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Developmental genetics of the Caenorhabditis elegans pharynx

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The Caenorhabditis elegans pharynx is a rhythmically pumping organ composed initially of 80 cells that, through fusions, amount to 62 cells in the adult worm. During the first 100 min of development, most future pharyngeal cells are born and gather into a double‐plate primordium surrounded by a basal lamina. All pharyngeal cells express the transcription factor PHA‐4, of which the concentration increases throughout development, triggering a sequential activation of genes with promoters responding differentially to PHA‐4 protein levels. The oblong‐shaped pharyngeal primordium becomes polarized, many cells taking on wedge shapes with their narrow ends toward the center, hence forming an epithelial cyst. The primordium then elongates, and reorientations of the cells at the anterior and posterior ends form the mouth and pharyngeal‐intestinal openings, respectively. The 20 pharyngeal neurons establish complex but reproducible trajectories using ‘fishing line’ and growth cone‐driven mechanisms, and the gland cells also similarly develop their processes. The genetics behind many fate decisions and morphogenetic processes are being elucidated, and reveal the pharynx to be a fruitful model for developmental biologists. WIREs Dev Biol 2014, 3:263–280. doi: 10.1002/wdev.139 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Invertebrate Organogenesis > Worms Nervous System Development > Worms
Pharyngeal anatomy. The figure outlines the main features of the Caenorhabditis elegans pharynx of relevance for this review. (a) Outline of the pharynx in which the trajectories of the M1, M2L, and NSML neurons are drawn, together with M3R of which only the cell body is shown for clarity. Except for the unique M1, each of these cells has a similar contralateral cell with a mirror shape. The pale gray lines outline the boundaries between muscle cell layers: for example, the pm4 and pm5 muscle cells make up the bulk of the metacorpus and isthmus, respectively (two of each per sector). (b) A cross section in the isthmus, revealing the threefold symmetry with two sublateral sectors, and a dorsal sector, as well as the relative positions of the M1, M2, and NSM processes. Unlabelled ovals in the muscle folds of each sector represent other axons or gland cell ducts. The anatomy is from Albertson et al. and Axäng et al.
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Model for the development of the M2 axon. Elongation of the pm5 cells likely separates the cell bodies of the sister cells M2 and M3. However, M2 remains attached to M3, such that their separation causes lengthening of the M2 axon proximal trajectory through the isthmus (a, b). Later, the distal end of the M2 cell forms a growth cone that requires a signal from M3 to be specified correctly. M3 depends on its expression of mnm‐2 to produce this signal (c). The M2 growth cone then interprets local cues and establishes the two arcs of the distal end (d), and meets its contralateral cellular homolog at the midline where they become connected by a gap junction. Anterior is up. (Reprinted with permission from Ref . Copyright 2007 Elsevier Ltd.)
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Isthmus defect in pha‐2 mutants. In wild‐type worms (left), pm5 (green) and other isthmus cells elongate anteriorly while their nuclei remain in the nascent posterior bulb. This drives the formation of the narrowed, nucleus‐free isthmus. Near the time of hatching, two cells fuse in each muscle sector to create the multinucleated pm5 cells that make up the mature isthmus. In pha‐2 mutants (right), the pm5 cells have a weak cytoskeleton that does not drive anterior elongation nor prevent movement of nuclei into the isthmus, which therefore does not form. The result is that the metacorpus and posterior bulb remain near each other, connected by a short abnormally thick isthmus containing many nuclei. The state of cellular fusion in the pha‐2 mutant is not known.
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Cellular origins and positions of pharyngeal cells. Color‐coded fates of the pharyngeal cells in the 28‐cell embryo (a) and location of their nuclei in the mature pharynx (b). Note the approximate preservation of spatial relationships in the mature pharynx, and absence of nuclei in the isthmus. The lineage is from Sulston et al., and the anatomy is from Albertson et al.
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Overview of pharyngeal development. All pharyngeal cells are descendants of the ABa and MS cells which are born after 2 and 3 cell divisions of the zygote, respectively (a). ABa will produce 49 pharyngeal cells while MS will produce 39. These cells are born and migrate during gastrulation (b) to form two plates of cells 6–8 cells deep along the dorsal–ventral axis (c). Many cells then constrict along the midline facing apical surfaces to create an elongated epithelial cyst depicted as a cross section in (d). Reorientation of the cells at the anterior and posterior ends of the primordium create openings toward the mouth and intestine, and retraction of the apical tips of marginal cells create a lumen that runs through the entire pharynx as it elongates; thin cellular processes, such as the axons of the M2 and other neurons, are drawn by cellular elongations such as that of the pm5 muscle cells in the isthmus (e). Pharyngeal cells then complete their differentiation and morphogenesis, including completing growth cone‐driven axon trajectories, to produce a mature pharynx that will grow in size during larval development and of which the muscle cells will gradually become disorganized during aging (f). See text for details. The lineage is from Sulston et al., and the developmental processes shown are mostly from Portereiko and Mango, Rasmussen et al. and Rauthan et al.
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