promotion of posterior fates in the ectoderm. We analyze expression and function of cyp26/P450RAI, a gene that encodes retinoic acid 4-hydroxylase, as a tool for investigating these events. Cyp26 is first expressed in the presumptive anterior neural ectoderm and the blastoderm margin at the late blastula. When the posterior neural gene hoxb1b is expressed during gastrulation, it shows a strikingly complementary pattern to cyp26. Using these two genes, as well as otx2 and meis3 as anterior and posterior markers, we show that Fgf and Wnt signals suppress expression of anterior genes, including cyp26. Overexpression of cyp26 suppresses posterior genes, suggesting that the anterior expression of cyp26 is important for restricting the expression of posterior genes. Consistent with this, knock-down of cyp26 by morpholino oligonucleotides leads to the anterior expansion of posterior genes. We further show that Fgf- and Wnt-dependent activation of posterior genes is mediated by RA, whereas suppression of anterior genes does not depend on RA signaling. Fgf and Wnt signals suppress cyp26 expression, while Cyp26 suppresses the RA signal. Thus, cyp26 has an important role in linking the Fgf, Wnt and RA signals to regulate AP patterning of the neural ectoderm in the late blastula to gastrula embryo in zebrafish.  (+info)

Embryogenesis and development of Epimenia babai (Mollusca Neomeniomorpha). (2/172)

Neomenioid aplacophorans (= Solenogastres) constitute one of the main lineages of molluscs. Developmental data of early embryogenesis and larval development of neomenioids are available for some species based on histological sections. I used other techniques to study the development of Epimenia babai Salvini-Plawen, 1997, and here I report new data on neomenioid development. The embryos of E. babai are lecithotrophic and cleavage is spiral, unequal, and holoblastic. Two polar lobes are formed, one at the first cleavage stage and one at the second cleavage stage. No evidence of external metameric iteration is visible through scanning electron microscopy or histology at any stage. A ciliated foot, a pedal pit, and aragonitic spicules develop from the definitive ectoderm. A spicule begins as a solid tip, continues to an open-ended hollow spicule, and finally becomes a closed-ended hollow spicule. The free-swimming trochophore larvae of E. babai have been considered unusual in lacking the characteristic neomenioid cellular test, an outer locomotory structure within which the entire definitive adult body develops. However, through the use of scanning electron and light microscopy, semithin sections, Hoechst nuclear staining, and programmed cell death staining to study the ontogeny and fate of the apical cells, I show that the entire pre-oral sphere (the apical cap) of the larvae is similar to the test of the other neomenioids. The results suggest that the test of the neomenioid larvae is an enlarged pre-oral sphere of a trochophore. The test morphologies of neomenioid larvae are compared to those of pericalymma larvae of protobranch bivalves, and the homology and evolution of molluscan larval tests is discussed.  (+info)

A prospective randomized comparison of sequential versus monoculture systems for in-vitro human blastocyst development. (3/172)

BACKGROUND: Extending the period of in-vitro culture to the blastocyst stage may improve implantation rates in IVF treatment. Recognition of the dynamic nature of early embryo metabolism has led to the development of commercially available sequential culture systems. However, their improved efficacy over monoculture systems remains to be demonstrated in prospective studies. METHODS: Embryos obtained from 158 women undergoing IVF treatment were randomized by sealed envelopes to culture in one of three systems: (A) culture for 5 days in our own monoculture medium (Rotterdam medium); (B) culture for 3 days in Rotterdam medium followed by 2 days in fresh Rotterdam medium; (C) culture for 5 days using the commercially available G1/G2 sequential culture system. RESULTS: There were no significant differences in blastulation, implantation or pregnancy rates between the three tested culture systems. CONCLUSION: The employed monoculture system is as effective as the G1/G2 sequential system for the culture of blastocysts for IVF.  (+info)

Establishment of the organizing activity of the lower endodermal half of the dorsal marginal zone is a primary and necessary event for dorsal axis formation in Cynops pyrrhogaster. (4/172)

The formation of the head and trunk-tail organizers in the dorsal marginal zone (DMZ) of an amphibian embryo is thought to require spatial and temporal interactions between the Nieuwkoop center and the DMZ. Recent studies of the Xenopus embryo suggested that intra-DMZ interaction is also needed to establish the regional specificity of the DMZ. However, it is not yet clarified when and how the final pattern of the head and trunk-tail organizers is established. To analyze the intra-DMZ interactions, we injected suramin into the blastocoel of the mid-blastula of the urodele, Cynops pyrrhogaster, at 6 h prior to the onset of gastrulation. The pigmented blastopore formed normally, but the convergent extension and involution of the DMZ and dorsal axis formation of the embryo were completely inhibited. Expression of gsc, chd and Lim-1 were not maintained, but noggin was unaffected in the suramin-treated embryos. Dorsal axis formation and the expression of these genes of the suramin-treated embryos were rescued by replacing the lower endodermal half of the DMZ (LDMZ) with normal LDMZ. The present results of embryological and molecular examinations indicate that organizing activity of the early Cynops gastrula DMZ is restricted to the LDMZ, and that the organizing activity of the LDMZ is established during the late blastula stages. The results also indicate that LDMZ triggers the sequential interaction within the DMZ that establishes the final pattern of the regional specificity of the DMZ, and that the formation of the LDMZ is a primary and necessary event for dorsal axis formation.  (+info)

