172. The Vestibulocochlear System: The Internal Ear – Bony Labyrinth

  1. Development of the peripheral mesenchyme surrounding the membranous labyrinth
    1. THE OSSEOUS OR BONY LABYRINTH: the membranous labyrinth is embedded in mesenchyme, and at some distance from the membranous labyrinth, the peripheral mesenchyme becomes organized into cartilage at about week 5, and begins to form bone or the so-called otic capsule, after week 8. Gradually, the entire membranous labyrinth is encased in. a bony shell.
    2. PERILYMPHATIC SPACES are derived from the mesenchyme between the membranous labyrinth and the osseous labyrinth
      1. As the osseous labyrinth develops, the mesenchyme is transformed into a large-meshed reticulum which contains the perilymphatic fluid
        1. The perilymphatic spaces corresponding to the cochlea are divided into a vestibular space or vestibule within which lie the saccule and utricle, the scala vestibuli (continuous with the vestibule), and the scala tympani
        2. A perilymphatic space also exists around the semicircular canals
        3. The scala vestibuli and scala tympani, both related to the cochlear duct, are independent of each other as a result of incomplete mesenchymal resorption
          1. Thus, the mesenchyme gives rise outside to the spiral ligament which gives rise to the basilar membrane, and inside to a bony plate called the spiral lamina or plate
          2. The 2 spaces (scala vestibuli and tympani) are united at the end of the bony cochlea by a small orifice, the helicotrema, which forms in the spiral plate at the end of month 3
        4. Thus, the modiolus and osseous spiral lamina (plate) of the cochlea are not preformed in cartilage, but are ossified directly from connective tissue
      2. The perilymphatic spaces are connected with the meningeal spaces (the subarachnoid space) by a fine duct, the cochlear or perilymphatic duct, which runs through the otic capsule opposite the saccul Resorption of perilymph takes place through this pathway
  2. Formation of the neural sensory fibers
    1. THE GANGLIONIC CELLS derived from the auditory placode form 2 clusters
      1. One cluster, the vestibular ganglion or ganglion of Scarpa, is joined to the vestibular portion of the labyrinth
      2. The other, the cochlear ganglion or ganglion of Corti, is joined to the cochlear duct
      3. The dendrites of these cells reach the sensory epithelium of the internal ear, and the axons pass toward the metencephalon of the brainstem, bunching together to form the statoacoustic (VIII) cranial nerve which leaves the bony labyrinth through the internal acoustic meatus by way of bony canals that form a channel in the bone called the modiolus
      4. After metencephalic connections (nuclei of nerve VIII), the vestibular fibers reach the cerebellum to control subconscious balance, and the cochlear fibers reach the internal (medial) geniculate body and then the temporal cortex for conscious sound sensations

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