Histology_Ross

Histology_Ross. stereocilia is depicted in the enlarged rectangle. The top link connects the lateral plasma membrane of the stereocilium shaft (where K transduction channels are located) with the tip of the shorter stereocilium (where the mechanoelectric transduction [MET] channel protein is located). Note that the proximal end of each stereocilium is tapered and its narrow rootlets are anchored within the terminal web (cuticular plate) of the hair cell. Several other fibrillar connectors between neighboring stereocilia are also shown.


Histology_Ross

Histology_Ross.  FOLDER 25.3 Clinical Correlation: Vertigo


Histology_Ross

Histology_Ross. The sensation of rotation without equilibrium (dizziness, vertigo) signifies dysfunction of the vestibular system. Causes of vertigo include viral infections, certain drugs, and tumors such as acoustic neuroma. Acoustic neuro-mas develop in or near the internal acoustic meatus and exert pressure on the vestibular division of cranial nerve VIII or branches of the labyrinthine artery. Vertigo can also be produced normally in individuals by excessively stimulating the semicircular ducts. Similarly, excessive stimulation of the utricle can produce motion sickness (seasickness, car-sickness, or airsickness) in some individuals. Some diseases of the internal ear affect both hearing and equilibrium. For example, people with Ménière’s syn-drome initially complain of episodes of dizziness and tinni-tus (ringing in the ears) and later develop a low-frequency hearing loss. The causes of Ménière’s syndrome are related to blockage of the cochlear aqueduct, which drains excess endolymph from the


Histology_Ross

Histology_Ross. (ringing in the ears) and later develop a low-frequency hearing loss. The causes of Ménière’s syndrome are related to blockage of the cochlear aqueduct, which drains excess endolymph from the membranous labyrinth. Blockage of this duct causes an increase in endolymphatic pressure and distension of the membranous labyrinth (endolymphatic hydrops).


Histology_Ross

Histology_Ross. FIGURE 25.11 • Diagram of the crista ampullaris within a semicircular duct. The cellular organization of the crista ampullaris of a semicircular duct is shown in the large diagram and the enlarged rectangle. The crista ampullaris is composed of both type I and type II sensory hair cells and supporting cells. The stereocilia and kinocilium of each hair are embedded in the cupula that projects toward the nonsensory wall of the ampulla. movement differential between the crista fixed to the wall of the duct and the endolymph. Deflection of the stereocilia in the narrow space between the hair cells and the cupula generates nerve impulses in the associated nerve endings. The maculae of the saccule and utricle are sensors of gravity and linear acceleration.


Histology_Ross

Histology_Ross. The maculae of the saccule and utricle are sensors of gravity and linear acceleration. The maculae of the saccule and the utricle are innervated sensory thickenings of the epithelium that face the endolymph of the saccule and utricle (see Fig. 25.8c). As in the cristae, each macula consists of type I and type II hair cells, supporting cells, and nerve endings associated with the hair cells. The maculae of the utricle and saccule are oriented at right angles to each another. When a person is standing, the macula of the utricle is in a horizontal plane, and the macula of the saccule is in a vertical plane. The gelatinous polysaccharide material that overlies the maculae is called the otolithic membrane (Fig. 25.12). Its outer surface contains 3-m to 5-m crystalline bodies of calcium carbonate and a protein (Fig. 25.13). Otoliths are heavier than the endolymph. The outer surface of the otolithic membrane


Histology_Ross

Histology_Ross. FIGURE 25.12 • Diagram of a macula within the utricle. A more detailed diagram of the cellular organization of the macula of the utricle is shown in the enlarged rectangle. Supporting cells can be seen lying between the two principal types of sensory hair cells (types I and II). The stereocilia and kinocilium of each sensory hair cell are embedded in the otolithic membrane on which otoconia lie. FIGURE 25.13 • Scanning electron micrograph of human otoconia. Each otoconium has a long cylindrical body with a three-headed facet on each end. 5,000.


