Editing Ear (section)

===Inner ear===
[[File:G1. Otic placode (V07a).png|thumb|The [[otic placode]] on a developing embryo (about four weeks old)]]
[[File:Gray62.png|thumb|The ear develops in the lower neck region and moves upwards as the [[mandible]] develops (six weeks).]]

Around its second to third week, the developing embryo consists of three layers: ectoderm, [[mesoderm]], and endoderm. The first part of the ear to develop is the inner ear,<ref name=moore2008/> which begins to form from the ectoderm around the embryo's 22nd day,<ref name=sadler2012/> derived from two thickenings called [[otic placode]]s on either side of the head. Each otic placode recedes below the ectoderm, forms an [[otic pit]] and then an [[otic vesicle]].<ref name="GRAYS40th_38">{{cite book|last1=Standring|first1=Susan|editor1-last=Borley|editor1-first=Neil R.|title=Gray's Anatomy: The Anatomical Basis of Clinical Practice|date=2008|publisher=[[Churchill Livingstone]]/[[Elsevier]]|location=[[Edinburgh]]|isbn=978-0-443-06684-9|edition=40|pages=Chapter 38. "Development of the ear", 651–653}}</ref> This entire mass is eventually surrounded by mesenchyme to form the bony labyrinth.<ref name=GRAYS40th_38 /><ref name=UNSWembryology>{{cite book |title= UNSW Embryology. Hearing-Inner Ear Development |url= http://php.med.unsw.edu.au/embryology/index.php?title=Hearing_-_Inner_Ear_Development |access-date= 20 April 2013 |url-status=dead |archive-url= https://web.archive.org/web/20120930063158/http://php.med.unsw.edu.au/embryology/index.php?title=Hearing_-_Inner_Ear_Development |archive-date= 30 September 2012}}</ref>

Around the 28th day, parts of the otic vesicle begin to form the vestibulocochlear nerve.<ref name="GRAYS40th_38" /><ref name="drake2010">{{cite book |last1=Drake |first1=Richard L. |author-link=Richard L. Drake |title=GRAY Anatomía para estudiantes |last2=Wayne |first2=A. |author-link2=A. Wayne |last3=Mitchell |first3=Adam |author-link3=Adam Mitchell (scientist) |year=2010 |pages=854–871}}</ref> These form [[bipolar neuron]]s, which supply sensation to parts of the inner ear (namely the sensory parts of the semicircular canals, macular of the utricle and saccule, and organ of Corti).<ref name="GRAYS40th_38" />

Around the 33rd day, the vesicles begin to differentiate. Posteriorly, they form what will become the utricle and semicircular canals. Anteriorly, the vesicles differentiate into a rudimentary saccule, which eventually becomes the saccule and cochlea. Part of the saccule eventually gives rise and connects to the cochlear duct, which appears approximately during the sixth week and connects to the saccule through the [[ductus reuniens]].<ref name="sadler2012" />

As the cochlear duct's mesenchyme begins to differentiate, three cavities are formed: the [[scala vestibuli]], the [[scala tympani]] and the [[scala media]].<ref name="sadler2012" /><ref name="UNSWembryology" /> Both the scala vestibuli and the scala tympani contain an extracellular fluid called [[perilymph]], while the scala media contains endolymph.<ref name="UNSWembryology" /> The [[vestibular membrane]] and the [[basilar membrane]] develop to separate the cochlear duct from the vestibular duct and the tympanic duct, respectively.<ref name="sadler2012" />

;Molecular regulation
Most of the genes responsible for the [[Regulation of gene expression|regulation]] of inner ear formation and its [[morphogenesis]] are members of the [[homeobox gene]] family such as [[Pax genes|Pax]], Msx and Otx homeobox genes. The development of inner ear structures such as the [[cochlea]] is regulated by [[DLX5 (gene)|Dlx5]]/[[DLX6 (gene)|Dlx6]], [[OTX1 (gene)|Otx1]]/[[OTX2 (gene)|Otx2]] and [[PAX2|Pax2]], which in turn are controlled by the master [[SHH (gene)|gene Shh]]. Shh is secreted by the [[notochord]].<ref name=sumantra2010>{{cite journal |last1=Chatterjee |first1=Sumantra |author-link=Sumantra Chatterjee |last2=Kraus |first2=Petra |author-link2=Petra Krauss |last3=Luftkin |first3=Thomas |author-link3=Thomas Luftkin |year=2010 |title= A symphony of inner ear developmental control genes |journal=BMC Genetics |volume=11 |pages=68 |doi=10.1186/1471-2156-11-68 |pmid=20637105 |pmc=2915946 |doi-access=free }}</ref>