Editing Germ layer

{{Short description|Primary layer of cells in embryonic development}}
{{More citations needed|date=September 2022}}
A '''germ layer''' is a primary layer  of [[cell (biology)|cell]]s that forms during  [[embryonic development]].<ref>{{cite book | title = Developmental Biology | chapter = The Epidermis and the Origin of Cutaneous Structures | first = Scott F | last = Gilbert | chapter-url = https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2929 | publisher = Sinauer Associates | year = 2003}}</ref> The three germ layers in [[vertebrate]]s are particularly pronounced; however, all [[eumetazoa]]ns ([[animal]]s that are sister taxa to the [[sponge]]s) produce two or three primary germ layers. Some animals, like [[cnidaria]]ns, produce two germ layers (the [[ectoderm]] and [[endoderm]]) making them [[diploblastic]]. Other animals such as [[bilaterians]] produce a third layer (the [[mesoderm]]) between these two layers, making them [[triploblastic]]. Germ layers eventually give rise to all of an animal's [[Tissue (biology)|tissues]] and [[organ (anatomy)|organ]]s through the process of [[organogenesis]].

== History ==
[[File:Remak "Untersuchungen uber die...", 1855 Wellcome L0014201.jpg|thumb|Cleavage and division of the cell of an egg of a vertebrate (Remak, 1855).]]

[[Caspar Friedrich Wolff]] observed organization of the early embryo in leaf-like layers. In 1817, [[Heinz Christian Pander]] discovered three primordial germ layers while studying chick embryos. Between 1850 and 1855, [[Robert Remak]] had further refined the germ cell layer (''Keimblatt'') concept, stating that the external, internal and middle layers form respectively the epidermis, the gut, and the intervening musculature and vasculature.<ref>Remak, R. (1855). ''Untersuchungen über die Entwickelung der Wirbelthiere''. Berlin: G. Reimer. [https://books.google.com/books?id=Kknf7ZRHzagC link].</ref><ref>{{Cite journal | doi=10.1002/ca.980080610| pmid=8713164|title = The terminology of early development: History, concepts, and current usage| journal=Clinical Anatomy| volume=8| issue=6| pages=418–425|year = 1995|last1 = Collins|first1 = P.| last2=Billett| first2=F. S.| s2cid=23450709}}</ref><ref>Weyers, Wolfgang (2002). 150 Years of cell division. ''Dermatopathology: Practical & Conceptual'', Vol. 8, No. 2. [https://www.derm101.com/dpc-archive/april-june-2002-volume-8-no-2/dpc0802a14-150-years-of-cell-division/ link] {{Webarchive|url=https://web.archive.org/web/20190402014854/https://www.derm101.com/dpc-archive/april-june-2002-volume-8-no-2/dpc0802a14-150-years-of-cell-division/ |date=2019-04-02 }}.</ref> The term "[[mesoderm]]" was introduced into English by [[T. H. Huxley|Huxley]] in 1871, and "[[ectoderm]]" and "[[endoderm]]" by [[Ray Lankester|Lankester]] in 1873.

== Evolution ==
[[Image:Gastrulation.png|thumb|300px|'''Gastrulation of a diploblast:''' The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).]]

Among [[animal]]s, [[sea sponge|sponges]] show the least amount of compartmentalization, having a single germ layer. Although they have [[Cellular differentiation|differentiated cells]] (e.g. [[Choanocyte|collar cell]]s), they lack true tissue coordination. [[Diploblastic]] animals, [[Cnidaria]] and [[Ctenophora]], show an increase in compartmentalization, having two germ layers, the [[endoderm]] and [[ectoderm]]. Diploblastic animals are organized into recognisable tissues. All [[bilaterian]] animals (from flatworms to humans) are [[triploblastic]], possessing a [[mesoderm]] in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs.

==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>

==The germ layers==
===Endoderm===
[[Image:Endoderm2.png|thumb|200px|The [[endoderm]] produces tissue within the [[lung]]s, [[thyroid]], and [[pancreas]].]]
{{main|Endoderm}}

The '''endoderm''' is one of the germ layers formed during animal [[embryonic development]]. Cells migrating inward along the [[archenteron]] form the inner layer of the [[gastrula]], which develops into the [[endoderm]].

The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the [[gastrointestinal tract|digestive tract]] except part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.

The endoderm forms: the [[pharynx]], the [[esophagus]], the [[stomach]], the [[small intestine]], the [[large intestine|colon]], the [[liver]], the [[pancreas]], the [[urinary bladder|bladder]], the [[epithelial]] parts of the [[trachea]] and [[bronchi]], the [[lungs]], the [[thyroid]], and the [[parathyroid]].