Histology_Ross

Histology_Ross. FIGURE 25.13 • Scanning electron micrograph of human otoconia. Each otoconium has a long cylindrical body with a three-headed facet on each end. 5,000. lies opposite the surface in which the stereocilia of the hair cells are embedded. The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista. Stereocilia of the hair cells are deflected by gravity in the stationary individual when the otolithic membrane and its otoliths pull on the stereocilia. They are also displaced during linear movement when the individual is moving in a straight line and the otolithic membrane drags on the stereocilia because of inertia. In both cases, movement of the otolithic membrane activates MET channels and depolarizes hair cells. The spiral organ of Corti is the sensor of sound vibrations.


Histology_Ross

Histology_Ross. The spiral organ of Corti is the sensor of sound vibrations. The cochlear duct divides the cochlear canal into three par allel compartments or scalae:  Scala media, the middle compartment in the cochlear canal  Scala vestibuli  Scala tympani The cochlear duct itself is the scala media (Figs. 25.14 and 25.15). The scala vestibuli and scala tympani are the spaces above and below, respectively, the scala media. The scala media is an endolymph-containing space that is continuous FIGURE 25.14 • Schematic diagram of the cochlea. a.


Histology_Ross

Histology_Ross. FIGURE 25.14 • Schematic diagram of the cochlea. a. Schematic diagram of a midmodiolar section of the cochlea that illustrates the position of the cochlear duct within the 2.75 turns of the bony cochlea. Observe that the scala vestibuli and scala tympani are continuous apically (helicotrema). b. Cross section of the basal turn of the cochlear duct. The cochlear duct and the osseous spiral lamina divide the cochlea into the scala vestibuli and the scala tympani, which contain perilymph. The scala media (the space within the cochlear duct) is filled with endolymph and contains the organ of Corti. (Modified from Goodhill V. Ear, Diseases, Deafness, and Dizziness. Hagerstown, MD: Harper & Row, 1979.)


Histology_Ross

Histology_Ross. FIGURE 25.15 • Photomicrograph of cochlear canal. This photomicrograph shows a section of the basal turn of the cochlear canal. The osseous spiral lamina (OSL) and its membranous continuation, the basilar membrane (BM), as well as the vestibular membrane (VM) divide the cochlear canal into three parallel compartments: The scala vestibuli, the cochlear duct (CD), and the scala tympani. Both the scala vestibuli and the scala tympani are filled with perilymph, whereas the cochlear canal is filled with endolymph. Note the three walls of the cochlear canal, which are formed by the basilar membrane inferiorly, the stria vascularis (SV ) and underlying spiral ligament (SL) laterally, and the vestibular membrane superiorly. The spiral organ of Corti resides on the inferior wall of the cochlear canal. Dendrites of the cochlear nerve (CN) fibers that originate in the spiral ganglion (SG) enter the spiral organ of Corti. The axons of the cochlear nerve form the cochlear part of the


Histology_Ross

Histology_Ross. cochlear canal. Dendrites of the cochlear nerve (CN) fibers that originate in the spiral ganglion (SG) enter the spiral organ of Corti. The axons of the cochlear nerve form the cochlear part of the vestibulocochlear nerve. 65.


Histology_Ross

Histology_Ross. with the lumen of the saccule and contains the spiral organ of Corti, which rests on its lower wall (see Fig. 25.15). The scala vestibuli and the scala tympani are perilymphcontaining spaces that communicate with each other at the apex of the cochlea through a small channel called the helicotrema (see Fig. 25.14). The scala vestibuli begins at the oval window, and the scala tympani ends at the round window. The scala media is a triangular space with its acute angle attached to the modiolus. In transverse section, the scala media appears as a triangular space with its most acute angle attached to a bony extension of the modiolus, the osseous spiral lamina (see Fig. 25.15). The upper wall of the scala media, which mesothelial cellperilymph of scala vestibuli endolymph of scala media epithelial cell


Histology_Ross

Histology_Ross. FIGURE 25.16 • Transmission electron micrograph of the vestibular (Reissner’s) membrane. Two cell types can be observed: a mesothelial cell, which faces the scala vestibuli and is bathed by perilymph, and an epithelial cell, which faces the scala media and is bathed by endolymph. 8,400. separates it from the scala vestibuli, is the vestibular (Reissner’s) membrane (Fig. 25.16). The lateral or outer wall of the scala media is bordered by a unique epithelium, the stria vascularis. It is responsible for production and maintenance of endolymph. The stria vascularis encloses a complex capillary network and contains three types of cells (Fig. 25.17). The marginal cells, primarily involved in K transport, line the endolymphatic space of the scala media.


