Microscopic Anatomy of the Mouse Inner Ear, Third Edition

Details

• Personal Author:
  Bohne BA ; Harding GW
• Description:
  The petrous portion of the temporal bone houses the organs for hearing, equilibrium and motion detection. The cochlea, which houses the sensory organ for hearing, consists of a triangular-shaped, fluid-filled channel, the membranous labyrinth, that is housed within the bony labyrinth (otic capsule). The membranous labyrinth spirals around a central bony canal, the modiolus, that contains the auditory division of the vestibulocochlear nerve (i.e., 8th cranial) and blood vessels to the cochlea. The membranous labyrinth is anchored to the bony labyrinth at the spiral ligament, the lateral edge of the triangle, and at the lip of the osseous spiral lamina medially. By anatomical convention, structures toward the modiolus are medial while those toward the spiral ligament are lateral. Except for its attachments laterally and medially, the membranous labyrinth is separated from the bony labyrinth by fluid-filled channels (i.e., perilymph). The vestibule houses the two static organs of equilibrium (saccule and utricle) as well as the cristae in the semicircular canals. The vestibular sensory areas contain sensory (hair) cells and supporting cells. Hair cells have a bundle of elongated microvilli called stereocilia that project from the apical membrane into an extracellular gelatinous material that overlies the sensory area in each vestibular organ. Within the utricle and saccule, the sensory cells are arranged in a flat plate of cells called a macula. The maculae of the utricle and saccule are nearly perpendicular to one another. The otolithic membrane overlies each macula and is composed of numerous filaments and has a high content of acid mucopolysaccharides. Embedded in the otolithic membrane are numerous crystalline bodies, otoliths or otoconia, that are composed of a protein core and calcium carbonate. There are three semicircular canals that are oriented at nearly right angles to one another and are named the anterior (superior), lateral (horizontal) and posterior canals. Each canal has one enlarged or ampullated end that contains the crista, a crest of sensory and supporting cells that is oriented perpendicular to the axis of its canal. The stereocilia on the hair cells project into the overlying gelatinous material called the cupula. The organ of Corti (hearing organ) is unique in that it need not be sectioned in order to be examined microscopically. Portions of the cochlear duct can be dissected free from the otic capsule, placed flat on microscope slides in a liquid medium and cover-slipped (e.g., Engström et al., 1966). The cochlea can also be embedded in plastic before dissection of the cochlear duct (Bohne and Harding, 1997). The various cells in the organ of Corti can be examined by 'optically sectioning' or using the z-axis (fine focus) of a microscope to focus at successively deeper layers within the epithelium. Tissues prepared this way are called 'surface' or 'flat' preparations. Because of its small size, the mouse cochlea presents some advantages and disadvantages for cochlear preparation and microscopic evaluation. Advantages: 1) The soft tissue in the temporal bone can readily be preserved by vascular perfusion because of the short diffusion distances from the cochlear vasculature to the organ of Corti; 2) The bony labyrinth is joined to the rest of the skull by fibrous tissue only. Thus, the cochleas and vestibules can be easily removed from the skull without damage; 3) The cochlear bone is extremely thin. Removal of the bony labyrinth from the membranous labyrinth requires a small sharp pick and manual dexterity only; 4) The mouse organ of Corti averages about 6 mm in length and contains about 700 inner hair cells and 2400 outer hair cells. Thus, the collection of quantitative data in a given cochlea requires much less time than for mammals with longer cochleas. Disadvantages: 1) The hair cells and supporting cells in the organ of Corti are very small. The small size makes it difficult to evaluate intracellular detail in surface preparations; 2) The frequency range of mouse hearing is about 1-100 kHz (Ehret, 