ABSTRACT
Current research and an overall review of 25 years of round window membrane studies are presented. The approach, rationale and concepts that have evolved from these studies are described. Ultrastructural studies of the round window membrane of humans, monkeys, felines and rodents have disclosed three basic layers: an outer epithelium, a middle core of connective tissue and an inner epithelium. Interspecies variations are mainly in terms of thickness, being thinnest in rodents and thickest in humans. Morphologic evidence suggests that the layers of the round window participate in resorption and secretion of substances to and from the inner ear, and that the membrane could play a role in the defense system of the ear. Different substances, including antibiotics and tracers, when placed in the middle ear side traverse the membrane. Tracers placed in perilymph become incorporated into the membrane by the inner epithelial cells. Permeability is selective and factors affecting permeability include size, concentration, electrical charge, thickness of the membrane and tacilitating agents. Passage of substances through the membrane is by different pathways, the nature of which is seemingly decided at the outer epithelium of the membrane. Round window membrane studies have provided increased knowledge of the anatomy and function of this structure, as well as new insights into pathology and pathogenesis. The concepts that have evolved from these studies are potentially useful for understanding middle and inner ear interactions, and for eventual drug delivery (based on permeability) to the inner ear.
Subject(s)
Basilar Membrane/pathology , Basilar Membrane/ultrastructure , Round Window, Ear/pathology , Round Window, Ear/ultrastructure , Animals , Cell Membrane Permeability/physiology , Chinchilla , Epithelium/physiology , Humans , Macaca mulatta , Microscopy, Electron , Temporal Bone/pathologyABSTRACT
Tadarida brasiliensis mexicana employs a broad-band sonar system at frequencies between 80 and 20 kHz and is characterized by non-specialized hearing capabilities. The cochlear frequency map was determined with extracellular horseradish peroxidase tracing in relation to quantitative morphological data obtained with light, scanning and transmission electron microscopy. These data reveal distinct species characteristic specializations clearly separate from the patterns observed in other bats with either broad-band or narrow-band sonar systems. The basilar membrane (BM) is coiled to 2.5 turns and about 12 mm long. Its thickness and width only change within the extreme basal and apical ends. The frequency range from about 30 to 80 kHz is represented in the lower basal turn with a typically mammalian mapping coefficient of about 3 mm/octave. This region exhibits morphological features correlated with non-specialized processing of high frequencies. (1) The BM is radially segmented by thickenings of pars tecta and pars pectinata. (2) The 3 rows of outer hair cells (OHCs) have similar morphology. Between 35 and 86% distance from base, frequencies between 30 and 12 kHz are represented with a slightly expanded mapping coefficient of about 6 mm/octave. In analogy to previous work, this cochlea region is termed acoustic fovea. It includes the frequency range of maximum sensitivity and sharpest tuning (21-27 kHz) but also frequencies below the sonar signals. The fovea is characterized by several morphological specializations. (1) The BM features a continuous radial thickening mainly composed of hyaline substance. (2) There is an increased number of layers of tension fibroblasts in the spiral ligament. (3) There are morphological differences in the arrangements of stereocilia bundles among the 3 rows of OHCs. The transitions between non-specialized and specialized cochlear regions occur gradually within a distance of about 600 microns. The gradients in stereocilia length of both receptor cell types and the gradations in length of the OHC bodies match specialized aspects of the frequency map.