Histology_Ross

Histology_Ross. FIGURE 25.17 • Transmission electron micrograph of the stria vascularis. The apical surfaces of the marginal cells (M) of the stria are bathed by endolymph (E) of the scala media. Intermediate cells (I) are positioned between the marginal cells and the basal cells (B). The basal cells separate the other cells of the stria vascularis from the spiral ligament (SpL). 4,700. Intermediate pigment-containing cells are scattered among capillaries. The basal cells separate stria vascularis from underlying spiral ligament. The lower wall or floor of the scala media is formed by a relatively flaccid basilar membrane that increases in width and decreases in stiffness as it coils from the base to apex of the cochlea. The spiral organ of Corti rests on the basilar membrane and is overlain by the tectorial membrane. The spiral organ of Corti is composed of hair cells, phalangeal cells, and pillar cells.


Histology_Ross

Histology_Ross. The spiral organ of Corti is composed of hair cells, phalangeal cells, and pillar cells. The spiral organ of Corti is a complex epithelial layer on the floor of the scala media (Fig. 25.18 and Plate 109, page 948). It is formed by the following:  Inner hair cells (close to the spiral lamina) and outer hair cells (farther from the spiral lamina) Inner phalangeal (supporting) cells and outer pha Several other named cell types of unknown function are also present in the spiral organ. The hair cells are arranged in inner and outer rows of cells. The inner hair cells form a single row of cells throughout all 2.75 turns of the cochlear duct. The number of cells forming the width of the continuous row of outer hair cells is variable. Three ranks of hair cells are found in the basal part of the coil (Fig. 25.19). The width of the row gradually increases to five ranks of cells at the apex of the cochlea. The phalangeal and pillar cells provide support for the hair cells.


Histology_Ross

Histology_Ross. The phalangeal and pillar cells provide support for the hair cells. Phalangeal cells are supporting cells for both rows of hair cells. The phalangeal cells associated with the inner hair cells surround the cells completely (Fig. 25.20a). The phalangeal cells associated with the outer hair cells surround only the basal portion of the hair cell completely and send apical processes toward the endolymphatic space (Fig. 25.20b). These processes flatten near the apical ends of the hair cells and collectively form a complete plate surrounding each hair cell (Fig. 25.21).


Histology_Ross

Histology_Ross. FIGURE 25.18 • Photomicrograph of the vestibular duct and spiral organ of Corti. This higher-magnification photomicrograph of the cochlear duct shows the structure of the spiral organ of Corti. Relate this structure to the inset, which labels the structural features of the spiral organ. 180. Inset. Diagram of the sensory and supporting cells of the spiral organ of Corti. The sensory cells are divided into an inner row of sensory hair cells and three rows of outer sensory hair cells. The supporting cells are the inner and outer pillar cells, inner and outer (Deiters’) phalangeal cells, outer border cells (Hensen’s cells), inner border cells, Claudius’ cells, and Btcher’s cells. (Modified from Goodhill V. Ear, Diseases, Deafness, and Dizziness. Hagerstown, MD: Harper & Row, 1979.)


Histology_Ross

Histology_Ross. The apical ends of the phalangeal cells are tightly bound to one another and to the hair cells by elaborate tight junctions. These junctions form the reticular lamina that seals the endolymphatic compartment from the true intercellular spaces of the organ of Corti (Figs. 25.18 and 25.20b). The extracellular fluid in this intercellular space is cortilymph. Its composition is similar to that of other extracellular fluids and to perilymph. Pillar cells have broad apical and basal surfaces that form plates and a narrowed cytoplasm. The inner pillar cells rest on the tympanic lip of the spiral lamina; the outer pillar cells rest on the basilar membrane. Between them, they form a triangular tunnel, the inner spiral tunnel (see Fig. 25.18). The tectorial membrane extends from the spiral limbus over the cells of the spiral organ of Corti.


