EXTERNAL EAR

The microscopic description of the auricle, or pinna, and the external acoustic meatus is included in Chapter 16.

MIDDLE EAR

The thin tympanic membrane delimits the external acoustic meatus from the tympanic cavity (Fig. 18-1). It is covered externally by stratified squamous epithelium and internally by simple squamous epithelium continuous with that of the tympanic cavity. Between these two epithelial sheets is a connective-tissue layer composed of a central region of circularly arranged collagen fibers and a peripheral region of radially arranged collagen fibers. Where the manubrium of the malleus attaches to the tympanic membrane, the connective tissue is somewhat thicker and contains blood vessels and nerves that course along the manubrium and spread radially. Collagen fibers are sparse or even absent in the dorsal portion of the membrane, referred to as the flaccid part.

Tympanic Cavity

The air-filled tympanic cavity contains small bones, the auditory ossicles, and their muscles and ligaments (Fig. 18-1). The cavity is lined with simple squamous or simple cuboidal epithelium, which covers the ossicles. The epithelium rests on a thin layer of connective tissue. A few epithelial cells are ciliated, particularly those on the floor of the cavity.

Auditory Ossicles and Muscles of the Middle Ear

The three auditory ossicles (malleus, incus, and stapes) traverse the middle ear, connecting the tympanic membrane to the membrane of the vestibular (oval) window of the internal ear (Fig. 18-1).

FIGURE 18-1 Schematic drawing of the right ear (dog), rostral aspect, showing the external ear with the auricle and external acoustic meatus; middle ear with the tympanic membrane; auditory ossicles comprising the malleus, incus, and stapes; tympanic cavity; auditory tube with connection to the nasopharynx; cochlear window; vestibular window, where the footplate of the stapes is located; internal ear with the bony labyrinth (bone cut away for easier viewing of the membranous labyrinth) comprising the anterior, lateral, and posterior semicircular canals, each with an osseous ampulla; vestibule; cochlea with the scala vestibule and the scala tympani; membranous labyrinth (dotted lines) comprising the anterior, lateral, and posterior semicircular ducts (each with a membranous ampulla), utricle, saccule, and cochlear duct. Moreover, the vestibulocochlear nerve, comprising the vestibular nerve and the cochlear nerve, is depicted entering the cranial cavity through the temporal bone.

These compact bones transmit vibrations across the middle ear cavity. The manubrium of the malleus is firmly attached to the tympanic membrane, and the small hooklike process on the neck of the malleus serves as an attachment for the tendon of the tensor tympani muscle. The head of the malleus articulates with the incus, which, in turn, articulates with the stapes. Ligaments hold these synovial articulations in place.

Muscles of the middle ear (tensor tympani and stapedius) are composed of skeletal muscle and function to dampen ossicle movement, protecting the inner ear structures from excessive vibration. The stapedius muscle attaches to the rostral crus of the stapes, and the footplate (base) of the stapes is attached to the vestibular window by an annular ligament.

Auditory Tube

The auditory tube connects the tympanic cavity to the nasopharynx (Fig. 18-1). The tube is lined by ciliated pseudostratified columnar epithelium (with goblet cells) resting on loose connective tissue. The lamina propria of the tube is thin and lacks glands in the osseous region of the tube; it becomes thicker in the cartilaginous region, containing seromucous glands and lymphatic nodules. Aggregated lymphatic nodules (the tubal tonsil) are present near the pharynx. The auditory tube is surrounded either by bone near the tympanic membrane and or by an incomplete cartilaginous tube toward the pharynx (Fig. 18-1). Hyaline cartilage is present in the proximal portion of the tube near the bone, but the tissue transitions to elastic cartilage toward the pharynx.

In the horse, the auditory tube expands ventrally to form the guttural pouch. The pouch has the same histologic features as the pharyngeal portion of the auditory tube but lacks cartilaginous support.

The function of the auditory tube is to ensure equal air pressure on both sides of the tympanic membrane. Usually, the auditory tube is closed, but it opens during yawning and swallowing.

INTERNAL EAR

The cavities of the labyrinth include the vestibule, three semicircular canals, and the cochlea (Fig. 18-1). The vestibule is a small oval space connecting the cochlea with the semicircular canals located near the medial wall of the tympanic cavity. Three semicircular canals (anterior, posterior, and lateral) lie at right angles to each other and all communicate with the vestibule. The cochlea is a bony tube wound in the shape of a spiral. The spiral canal of the cochlea makes several turns around an axis of spongy bone, the modiolus. The modiolus is a cone-shaped hollow osseous structure in which the cochlear nerve and its spiral ganglion are located. The number of coils varies from species to species (e.g., dog, 3¹⁄₄; cat, 3; horse, 2¹⁄₄; pig, 4; guinea pig, 4¹⁄₂; cow, 3¹⁄₂; man, 2³⁄₄). The base of the modiolus forms the rostral part of the internal acoustic meatus, where the cochlear nerve and blood vessels enter the cochlea. The bony canal is partially divided by a hollow bony projection, the osseous spiral lamina, which contains the branches of the cochlear nerve coursing to the spiral organ. The width of this lamina is largest at the cochlear window and diminishes toward the apex of the cochlea.

The canals and cavities of the bony labyrinth are lined by periosteum. A clear fluid, perilymph, fills the perilymphatic space between the periosteum and the membranous labyrinth (Figs. 18-2 and 18-3). Perilymph is similar in ionic composition to cerebrospinal fluid and plasma, with sodium as the main cation. Perilymph flows from the subarachnoid space through the cochlear canaliculus into the cavities of the bony labyrinth. The vestibular aqueduct, a space surrounding the endolymphatic duct, also contains perilymph.

