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[
1983]
More than 100 years ago, early European embryologists had posed the two central questions of animal development: First, how is the sameness of cells and organisms maintained during development and reproduction, and what factors transmit this hereditary information? Second, how do the cells of an embryo become different; what factors dictate that a particular cell at a particular time and position becomes committed to a particular developmental pathway? In the intervening century, we have largely answered the first question, acquiring extensive information about the genetic machinery and how it works. By contrast, we have gained little new understanding of the epigenetic process responsible for temporal and positional control of cell determination in embryos. How this process operates remains a central problem of contemporary
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[
WormBook,
2005]
Cell-cell interactions mediated by the Notch signaling pathway occur throughout C. elegans embryogenesis. These interactions have major roles in specifying cell fates and in tissue morphogenesis. The network of Notch interactions is linked in part through the Notch-regulated expression of components of the pathway, allowing one interaction to pattern subsequent ones. The Notch signal transduction pathway is highly conserved in animal embryogenesis. The REF-1 family of bHLH transcription factors are major targets of Notch signaling in the C. elegans embryo, and are distantly related to HES proteins that are targets of Notch signaling in Drosophila and vertebrates.
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[
WormBook,
2005]
Asymmetric cell divisions play an important role in generating diversity during metazoan development. In the early C. elegans embryo, a series of asymmetric divisions are crucial for establishing the three principal axes of the body plan (AP, DV, LR) and for segregating determinants that specify cell fates. In this review, we focus on events in the one-cell embryo that result in the establishment of the AP axis and the first asymmetric division. We first describe how the sperm-derived centrosome initiates movements of the cortical actomyosin network that result in the polarized distribution of PAR proteins. We then briefly discuss how components acting downstream of the PAR proteins mediate unequal segregation of cell fate determinants to the anterior blastomere AB and the posterior blastomere P 1 . We also review how a heterotrimeric G protein pathway generates cortically based pulling forces acting on astral microtubules, thus mediating centrosome and spindle positioning in response to AP polarity cues. In addition, we briefly highlight events involved in establishing the DV and LR axes. The DV axis is established at the four-cell stage, following specific cell-cell interactions that occur between P 2 and EMS , the two daughters of P 1 , as well as between P 2 and ABp , a daughter of AB . The LR axis is established shortly thereafter by the division pattern of ABa and ABp . We conclude by mentioning how findings made in early C. elegans embryos are relevant to understanding asymmetric cell division and pattern formation across metazoan evolution.
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[
1984]
Developmental fates of blastomeres in early C. elegans embryos appear to be governed by internally segregating, cell-autonomous determinants. To ascertain whether previously described gut-lineage dterminants are nuclear or cytoplasmic, laser microsurgery was used to show that exposing the nucleus of a non-gut-precursor cell to gut-precursor cytoplasm can cause the progeny of the resulting hybrid cell to express gut-specific differentiation markers, supporting the view that the determinants are cytoplasmic. In attempts to obtain molecular probes for such determinants, a library of monoclonal antibodies to early embryonic antigens was generated and screened by immunofluorescence microscopy for antibodies reacting with lineage-specific components. Three of the antibodies react with cytoplasmic granules (P granules) that segregate specifically with the germ line in early cleavages and are found uniquely in germ-line cells throughout the life cycle. Experiments on unfertilized eggs, on mutant embryos with defects in early cleavage, and on normal embryos treated with various cytoskeletal inhibitors indicate that P-granule segregation depends upon fertilization and requires the function of actin microfilaments, but is independent of spindle and microtubule functions. Work on the biochemical nature and function of the P granules is in progress.
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[
1987]
Work in our laboratory over the past several years has focused on the nature of early determinative decisions in embryos of the free-living nematode Caenorhabditis elegans. Two of these decisions regard determination of sex and determination of the level of X-chromosome expression. C. elegans has two sexes, self-fertilizing hermaphrodites and males. Hermaphrodites normally have two X chromosomes, and males have only one (there is no Y chromosome). Genetic and molecular evidence suggest that C. elegans compensates for this difference in X dosage, not by X inactivation as in mammals, but rather by global regulation of the X chromosome as in Drosophila; that is, X-linked genes are expressed at a higher level per chromosome in 1X than 2X animals, so that levels of X expression are similar in the two sexes. Also as in Drosophila, the primary signal that dictates both sex determination and level of X expression in C. elegans is the ration of the number of X chromosomes to the number of sets of autosomes (X/A ratio) rather than the absolute number of X chromosomes.|
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[
Methods Cell Biol,
1995]
In situ hybridization to RNA is an effective tool for the analysis of gene expression during development. This technique is particularly important for Caenorhabditis elegans, as isolation of RNA from specific tissues or developmental stages is generally not possible in this organism (see Chapter 20 in this volume). In addition, the availability of the complete cell lineage and the reproducibility of cell positions from one animal to the next allow RNA expression patterns to be analyzed at the level of individual cells. A number of in situ hybridization protocols have been developed for the detection of RNA in squashed, dissected, or sectioned tissues of C. elegans. More recently, protocols using whole-mount preparations of C. elegans have also been described. By preserving the three-dimensional structure of the specimen, whole-mount preparations facilitate the identification of specific cells and the analysis of complex expression patterns. In this chapter, we describe a protocol for detection of RNA in whole-mount C. elegans embryos. This procedure is based on protocols for Drosophila, which make use of highly sensitive digoxigenin-labeled probes...
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[
Methods Cell Biol,
2008]
The Caenorhabditis elegans gonad and early embryo have recently emerged as an attractive metazoan model system for studying cell and developmental biology. The success of this system is attributable to the stereotypical architecture and reproducible cell divisions of the gonad/early embryo, coupled with penetrant RNAi-mediated protein depletion. These features have facilitated the development of visual assays with high spatiotemporal resolution to monitor specific subcellular processes. Assay development has relied heavily on the emergence of methods to circumvent germline silencing to allow the expression of transgenes encoding fluorescent fusion proteins. In this chapter, we discuss methods for the expression and imaging of fluorescent proteins in the C. elegans germline, including the design of transgenes for optimal expression, the generation of transgenic worm lines by ballistic bombardment, the construction of multimarker lines by mating, and methods for live imaging of the gonad and early embryo.