This drawing illustrates the normal organ of Corti (i.e., hearing organ) in cross-section. The organ is attached to the vibratory basilar membrane (BM) and is surrounded by fluid spaces that contain endolymph (purple) and perilymph (orange). The tectorial membrane (TM) covers the surface (i.e., reticular lamina) of the organ of Corti and is closely applied to supporting cells laterally (yellow arrow). The organ of Corti contains two types of sensory cells (pink) [a single row of inner hair cells (IHC) & three rows of outer hair cells (OHC 1, 2, 3)], supporting cells (e.g., IP; OP - blue-purple), nerve fibers (yellow-orange) and fluid spaces (e.g., tunnel - transparent orange). Elongated microvilli (i.e., stereocilia - green) project into the TM from the apical surface of each OHC. The IP, OP and three rows of Deiters' cells (one below each OHC) contain parallel bundles of microtubules that maintain the shape of the organ of Corti and keep the stereocilia in contact with the TM. Ordinarily, the reticular lamina provides a barrier to prevent the intermixing of endolymph and the fluid (i.e., Cortilymph) within the organ of Corti.
Exposure to excess noise, drugs such as aminoglycoside antibiotics or platinum compounds, or simply aging may result in the loss of sensory cells in the organ of Corti. If the loss of cells is concentrated in a particular region of the cochlea, the individual will sustain a permanent hearing loss. The hearing loss may be severe enough to have an adverse effect on the person's ability to communicate orally, maintain employment and enjoy some of life's greatest pleasures such as music and the sounds of nature. One research goal in our laboratory is to determine how temporary hearing loss or temporary threshold shift (TTS) and permanent threshold shift (PTS) are related and how repeated exposure to a TTS-producing noise eventually destroys sensory cells.
This drawing illustrates our theory of the mechanism for continuing cell loss in the organ of Corti after termination of a severe noise exposure. The initial effect of a severe exposure is to injure and destroy outer hair cells. For a short period of time after the outer hair cells have degenerated, OHC-sized holes are left in the reticular lamina. These holes allow endolymph to gain access to the Cortilymph spaces. (Cortilymph is similar in ionic composition to perilymph in scala tympani.) The contamination of Cortilymph by endolymph (i.e., high K+) (purple arrows) causes secondary damage to the organ of Corti and leads to degeneration of inner hair cells (IHC), additional OHCs, supporting cells and nerve fibers. This theory is being tested by injecting tracer particles into the endolymph space in control and noise-damaged cochleas, then determining if, how and when the tracer enters the organ of Corti after noise exposure.
Scope of work: The research focus of this laboratory is on the phenomena of degeneration and repair in the mammalian peripheral auditory system. Quantitative and qualitative anatomical studies of the normal and damaged inner ear are conducted. Exposure to noise and "natural" aging are the agents which are used to create inner-ear lesions in order to study these phenomena. Auditory function is determined before and several times after the experimental treatment. In all animals, auditory brainstem responses (ABRs) in response to tone pips (0.5-20 kHz) and clicks and distortion product otoacoustic emissions (DPOAEs) in response to pairs of tones are recorded. After completion of the functional studies, the animals' ears are prepared for microscopic examination, including transmission electron microscopy (TEM). Quantitative data on the magnitude and pattern of hair cell, supporting cell and nerve fiber loss are collected from plastic-embedded flat preparations of the cochlear duct. These data are correlated with the post-exposure changes in the measures of auditory function. Selected areas in the damaged organ of Corti are subsequently semi-thin and thin-sectioned for bright-field and transmission electron microscopy, respectively
Experimental subjects: The chinchilla is used for studies of degeneration and repair because: 1) The animal is small and easy to maintain in a laboratory setting, yet is large enough to tolerate survival surgery on its ears; 2) The animal is generally quite healthy and free of spontaneous middle ear infection; 3) It has a maximum life-span of 15-20 years so that aging need not be a complicating factor in long-term studies; 4) It can be successfully trained to perform a variety of listening tasks so that various measures of auditory function can be obtained; 5) Its hearing range and auditory sensitivity are similar to those of humans.
