Quantitative imaging of cochlear soft tissues in wild-type and hearing-impaired transgenic mice by spectral domain optical coherence tomography

Abstract

Human hearing loss often occurs as a result of damage or malformations to the functional soft tissues within the cochlea, but these changes are not appreciable with current medical imaging modalities. We sought to determine whether optical coherence tomography (OCT) could assess the soft tissue structures relevant to hearing using mouse models. We imaged excised cochleae with an altered tectorial membrane and during normal development. The soft tissue structures and expected anatomical variations were visible using OCT, and quantitative measurements confirmed the ability to detect critical changes relevant to hearing.

Fig. 1

(A) The cochlear has three chambers, scala vestibuli (SV), scala media (SM), and scala tympani (ST). The auditory neurons (AN) sit within the central core, the modiolus. The structure within the box is expanded in (B). (B) The organ of Corti contains three rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). The hair cells sit along with supporting cells on the basilar membrane (BM), which is fixed at the spiral limbus and osseous spiral lamina (OSL) medially and the spiral ligament (SL) laterally. Hair cell stereocilia are deflected when shearing forces develop between the apical surface of the hair cells and the tectorial membrane (TM) during sound transduction. Auditory nerve fibers (AN) connect the hair cells with the brainstem.

Fig. 2

(A) CT and (B) MRI images of four different deaf patients. On the left are cochleae (arrows) that appear normal, but are likely to have important anatomic changes if post-mortem histopathologic studies were performed. On the right are examples of the types of gross malformations that can be detected with the latest imaging techniques (arrows).

Fig. 3

Schematic of the spectral domain OCT system.

Fig. 4

(A) OCT image from a P15 mouse cochlea. The bone and soft tissue structures scatter light and produce a signal that is visible with OCT. In contrast, the surrounding fluid does not produce a visible signal. (B) Magnitude and depth plot of the A-line highlighted in yellow in (A).

Fig. 5

(A) Spectral domain OCT image of an adult mouse organ of Corti (unaveraged) as viewed with the apical otic capsule removed. (B) Paraffin-embedded histological section from a P15 mouse cochlea shown for comparison. The box encompasses what is not present in (A). In both cases, the basilar membrane (BM), inner hair cells (IHCs), internal spiral sulcus, outer hair cells (OHCs), modiolus, Reissner's membrane (RM), tectorial membrane (TM), and tunnel of Corti are visible.

Fig. 6

OCT (left) and paraffin-embedded histological sections (right) of cochleae from (A) Tecta ^+/+ ( Media 1), (B) Tecta ^+/C1509G ( Media 2), and (C) Tecta ^{C1509G/C1509G} ( Media 3). The videos show the image stacks which have been cropped but remain unadjusted. The OCT image of the Tecta ^+/+ gives the locations of where we made the measurements for (1) the area of the tectorial membrane (TM), (2) thickness of the hair cell epithelium, (3) the distance between the TM and hair cell epithelium, (4) the thickness of the spiral limbus and OSL, (5) the thickness of the RM, and (6) the thickness of the bone and SL at its junction with Reissner's membrane (RM). Since the bone was opened in the OCT images, the actual measurement of the bone and SL thickness was made at another slice of the image stack. Depicted is the approximation of that thickness in the current slice.

Fig. 7

Unaltered, cross-sectional spectral domain OCT images of the cochlea from (A) P3, (B) P15, and (C) adult mice. In all cases, the Reissner's membrane (RM), basilar membrane (BM), and modiolus are visible. An example of the vertical line chosen for measuring the amount of tissue imaged is shown in yellow. Examples of the 10 by 10 pixel boxes used to calculate the signal intensity in a given region are also in yellow. Graphs showing (D) the amount of tissue imaged, (E) the average signal intensity in different regions, and (F) the contrast percentage of regions 1, 2, and 3. Number of samples is noted in (D), and statistical significance is noted in each of the graphs by a paired * or ¥.

Fig. 8

Average intensity OCT images from five slices from the cochlea of the P3 (n = 7), P15 (n = 8), and adult mouse (n = 6). The pixel intensity across the internal spiral sulcus, depicted by the yellow line (100 µm), is graphed. The zero position is closer to the basilar membrane (BM). Statistical significance is noted in each of the graphs by a paired * or ¥.