section_path: Development/Inner ear
section_name: Inner ear
paragraph_text: Inner ear
  
  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, which begins to form from the ectoderm around the embryo's 22nd day, derived from two thickenings called otic placodes on either side of the head. Each otic placode recedes below the ectoderm, forms an otic pit and then an otic vesicle. This entire mass is eventually surrounded by mesenchyme to form the bony labyrinth.
  
  Around the 28th day, parts of the otic vesicle begin to form the vestibulocochlear nerve. These form bipolar neurons, 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).
  
  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.
  
  As the cochlear duct's mesenchyme begins to differentiate, three cavities are formed: the scala vestibuli, the scala tympani and the scala media. Both the scala vestibuli and the scala tympani contain an extracellular fluid called perilymph, while the scala media contains endolymph. The vestibular membrane and the basilar membrane develop to separate the cochlear duct from the vestibular duct and the tympanic duct, respectively.
  
  Most of the genes responsible for the regulation of inner ear formation and its morphogenesis are members of the homeobox gene family such as Pax, Msx and Otx homeobox genes. The development of inner ear structures such as the cochlea is regulated by Dlx5 / Dlx6, Otx1 / Otx2 and Pax2, which in turn are controlled by the master gene Shh. Shh is secreted by the notochord.