Editing Gastrulation (section)

==Amphibians==

The [[frog]] [[genus]] ''[[Xenopus]]'' has been used as a [[model organism]] for the study of gastrulation.<ref name="model">{{cite journal |last1=Blum |first1=Martin |last2=Beyer |first2=Tina |last3=Weber |first3=Thomas |last4=Vick |first4=Philipp |last5=Andre |first5=Philipp |last6=Bitzer |first6=Eva |last7=Schweickert |first7=Axel |title=Xenopus , an ideal model system to study vertebrate left-right asymmetry |journal=Developmental Dynamics |date=June 2009 |volume=238 |issue=6 |pages=1215–1225 |doi=10.1002/dvdy.21855 |pmid=19208433 |s2cid=39348233 |language=en|doi-access=free }}</ref>

===Symmetry breaking===
The sperm contributes one of the two [[Spindle apparatus|mitotic asters]] needed to complete first cleavage. The sperm can enter anywhere in the [[animal pole|animal half]] of the egg but its exact point of entry will break the egg's radial symmetry by organizing the [[cytoskeleton]]. Prior to first cleavage, the egg's cortex rotates relative to the internal [[cytoplasm]] by the coordinated action of [[microtubules]], in a process known as cortical rotation. This displacement brings maternally loaded determinants of cell fate from the equatorial cytoplasm and vegetal cortex into contact, and together these determinants set up the [[Primitive knot|organizer]]. Thus, the area on the vegetal side opposite the sperm entry point will become the organizer.<ref name = "Gilbert-AmpAxis">{{cite book |last=Gilbert |first=Scott F. |title=Developmental Biology |publisher=Sinauer Associates |date=2000|chapter=Axis Formation in Amphibians: The Phenomenon of the Organizer, The Progressive Determination of the Amphibian Axes|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10101/#A2296}}</ref> [[Hilde Mangold]], working in the lab of [[Hans Spemann]], demonstrated that this special "organizer" of the embryo is [[Necessity and sufficiency|necessary and sufficient]] to induce gastrulation.<ref name="Gilbert">{{cite web |last1=Gilbert |first1=Scott F. |title=Figure 10.20, [Organization of a secondary axis...]. |url=https://www.ncbi.nlm.nih.gov/books/NBK10101/figure/A2302/?report=objectonly |website=www.ncbi.nlm.nih.gov |access-date=1 June 2020 |language=en |date=2000}}</ref><ref>{{cite journal | author = Spemann H., Mangold H. | year = 1924 | title = Über Induktion von Embryonanlagen durch Implantation artfremder Organisatoren | journal = Roux' Arch. F. Entw. Mech | volume = 100 | issue = 3–4| pages = 599–638 | doi=10.1007/bf02108133| s2cid = 12605303 }}</ref><ref>{{cite journal | author = De Robertis Edward | year = 2006 | title = Spemann's organizer and self-regulation in amphibian embryos | journal = Nature Reviews Molecular Cell Biology | volume = 7 | issue = 4| pages = 296–302 | doi = 10.1038/nrm1855 | pmid = 16482093 | pmc = 2464568 }}</ref>

The [[dorsal lip]] of the blastopore is the mechanical driver of gastrulation, and the first sign of invagination seen in the frog.{{cn|date=April 2022}}

===Germ layer differentiation===
Specification of endoderm depends on rearrangement of maternally deposited determinants, leading to nuclearization of [[Beta-catenin]]. Mesoderm is [[Cellular differentiation|induced]] by signaling from the presumptive endoderm to cells that would otherwise become ectoderm.<ref name="Gilbert-AmpAxis"/>

=== Cell signaling ===
In the frog, ''Xenopus,'' one of the signals is [[retinoic acid]] (RA).<ref name="Zorn A-2009">{{Cite journal|last=Zorn A|first=Wells J|date=2009|title=Vertebrate Endoderm Development and Organ Formation|journal=Annu Rev Cell Dev Biol|volume=25|pages=221–251|doi=10.1146/annurev.cellbio.042308.113344|pmid=19575677|pmc=2861293}}</ref> RA signaling in this organism can affect the formation of the endoderm and depending on the timing of the signaling, it can determine the fate whether its pancreatic, intestinal, or respiratory. Other signals such as Wnt and BMP also play a role in respiratory fate of the ''Xenopus'' by activating cell lineage tracers.<ref name="Zorn A-2009"/>