Human Anatomy - Kotsan I. Ya. 2009

Vestibulocochlear organ (organ of hearing and equilibrium)
Development of the organ of hearing and equilibrium

In the course of evolution, the vestibular organ (The Organ of static sense) appeared first. As a primitively structured formation—the static vesicle (otocyst or statocyst)—it is already present in aquatic invertebrates (such as Mollusks). Genetically, the vesicle develops from the ectoderm via invagination followed by detachment. Later, specialized tubular appendages of the static apparatus, known as semicircular ducts, begin to separate. Myxine have a single semicircular duct, cyclostomes have two, and fish along with all other vertebrates already possess three. In terrestrial vertebrates, driven by their complex movements, an apparatus evolved that in humans is represented by the vestibule and 3 semicircular canals located in three mutually perpendicular planes. This apparatus perceives not only the body's position in space and its linear displacement, but also movements (body rotations) in any plane.

The Organ of Hearing emerged later by segregating from the Organ of Balance. In aquatic vertebrates, it is poorly developed. With the transition of vertebrates to land, the organ of hearing underwent significant progressive development and formed largely through the remodeling of the gill apparatus. Alongside the sound-perceiving apparatus belonging to the Inner ear, a sound-conducting apparatus appeared, comprising the Middle ear (the tympanic cavity with its auditory ossicles and the auditory tube). The External ear also formed, featuring a sound-collecting device—the auricle (pinna)—which in mammals is capable of moving and turning toward the sound. Subcortical and cortical hearing centers emerged, reaching their highest development in the human Cerebral Cortex, where not only the analysis of nerve impulses arriving from the auditory Organs takes place, but also Abstract "auditory" thinking associated with the Features of the second signaling system.

The Organ of Hearing and Balance develops quite early in human Embryogenesis. The primordium of the membranous labyrinth appears at the beginning of the 3rd week of intrauterine life as an ectodermal thickening On the surface of the embryo's HEAD region, lateral to the neural plate. By the 4th week, the ectodermal plate invaginates, deepens, and pinches off from the ectoderm, forming the otic vesicle (auditory vesicle)—the precursor of the labyrinth. By the end of the 4th week, through complex differentiation, three semicircular ducts and the endolymphatic duct are formed from the otic vesicle. In the 5th week, the cochlear duct sprouts from the anterior part of the otic vesicle and coils into a spiral. By the third month of intrauterine development, the membranous labyrinth is largely formed, although its walls are covered with an undifferentiated epithelial layer. Only after nerve Cell processes grow in from the vestibular and cochlear ganglia do some of the undifferentiated epithelial Cells of the labyrinth transform into the sensory cells of the spiral organ, vestibular maculae, and ampullary crests. By the sixth month of intrauterine life, the bony labyrinth arises around the membranous labyrinth with its perilymphatic spaces through perichordal ossification from the perichondrium of the cartilaginous otic capsule of the Skull, generally mirroring the shape of the membranous labyrinth.

The middle ear—the tympanic cavity with the auditory tube—develops from the first pharyngeal pouch and the lateral wall of the Pharynx; accordingly, the epithelium of the mucous membrane lining the middle ear cavities originates from the ectoderm. The auditory ossicles located within the tympanic cavity are formed from the Cartilage of the first (malleus and incus) and second (stapes) branchial arches. The external ear develops from the first pharyngeal cleft.