Editing Sea urchin (section)

=== Development ===
[[File:Sea Urchin Blastula.jpg|thumb|Sea urchin blastula]]

During early development, the sea urchin [[embryo]] undergoes 10 cycles of [[cell division]],<ref>{{cite journal|title =Arsenic Exposure Affects Embryo Development of Sea Urchin, Paracentrotus lividus (Lamarck, 1816) | journal = Bulletin of Environmental Contamination and Toxicology| volume = 39| issue = 2| pages = 124–8|date =2013 | last= A. Gaion|first = A. Scuderi|author2= D. Pellegrini|author3= D. Sartori | doi = 10.3109/01480545.2015.1041602| pmid = 25945412| s2cid = 207437380}}</ref> resulting in a single [[epithelial]] layer enveloping the [[blastocoel]]. The embryo then begins [[gastrulation]], a multipart process which dramatically rearranges its structure by [[invagination]] to produce the three [[germ layer]]s, involving an [[epithelial-mesenchymal transition]]; primary [[mesenchyme]] cells move into the blastocoel<ref name=pmid15367199>{{cite journal |last1=Kominami |first1=Tetsuya |last2=Takata |first2=Hiromi |title=Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium |journal=Development, Growth & Differentiation |volume=46 |issue=4 |pages=309–26 |year=2004 |doi=10.1111/j.1440-169x.2004.00755.x|pmid=15367199 |s2cid=23988213 }}</ref> and become [[mesoderm]].<ref>{{cite journal |doi=10.1016/j.mod.2003.06.005 |last1=Shook |first1=D |last2=Keller |first2=R |title=Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development |journal=Mechanisms of Development |volume=120 |issue=11 |pages=1351–83 |year=2003 |pmid=14623443|s2cid=15509972 |doi-access=free }}; {{cite journal |last1=Katow |first1=Hideki |last2=Solursh |first2=Michael |title=Ultrastructure of primary mesenchyme cell ingression in the sea urchinLytechinus pictus |journal=Journal of Experimental Zoology |volume=213 |pages=231–246 |year=1980 |doi=10.1002/jez.1402130211 |issue=2|bibcode=1980JEZ...213..231K }}; {{cite journal |doi=10.1016/0014-4827(59)90275-7 |last1=Balinsky |first1=BI |title=An electro microscopic investigation of the mechanisms of adhesion of the cells in a sea urchin blastula and gastrula |journal=Experimental Cell Research |pmid=13653007 |volume=16 |issue=2 |pages=429–33 |year=1959}}; {{cite journal |last1=Hertzler |first1=PL |last2=McClay |first2=DR |title=alphaSU2, an epithelial integrin that binds laminin in the sea urchin embryo |journal=Developmental Biology |volume=207 |issue=1 |pages=1–13 |year=1999 |pmid=10049560 |doi=10.1006/dbio.1998.9165|doi-access=free }}; {{cite journal |doi=10.1016/0012-1606(85)90376-8 |last1=Fink |first1=RD |last2=McClay |first2=DR |title=Three cell recognition changes accompany the ingression of sea urchin primary mesenchyme cells |pmid=2578117 |journal=Developmental Biology |volume=107 |issue=1 |pages=66–74 |year=1985}}; {{cite journal |doi=10.1016/0012-1606(91)90425-3 |last1=Burdsal |first1=CA |last2=Alliegro |first2=MC |last3=McClay |first3=DR |title=Tissue-specific, temporal changes in cell adhesion to echinonectin in the sea urchin embryo |pmid=1707016 |journal=Developmental Biology |volume=144 |issue=2 |pages=327–34 |year=1991}}; {{cite journal |last1=Miller |first1=JR |last2=McClay |first2=DR |title=Characterization of the Role of Cadherin in Regulating Cell Adhesion during Sea Urchin Development |journal=Developmental Biology |volume=192 |issue=2 |pages=323–39 |year=1997 |pmid=9441671 |doi=10.1006/dbio.1997.8740|doi-access=free }}; {{cite journal |last1=Miller |first1=JR |last2=McClay |first2=DR |title=Changes in the pattern of adherens junction-associated beta-catenin accompany morphogenesis in the sea urchin embryo |journal=Developmental Biology |volume=192 |issue=2 |pages=310–22 |year=1997 |pmid=9441670 |doi=10.1006/dbio.1997.8739|doi-access=free }}; {{cite journal |doi=10.1016/S0012-1606(89)80058-2 |last1=Anstrom |first1=JA |title=Sea urchin primary mesenchyme cells: ingression occurs independent of microtubules |journal=Developmental Biology |pmid=2562830 |volume=131 |issue=1 |pages=269–75 |year=1989}}; {{cite journal |last1=Anstrom |first1=JA |title=Microfilaments, cell shape changes, and the formation of primary mesenchyme in sea urchin embryos |journal=The Journal of Experimental Zoology |volume=264 |issue=3 |pages=312–22 |year=1992 |pmid=1358997 |doi=10.1002/jez.1402640310}}</ref> It has been suggested that [[epithelial polarity]] together with planar cell polarity might be sufficient to drive gastrulation in sea urchins.<ref>{{cite journal | last1 = Nissen | first1 = Silas Boye | last2 = Rønhild | first2 = Steven | last3 = Trusina | first3 = Ala | last4 = Sneppen | first4 = Kim | title=Theoretical tool bridging cell polarities with development of robust morphologies | journal=eLife | date=November 27, 2018 | volume=7 |pages=e38407 |doi=10.7554/eLife.38407 | pmid = 30477635 | pmc = 6286147 | doi-access = free }}</ref>

[[File:Left-right asymmetry in the sea urchin - journal.pbio.1001404.g001.png|thumb|center|upright=2.7|The development of a regular sea urchin]]

An unusual feature of sea urchin development is the replacement of the larva's [[bilateral symmetry]] by the adult's broadly fivefold symmetry. During cleavage, mesoderm and small micromeres are specified. At the end of gastrulation, cells of these two types form [[coelom]]ic pouches. In the larval stages, the adult rudiment grows from the left coelomic pouch; after metamorphosis, that rudiment grows to become the adult. The [[animal-vegetal axis]] is established before the egg is fertilized. The oral-aboral axis is specified early in cleavage, and the left-right axis appears at the late gastrula stage.<ref name="WarnerLyons2012">{{cite journal |last1=Warner |first1=Jacob F. |last2=Lyons |first2=Deirdre C. |last3=McClay |first3=David R. |title=Left-Right Asymmetry in the Sea Urchin Embryo: BMP and the Asymmetrical Origins of the Adult |journal=PLOS Biology |volume=10 |issue=10 |year=2012 |pages=e1001404 |doi=10.1371/journal.pbio.1001404|pmid=23055829 |pmc=3467244 |doi-access=free }}</ref>