Cochlea, Anatomy

Synonyms

Morphology of cochlea

Definition

The labyrinth is a fluid-filled system containing the inner ear hearing (located anterior, cochlea) and balance (located posterior, vestibule and three semicircular canals) sensory organs. The labyrinth has a layered structure: The bony labyrinth surrounds the fluid-filled spaces within and is lined with an endosteal membrane. The membranous labyrinth is located within the bony labyrinth and filled with endolymph. The space between bony and membranous labyrinth is filled with perilymphatic fluid.

The cochlea (Greek for snail) is the auditory portion of the inner ear. In the human, the cochlea comprises of 2½ to 2¾ turns. The cochlea is subdivided into various fluid spaces demonstrating different ion concentrations. The critical portion facilitating the mechanoelectrical transduction of mechanical energy (sound pressure waves) into electrical signals (action potentials) is the organ of Corti, which is embedded within the basilar membrane. The action potentials are then sent via peripheral dendrites of the spiral ganglion cell bodies in the center of the cochlea, the modiolus. The central axons form the cochlear portion of the vestibulocochlear nerve.

Developmental Anatomy

Development of the ear begins with preliminary inductions of the surface ectoderm, first by the notochord (chordamesoderm) and then by the paraxial mesoderm. These inductions prepare the ectoderm for a third induction, in which the rhombencephalon induces the adjacent surface ectoderm to thicken and form the otic placode. Late in the fourth week, the otic placode invaginates and then separates from the surface ectoderm to form the otic vesicle, or otocyst (Rinkwitz et al. 2001) (Fig. 1).

Cochlea, Anatomy, Fig. 1

Evolution of the membranous labyrinth: (a) 22 day, (b) 4 weeks, (c) 4½ weeks, (d) 5½ weeks, (e) 6 weeks, (f) from the eighth week forward (Reproduced with permission from Gulya AJ, Schuknecht HF. Anatomy of the temporal bone with surgical implications. 2nd edition. Pearl River (NY): Parthenon Publishing Group; 1995)

The otic vesicle soon begins to elongate, forming a dorsal (utricle) vestibular and a ventral (saccule) cochlear region. At about 5-week gestational age, two ridges appear in the vestibular portion (Fig. 2). Ultimately, two of the three semicircular canals will develop from these two ridges. Altogether, these epithelial structures form the membranous labyrinth (Fig. 3). The sensory neurons that make up the eighth cranial nerve (statoacoustic ganglion) arise from cells that migrate from a portion of the medial wall of the otocyst. The cochlear portion (spiral ganglion) of the eighth cranial nerve fans out in association with the sensory cells of the organ of Corti. Neural crest cells invade the developing statoacoustic ganglion and ultimately form the supporting cells. The sensory cells of the organ of Corti are derived from the epithelium of the otocyst.

Cochlea, Anatomy, Fig. 2

The membranous labyrinth at about 6-week gestational age. Folds I, II, and III begin to indent into the otocyst (After Bast and Anson. Reproduced with permission from Gulya AJ, Schuknecht HF Anatomy of the temporal bone with surgical implications. 2nd edition. Pearl River (NY): Parthenon Publishing Group; 1995)

Cochlea, Anatomy, Fig. 3

The mature membranous labyrinth, as viewed from medial (After Anson and Donaldson. Reproduced with permission from Gulya AJ, Schuknecht HF. Anatomy of the temporal bone with surgical implications. 2nd edition. Pearl River (NY): Parthenon Publishing Group; 1995)

In the sixth week of development, the saccule forms a tubular elongation at its lower pole, which will eventually give rise to the cochlear duct. The cochlear duct penetrates the surrounding mesenchyme to form the human cochlea with its 2½ turns. The ductus reuniens remains as the residual connection of the cochlear duct with the saccule. Mesenchyme surrounding the cochlear duct soon differentiates into cartilage. This cartilaginous shell (with the cochlear duct in its center) undergoes vacuolation to form two perilymph spaces on both sides of the duct, the scala vestibuli and the scala tympani. The cochlear duct is partitioned from the scala vestibuli and the scala tympani by the vestibular (Reissner’s) and basilar membranes, respectively. At the lateral, wide wall of the cochlear duct is the spiral ligament and at its medial, narrow part is the modiolus, the future axis of the bony cochlea. With further development, epithelial cells of the duct form two ridges: an inner ridge (spiral limbus) and an outer ridge. The outer ridge forms one row of inner and three to four rows of outer hair cells, which are sensory.

In summary, the embryological development of the cochlea is quite complex. Despite it’s completion well before birth, knowledge on various stages seems relevant in order to understand the clinical appearance of various inner ear malformations (Buchman et al. 2004). Also, the lack of postnatal growth of the labyrinth has clinical implications mainly facilitating pediatric cochlear implantation.

Anatomical Overview of the Cochlea

The human cochlea features 2½ to 2¾ turns and spirals around the modiolus, its axis (Fig. 4). The spiral ganglion cell bodies are located in the modiolus and the dendrites project to the organ of Corti via the cribrose area of the basal cochlear turn and the osseous spiral lamina. The apex of the cochlea is located medial to the cochleariform process and the tensor tympani muscle. As detailed previously, the development of the cochlea has been completed long before birth. Thus, there is no postnatal growth, which is important for cochlear implantation. However, there are substantial interindividual differences in shape and size of the cochlea that deserve consideration and that have received recent attention.