===Mesoderm===
[[Image:Mesoderm.png|thumb|275px|The [[mesoderm]] aids in the production of [[cardiac muscle]], [[skeletal muscle]], [[smooth muscle]], tissues within the [[kidney]]s, and [[red blood cell]]s.]]
{{main|Mesoderm}}
The '''mesoderm''' germ layer forms in the [[embryo]]s of [[triploblastic]] [[animal]]s. During [[gastrulation]], some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the [[ectoderm]].<ref>{{Citation |last1=Muhr |first1=Jeremy |title=Embryology, Gastrulation |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK554394/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=32119281 |access-date=2022-02-27 |last2=Ackerman |first2=Kristin M.}}</ref> The formation of a mesoderm leads to the development of a [[coelom]]. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protects them from shocks.<ref>{{Cite web |date=2017-06-07 |title=Coelom |url=https://biologydictionary.net/coelom/ |access-date=2022-02-23 |website=Biology Dictionary}}</ref>

The mesoderm has several components which develop into tissues: [[intermediate mesoderm]], [[paraxial mesoderm]], [[lateral plate mesoderm]], and [[Axial mesoderm|chorda-mesoderm]]. The chorda-mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, and wall of the human body.<ref>{{cite book | title = Developmental Biology | chapter = Paraxial and Intermediate Mesoderm | first = Scott F | last = Gilbert | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK9997/ | publisher = Sinauer Associates | year = 2003}}</ref>

Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of [[cellular differentiation|differentiation]].<ref name="Brand 2003">{{cite journal|last=Brand|first=Thomas|title=Heart development: molecular insights into cardiac specification and early morphogenesis|journal=Developmental Biology|date=1 June 2003|volume=258|issue=1|pages=1–19|doi=10.1016/S0012-1606(03)00112-X|pmid=12781678|doi-access=free}}</ref>

The mesoderm forms: muscle ([[Smooth muscle tissue|smooth]] and [[Striated muscle tissue|striated]]), [[bone]], [[cartilage]], [[connective tissue]], [[adipose tissue]], [[circulatory system]], [[lymphatic system]], [[dermis]], [[dentine]] of teeth, [[genitourinary system]], [[serous membrane]]s, [[spleen]] and [[notochord]].

===Ectoderm===
[[Image:Ectoderm.png|thumb|250px|The [[ectoderm]] produces tissues within the [[Epidermis (skin)|epidermis]], aids in the formation of [[neuron]]s within the brain, and constructs [[melanocytes]].]]
{{main|Ectoderm}}
The '''ectoderm''' generates the outer layer of the embryo, and it forms from the embryo's [[epiblast]].<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 = 2003}}</ref> The [[ectoderm]] develops into the [[surface ectoderm]], [[neural crest]], and the [[neural tube]].<ref>{{cite book | title = Developmental Biology | chapter = The Central Nervous System and The Epidermis | first = Scott F | last = Gilbert | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK10026/figure/A2869/ | publisher = Sinauer Associates | year = 2003}}</ref>

The surface ectoderm develops into: [[epidermis (skin)|epidermis]], [[hair]], [[nail (anatomy)|nails]], [[lens of the eye]], [[sebaceous glands]], [[cornea]], [[tooth enamel]], the epithelium of the [[mouth]] and [[nose]].

The neural crest of the ectoderm develops into: [[peripheral nervous system]], [[adrenal medulla]], [[melanocytes]], facial cartilage.

The neural tube of the ectoderm develops into: [[brain]], [[spinal cord]], [[posterior pituitary]], [[motor neurons]], [[retina]].

Note: The anterior pituitary develops from the ectodermal tissue of [[Rathke's pouch]].

====Neural crest====

Because of its great importance, the neural crest is sometimes considered a fourth germ layer.<ref>{{cite journal |author=Hall BK |s2cid=27150120 |year=2000 |title=The neural crest as a fourth germ layer and vertebrates as quadroblastic not triploblastic |journal=Evolution & Development |pmid=11256415 |volume=2 |issue=1 |pages=3–5 |doi=10.1046/j.1525-142x.2000.00032.x}}</ref> It is, however, derived from the ectoderm.

== See also ==

[[Image:Teratoma 2 low mag.jpg|thumb|right|[[Micrograph]] of a [[teratoma]], a tumour that characteristically has tissue from all three '''germ layers'''. The image shows tissue derived from the [[mesoderm]] (immature [[cartilage]] - left-upper corner of image), [[endoderm]] (gastrointestinal glands - center-bottom of image) and [[ectoderm]] (epidermis - right of image). [[H&E stain]].]]
* [[Germ cell]]
* [[Histogenesis]]
* [[Neurulation]]
* [[List of human cell types derived from the germ layers]]

== References ==
{{Reflist}}

{{Embryology}}
{{Authority control}}

{{DEFAULTSORT:Germ Layer}}
[[Category:Germ layers| ]]
[[Category:Embryology]]
[[Category:Developmental biology]]
[[Category:Gastrulation]]