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Caenorhabditis elegans
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=== Embryonic development === The fertilized zygote undergoes rotational holoblastic [[Cleavage (embryo)|cleavage]]. Sperm entry into the oocyte commences formation of an anterior-posterior axis.<ref name="pmid8625834">{{cite journal | vauthors = Goldstein B, Hird SN | title = Specification of the anteroposterior axis in Caenorhabditis elegans | journal = Development | volume = 122 | issue = 5 | pages = 1467β74 | date = May 1996 | pmid = 8625834 | doi = 10.1242/dev.122.5.1467 | url = https://pubmed.ncbi.nlm.nih.gov/8625834/ }}</ref> The sperm [[microtubule organizing center]] directs the movement of the sperm [[pronucleus]] to the future posterior pole of the embryo, while also inciting the movement of [[Cell polarity|PAR proteins]], a group of cytoplasmic determination factors, to their proper respective locations.<ref>{{cite book | title=Developmental biology| edition=11th| author=Gilbert SF| year=2016| pages=268| publisher=Sinauer| isbn=9781605354705}}</ref> As a result of the difference in PAR protein distribution, the first cell division is highly [[asymmetric cell division|asymmetric]].<ref name="pmid7758115">{{cite journal | vauthors = Guo S, Kemphues KJ | title = par-1, a gene required for establishing polarity in ''C. elegans'' embryos, encodes a putative Ser/Thr kinase that is asymmetrically distributed | journal = Cell | volume = 81 | issue = 4 | pages = 611β20 | date = May 1995 | pmid = 7758115 | doi = 10.1016/0092-8674(95)90082-9 | doi-access = free }}</ref> ''C. elegans'' [[embryogenesis]] is among the best understood examples of asymmetric cell division.<ref name="ReferenceA">{{cite journal | vauthors = GΓΆnczy P, Rose LS | title = Asymmetric cell division and axis formation in the embryo | journal = WormBook | pages = 1β20 | date = October 2005 | pmid = 18050411 | pmc = 4780927 | doi = 10.1895/wormbook.1.30.1 }}</ref> All cells of the [[germline]] arise from a single [[primordial germ cell]], called the ''P4'' cell, established early in [[embryogenesis]].<ref>Kimble J, Crittenden SL. Germline proliferation and its control. 2005 Aug 15. In: WormBook: The Online Review of C. elegans Biology [Internet]. Pasadena (CA): WormBook; 2005-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK19769/</ref><ref name="wormbase0006773">{{cite encyclopedia | title=WBbt:0006773 (anatomy term) | encyclopedia=[[WormBase]] |edition=WS242 |id=WBbt:0006773 |date=May 14, 2014}}</ref> This primordial cell divides to generate two germline precursors that do not divide further until after hatching.<ref name="wormbase0006773"/> ==== Axis formation ==== The resulting daughter cells of the first cell division are called the AB cell (containing PAR-6 and PAR-3) and the P1 cell (containing PAR-1 and PAR-2). A second cell division produces the ABp and ABa cells from the AB cell, and the EMS and P2 cells from the P1 cell. This division establishes the dorsal-ventral axis, with the ABp cell forming the dorsal side and the EMS cell marking the ventral side.<ref>{{cite book | title=Developmental biology| edition=11th| author=Gilbert SF| year=2016| pages=272| publisher=Sinauer| isbn=9781605354705}}</ref> Through [[Wnt signaling pathway|Wnt signaling]], the P2 cell instructs the EMS cell to divide along the anterior-posterior axis.<ref name="pmid9288749">{{cite journal | vauthors = Thorpe CJ, Schlesinger A, Carter JC, Bowerman B | title = Wnt signaling polarizes an early ''C. elegans'' blastomere to distinguish endoderm from mesoderm | journal = Cell | volume = 90 | issue = 4 | pages = 695β705 | date = August 1997 | pmid = 9288749 | doi = 10.1016/s0092-8674(00)80530-9 | doi-access = free }}</ref> Through [[Notch signaling pathway|Notch signaling]], the P2 cell differentially specifies the ABp and ABa cells, which further defines the dorsal-ventral axis. The left-right axis also becomes apparent early in embryogenesis, although it is unclear exactly when specifically the axis is determined. However, most theories of the L-R axis development involve some kind of differences in cells derived from the AB cell.<ref>{{cite journal | vauthors = Pohl C, Bao Z | title = Chiral forces organize left-right patterning in ''C. elegans'' by uncoupling midline and anteroposterior axis | journal = Developmental Cell | volume = 19 | issue = 3 | pages = 402β12 | date = September 2010 | pmid = 20833362 | pmc = 2952354 | doi = 10.1016/j.devcel.2010.08.014 }} {{cite journal | pmid = 3073266 | volume=16 | title=[Quantification of sebaceous excretion in volunteers: influence of chronological age, sex and race] | year=1988 | journal=Med Cutan Ibero Lat Am | pages=439β44 | vauthors=Villares JC, Carlini EA| issue=6 }} {{cite book | title=Developmental biology| edition=11th| author=Gilbert SF| year=2016| pages=269| publisher=Sinauer| isbn=9781605354705}}</ref> ==== Gastrulation ==== Gastrulation occurs after the embryo reaches the 24-cell stage.<ref name="pmid1601187">{{cite journal | vauthors = Skiba F, Schierenberg E | title = Cell lineages, developmental timing, and spatial pattern formation in embryos of free-living soil nematodes | journal = Developmental Biology | volume = 151 | issue = 2 | pages = 597β610 | date = June 1992 | pmid = 1601187 | doi = 10.1016/0012-1606(92)90197-o | url = https://pubmed.ncbi.nlm.nih.gov/1601187 }}</ref> ''C. elegans'' are a species of [[protostome]]s, so the blastopore eventually forms the mouth. Involution into the blastopore begins with movement of the [[endoderm]] cells and subsequent formation of the gut, followed by the P4 germline precursor, and finally the [[mesoderm]] cells, including the cells that eventually form the pharynx. Gastrulation ends when [[epiboly]] of the hypoblasts closes the blastopore.<ref>{{cite book | title=Developmental biology| edition=11th| author=Gilbert SF| year=2016| pages=273| publisher=Sinauer| isbn=9781605354705}}</ref>
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