Talk:Frog Development

From Embryology

Background Reading

Organizing early for left-right asymmetry

Organizing early for left-right asymmetry Development 2010 137:e702

http://dev.biologists.org/content/137/7.toc

"Although the overall vertebrate body plan is bilaterally symmetrical, internal organs show consistent left-right (LR) asymmetry. One model proposes that the motion of nodal cilia during gastrulation sets LR asymmetry. However, on p. 1095, Laura Vandenberg and Michael Levin show that in Xenopus embryos the crucial LR asymmetry events occur shortly after fertilization, well before ciliogenesis. The researchers ablate the primary dorsal organizer by UV irradiation, induce a new organizer either early (before cleavage) or late (after the 32-cell stage), and then examine the position of the heart, stomach and gall bladder. The LR axis is usually properly oriented when the new organizer is induced early, they report, but not when it is induced late. Intriguingly, late-induced organizers can correctly orient asymmetry if instructed by a conjoined twin arising from an organizer that was present during the first cleavages. Together, these results suggest that very early symmetry-breaking events, rather than events happening at the node during gastrulation, are of prime importance in establishing LR asymmetry in Xenopus embryos."


Consistent left-right asymmetry cannot be established by late organizers in Xenopus unless the late organizer is a conjoined twin

Vandenberg LN, Levin M. Development. 2010 Apr;137(7):1095-105. PMID: 20215347

http://dev.biologists.org/content/137/7/1095.full

"How embryos consistently orient asymmetries of the left-right (LR) axis is an intriguing question, as no macroscopic environmental cues reliably distinguish left from right. Especially unclear are the events coordinating LR patterning with the establishment of the dorsoventral (DV) axes and midline determination in early embryos. In frog embryos, consistent physiological and molecular asymmetries manifest by the second cell cleavage; however, models based on extracellular fluid flow at the node predict correct de novo asymmetry orientation during neurulation. We addressed these issues in Xenopus embryos by manipulating the timing and location of dorsal organizer induction: the primary dorsal organizer was ablated by UV irradiation, and a new organizer was induced at various locations, either early, by mechanical rotation, or late, by injection of lithium chloride (at 32 cells) or of the transcription factor XSiamois (which functions after mid-blastula transition). These embryos were then analyzed for the position of three asymmetric organs. Whereas organizers rescued before cleavage properly oriented the LR axis 90% of the time, organizers induced in any position at any time after the 32-cell stage exhibited randomized laterality. Late organizers were unable to correctly orient the LR axis even when placed back in their endogenous location. Strikingly, conjoined twins produced by late induction of ectopic organizers did have normal asymmetry. These data reveal that although correct LR orientation must occur no later than early cleavage stages in singleton embryos, a novel instructive influence from an early organizer can impose normal asymmetry upon late organizers in the same cell field."


PLoS Biology

  • Genetic Screens for Mutations Affecting Development of Xenopus tropicalis Tadahiro Goda, Anita Abu-Daya, Samantha Carruthers, Matthew D Clark, Derek L Stemple, and Lyle B Zimmerman PLoS Genet. 2006 June; 2(6): e91. Prepublished online 2006 April 28. Published online 2006 June 9. doi: 10.1371/journal.pgen.0020091. PMCID: PMC1475704

Frog Mitochondrial Genome

The complete nucleotide sequence of the Xenopus laevis mitochondrial genome.[1] "The complete sequence of the 17,553-nucleotide Xenopus laevis mitochondrial genome has been determined. A comparison of this amphibian mitochondrial genomic sequence with those of the mammalian mitochondrial genomes reveals a similar gene order and compact genomic organization."

Xenopus laevis mitochondrial DNA, complete genome - 17553 bp GenBank report

Taxon

Xenopus Laevis

  • Eukaryotae; mitochondrial eukaryotes; Metazoa; Chordata;Vertebrata; Amphibia; Batrachia; Anura; Mesobatrachia; Pipoidea;Pipidae; Xenopodinae; Xenopus

Rana pipiens

Taxonomy Id: 8404 Preferred common name: northern leopard frog Rank: species

Genetic code: Translation table 1 (Standard) Mitochondrial genetic code: Translation table 2 Lineage( abbreviated ):

  • Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Amphibia; Batrachia; Anura; Neobatrachia; Ranoidea; Ranidae; Raninae; Rana

External WWW Links

Note the dynamic developmental nature of the Internet means that some links may not always work (search using the link term).

