Editing Germ layer (section)

==Development==
[[fertilisation|Fertilization]] leads to the formation of a [[zygote]]. During the next stage, [[cleavage (embryo)|cleavage]], [[mitosis|mitotic]] cell divisions transform the zygote into a hollow ball of cells, a [[blastula]]. This early embryonic form undergoes [[gastrulation]], forming a [[gastrula]] with either two or three layers (the germ layers). In all [[vertebrate]]s, these progenitor cells differentiate into all adult tissues and organs.<ref>{{cite book | title = Developmental Biology | chapter = Comparative Embryology | first = Scott F | last = Gilbert | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK9974/#A39 | publisher = Sinauer Associates | year = 2000}}</ref>

In the [[human embryogenesis|human embryo]], after about three days, the zygote forms a solid mass of cells by mitotic division, called a [[morula]]. This then changes to a [[blastocyst]], consisting of an outer layer called a [[trophoblast]], and an inner cell mass called the [[embryoblast]]. Filled with uterine fluid, the blastocyst breaks out of the [[zona pellucida]] and undergoes [[Implantation (human embryo)|implantation]]. The inner cell mass initially has two layers: the [[hypoblast]] and [[epiblast]]. At the end of the second week, a [[primitive streak]] appears. The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called the [[endoderm]], pushing the hypoblast out of the way (this goes on to form the [[amnion]].) The epiblast keeps moving and forms a second layer, the [[mesoderm]]. The top layer is now called the [[ectoderm]].<ref>{{cite book | title = Developmental Biology | chapter = Early Mammalian Development | first = Scott F | last = Gilbert | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK10052/#A2620 | publisher = Sinauer Associates | year = 2000}}</ref>

Gastrulation occurs in reference to the primary [[Anatomical terms of location|body axis]]. Germ layer formation is linked to the primary body axis as well, however it is less reliant on it than gastrulation is. ''[[Hydractinia]]'' shows that germ layer formation that transpires as a mixed delamination.<ref>{{Cite journal |last=Technau |first=Ulrich |date=September 2020 |title=Gastrulation and germ layer formation in the sea anemone Nematostella vectensis and other cnidarians |journal=Mechanisms of Development |volume=163 |pages=103628 |doi=10.1016/j.mod.2020.103628 |pmid=32603823 |s2cid=220121520 |issn=0925-4773|doi-access=free }}</ref>

In mice, germ layer differentiation is controlled by two [[transcription factor]]s: [[SOX2|Sox2]] and [[Oct-4|Oct4]] proteins. These transcription factors cause the [[pluripotent]] mouse [[embryonic stem cell]]s to select a germ layer fate. Sox2 promotes ectodermal differentiation, while Oct4 promotes mesendodermal differentiation. Each gene inhibits what the other promotes. Amounts of each protein are different throughout the genome, causing the embryonic stem cells to select their fate.<ref>{{Cite journal |last1=Thomson |first1=Matt |last2=Liu |first2=Siyuan John |last3=Zou |first3=Ling-Nan |last4=Smith |first4=Zack |last5=Meissner |first5=Alexander |last6=Ramanathan |first6=Sharad |date=June 2011 |title=Pluripotency Factors in Embryonic Stem Cells Regulate Differentiation into Germ Layers |url=|journal=Cell |volume=145 |issue=6 |pages=875–889 |doi=10.1016/j.cell.2011.05.017 |pmid=21663792 |pmc=5603300 |issn=0092-8674}}</ref>