Rat eustachian tube and its musculature

The rat eustachian tube: Anatomical, histological, and radiological features

Purpose

This study investigated the anatomical and histological characteristics of the rat Eustachian tube (E-tube) and the feasibility of Eustachian tubography in a rat model.

Materials and methods

Fifteen male Wistar rats were used in this study, and the bilateral E-tubes of each rat were examined. Ten E-tubes were used for anatomical studies, another ten for histological analysis, and the other ten for Eustachian tubography. Five rats were euthanized and decapitated, and ten E-tubes were dissected to describe the anatomy of the E-tube. Ten E-tube specimens obtained from five other rats were sectioned to investigate E-tube histology. Eustachian tubography was performed on the bilateral E-tubes of the other five rats using the trans-tympanic approach.

Results

The rat E-tubes consisted of bony and membranous parts. Cartilage and bone tissue covered only the bony part. The E-tubes' mean diameter and overall length were 2.97 ​mm and 4.96 ​mm, respectively. The tympanic orifices' mean diameter was 1.21 ​mm. The epithelium of E-tubes was mainly composed of pseudostratified ciliated and goblet cells. Eustachian tubography was successfully performed on both sides of the E-tube for each rat. The technical success rate was 100%, the average running time was 4.9 ​min, and no procedure-related complications occurred. On tubography images, the E-tube, tympanic cavity, and nasopharynx could be identified because of the visualization of bony landmarks.

Conclusion

In this study, we described the anatomical and histological features of rat E-tubes. With the aid of these findings, E-tube angiography was successfully performed using a transtympanic approach. These results will facilitate further investigation of E-tube dysfunction.

Contralateral spreading of substances following intratympanic nanoparticle-conjugated gentamicin injection in a rat model

This study was performed to investigate the Eustachian tube as a potential route for contralateral spreading following intratympanic nanoparticle (NP)-conjugated gentamicin injection in a rat model. Sprague–Dawley rats were divided into three groups and substances were injected in the right ear: group 1 (fluorescent magnetic nanoparticles [F-MNPs], n = 4), group 2 (F-MNP-conjugated gentamicin [F-MNP@GM], n = 2), and control group (no injections, n = 2). T2-weighted sequences corresponding to the regions of interest at 1, 2, and 3 h after intratympanic injection were evaluated, along with immunostaining fluorescence of both side cochlea. The heterogeneous signal intensity of F-MNPs and F-MNP@GM on T2-weighted images, observed in the ipsilateral tympanum, was also detected in the contralateral tympanum in 4 out of 6 rats, recapitulating fluorescent nanoparticles in the contralateral cochlear hair cells. Computational simulations demonstrate the contralateral spreading of particles by gravity force following intratympanic injection in a rat model. The diffusion rate of the contralateral spreading relies on the sizes and surface charges of particles. Collectively, the Eustachian tube could be a route for contralateral spreading following intratympanic injection. Caution should be taken when using the contralateral ear as a control study investigating inner-ear drug delivery through the transtympanic approach.

Development of a rat model of patulous eustachian tube by mandibular nerve resection

OBJECTIVE

Patulous eustachian tube (PET) is currently treated using a variety of conservative or surgical approaches. To further elucidate the pathology of PET and to establish new therapies, the development of an animal model is necessary. The objective of this study was to develop a highly reproducible and sustainable rat model of PET by mandibular nerve resection.

METHODS

Sixteen rats underwent mandibular nerve resection. Following an external incision, the main trunk of the mandibular nerve at the foramen ovale was identified in the pterygoid fossa, and its branches were resected. This surgery was performed on the right side, with the unoperated left side used as the control. To determine eustachian tube (ET) function, passive opening pressure (POP) was measured using inflation method up to Week 16 post-surgery. Changes in POP were statistically compared to the preoperative level on the operated and control sides. In addition, specimens of the ET and its surrounding tissue at Week 16 post-surgery were prepared for morphological evaluation in eight rats.

RESULTS

On the control side, POP did not significantly decrease across the 16 weeks post-surgery compared to the preoperative level. In contrast, on the operated side, POP was significantly decreased at Week 2 and continued to be lower than the preoperative level until at least Week 16 post-surgery. POP dropped 10% or more on the operated side examined in thirteen rats. Histologically, the medial pterygoid muscle was atrophied on the operated side mainly.

CONCLUSION

Mandibular nerve resection in rats may be an effective method for generating an animal model of PET. It was suggested that this rat model may be useful for verifying new treatments for PET.

A rat model for muscle regeneration in the soft palate

Background

Children with a cleft in the soft palate have difficulties with speech, swallowing, and sucking. Despite successful surgical repositioning of the muscles, optimal function is often not achieved. Scar formation and defective regeneration may hamper the functional recovery of the muscles after cleft palate repair. Therefore, the aim of this study is to investigate the anatomy and histology of the soft palate in rats, and to establish an in vivo model for muscle regeneration after surgical injury.

Methods

Fourteen adult male Sprague Dawley rats were divided into four groups. Groups 1 (n = 4) and 2 (n = 2) were used to investigate the anatomy and histology of the soft palate, respectively. Group 3 (n = 6) was used for surgical wounding of the soft palate, and group 4 (n = 2) was used as unwounded control group. The wounds (1 mm) were evaluated by (immuno)histochemistry (AZAN staining, Pax7, MyoD, MyoG, MyHC, and ASMA) after 7 days.