FIGURE 18-2 Schematic drawing of the right internal ear with its connections to the cranial cavity, intradural space, and subarachnoid space (rostral aspect). The cochlea with its contents is drawn uncoiled to show the helicotrema, where there is an open connection between the scala vestibuli and scala tympani. Note especially that the cochlear canaliculus connects with the subarachnoid space, which contains cerebrospinal fluid (light gray), and the scala tympani. Perilymph (dark gray) fills the space between the membranous labyrinth and the bony labyrinth. The other connection to the cranial cavity is the vestibular aqueduct, which houses portions of the endolymphatic duct and endolymphatic sac. Structures of the membranous labyrinth containing endolymph are shown in white. Dura mater, the utriculosaccular duct, ductus reuniens, saccule, utricle, and stapes in the vestibular window are also indicated.

FIGURE 18-3 Semicircular canal (cat). The semicircular duct, an epithelium-lined membranous tube (A), lies within the semicircular canal of the osseous labyrinth (B). The perilymphatic space (C) contains delicate connective tissue trabeculae (arrowhead). The spaces between the trabeculae are lined with mesothelium. Hematoxylin and eosin (×400).

Membranous Labyrinth

The membranous labyrinth comprises the semicircular ducts, utricle, saccule, cochlear duct, endolymphatic duct, and endolymphatic sac (Fig. 18-2). The membranous labyrinth is lined with simple squamous epithelium and filled with a fluid called endolymph, which is more viscous than perilymph. Endolymph contains high levels of potassium and low to minimal levels of sodium when compared to perilymph. The protein profile of endolymph is similar to that of perilymph but different from proteins found in plasma. Epithelial cells of the vestibule and the stria vascularis, which is described later in this chapter, are thought to produce endolymph. The connective tissue underlying the epithelium of the membranous labyrinth forms fine trabeculae (Fig. 18-3). The trabeculae span the adjacent perilymphatic space and anchor the suspended membranous labyrinth to the periosteum of the bony wall. The periosteum and the spaces between the trabeculae are lined by flattened mesothelial cells.

Semicircular Ducts

The semicircular ducts lie within the semicircular canals (Fig. 18-1). One end of each duct is enlarged as an ampulla. The ducts are lined by simple squamous epithelium. Connection of the ducts with the utricle is shown in Figure 18-2.

Utricle and Saccule

The medial wall of the vestibule has two depressions in which the utricle (caudodorsal) and the saccule (rostroventral) are housed (Fig. 18-2). Two parts of the utriculosaccular duct extend from the utricle and saccule respectively and converge to form the endolymphatic duct, which terminates as the endolymphatic sac. Part of the endolymphatic duct lies within the vestibular aqueduct. The endolymphatic sac lies partially within the vestibular aqueduct, and partially between two laminae of the dura. The function of the sac is to regulate the pressure and volume of endolymph.

Cochlear Duct

The cochlear duct is connected to the saccule by a small duct, the ductus reuniens, and ends as a blind sac at the apex of the cochlea. The triangular cochlear duct lies between two additional compartments of the cochlea (Fig. 18-4). The dorsal compartment, or scala vestibuli, extends from the region of the vestibular (oval) window to the apex of the cochlea, where it becomes confluent with the ventral compartment, the scala tympani, through an opening called the helicotrema (Fig. 18-2). The scala tympani ends at the cochlear (round) window. The cochlear duct is separated from the scala vestibuli by the vestibular membrane (Reissner’s membrane) and from the scala tympani by the basilar membrane (Figs. 18-4 and 18-5). Scant collagen fibers form the vestibular membrane, which is covered with simple squamous epithelium on both surfaces. Basement membranes separate the epithelia from the connective tissue. The basilar membrane is attached to the outer osseous cochlea by the spiral ligament and extends to the spiral lamina of the modiolus. The membrane is composed of collagen fibers embedded in homogeneous ground substance; it increases in thickness as it progresses from the cochlear window to the helicotrema. Width of the basilar membrane increases continuously from the cochlear window, where it is narrowest, to the helicotrema, where it is widest. On the side facing the scala tympani, the basilar membrane is covered with simple squamous epithelium, while the spiral organ (organ of Corti) is present on the cochlear duct surface.

Stria Vascularis

The third wall of the triangular-shaped cochlear duct contains the stria vascularis, which contributes to the production of endolymph and regulates its unique ion content (Figs. 18-4, 18-5, and 18-6). Stratified cuboidal cells of the stria rest directly on a layer of connective tissue without an intervening basal lamina (Fig. 18-7). Three epithelial cell types (basal, intermediate, and marginal cells) appear similar with electron microscopy. Dark-staining marginal cells lie adjacent to the cochlear duct lumen and are known to pump Na⁺ out of the endolymph. Light-staining intermediate and basal cells are found between the marginal cells and the spiral ligament. The intermediate cells contain melanin and also play a role in the generation of electrical potentials in the inner ear. Basal cells are flat cells that form a barrier between the stria vascularis and the spiral ligament. While epithelium in general is considered to be avascular, many capillaries are present between the epithelial cells of the stria vascularis (Fig. 18-6). At the junction of the stria vascularis with the spiral organ, the stratified epithelium changes abruptly to simple cuboidal. This region is called the spiral prominence (Fig. 18-4).

FIGURE 18-4 Schematic representation of the structure of the scala vestibuli, cochlear duct, and scala tympani. Note the structure of the spiral organ, which includes many cell types that lie under the tectorial membrane above the basilar membrane. Other major structures include the stria vascularis, vestibular membrane, spiral ligament, cochlear nerve, and spiral ganglion.

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