In recent years, it has been found that different inbred strains of the mouse (e.g., CBA, C57BL) have different susceptibilities to aging-related (i.e., presbycusis) and noise-induced hearing loss. For these reasons, the mouse in being used to examine the role of genetic variation and environmental factors in the development and rate of progression of presbycusis and in susceptibility to noise-induced hearing loss.
Methods: The general protocol for studies in this laboratory is as follows: 1) Determine ABR thresholds and DPOAE levels pre-exposure; 2) Expose intact animal to experimental treatment; 3) Determine ABR thresholds and DPOAE levels immediately post-exposure and at specified intervals thereafter until the animals are terminated for histopathological analysis; 4) After a pre-determined recovery period (i.e., hours to months), preserve the soft tissues of the inner ear by circulating fixative through the vascular system and/or the perilymphatic spaces of the cochlea; 5) After in-vivo fixation of both cochleas has been completed, remove the temporal bones, then dehydrate and passively infiltrate the specimens with plastic; 6) After polymerization of the plastic, dissect the specimens and prepare the cochlear ducts and/or vestibular sensory organs for examination as flat (surface) preparations; 7) Evaluate cochlea from apex to base by phase contrast microscopy (magnification of 625-1250X) and collect quantitative data, including the number of degenerated sensory and supporting cells, etc.; 8) If necessary, divide the 5 sensory regions of the vestibular system into several 50-100 micrometer-thick slices, re-embed the thick sections flat and examine by phase-contrast microscopy; 9) In the flat preparations, identify regions of the specimens which require more detailed morphological evaluation; 10) Using an ultramicrotome, section these regions at a radial, tangential or horizontal angle and evaluate sections by bright-field or TEM.
Research skills: Skills required for the anatomical/pathological study of the inner ear include: experimental design, small animal anesthesia and surgery, recording of ABRs and DPOAEs, histological processing, microdissection, light and transmission electron microscopy, photomicrography, image processing, computer analyses.
I. Relation between temporary (TTS) and
permanent (PTS) threshold shifts:
A novel 'survival-fixation' technique developed in this laboratory (Bohne et al., 1999) allows the two cochleas of a particular animal to be preserved for histopathological examination at two different time points post-exposure, separated by up to 14 days. Chinchillas can be exposed to noise, functionally tested immediately post-exposure and one cochlea fixed when a sizeable TTS is present. Function can then be followed in the other cochlea until it recovers or stabilizes at a reduced level [i.e., permanent threshold shift (PTS)]. This paradigm is allowing us to determine if the histopathological correlates of PTS are present immediately post-exposure when the ear has a sizeable TTS, or if they develop belatedly as the ear is recovering from its TTS.
II. Degeneration in the mammalian peripheral auditory system after
Does severe noise exposure temporarily disrupt the endolymphatic surface of the organ of Corti and lead to the intermixing of endolymph and Cortilymph?
Do the OHCs in the region of a TTS sustain microlesions in their apical membranes?
IV. Presbycusis or age-related hearing loss is very common in Americans. Three-quarters of the population over the age of 75 has a hearing loss significant enough to affect their daily lives. Several different animal models are being used to determine the etiology of presbycusis, including different inbred strains of mice that develop presbycusis at different ages and have variable rates of progression.
Research Projects Completed or Ongoing and Funding Sources
NIH - NIDCD "Inner Ear Fluid Interactions" A.N. Salt, PI, 1/92-12/99.
NIH - NIDCD "Genes involved in the development of vestibular otoconia" D.M. Ornitz (Pharmacology), PI, 1/95-12/98.
NOHR "Development of a frequency-place map for the mouse organ of Corti" G.W. Harding, PI, 1/97-1/98.
NIH - NIA "Inducing nerve-fiber regeneration for cochlear implants" G.W. Harding, PI, 7/97-6/98.
DRF " Defining the relationship between temporary and permanent threshold shift using a within-animal paradigm" B.A. Bohne, PI, 1/98-12/99.
NOHR "Changes in the boundary of the endolymphatic space in the severely noise damage inner ear" B.A. Bohne, PI, 1/99-1/01.
CDC - NIOSH "Adverse effects of noise on hearing: Basic mechanisms" B.A. Bohne, PI, 5/01-4/06.
Link to articles on Arnold L. Towe's Legacy
Last updated 2/6/02.
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