Cochlea, Anatomy, Fig. 4

Cross section through the fluid spaces of the cochlea. Sc. Vest. scala vestibuli, Sc. Tymp. Scala tympani, Sc. Media Scala media, TM tectorial membrane, BM basilar membrane, RM Reissner’s membrane, OSL osseous spiral lamina, SV stria vascularis, SL spiral ligament (Adapted from Snow and Ballenger (2002))

The cochlea has three main fluid compartments: scala tympani and vestibuli, which contain sodium-rich perilymph and the scala media (also known as the cochlear duct), which contains endolymph and houses the organ of Cori. The organ of Corti rests on the basilar membrane, which spans between the osseous spiral lamina and the spiral ligament on the outer wall. The spiral ligament also contains the stria vascularis in its upper portion, which contains a rich vascular network and produces potassium-rich endolymph. Reissner’s (vestibular) membrane is a fragile two-cell-layered structure, which separates scala vestibuli and media. The more robust basilar membrane divides scala tympani and media. The organ of Corti contains the inner and outer auditory hair cells and thus carries the central sensory elements of hearing.

Organ of Corti

The organ of Corti rests on the basilar membrane within scala media of the cochlea (Fig. 5). It contains two types of cells: supporting cells and hair cells. Hair cells are the receptor cells transducing mechanical sound information into action potentials. As their name suggests, supporting cells take on a supporting role for hair cells. Each hair cell’s top portion is formed by the reticular lamina, which isolates the hair cells’ stereocilia from their cell bodies. It provides a solid surface so that the top portion of the hair cells penetrates into the endolymphatic space but the remainder of the hair cell body is embedded in perilymph (Anniko and Wroblewski 1986). One type of support cells, the Deiter’s cells, fills the gaps between the top parts of the hair cells and thus helps form the reticular lamina.

Cochlea, Anatomy, Fig. 5

Detailed schematic of the organ of Corti: RM Reissner’s membrane, TM tectorial membrane, IS inner sulcus, BC border cells, IHC inner hair cells, TC tunnel of Corti, OHC outer hair cells, PHC phalangeal cells, CH cells of Hensen, CC cells of Claudius, BM basilar membrane (Adapted from Snow and Ballenger (2002))

There are three to five rows of hair cells, one on the inner (modiolar) side of the tunnel of Corti formed by the pillar cells; these are the inner hair cells. Three rows are located on the outer side of the tunnel of Corti; thus, these are the outer hair cells. Overall, the organ of Corti contains about 15,000 hair cells; about 3,500 being inner hair cells and 12,000 being outer hair cells. Stereocilia on inner and outer hair cells are arranged in curved or v-shaped rows that face toward the tectorial membrane. Each row of stereocilia has its own height and each row is taller than the previous one. The tip of each stereocilium is linked to the side of the stereocilium in the previous row via a tip link.

The inner and outer hair cells differ morphologically in that the inner hair cells are more flash shaped and tightly surrounded by supporting cells. Their stereocilia are arranged in a linear fashion. The outer hair cells, on the other hand, are columnar in shape and are surrounded incompletely by phalangeal or supporting cells lying free in the perilymph of the organ of Corti. The stereocilia of outer hair cells are arranged in a special fashion and a basal body representing a rudimentary kinocilium is located on the spiral ligament side of the ciliary tuft. The inner hair cells are supported by interphalangeal cells, whereas the outer hair cells are supported by Deiter’s cells inferiorly and laterally by Hensen’s cells.

The tectorial membrane is a glycoprotein containing membrane that covers the organ of Corti. It is anchored at the limbus of the spiral lamina (spiral limbus) and the longest stereocilia of outer hair cells are embedded in its outer portion. Laterally, the tectorial membrane is attached to the Hensen’s cells via a fibrous net. Although the tectorial membrane extends over the top of the inner hair cells, their stereocilia are free and not embedded in the membrane. The fulcrum of the tectorial membrane and the basilar membrane is separate. They are both displaced vertically by the travelling wave created by sound energy but due to their different attachments, they will slide horizontally, thus creating a shearing action which is then translated to a displacement of hair cell stereocilia. This initiates the action potential and subsequent auditory stimulation.

Spiral Ligament

The outer wall of the cochlea hosts the spiral ligament, a thickening of the cochlear periosteum (endosteum). In its upper portion, the spiral ligament features capillaries and small blood vessels as well as pigment-containing melanocytes and endothelial cells. This portion of the spiral ligament is termed the stria vascularis. It receives the majority of the blood supply to the cochlea and is responsible for producing endolymph (from perilymph) and maintaining its ion composition. Thus, the stria vascularis creates the endocochlear potential.

The ion transporters in the stria vascularis are the same as those found in the kidney and this seems to be the reason for the ototoxic nature of drugs impairing renal function (Anniko 1985; Walker et al. 1990; van Ruijven et al. 2005).