Histology_Ross

Histology_Ross. The tectorial membrane extends from the spiral limbus over the cells of the spiral organ of Corti. The tectorial membrane is attached medially to the modiolus. Its lateral free edge projects over and attaches to the organ of Corti by the stereocilia of the hair cells. It is formed from the radially oriented bundles of collagen types II, V, and IX embedded in a dense amorphous ground substance. Glycoproteins unique to the internal ear, called otogelin and tectorin, are associated with the collagen bundles. These proteins are also present in the otolithic membranes overlying the maculae of the utricle and saccule, as well as in the cupulae of the cristae in the semicircular canals. As described on page 929, sound waves striking the tympanic membrane are translated into simple mechanical vibrations. The ossicles of the middle ear convey these vibrations to the cochlea. In the internal ear, the vibrations of the ossicles are transformed into waves in the perilymph.


Histology_Ross

Histology_Ross. In the internal ear, the vibrations of the ossicles are transformed into waves in the perilymph. Movement of the stapes in the oval window of the vestibule sets up vibrations or traveling waves in the perilymph of the scala vestibuli. The vibrations are transmitted through FIGURE 25.19 • Scanning electron micrograph of the spiral organ of Corti. This electron micrograph illustrates the configuration of stereocilia on the apical surfaces of the inner row and three outer rows of the cochlear sensory hair cells. 3,250. the vestibular membrane to the scala media (cochlear duct), which contains endolymph, and are also propagated to the perilymph of the scala tympani. Pressure changes in this closed perilymphatic–endolymphatic system are reflected in movements of the membrane that covers the round window in the base of the cochlea.


Histology_Ross

Histology_Ross. As a result of sound vibrations entering the internal ear, a traveling wave is set up in the basilar membrane (Fig. 25.22). A sound of specified frequency causes displacement of a relatively long segment of the basilar membrane, but the region of maximal displacement is narrow. The point of maximal displacement of the basilar membrane is specified for a given frequency of sound and is the morphologic basis of frequency discrimination. High-frequency sounds cause maximal vibration of the basilar membrane near the base of the cochlea; low-frequency sounds cause maximal displacement nearer the apex. Amplitude discrimination (i.e., perception of sound intensity or loudness) depends on the degree of displacement of the basilar membrane at any given frequency range. Thus, coding acoustic information into nerve impulses depends on the vibratory pattern of the basilar membrane. Movement of the stereocilia of the hair cells in the cochlea initiates neuronal transduction.


Histology_Ross

Histology_Ross. Movement of the stereocilia of the hair cells in the cochlea initiates neuronal transduction. Hair cells are attached through the phalangeal cells to the basilar membrane, which vibrates during sound reception. The stereocilia of these hair cells are, in turn, attached to the tectorial membrane, which also vibrates. However, the tectorial membrane and the basilar membrane are hinged at different points. Thus, a shearing effect occurs between the basilar membrane (and the cells attached to it) and the tectorial membrane when sound vibrations impinge on the internal ear.


Histology_Ross

Histology_Ross. Because they are inserted into the tectorial membrane, the stereocilia of the hair cells are the only structures that connect the basilar membrane and its complex epithelial layer to the tectorial membrane. The shearing effect between the basilar membrane and the tectorial membrane deflects the stereocilia and thus the apical portion of the hair cells. This deflection activates MET channels located at the tips of stereocilia and generates action potentials that are conveyed to the brain via the cochlear nerve (cochlear division of the vestibulocochlear nerve, cranial nerve VIII). Innervation of the Internal Ear The vestibular nerve originates from the sensory receptors associated with the vestibular labyrinth.


Histology_Ross

Histology_Ross. Innervation of the Internal Ear The vestibular nerve originates from the sensory receptors associated with the vestibular labyrinth. The vestibulocochlear nerve (cranial nerve VIII) is a special sensory nerve and is composed of two divisions: A vestibular division called the vestibular nerve and a cochlear division called the cochlear nerve. The vestibular nerve is associated with equilibrium and carries impulses from the sensory receptors located within the vestibular labyrinth. The cochlear nerve is associated with hearing and conveys impulses from the sensory receptors within the cochlear labyrinth (Fig. 25.23).


