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[
WormBook,
2005]
The C. elegans germ line proliferates from one primordial germ cell (PGC) set aside in the early embryo to over a thousand cells in the adult. Most germline proliferation is controlled by the somatic distal tip cell, which provides a stem cell niche at the distal end of the adult gonad. The distal tip cell signals to the germ line via the Notch signaling pathway, which in turn controls a network of RNA regulators. The FBF-1 and FBF-2 RNA-binding proteins promote continued mitoses in germ cells located close to the distal tip cell, while the GLD-1 , GLD-2 , GLD-3 , and NOS-3 RNA regulators promote entry into meiosis as germ cells leave the stem cell niche. In addition to these key regulators, many other genes affect germline proliferation.
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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]
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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[
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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[
2007]
In the pair of the nematode Caenorhabiditis elegans serotonergic chemosensory neurons ADF, the TRPV channel protein OCR-2 interacts with another TRPV protein, OSM-9, to control the production of the neurotransmitter serotonin. The activity and specificity of OCR-2 in the serotonergic neurons is governed by structural determinants within the channel protein in concert with defined cellular components. The dynamic gating mechanisms, multiple sensory modalities, and functional conservation in diverse organisms make TRPV channels ideal candidates for the long-awaited molecular sensors that underscore the ancient role of the serotonergic system in coupling sensory cues and internal milieu to behavior and physiology.
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[
WormBook,
2005]
The mitochondrial genome is vital for Caenorhabditis elegans metabolism, physiology, and development. The C. elegans mitochondrial DNA is typical of animal mitochondrial genomes in its size and gene content. It is 13,794 nucleotides in length and encodes 36 genes: 2 ribosomal RNAs, 22 transfer RNAs, and 12 protein subunits of the mitochondrial respiratory chain. Although it represents only a small number of genes, an elaborate cellular machinery comprised of over 200 nuclear genes is needed to replicate, transcribe, and maintain the mitochondrial chromosome and to assemble the translation machinery needed to express this dozen proteins. Mitochondrial genetics is peculiar and complex because mitochondrial DNA is maternally inherited and can be present at tens to tens of thousands of copies per cell. The mitochondrial genome content of the developing nematode is developmentally regulated; it increases about 30-fold between the L1 and the adult stages and blocking the increase leads to larval arrest. Energy metabolism is also intimately linked to aging and lifespan determination. The nematode model system offers numerous advantages for understanding the full importance and scope of the mitochondrial genome in animal life.
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[
1990]
Single-gene mutants and genetic lines of the simple roundworm Caenorhabditis elegans that have mean life spans of about 35 days as compared with the wild type (20 days) have been developed. One recessive gene,
age-1, has been mapped to a well-studied region of linkage group II, affects both males and hermaphrodites, and is responsible for a five-fold decrease in hermaphrodite self-fertility. This gene is being more precisely mapped by means of multi-factor crosses and deficiency analysis. We are currently cloning the
age-1 gene using strategies including the construction of congenic strains carrying multiple transposon-mediated RFLPs flanking
age-1 and deficiencies