171. The Vestibulocochlear System: The Internal Ear – Membranous Labyrinth

  1. The auditory placode appears toward week 3 in the human embryo, in the area or the rhombencephalon. Primary induction comes from the anterior chordomesoderm and secondarily from the rhombencephalon
    1. THE PLACODE gradually invaginates as a result of the fact that cellular proliferation is more intense internally than it is externally. As a result, the auditory or otic vesicle is formed, isolated from the superficial ectoderm by its active invagination and by the thrust of the lateral mesenchyme. The vesicle forms the membranous labyrinth
    2. THE SIMPLE OTIC VESICLE gives rise to the basic internal ear structure in about 50 days. The liquid it contains is found in all its derivatives and is called endolymph, which is supplied by specific vessels adjacent to the epithelium
    3. THE STATOACOUSTIC GANGLION CELLS arise from the inferior internal surface of the vesicle. Their dendrites are in contact with the sensory structures of the internal ear, and their axons conduct impulses toward the central nervous system
  2. The primary auditory vesicle constricts to form the utricle, semicircular canals, and endolymphatic duct, dorsally, and the saccule and cochlear duct, ventrally
    1. THE ENDOLYMPHATIC DUCT OR SAC is an evagination, which is seen about day 30 on the internal surface of the vesicle and gradually elongates dorsally. During the second half of pregnancy, it elongates to reach the dura mater across the surrounding mesenchyme. It is important in the resorption of endolymphatic fluid
      1. The semicircular canals are derivatives of the utricl The external half of the utricle gives rise to 2 flat evaginations. One is vertical and sagittal with respect to the embryo, and the other is horizontal (lateral), with its apex toward the exterior
        1. The vertical primordium gives rise to the anterior (superior) and posterior semicircular canals. The anterior (superior) canal differentiates first at about day 3 Its central portion is rapidly resorbed to form the canal. The posterior primordium undergoes a similar type of development after several hours. The common median portion of the 2 primordia persists to form the common trunk of the 2 canals
        2. The horizontal (lateral) primordium is perpendicular to the first 2 and undergoes a similar development after 3 or 4 days. Morphogenesis of all 3 canals is complete by about day 50
      2. One of the ends of each canal opens into the utricle via a swelling, the ampulla, wherein the sensory organs of balance develop
      3. The final transformation of this canal system consists of a 90? displacement of the anterior canal to the outside
        1. Thus, the 3 canals are placed in the 3 planes of space, and from here on they show only an increase in size until about day 70
      1. The saccule shows a ventral evagination, the cochlear duct, at about day 3 The duct elongates progressively. It is straight at first, but soon spirals as a result of unequal growth of its internal and external surfaces
      2. By day 70 of gestation, the cochlear duct has 2 1/2 spiral turns and its morphogenesis is finished
      3. The saccule, like the utricle, contains sensory organs for balance, whereas the cochlea develops cells especially adapted for hearing
    4. THE SEMICIRCULAR CANALS, the utricle and saccule, the endolymphatic duct, and the cochlear duct comprise the totally enclosed system called the membranous labyrinth. The latter is surrounded by mesenchyme which will give rise to the osseous labyrinth
      1. The membranous labyrinth is filled with endolymph; the bony labyrinth is filled with perilymph

the vestibulocochlear system: the internal ear – membranous labyrinth: image #1