Spiral Ganglion

Hair cells are innervated by afferent and efferent neurons in a complex but orderly manner (Tylstedt et al. 1997). The cell bodies of the first neuron in the auditory nerve are located in the spiral ganglion, which is hosted within the bony modiolus. In fact, the collection of cell bodies is termed the spiral ganglion. The spiral ganglion features clusters of ganglion cells spanning throughout the entire of the length of the cochlea. The dendrites project distally to the base of hair cells and the axon form the cochlear portion of the vestibulocochlear nerve. Like other craniospinal ganglia, most spiral ganglion cells are classified as pseudounipolar in structure (except for Type II cells which are unipolar, see below). A healthy human auditory system features about 35,000 spiral ganglion cells, a number that typically decreases with age (Adams and Schulte 1997).

Two main cell types can be differentiated within the ganglion: Type I and Type II cells (Nadol 1990). About 90% of the spiral ganglion is comprised of myelinated Type I cells, which innervate inner hair cells. Specifically, each inner hair cell is often innervated by many afferent Type I cells. Type II cells, on the other hand, are mostly non-myelinated and have a unipolar structure. Also, Type II cells innervate about 20 outer hair cells and they seem to carry both afferent and efferent fibers. Efferent synapses form large calyx-shaped contact on the outer hair cell body, whereas afferent synapses feature a small button-like contact.

The efferent information is mostly generated in the brainstem, more specifically in the superior olivary complex (Warr 1980). Fibers from both sides of the brain innervate both inner and outer hair cells but the fibers innervating the two types of HC originate in different places. One recent study suggests that the SOC receives input from auditory cortex – so fairly high level processing. The fiber tract containing the efferent fibers is known as the olivocochlear bundle. The tract from the same side of the brain is called the uncrossed olivocochlear bundle and the tract from the opposite side of the brain is called the crossed olivocochlear bundle.

Cochlear Aqueduct

The perilymphatic duct (cochlear aqueduct) is a small opening at the basal end of scala tympani. It traverses inferiorly and connects to the posterior fossa anterior to the jugular fossa. Its superior part is usually obliterated by connective tissue. The inferior part, however, is lined by dura and contains CSF (Aslan et al. 1998). The cochlear aqueduct can serve as a landmark for the pars nervosa of the jugular foramen during a translabyrinthine craniotomy (drilling inferior to the aqueduct might compromise the lower cranial nerves). Also, cerebrospinal fluid often gushes from the opening indicating the presence of the aqueduct. The cochlear aqueduct frequently hosts a cochlear vein (Rask-Andersen et al. 1977).

Round Window

The scala tympani terminates at the round window (Fig. 6). Similar to the tympanic membrane, the round window features a conical shape with its base projecting inward into scala tympani. It is subject to considerable size and position variations and has recently been rediscovered as a route for cochlear implant electrode insertion and cochlear opening. Despite the fact that the round window provides a direct opening into scala tympani, the resulting trajectory for cochlear implantation is often suboptimal and a deflection of the electrode off the modiolus has been described. However, in contrast to a more blind cochleostomy, the round window provides a variable, yet reliable landmark for scala tympani.

Cochlea, Anatomy, Fig. 6

Microscopic image of a cross section taken at the level of the round window. The conical shape of the round window with its bony attachment (annulus) can be seen as well as the very basal end of the basilar membrane and osseous spiral lamina. The close anatomic relationship of the posterior portion of the round window and the basal aspect of the basilar membrane can be appreciated

Fissures of the Bony Labyrinth

Fissures are bony dehiscences of the osseous labyrinth. The fissula ante fenestram seems clinically relevant as otosclerotic bone remodeling might start in this area. As suggested by its name, it can be found directly anterior to the oval window. In the normal temporal bone, this fissula is usually filled with fibrous and cartilaginous tissue. The fissula post fenestram and Hyrtl’s fissure are less constant features and are of unknown clinical significance.

Implications for Cochlear Implantation

Despite the fact that the cochlear size remains unchanged after birth, evidence suggests a change in position of the cochlea within the temporal bone over the first few years of life (Fig. 7). This can have a surgical effect on the insertion angle of cochlear implant electrodes. Specifically, in infants and toddlers, the insertion trajectory of the basal scala tympani can assume a more superior angle compared to the adult situation (Meshik et al. 2010).

Cochlea, Anatomy, Fig. 7

Corrosion cast of the human cochlea demonstrating measurements and considerable interindividual size variations of the human cochlea (Taken from Erixon et al. (2009))

More importantly, however, the size of the cochlea and its shape are subject to interindividual variations. This has been demonstrated in various publications as early as the early 1980s (Zrunek et al. 1980). This has further been observed in clinical investigations of variable angular insertion depths with similar linear electrode insertion lengths of free fitting (lateral wall) electrodes. Clinical implications include considerations for different electrode lengths in a residual hearing setting when combined electric acoustic stimulation of the auditory system is intended (Adunka et al. 2005).

Cross-References

Cochlear Implant

Hearing Exam

Physiology of Cochlea

Sensorineural Hearing Loss