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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.
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WormBook,
2005]
Early development of many species depends on the temporal and spatial control of maternal gene products. This review discusses the control of maternal mRNAs that encode regulators of C. elegans embryogenesis. In the C. elegans embryo, maternal mRNA regulation is crucial to the patterning of early cell fates. Translational control of key mRNAs spatially organizes cell signaling pathways, localizes transcription factor activities, and controls germ cell precursor development. From the few mRNAs studied thus far, some themes are beginning to emerge. Control of maternal mRNA translation begins in the hermaphrodite germ line. Distinct regulatory systems keep mRNAs silent during different stages of oogenesis, and lead to precise temporal and spatial patterns of translation in the embryo. In the embryo, cell polarity factors control the localization of translational regulators. Each maternal mRNA contains multiple elements in its 3'' untranslated region (3'' UTR) that specify the timing and localization of translation. A relatively small number of RNA-binding proteins likely control many mRNAs through these 3'' UTR elements. Therefore, the combination of RNA elements and the regulatory complexes recruited to them specify unique patterns of translation for different mRNAs. The mechanisms of translational control are only beginning to be explored, but are likely to regulate diverse developmental and cellular events in metazoans.
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[
WormBook,
2005]
The morphogenesis of the C. elegans embryo is largely controlled by the development of the epidermis, also known as the hypodermis, a single epithelial layer that surrounds the animal. Morphogenesis of the epidermis involves cell-cell interactions with internal tissues, such as the developing nervous system and musculature. Genetic analysis of mutants with aberrant epidermal morphology has defined multiple steps in epidermal morphogenesis. In the wild type, epidermal cells are generated on the dorsal side of the embryo among the progeny of four early embryonic blastomeres. Specification of epidermal fate is regulated by a hierarchy of transcription factors. After specification, dorsal epidermal cells rearrange, a process known as dorsal intercalation. Most epidermal cells fuse to generate multinucleate syncytia. The dorsally located epidermal sheet undergoes epiboly to enclose the rest of the embryo in a process known as ventral enclosure; this movement requires both an intact epidermal layer and substrate neuroblasts. At least three distinct types of cellular behavior underlie the enclosure of different regions of the epidermis. Following enclosure, the epidermis elongates, a process driven by coordinated cell shape changes. Epidermal actin microfilaments, microtubules, and intermediate filaments all play roles in elongation, as do body wall muscles. The final shape of the epidermis is maintained by the collagenous exoskeleton, secreted by the apical surface of the epidermis.
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WormBook,
2006]
The C. elegans embryo is a powerful model system for studying the mechanics of metazoan cell division. Its primary advantage is that the architecture of the syncytial gonad makes it possible to use RNAi to generate oocytes whose cytoplasm is reproducibly (typically > 95%) depleted of targeted essential gene products via a process that does not depend exclusively on intrinsic protein turnover. The depleted oocytes can then be analyzed as they attempt their first mitotic division following fertilization. Here we outline the characteristics that contribute to the usefulness of the C. elegans embryo for cell division studies. We provide a timeline for the first embryonic mitosis and highlight some of its key features. We also summarize some of the recent discoveries made using this system, particularly in the areas of nuclear envelope assembly/ dissassembly, centrosome dynamics, formation of the mitotic spindle, kinetochore assembly, chromosome segregation, and cytokinesis.
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WormBook,
2006]
A distinctive feature of polarized epithelial cells is their specialized junctions, which contribute to cell integrity and provide platforms to orchestrate cell shape changes. The chapter discusses the composition and assembly of C. elegans cell-cell and cell-extracellular matrix junctions, proteins that anchor the cytoskeleton and mechanisms involved in establishing epithelial polarity. The focus remains cellular and does not properly deal with epithelial cells in the context of the developing embryo.
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[
WormBook,
2005]
Gastrulation is the process by which the germ layers become positioned in an embryo. C. elegans gastrulation serves as a model for studying the molecular mechanisms of diverse cellular and developmental phenomena, including morphogenesis, cell polarization, cell-cell signaling, actomyosin contraction and cell-cell adhesion. One distinct advantage of studying these phenomena in C. elegans is that genetic tools can be combined with high resolution live cell imaging and direct manipulations of the cells involved. Here we review what is known to date about the cellular and molecular mechanisms that function in C. elegans gastrulation.
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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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[
1987]
Ascaris and several other parasitic nematodes undergo chromatin diminution in the somatic cell precursors of the early embryo. In 1910 Boveri hypothesized that the chromatin lost might include genes essential to the function of the germ line. We have cloned a germ line-specific cDNA which codes for the major sperm protein. Using this clone as a probe we found that these genes show no loss or rearrangement of DNA in somatic cells which have undergone chromatin diminution. Actin and a-tubulin genes from Ascaris are also unchanged following diminution. Ascaris and the free-living nematode Caenorhabditis elegans differ substantially in the numbers of actin and major sperm protein genes, in spite of conservation of gene
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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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[
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