Editing Human brain (section)

==Development==
{{Main |Development of the nervous system in humans}}
{{Further|Development of the human brain}}
[[File:Embryonic Development CNS.png|thumb|upright=1.25|Neurulation and neural crest cells]]
[[File:1302 Brain Vesicle DevN.jpg|thumb|upright=1.25|alt= Simple drawing of the lateral view of the three primary vesicle stage of the three to four week old embryo shown in different colors, and the five secondary vesicle stage of the five week old embryo shown in different colors and a lateral view of this |Primary and secondary [[brain vesicle|vesicle]] stages of development in the early embryo to the fifth week]]
[[File:6 week embryo brain.jpg|thumb|alt=Very simple drawing of the front end of a human embryo, showing each vesicle of the developing brain in a different color. |Brain of a human embryo in the sixth week of development]]

At the beginning of the third week of [[human embryonic development|development]], the [[embryo]]nic [[ectoderm]] forms a thickened strip called the [[neural plate]].<ref name="Sadler">{{cite book |last1=Sadler |first1=T. |title=Langman's medical embryology |date=2010 |publisher=Lippincott Williams & Wilkins |location=Philadelphia |isbn=978-0-7817-9069-7 |page=293 |edition=11th}}</ref> By the fourth week of development the neural plate has widened to give a broad [[cephalization|cephalic]] end, a less broad middle part and a narrow caudal end. These swellings are known as the [[Brain vesicle|primary brain vesicles]] and represent the beginnings of the [[forebrain]] (prosencephalon), [[midbrain]] (mesencephalon), and [[hindbrain]] (rhombencephalon).{{sfn|Larsen|2001|p=419}}<ref>{{Cite journal |last1=Zhou |first1=Yi |last2=Song |first2=Hongjun |last3=Ming |first3=Guo-Li |date=2023-07-28 |title=Genetics of human brain development |journal=Nature Reviews. Genetics |volume=25 |issue=1 |pages=26–45 |doi=10.1038/s41576-023-00626-5 |issn=1471-0064 |pmid=37507490|pmc=10926850 |s2cid=260286574 }}</ref>

[[Neural crest|Neural crest cells]] (derived from the ectoderm) populate the lateral edges of the plate at the [[neural fold]]s. In the fourth week{{Em dash}}during the [[neurulation |neurulation stage]]{{Em dash}}the [[Neural fold#Folding mechanism|neural folds close]] to form the [[neural tube]], bringing together the neural crest cells at the [[neural crest]].{{sfn|Larsen|2001|pp=85–88}} The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail. Cells detach from the crest and [[cell migration|migrate]] in a craniocaudal (head to tail) wave inside the tube.{{sfn|Larsen|2001|pp=85–88}} Cells at the cephalic end give rise to the brain, and cells at the caudal end give rise to the spinal cord.{{sfn|Purves|2012|pp=480–482}}

The tube [[Flexure (embryology)|flexes]] as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32.{{sfn|Larsen|2001|pp=445–446}}
Early in the fourth week, the cephalic part bends sharply forward in a [[cephalic flexure]].{{sfn|Larsen|2001|pp=85–88}} This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure becomes the hindbrain (rhombencephalon). These areas are formed as swellings known as the three [[brain vesicle|primary brain vesicles]]. In the fifth week of development five [[brain vesicle|secondary brain vesicles]] have formed.<ref>{{Cite web|title = OpenStax CNX|url = http://cnx.org/contents/b037bde2-ea37-43a5-9102-8d4fcbc623d1@3/The_Embryologic_Perspective|website = cnx.org|access-date = May 5, 2015|url-status = live|archive-url = https://web.archive.org/web/20150505054856/http://cnx.org/contents/b037bde2-ea37-43a5-9102-8d4fcbc623d1@3/The_Embryologic_Perspective|archive-date = May 5, 2015|df = mdy-all}}</ref> The forebrain separates into two vesicles – an anterior [[telencephalon]] and a posterior [[diencephalon]]. The telencephalon gives rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon gives rise to the thalamus and hypothalamus. The hindbrain also splits into two areas – the [[metencephalon]] and the [[myelencephalon]]. The metencephalon gives rise to the cerebellum and pons. The myelencephalon gives rise to the medulla oblongata.{{sfn|Larsen|2001|pp=85–87}} Also during the fifth week, the brain divides into [[segmentation (biology)|repeating segments]] called [[neuromere]]s.{{sfn|Larsen|2001|p=419}}{{sfn|Purves|2012|pp=481–484}} In the [[hindbrain]] these are known as [[rhombomere]]s.<ref name=Neuro>{{cite book |editor1-first=Dale |editor1-last=Purves |editor2-first=George J |editor2-last=Augustine |editor3-first=David |editor3-last=Fitzpatrick |editor4-first=Lawrence C |editor4-last=Katz |editor5-first=Anthony-Samuel |editor5-last=LaMantia |editor6-first=James O |editor6-last=McNamara |editor7-first=S Mark |editor7-last=Williams |year=2001 |chapter=Rhombomeres |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10954/box/A1478/ |title=Neuroscience |publisher=Palgrave Macmillan |edition=2nd |isbn=978-0-87893-742-4}}</ref>

