Ultrastructural Characteristics of the Round Window Membrane During Pneumococcal Otitis Media in Rat

Abstract

To understand better the pathogenesis of inner ear (IE) damage caused by otitis media (OM), the round window membrane(RWM) structure was investigated in a rat model for pneumococcal otitis media (POM). The RWM of 25 rats were evaluated light and electron microscopically on 1 day, 3 days, 6 days, 10 days, and 20 days after the unilateral inoculation of type 3 pneumococcus suspension into their middle ear cavities. The thickness of the RWM increased in various stages of the pneumococcus-evoked otitis media, compared with that of the normal. The thickening was most pronounced on day 1, being about 4 to 5 times greater than that of the normal RWM. All layers of the RWM were affected by the pneumococcal infection, but the major changes were confined to the subepithelial space close to the basement membrane (BM). Together with alterations to the BM, the most distinct pathological features were characterized by an increase and hypertrophy of fibroblasts in association with abundant collagen fibers. Elastic fibers observed close to the inner mesothelial layer under a high power magnification also increased during the experiment. These results will be relevant to a better understanding of the histologic implication of RWM in stages of acute otitis media involving pneumococcus-evoked otitis media.

Evaluation of the mouse model for acute otitis media

“…While it does thicken with inflammation, the integrity of the membrane in such cases probably is not sufficient to prevent movement of mediators through it. Changes in the rat round window membrane after exposure to S. pneumo have been described, including thickening of the membrane with major changes confined to the subepithelial space close to the basement membrane (Yoon and Hellstrom, 2002). The transport of infectious or inflammatory agents from the middle ear into the inner ear has been demonstrated in several studies (Lim et al, 1990;Engel et al, 1995;Goycoolea, 1995;Schachern et al, 1992).…”

Complex modulus of round window membrane over auditory frequencies in normal and otitis media chinchilla ears

“…AOM induces morphological changes of the middle ear tissues, including the tympanic membrane (TM), stapedial annular ligament and RWM, which have been observed in patients and experimental animal models (Sahni et al, 1987;Goycoolea, 1995;Schachern et al, 2008;Yoon and Hellstrom, 2002). Recently, an AOM model of guinea pig induced by Streptococcus pneumonia was created and the changes of RWM mechanical properties were reported by Gan et al (2013).…”

Dynamic properties of round window membrane in guinea pig otitis media model measured with electromagnetic stimulation

“…AOM causes morphological and material property changes of middle ear tissues including the tympanic membrane and RWM, and it results in conductive hearing loss. Animal experiments also show that inflammatory mediators and bacterial products from the middle ear can enter the inner ear via the RWM and cause inner ear damage (Ikeda and Morizono, 1988; Goycoolea and Lundman, 1997; Yoon and Hellstrom, 2002; Shacheern et al, 2008). …”

“…The RWM of animals (e.g., rodents such as the rat, chinchilla, guinea pig) generally is much thinner than that of humans. For example, RWM mean thickness is 11.5 μm in rats (Yoon and Hellstrom, 2002) and 11–14 μm in chinchillas (Goycoolea et al, 1988). …”

“…Changes in the thickness and ultrastructure of the RWM associated with otitis media have been observed both in human patients and in experimental animal models (Sahni et al, 1987; Goycoolea, 1995; Yoon and Hellstrom; 2002). Yoon and Hellstrom reported that the thickness of rat RWMs increased 3-fold by the third day post challenge with Streptococcus pneumoniae when compared to uninfected control animals.…”