Editing Gastrulation (section)

==Sea urchins==
{{Further|Sea urchin#Development}}
[[Sea urchin]]s have been important [[model organism]]s in [[developmental biology]] since the 19th century.<ref>Laubichler, M.D. and Davidson, E. H. (2008). "Boveri's long experiment: sea urchin merogones and the establishment of the role of nuclear chromosomes in development". ''Developmental Biology''. 314(1):1–11. {{doi|10.1016/j.ydbio.2007.11.024}}.</ref> Their gastrulation is often considered the archetype for invertebrate deuterostomes.<ref name="gast-urchin">{{cite book |last1=McClay |first1=David R.|last2=Gross |first2=J.M. |last3=Range|first3=Ryan |last4=Peterson |first4=R.E. |last5=Bradham |first5=Cynthia |editor-last=Stern |editor-first=Claudio D. |title=Gastrulation: From Cells to Embryos |publisher=Cold Spring Harbor Laboratory Press |date=2004 |pages=123–137|chapter=Chapter 9: Sea Urchin Gastrulation |isbn=978-0-87969-707-5}}</ref> 

Sea urchins exhibit highly stereotyped cleavage patterns and cell fates. Maternally deposited [[mRNA]]s establish the organizing center of the sea urchin embryo. Canonical [[Wnt signaling pathway|Wnt]] and [[Notch signaling pathway|Delta-Notch]] signaling progressively segregate progressive endoderm and mesoderm.<ref>McClay, D. R. 2009. Cleavage and Gastrulation in Sea Urchin. eLS. {{doi|10.1002/9780470015902.a0001073.pub2}}</ref>

The first cells to internalize are the primary [[mesenchyme]] cells (PMCs), which have a [[Sea urchin skeletogenesis|skeletogenic]] fate, which ingress during the blastula stage. Gastrulation – internalization of the prospective [[endoderm]] and non-skeletogenic [[mesoderm]] – begins shortly thereafter with invagination and other cell rearrangements the [[vegetal pole]], which contribute approximately 30% to the final [[archenteron]] length. The [https://www.ncbi.nlm.nih.gov/books/NBK9987/figure/A1730/?report=objectonly gut's final length] depends on cell rearrangements within the archenteron.<ref>{{cite journal | author = Hardin J D | year = 1990 | title = Context-sensitive cell behaviors during gastrulation. | url = http://worms.zoology.wisc.edu/reprints/hardin_sem_DB_1990.pdf | journal = Semin. Dev. Biol. | volume = 1 | pages = 335–345 }}</ref>