A characteristic of the brain is the cortical folding known as [[gyrification]]. For just over five months of [[prenatal development]] the cortex is smooth. By the gestational age of 24 weeks, the wrinkled morphology showing the fissures that begin to mark out the lobes of the brain is evident.<ref name="Chen">{{cite book |url=https://books.google.com/books?id=94aPR_Oh40oC&pg=PA188 |title=Mechanical Self-Assembly: Science and Applications |publisher=[[Springer Science & Business Media]] |year=2012 |isbn=978-1-4614-4562-3 |pages=188–189 |last=Chen |first=X.}}</ref> Why the cortex wrinkles and folds is not well-understood, but gyrification has been linked to intelligence and [[neurological disorder]]s, and a [[Gyrification#Theories on causality in gyrification|number of gyrification theories]] have been proposed.<ref name="Chen"/> These theories include those based on [[Gyrification#Mechanical buckling|mechanical buckling]],<ref name="Ronan">{{cite journal |last1=Ronan |first1=L |last2=Voets |first2=N |last3=Rua |first3=C |last4=Alexander-Bloch |first4=A |last5=Hough |first5=M |last6=Mackay |first6=C |last7=Crow |first7=TJ |last8=James |first8=A |last9=Giedd |first9=JN |last10=Fletcher |first10=PC |title=Differential tangential expansion as a mechanism for cortical gyrification. |journal=Cerebral Cortex |date=August 2014 |volume=24 |issue=8 |pages=2219–28 |doi=10.1093/cercor/bht082 |pmid=23542881|pmc=4089386 }}</ref><ref name="Ackerman">{{cite book |last1=Ackerman |first1=S. |title=Discovering the brain |url=https://archive.org/details/discoveringbrain00acke |url-access=registration |date=1992 |publisher=National Academy Press |location=Washington, D.C. |isbn=978-0-309-04529-2 |pages=[https://archive.org/details/discoveringbrain00acke/page/22 22–25]}}</ref> [[Gyrification#Axonal tension|axonal tension]],<ref name="Van Essen">{{cite journal |last1=Van Essen |first1=DC |title=A tension-based theory of morphogenesis and compact wiring in the central nervous system. |journal=Nature |date=January 23, 1997 |volume=385 |issue=6614 |pages=313–8 |doi=10.1038/385313a0 |pmid=9002514|bibcode=1997Natur.385..313E |s2cid=4355025 }}</ref> and [[Gyrification#Differential tangential expansion|differential tangential expansion]].<ref name="Ronan"/> What is clear is that gyrification is not a random process, but rather a complex developmentally predetermined process which generates patterns of folds that are consistent between individuals and most species.<ref name="Ronan"/><ref name="Borrell">{{cite journal |last1=Borrell |first1=V |title=How Cells Fold the Cerebral Cortex. |journal=The Journal of Neuroscience |date=24 January 2018 |volume=38 |issue=4 |pages=776–783 |doi=10.1523/JNEUROSCI.1106-17.2017 |pmid=29367288|pmc=6596235 }}</ref>

The first groove to appear in the fourth month is the lateral cerebral fossa.{{sfn|Larsen|2001|pp=445–446}} The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space. This covers the fossa and turns it into a much deeper ridge known as the [[lateral sulcus]] and this marks out the temporal lobe.{{sfn|Larsen|2001|pp=445–446}} By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes.{{sfn|Larsen|2001|pp=445–446}} A gene present in the human genome ([[ARHGAP11B]]) may play a major role in gyrification and encephalisation.<ref>{{cite journal |last1=Florio |first1=M.|display-authors=etal |title=Human-specific gene ARHGAP11B promotes basal progenitor amplification and neocortex expansion |journal=Science |date=March 27, 2015 |volume=347 |issue=6229 |pages=1465–70 |pmid=25721503 |doi=10.1126/science.aaa1975|bibcode=2015Sci...347.1465F|s2cid=34506325|doi-access=free }}</ref>
{{Gallery
 | title=
 | width=200
 | height=180
 |File:Gray651.png |Brain of human embryo at 4.5 weeks, showing interior of forebrain
 |File:Gray653.png |Brain interior at 5 weeks
 |File:Gray654.png |Brain viewed at midline at 3 months
}}