Results

The present study shows that the anatomy and histology of the soft palate muscles of the rat is largely comparable with that in humans. All wounds showed clinical evidence of healing after 7 days. AZAN staining demonstrated extensive collagen deposition in the wound area, and initial regeneration of muscle fibers and salivary glands. Proliferating and differentiating satellite cells were identified in the wound area by antibody staining.

Conclusions

This model is the first, suitable for studying muscle regeneration in the rat soft palate, and allows the development of novel adjuvant strategies to promote muscle regeneration after cleft palate surgery.

Antigenic as well as nonantigenic stimuli induce similar middle ear responses in the rat

OBJECTIVES/HYPOTHESIS

The observation that during otitis media many different types of micro-organisms have been cultured from effusions indicate that, once present in the middle ear cavity, most types of micro-organisms are able to trigger an inflammatory reaction leading to otitis media. The present study was designed to determine the middle ear response after injection of different substances into the middle ear cavity.

STUDY DESIGN

To determine whether and to what extent an inflammatory response of the middle ear depends on the entering agent, the response in the tympanic cavity was studied by otomicroscopy and histological examination after inoculation of various substances.

METHODS

Lewis rats were inoculated in transtympanic fashion either with live or heat-killed bacteria (pathogenic and nonpathogenic), Keyhole limpet hemocyanin, active charcoal, or saline. The mucosal response of the challenged middle ears was studied histologically.

RESULTS

Irrespective of the inoculated substance, no essential differences in the mucosal response were found. The intensity of the inflammatory response was greater when live bacteria were inoculated.

CONCLUSIONS

The present study demonstrates that any substance reaching the middle ear cavity is likely to induce otitis media. These observations emphasize the role of the eustachian tube as "porte d'entrée" in the pathogenesis of this disorder. Determination of specific aspects of the eustachian tube involved in protection or in facilitating bacterial translocation will be important for the understanding of the pathogenesis of otitis media and the subsequent development of new therapeutic strategies. In addition, elucidation of bacterial factors involved in the process of colonization and translocation will be of equal importance.

Effect of exogenous surfactant on ventilatory and clearance function of the rat's eustachian tube

HYPOTHESIS AND BACKGROUND

The Eustachian tube has three important functions with respect to the middle ear: ventilation, clearance, and protection. Surfactants are assumed to be important to maintain these functions. The administration of exogenous surfactant may therefore be effective to improve the function of the Eustachian tube. This randomized, double-blind, placebo-controlled study was designed to investigate the effect of exogenous surfactant on the function of the Eustachian tube in rats.

MATERIALS AND METHODS

Exogenous surfactant was administered into the middle ear of 10 otologically healthy rats, and 10 other rats received placebo. The effect on the opening and closing pressure (passive ventilatory function) and the dye clearance time (clearance function) of the rat's Eustachian tube was measured.

RESULTS

A significant decrease in the opening pressure was seen after the administration of surfactant. Both surfactant and placebo caused an increase in the closing pressure. A serious disturbance of the dye clearance time was induced in 13 rats, and the test failed in 1 rat. In the remaining 6 rats, no significant differences in the dye clearance time were observed between the two groups.

CONCLUSIONS

Exogenous surfactant decreased the closing forces of the Eustachian tube even in otologically healthy rats. No significant effect on the mucociliary clearance was observed, but this may have resulted from the small number of rats. Additional randomized, double-blind, placebo-controlled trials should be conducted to determine the clinical relevance of these changes and to further assess the effect of surfactant on the function of the Eustachian tube.

Development of the tubotympanum in the rat

OBJECTIVE

To investigate the relationship between the anatomical maturation of the middle ear and that of the eustachian tube and paratubal muscles in the rat.

DESIGN

Wistar rats ranging from gestational day 12 to postnatal day 40 were used.

METHODS

Tissue specimens were examined with routine light microscopy and electron microscopy. Epithelial differentiation was studied immunohistochemically with antibodies to different cytokeratins.

RESULTS

The epithelial lining of the tubotympanum showed differentiation-related cytokeratin expression throughout the whole developmental period. The mucociliary epithelium reached mature features around birth. A dorsal extension and its framing cartilage started forming around 5 days after birth. This extension became lined by stratified nonciliated epithelium and attained maturity around 10 days after birth concurrently with the attachment of the dilatory muscles. This process was immediately followed by aeration of the middle ear cavity.

CONCLUSIONS

The continuous expression of cytokeratins demonstrates that the epithelial lining of the tubotympanum is only derived from the embryonal endoderm. Furthermore, this study demonstrates that the eustachian tube shows a two-stage postnatal development. First, the mucociliary system matures, providing protection/clearance when the animal starts respiration and swallowing. Subsequently, the dorsal part attains maturity. The features of the epithelial lining of the dorsal part of the eustachian tube and the coincidence of the maturation of this part with the attachment of the dilating muscle fibers and the aeration of the middle ear indicates that this part provides ventilation. These findings support the authors' hypothesis that different parts of the eustachian tube serve different purposes: clearance, protection and ventilation.