References

PubMed

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Search PubMed:term=frog+development | term=xenopus+development

Developmental Biology (6th ed) Gilbert: Frog Life Cycle

Molecular Cell Biology (4th ed.)Lodish Figure 23-5. Early embryogenesis of the frog Xenopus laevis

  • Organisation of Xenopus oocyte and egg cortices. Chang P, Perez-Mongiovi D, Houliston E Microsc Res Tech 1999 Mar 15;44(6):415-29
    • Abstract: The division of the Xenopus oocyte cortex into structurally and functionally distinct "animal" and "vegetal" regions during oogenesis provides the basis of the organisation of the early embryo. The vegetal region of the cortex accummulates specific maternal mRNAs that specify the development of the endoderm and mesoderm, as well as functionally-defined "determinants" of dorso-anterior development, and recognisable "germ plasm" determinants that segregate into primary germ cells. These localised elements on the vegetal cortex underlie both the primary animal-vegetal polarity of the egg and the organisation of the developing embryo. The animal cortex meanwhile becomes specialised for the events associated with fertilisation: sperm entry, calcium release into the cytoplasm, cortical granule exocytosis, and polarised cortical contraction. Cortical and subcortical reorganisations associated with meiotic maturation, fertilisation, cortical rotation, and the first mitotic cleavage divisions redistribute the vegetal cortical determinants, contributing to the specification of dorso-anterior axis and segregation of the germ line. In this article we consider what is known about the changing organisation of the oocyte and egg cortex in relation to the mechanisms of determinant localisation, anchorage, and redistribution, and show novel ultrastructural views of cortices isolated at different stages and processed by the rapid-freeze deep-etch method. Cortical organisation involves interactions between the different cytoskeletal filament systems and internal membranes. Associated proteins and cytoplasmic signals probably modulate these interactions in stage-specific ways, leaving much to be understood.
  • Translocation of a localized maternal mRNA to the vegetal pole of Xenopus oocytes. Melton DA Nature 1987 Jul 2-8;328(6125):80-2
    • A key paper in establishing localization of maternal mRNAs as regulators of developmental pattern formation. This localization of mRNAs has also been found for many different drosophila (fly) mRNAs with many different patterns.
    • Abstract: A prominent hypothesis in embryology is that localized maternal factors are important in specifying cell fate. There are, however, only a few examples of maternal molecules that have been shown to be localized and very little is known about how such factors are physically localized within an egg (for review see ref. 1). Previously, cDNA clones were obtained for a class of localized maternal mRNAs from Xenopus laevis. These mRNAs are unusual in that they are concentrated at either the animal or vegetal pole of unfertilized eggs. In the present study the synthesis and intracellular distribution of one of them, Vg1, has been examined during oogenesis. The results show that Vg1 mRNA is localized as a crescent at the vegetal pole of mature oocytes. Surprisingly, this mRNA is uniformly distributed in the cytoplasm of immature oocytes. These findings suggest that a single cell, the frog oocyte, has some mechanism for translocating specific RNAs like Vg1. The process that moves Vg1 mRNA is evidently a cytoplasmic localization machinery which is not directly coupled to the synthesis of Vg1 RNA.
  • A two-step model for the localization of maternal mRNA in Xenopus oocytes: involvement of microtubules and microfilaments in the translocation and anchoring of Vg1 mRNA. Yisraeli JK, Sokol S, Melton DA Development 1990 Feb;108(2):289-98.
    • Abstract: In an effort to understand how polarity is established in Xenopus oocytes, we have analyzed the process of localization of the maternal mRNA, Vg1. In fully grown oocytes, Vg1 mRNA is tightly localized at the vegetal cortex. Biochemical fractionation shows that the mRNA is preferentially associated with a detergent-insoluble subcellular fraction. The use of cytoskeletal inhibitors suggests that (1) microtubules are involved in the translocation of the message to the vegetal hemisphere and (2) microfilaments are important for the anchoring of the message at the cortex. Furthermore, immunohistochemistry reveals that a cytoplasmic microtubule array exists during translocation. These results suggest a role for the cytoskeleton in localizing information in the oocyte.
    • RNA sorting in Xenopus oocytes and embryos. Mowry KL, Cote CA FASEB J 1999 Mar;13(3):435-45
    • mRNA localisation during development. Micklem DR Dev Biol 1995 Dec;172(2):377-95.
  • Thyroid hormone-dependent metamorphosis in a direct developing frog. Callery, EM and Elinson RP, Proc. Natl. Acad. Sci. USA, Vol. 97, Issue 6, 2615-2620, March 14, 2000
    • The direct developing anuran, Eleutherodactylus coqui, lacks a tadpole, hatching as a tiny frog. We investigated the role of the metamorphic trigger, thyroid hormone (TH), in this unusual ontogeny. Expression patterns of the thyroid hormone receptors, TR and TR, were similar to those of indirect developers. TR mRNA levels increased dramatically around the time of thyroid maturation, when remodeling events reminiscent of metamorphosis occur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. Despite their radically altered ontogeny, direct developers still undergo a TH-dependent metamorphosis, which occurs before hatching. We propose a new model for the evolution of anuran direct development.
  1. ↑ <pubmed>85261388</pubmed>
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