Histology_Ross

Histology_Ross. The cell bodies of the bipolar neurons of the vestibular nerve are located in the vestibular ganglion (of Scarpa) in the internal acoustic meatus. Dendritic processes of the vestibular ganglion cells originate in the cristae ampullaris of the three semicircular ducts, the macula of the utricle, and the macula of the saccule. They synapse at the base of the vestibular sensory hair cells, either as a chalice around a type I hair cell or as a bouton associated with a type II hair cell. The axons of the vestibular nerve originate from the vestibular ganglion, enter the brain stem and terminate in four vestibular nuclei. Some secondary neuronal fibers travel to the cerebellum and to the nuclei of cranial nerves III, IV, and VI, which innervate the muscles of the eye. The cochlear nerve originates from the sensory receptors of the spiral organ of Corti.


Histology_Ross

Histology_Ross. The cochlear nerve originates from the sensory receptors of the spiral organ of Corti. Neurons of the cochlear nerve are also bipolar, and their cell bodies are located in the spiral ganglion within the modiolus. Dendritic processes of spiral ganglion cells exit the


Histology_Ross

Histology_Ross. FIGURE 25.20 • Electron micrograph of an inner and outer hair cell. a. Observe the rounded base and constricted neck of the inner hair cell. Nerve endings (NE) from afferent nerve fibers (AF ) to the inner hair cells are seen basally. IP, inner pillar cell; IPH, inner phalangeal cell. 6,300. b. Afferent (AF ) and efferent (EF ) nerve fiber endings on the base of an outer sensory hair cell are evident. Outer phalangeal cells (OPH) surround the outer hair cells basally. Their apical projections form the apical cuticular plate (ACP). Note that the lateral domains in the middle third of the outer hair cells are not surrounded by supporting cells. 6,300. (Reprinted with permission from Kimura RS. Sensory and accessory epithelia of the cochlea. In: Friedmann I, Ballantyne J, eds. Ultrastructural Atlas of the Inner Ear. London: Butterworth, 1984.)


Histology_Ross

Histology_Ross. FIGURE 25.21 • Structure of the outer phalangeal cell. a. This scanning electron micrograph illustrates the architecture of the outer phalangeal (Deiters’) cells. Each phalangeal cell cups the basal surface of an outer hair cell and extends its phalangeal process apically to form an apical cuticular plate that supports the outer sensory hair cells. 2,400. b. Schematic drawing showing the relationship of an outer phalangeal cell to an outer hair cell.


Histology_Ross

Histology_Ross. FIGURE 25.22 • Schematic diagram illustrating the dynamics of the three divisions of the ear. The cochlear duct is shown here as if straightened. Sound waves are collected and transmitted from the external ear to the middle ear, where they are converted into mechanical vibrations. The mechanical vibrations are then converted at the oval window into fluid vibrations within the internal ear. Fluid vibrations cause displacement of the basilar membrane (traveling wave) on which rest the auditory sensory hair cells. Such displacement leads to stimulation of the hair cells and a discharge of neural impulses from them. Note that high-frequency sounds cause vibrations of the narrow, thick portion of the basilar membrane at the base of the cochlea, whereas low-frequency sounds displace basilar membrane toward the apex of the cochlea near its helicotrema.


Histology_Ross

Histology_Ross. modiolus through the small openings in the bony spiral lam-the spiral ganglion cells form the cochlear nerve, which enina and enter the spiral organ. About 90% of dendrites orig-ters the bony cochlea through the modiolus to appear in the inating from the spiral ganglion cells synapse with the inner internal acoustic meatus (see Fig. 25.23). From the internal hair cells; the remaining 10% of dendrites synapse with acoustic meatus, the cochlear nerve enters the brain stem and outer hair cells of the spiral ganglion of Corti. The axons of terminates in the cochlear nuclei of the medulla. Nerve fibers superior part of ventral vestibular nerve cochlear nuclei inferior part of vestibular nerve


