The effect of cell size on auditory morphology and tuning in gray treefrogs

As it travels through the ears sound is modified before it is converted into neural signals. The type and extent of modification depends on the structure of the tissues. If the sizes of the cells vary, the tuning of the auditory structures formed by them might be affected. Two gray treefrogs: Hyla versicolor and Hyla chrysoscelis, look identical, but the former is a tetraploid derived from the later (diploid), and polyploidy always increases cell size. We hypothesized that the larger cells of auditory elements in the tetraploid H. versicolor make it more sensitive to lower frequencies than diploid H. chrysoscelis. In order to obtain quantitative results, we have been dissecting the ears, which were then decalcified and embedded in paraffin, sectioned in the microtome and stained with hematoxylin-eosin. We then took pictures of the complete series of stained slides under the dissection microscope. These pictures were then stacked and aligned, and run through a 3D modeling program called 3dmod. Through this program, we were able to obtain measurements of the dimensions of bones, cartilages, and epithelial cells in H. chrysoscelis and H. versicolor. These results then were compared to hearing performance data obtained by other students in our lab. Our analyses show that while the tetraploid does have larger cells, the cartilages and bones in the middle ear have the same size and their middle ears have the same tuning. Tissue reorganization thus allows the tetraploid middle ear to operate within the same range of frequencies as the diploid ear