Editing Germ layer (section)

==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.