T-brain homologue (HpTb) is involved in the archenteron induction signals of micromere descendant cells in the sea urchin embryo. (5/172)

Signals from micromere descendants play a crucial role in sea urchin development. In this study, we demonstrate that these micromere descendants express HpTb, a T-brain homolog of Hemicentrotus pulcherrimus. HpTb is expressed transiently from the hatched blastula stage through the mesenchyme blastula stage to the gastrula stage. By a combination of embryo microsurgery and antisense morpholino experiments, we show that HpTb is involved in the production of archenteron induction signals. However, HpTb is not involved in the production of signals responsible for the specification of secondary mesenchyme cells, the initial specification of primary mesenchyme cells, or the specification of endoderm. HpTb expression is controlled by nuclear localization of beta-catenin, suggesting that HpTb is in a downstream component of the Wnt signaling cascade. We also propose the possibility that HpTb is involved in the cascade responsible for the production of signals required for the spicule formation as well as signals from the vegetal hemisphere required for the differentiation of aboral ectoderm.  (+info)

Experimental analysis of gravitational effects on amphibian gastrulation. (6/172)

The effects of simulated microgravity on blastopore (Bp) formation were analysed in Xenopus laevis and Cynops pyrrhogaster embryos. Simulated microgravity produced by clinostat rotation shifted the Bp-forming region toward the vegetal pole, more markedly in Cynops embryos than in Xenopus embryos. The simulated microgravity induced aggregation of endoderm cells at the center of the embryo and separation between the endoderm and presumptive mesoderm (PM). These findings suggest that clinostat treatment disrupts cell-to-cell interaction between endoderm and PM by increasing the separation between them and, as a result, Bp formation may be shifted towards the vegetal pole.  (+info)

Beta-catenin/Tcf-regulated transcription prior to the midblastula transition. (7/172)

Following fertilization, the zygotic genome in many organisms is quiescent until the midblastula transition (MBT), when large-scale transcription begins. In Xenopus embryos, for example, transcription is believed to be repressed until the twelfth cell division. Thus, although dorsal-ventral patterning begins during the first cell cycle, little attention has been given to transcriptional regulation in pre-MBT development. We present evidence that regulated transcription begins during early cleavage stages and that the beta-catenin-Tcf complex is required for the transcription of the Xenopus nodal genes Xnr5 and Xnr6 as early as the 256-cell stage. Moreover, inhibition of beta-catenin/Tcf function can block dorsal development, but only if the inhibition begins early and is maintained throughout pre-MBT stages. Dorsal development can be rescued in ventralized embryos if Tcf-dependent transcription is activated prior to MBT, but activation of Tcf after MBT cannot rescue ventralized embryos, suggesting that beta-catenin/Tcf-dependent transcription is required prior to MBT for dorsal-ventral patterning in Xenopus.  (+info)

Activin A induces craniofacial cartilage from undifferentiated Xenopus ectoderm in vitro. (8/172)

Activin A has potent mesoderm-inducing activity in amphibian embryos and induces various mesodermal tissues in vitro from the isolated presumptive ectoderm. By using a sandwich culture method established to examine activin A activity, we previously demonstrated that activin-treated ectoderm can function as both a head and trunk-tail organizer, depending on the concentration of activin A. By using activin A and undifferentiated presumptive ectoderm, it is theoretically possible to reproduce embryonic induction. Here, we test this hypothesis by studying the induction of cartilage tissue by using the sandwich-culture method. In the sandwiched explants, the mesenchymal cell condensation expressed type II collagen and cartilage homeoprotein-1 mRNA, and subsequently, cartilage were induced as they are in vivo. goosecoid (gsc) mRNA was prominently expressed in the cartilage in the explants. Xenopus distal-less 4 (X-dll4) mRNA was expressed throughout the explants. In Xenopus embryos, gsc expression is restricted to the cartilage of the lower jaw, and X-dll4 is widely expressed in the ventral head region, including craniofacial cartilage. These finding suggest that the craniofacial cartilage, especially lower jaw cartilage, was induced in the activin-treated sandwiched explants. In addition, a normal developmental pattern was recapitulated at the histological and genetic level. This work also suggests that the craniofacial cartilage-induction pathway is downstream of activin A. This study presents a model system suitable for the in vitro analysis of craniofacial cartilage induction in vertebrates.  (+info)

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