1974; Mikaelian et al., 1974). The functional testing of frequencies above 50 kHz requires expensive, specialized equipment and, therefore, is rarely performed. In most laboratories, 32, 40 or 50 kHz is the highest frequency that is routinely tested (e.g., Davis et al., 1999; Jimenez et al, 2001; Ohlemiller et al., 2000; Ou et al, 2000a; Vazquez et al., 2004). Based on the frequency- place map of the mouse cochlea by Ou et al. (2000b), 32 kHz and 50 kHz are located about 76% and 85% distance from the apex, respectively. Thus, beginning damage in the basal 15-24% of the organ of Corti cannot be detected if 32-50 kHz is the highest frequency that is functionally tested; 3) It is more difficult to perform survival surgery on the inner ear of a mouse compared to other rodents (e.g., chinchilla, guinea pig) (Bohne et al., 2001). Fixation and preparation of the specimens illustrated in this Atlas was as follows. The heads of the 18.5-day embryonic mice were fixed by immersion in 4% buffered paraformaldehyde for several days. After washing in buffer, the heads were bisected, the brain removed and the two halves of the skull immersed for one hour in a buffered solution of cold 1% osmium tetroxide. After washing out the osmium, the temporal bones were separated from the remainder of the skull, dehydrated in a graded series of ethanol and propylene oxide then infiltrated with plastic. Each infiltrated temporal bone was carefully held in a small pool of liquid plastic and divided with razor blades into 4-5 thick sections. The thick sections were re-embedded flat in thin layers of plastic for subsequent microscopic examination (Nikon dissection and Wild phase-contrast microscopes). The temporal bones of the 9-day-old and adult mice were fixed by perfusing buffered 1% osmium tetroxide through the vascular system. The temporal bones were removed and immersed in cold fixative for two hours. The specimens were washed in Hank's balanced salt solution, dehydrated in ethanol and propylene oxide then embedded in plastic. After the plastic polymerized, the cochleas were dissected as surface preparations. The dissected segments of the cochlear duct were re-embedded in thin layers of plastic for examination by phase-contrast microscopy (Wild, M-20) (Bohne and Harding, 1997). Selected organ-of- Corti segments were sectioned radially with an RMC ultramicrotome. One-micrometer-thick sections were mounted on glass slides and either stained with methylene blue-azure II (Richardson et al., 1960) or left unstained, cover-slipped and examined by phase-contrast microscopy. Thinner sections were mounted on grids and examined by transmission electron microscopy (Hitachi H7500). The thick and semi-thick sections of inner ear and organ of Corti and flat preparations of the cochlear duct were photographed with a Micropublisher digital camera, then color-balanced, cropped and labeled using Adobe Photoshop (version 7). [Description provided by NIOSH]
• Subjects:
  Anatomy Ear Laboratories Microbiology Occupational Health Safety Sense Organs
• Keywords:
  Auditory System Ears Laboratory Animals Microscopic Analysis
• Publisher:
  Washington University School of Medicine
• Document Type:
  Text
• Funding:
  Grant
• Genre:
  Report
• Place as Subject:
  Missouri ; OSHA Region 7
• CIO:
  National Institute for Occupational Safety and Health (NIOSH)
• Topic:
  Workplace Safety & Health
• Location:
  North America
• Pages in Document:
  1-53
• NIOSHTIC Number:
  nn:20055536
• Citation:
  St. Louis, MO: Washington University School of Medicine, 2011 Sep; :1-53
• Federal Fiscal Year:
  2011
• NORA Priority Area:
  Manufacturing ; Wholesale and Retail Trade
• Performing Organization:
  Washington University, St. Louis
• Peer Reviewed:
  False
• Start Date:
  20000401
• Source Full Name:
  Microscopic anatomy of the mouse inner ear, third edition
• End Date:
  20130914
• Collection(s):
  National Institute for Occupational Safety and Health
• Main Document Checksum:
  urn:sha-512:5fbf6bb433355eb4f4de1a3e31d5dbc4b05e2fea0636659fc7c4acae1624a93d8bb34b4438719c4a3867464ee6fd3404b8ddff2e80729f4d04915940ec97f4bb
• Download URL:
  https://stacks.cdc.gov/view/cdc/213366/cdc_213366_DS1.pdf
• File Type:
  [PDF - 2.27 MB ]