Histology_Ross

Histology_Ross. FIGURE 25.23 • Diagram illustrating the innervation of the sensory regions of the membranous labyrinth. Note the two parts of the vestibulocochlear nerve. The cochlear nerve carries the hearing impulses from the cochlear duct; the vestibular nerve carries balance information from the semicircular canals. The cell bodies of these sensory fibers are located in the spiral ganglion (for hearing) and vestibular ganglion (for equilibrium). (Modified from Hawke M, Keene M, Alberti PW. Clinical Otoscopy: A Text and Colour Atlas. Edinburgh: Churchill Livingstone, 1984.) from these nuclei pass to the geniculate nucleus of the thalamus and then to the auditory cortex of the temporal lobe.


Histology_Ross

Histology_Ross. It is interesting that the organ of Corti also receives small amounts of efferent fibers conveying impulses from the brain pass parallel to the afferent nerve fibers of the vestibulocochlear nerve (olivocochlear tract, cochlear efferents of Rasmussen). Efferent nerve fibers from the brain stem pass through the vestibular nerve. They synapse either on afferent endings of the inner hair cell or on the basal aspect of an outer hair cell. Efferent fibers are thought to affect control of auditory and vestibular input to the central nervous system, presumably by enhancing some afferent signals while suppressing other signals. Damage to the organ of Corti, cochlear nerve, nerve pathways, or auditory cortex is responsible for permanent sensorineural hearing loss. Blood Vessels of the Membranous Labyrinth Arterial blood is supplied to the membranous labyrinth by the labyrinthine artery; venous blood drainage is to the venous dural sinuses.


Histology_Ross

Histology_Ross. Blood Vessels of the Membranous Labyrinth Arterial blood is supplied to the membranous labyrinth by the labyrinthine artery; venous blood drainage is to the venous dural sinuses. The blood supply to the external ear, middle ear, and bony labyrinth of the internal ear is derived from vessels associated with the external carotid arteries. The arterial blood supply to tissues of the membranous labyrinth of the internal ear is derived intracranially from the labyrinthine artery, a common branch of the anterior inferior cerebellar or basilar artery (Fig. 25.24). The labyrinthine artery is a terminal artery: It has no anastomoses with other surrounding arteries. Branches of this artery exactly parallel the distribution of the superior and inferior parts of the vestibular nerve.


Histology_Ross

Histology_Ross. FIGURE 25.24 • Diagram of the arterial supply of the membranous labyrinth of the internal ear. The blood supply to the membranous labyrinth of the internal ear is derived from the labyrinthine artery, a branch of the anterior inferior cerebellar or basilar artery. (Modified from Schuknecht HF. Pathology of the Ear. Cambridge, MA: Harvard University Press, 1974.) Venous drainage from the cochlear labyrinth is via the posterior and anterior spiral modiolar veins that form the common modiolar vein. The common modiolar vein and the vestibulocochlear vein form the vein of the cochlear aqueduct, which empties into the inferior petrosal sinus. Venous drainage from the vestibular labyrinth is via vestibular veins that join the vein of the cochlear aqueduct and by the vein of vestibular aqueduct, which drains into the sigmoid sinus.


Histology_Ross

Histology_Ross. The internal ear, located in the temporal bone, consists of a system of chambers and canals that contain a network of membranous channels. These are referred to, respectively, as the bony labyrinth and membranous labyrinth. In places, the membranous labyrinth forms the lining of the bony labyrinth; in other places, there is a separation of the two. Within the space lined by the membranous labyrinth is a watery fluid called endolymph. External to the membranous labyrinth, i.e., between the membranous and bony labyrinths, is additional fluid called perilymph. The bony labyrinth is divided into three parts: cochlea, semicircular canals, and vestibule. The cochlea and semicircular canals contain membranous counterparts of the same shape; however, the membranous components of the vestibule are more complex in form, being composed of ducts and two chambers, the utricle and saccule. The cochlea contains the receptors for hearing, i.e., the organ of Corti, the semicircular canals contain the


Histology_Ross

Histology_Ross. more complex in form, being composed of ducts and two chambers, the utricle and saccule. The cochlea contains the receptors for hearing, i.e., the organ of Corti, the semicircular canals contain the receptors for movement of the head, and the utricle and saccule contain receptors for position of the head.


