Access Type

Open Access Dissertation

Date of Award

1-1-1997

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Communication Sciences and Disorders

First Advisor

Dr. James Kaltenbach

Abstract

An understanding of auditory development requires description of anatomical events underlying the onset and maturation of function. Relatively little is known about the anatomical events responsible for subsequent maturation of auditory sensitivity. The present project will attempt to fill this gap by focusing on ultrastructural changes in the type I spiral ganglion cells (SGC) of the cochlea during the period of functional maturation using the Syrian golden hamster (LVG strain). Cochleas were taken from 6 day old to 30 day old animals in two day intervals, inclusively. Mature 36 day old hamsters were studied for comparison. The cochlear tissue was viewed at the light microscopic level with images recorded with an IBM PC equipped with a Matrax Frame Grabber to facilitate precise morphometric measurements. Photo-electron micrographs were scanned using an UMAX 840 scanner and an IBM PC equipped with a Dage/MT-1 video camera and the Adobe Photoshop 6.0 computer package. These images were then analyzed with the Image-l Metamorph Imaging System. All data collected were further analyzed using the BMDP Statistical Software and Statistical Package for the Social Sciences (SPSS), Version 5.0 for Windows. Systematic quantitative evaluations were conducted including cell size, numbers, thickness and number of myelin sheaths surrounding the cell somas and their axons at the light and electron microscopy levels. The results were compared with time course plots of functional development reported in previous studies. The results indicate that there were no significant increases in the cell soma and nuclear sizes as the hamsters mature. The mean type I SGC width, height, area, perimeter, and nucleus:cell ratio of the mature hamster was 11.7 µm, 11.0 µm, 91.2 µm2 , 36.6 µm, and 0.32 respectively. In the youngest age group studied (6 DAB), at the light microscopic level, there appear to be decreases in cell number per viewing field and cell density which was suggestive of apoptosis. At the electron microscopic level, there were apparently "apoptotic bodies" observed only in the day 6 old animals. The process of myelination begins at 8 DAB around axons and at 12 DAB around the cell somas of hamster spiral ganglion cells and take ~ 4 days to be completed. Mean myelin thicknesses around the mature cell soma and axon were 77.1 nm and 283.5 nm, respectively. Myelin sheath thickness was a function of number of lamellae which increased to a mean of 7.2 and 27 lamellae in the mature cell soma and axon, respectively. The mean thickness of one compact lamella remains constant at 10.5 nm. Completion of myelination around the cell soma coincides with the initiation of BAEP electrophysiological peaks reported in hamsters. There were no further significant structural changes in myelin after 16 DAB when the maturation of the myelination process was completed around the cell somas.

Download

Share

COinS