Histology_Ross

Histology_Ross. Internal ear, ear, guinea pig, H&E 20. In this section through the internal ear, bone surrounds the entire internal ear cavity. Because of its labyrinthine character, sections of the internal ear appear as a number of separate chambers and ducts. These, however, are all interconnected (except that the perilymphatic and endolymphatic spaces remain separate). The largest chamber is the vestibule (V ). The left side of this chamber (black arrow) leads into the cochlea (C). Just below the black arrow and to the right is the oval ligament (OL) surrounding the base of the stapes (S). Both structures have been cut obliquely and are not seen in their entirety. The facial nerve (FN ) is in an osseous tunnel to the left of the oval ligament. The com munication of the vestibule with one of the semicircular canals is marked by the white arrow. At the upper right are cross sections of the membranous labyrinth passing through components of the semicircular duct system (DS).


Histology_Ross

Histology_Ross. The cochlea is a spiral structure having the general shape of a cone. The specimen illustrated here makes 31⁄2 turns (in humans, there are 23⁄4 turns). The section goes through the central axis of the cochlea. This consists of a bony stem called the modiolus (M). It contains the beginning of the cochlear nerve (CN ) and the spiral ganglion (SG). Because of the plane of section and the spiral arrangement of the cochlear tunnel, the tunnel is cut crosswise in seven places (note 31⁄2 turns). A more detailed examination of the cochlea and the organ of Corti is provided in Plate 109. Semicircular canal, ear, guinea pig, H&E 225.


Histology_Ross

Histology_Ross. A higher magnification of one of the semicircular canals and of the crista ampullaris (CA) within the canal seen in the lower right corner of figure above is provided here. The receptor for movement, the crista ampullaris (note its relationships in figure above), is present in each of the semicircular canals. The epithelial (EP) surface of the crista consists of two cell types, sustentacular (supporting) cells and hair (receptor) cells. (Two types of hair cells are distinguished with the electron microscope.) It is difficult to identify the hair and sustentacular cells on the basis of specific characteristics; they can, however, be distinguished on the basis of location (see inset), as the hair cells (HC) are situated in a more superficial location than the sustentacular cells (SC). A gelatinous mass, the cupula (Cu), surmounts the epithelium of the crista ampullaris. Each receptor cell sends a hair-like projection deep into the substance of the cupula.


Histology_Ross

Histology_Ross. The epithelium rests on a loose, cellular connective tissue (CT ) that also contains the nerve fibers associated with the receptor cells. The nerve fibers are difficult to identify because they are not organized into a discrete bundle. KEY C, cochlea CA, crista ampullaris CN, cochlear nerve CT, connective tissue Cu, cupula DS, duct system (of membranous labyrinth) EP, epithelium FN, facial nerve HC, hair cell M, modiolus OL, oval ligament S, stapes SC, sustentacular cell SG, spiral ganglion V, vestibule black arrow, entry to cochlea white arrow, entry to semicircular canal PLATE 109 Cochlear Canal and Organ of Corti


Histology_Ross

Histology_Ross. PLATE 109 Cochlear Canal and Organ of Corti The hair cell, a nonneuronal mechanoreceptor, is the common receptor cell of the vestibulocochlear system. Hair cells are epithelial cells that possess numerous stereocilia, modified microvilli also called sensory hairs. They convert mechanical energy to electrical energy that is transmitted via the vestibulocochlear nerve (cranial nerve VIII) to the brain. Hair cells are associated with afferent, as well as efferent, nerve endings. All hair cells have a common basis of receptor cell function that involves bending or flexing of their stereocilia. The specific means by which the stereocilia are bent varies from receptor to receptor, but in each case, stretching of the plasma membrane caused by the bending of the stereocilia generates transmembrane potential changes that are transmitted to the afferent nerve endings associated with each cell. Efferent nerve endings on the hair cells serve to regulate their sensitivity.


Histology_Ross

Histology_Ross. Cochlear canal, ear, guinea pig, H&E 65; inset 380.


Histology_Ross

Histology_Ross. A section through one of the turns of the cochlea is shown here. The most important functional component of the cochlea is the organ of Corti, enclosed by the rectangle and shown at higher magnification in figure below. Other structures are included in this figure. The spiral ligament (SL) is a948 thickening of the periosteum on the outer part of the tunnel. Two membranes, the basilar membrane (BM ) and the vestibular membrane (VM), join with the spiral ligament and divide the cochlear tunnel into three parallel canals, namely, the scala vestibuli (SV ), the scala tympani (ST ), and the cochlear duct (CD). Both the scala vestibuli and the scala tympani are perilymphatic spaces; these communicate at the apex of the cochlea. The cochlear duct, on the other hand, is the space of the membranous labyrinth and is filled with endolymph. It is thought that the endolymph is formed by the portion of the spiral ligament that faces the cochlear duct, the stria vascularis (StV ). This is highly


Histology_Ross

Histology_Ross. labyrinth and is filled with endolymph. It is thought that the endolymph is formed by the portion of the spiral ligament that faces the cochlear duct, the stria vascularis (StV ). This is highly vascularized and contains specialized “secretory” cells.


Histology_Ross

Histology_Ross. A shelf of bone, the osseous spiral lamina (OSL), extends from the modiolus to the basilar membrane. Branches of the cochlear nerve (CN) travel along the spiral lamina to the modiolus, where the main trunk of the nerve is formed. The components of the cochlear nerve are bipolar neurons whose cell bodies constitute the spiral ganglion (SG). These cell bodies are shown at higher magnification in the inset (upper right). The spiral lamina supports an elevation of cells, the limbus spiralis (LS). The surface of the limbus is composed of columnar cells. Organ of Corti, ear, guinea pig, H&E 180; inset 380.


Histology_Ross

Histology_Ross. Organ of Corti, ear, guinea pig, H&E 180; inset 380. The components of the organ of Corti, beginning at the limbus spiralis (LS), are as follows: inner border cells (IBC ); inner phalangeal and hair cells (IP&HC); inner pillar cells (IPC ); (the sequence continues, repeating itself in reverse) outer pillar cells (OPC ); hair cells (HC ) and outer phalangeal cells (OP); and outer border cells or cells of Hensen (CH ). Hair cells are receptor cells; the other cells are collectively referred to as supporting cells. The hair and outer phalangeal cells can be distinguished in this figure by their location (see inset) and because their nuclei are well aligned. Because the hair cells rest on the phalangeal cells, it can be concluded that the upper three nuclei belong to outer hair cells, whereas the lower three nuclei belong to outer phalangeal cells.


Histology_Ross

Histology_Ross. The supporting cells extend from the basilar membrane (BM) to the surface of the organ of Corti (this is not evident here but can be seen in the inset), where they form a reticular membrane (RM). The free surface of the receptor cells fits into openings in the reticular membrane, and the “hairs” of these cells project toward, and make contact with, the tectorial membrane (TM ). The latter is a cuticular extension from the columnar cells of the limbus spiralis. In ideal preparations, nerve fibers can be traced from the hair cells to the cochlear nerve (CN). In their course from the basilar membrane to the reticular membrane, groups of supporting cells are separated from other groups by spaces that form spiral tunnels. These tunnels are named the inner tunnel (IT ), the outer tunnel (OT ), and the internal spiral tunnel (IST ). Beyond the supporting cells are two additional groups of cells, the cells of Claudius (CC) and the cells of Btcher (CB).


Histology_Ross

Histology_Ross. KEY BM, basilar membrane CB, cells of Btcher CC, cells of Claudius CD, cochlear duct CH, cells of Hensen CN, cochlear nerve HC, hair cells IBC, inner border cells IPHC, inner phalangeal and hair cells IPC, inner pillar cells IST, internal spiral tunnel IT, inner tunnel LS, limbus spiralis OP, outer phalangeal cells OPC, outer pillar cells OSL, osseous spiral lamina OT, outer tunnel RM, reticular membrane SG, spiral ganglion SL, spiral ligament ST, scala tympani StV, stria vascularis SV, scala vestibuli TM, tectorial membrane VM, vestibular membrane