The walls of the membranous labyrinth are made up of outer thin tunica propria and an inner lining epithelium. The tunica propria is composed of delicate fibrous tissue and stellate fibroblasts. The epithelium of the membranous labyrinth is a single layer of flat squamous cells resting on a basement membrane.
- In certain regions, the wall of the membranous labyrinth is considerably modified and is lined by neuroepithelium. These are the macula utriculi, macula sacculi, the crista ampullaris and the organ of Corti.
- Maculae: These represent local thickenings of the membranous wall in the utricle and saccule forming an elevation into the endolymphatic space. The epithelium is columnar type in which two kinds of cells may be distinguished, the sustentacular and hair cells. The tunica propria in the macular region is firm, thickened and is closely united to the underlying periosteum of the bony vestibule.
- The sustentacular or supporting cells are tall columnar elements resting on the basement membrane. Their basal portions containing the oval granular nuclei and are broader than their upper portions that pass between the hair cells. The free surface of each sustentacular cell bears a cuticular plate. At the edge of the macula, the sustentacular cells show a gradual transition into the simple squamous epithelium characteristic of the remainder of membranous labyrinth.
- The hair cells occupy the outer part of the epithelium. Each hair cell has the shape of rounded flask. The large, oval and deeply staining nucleus lies towards the base of the cells. The free surface is covered by cuticular plate, through which passes a single long tapering process the so called hair of the hair cells. The hair is made of fine nonmotile cilia. The hairs penetrate a membrane called the otolithic membrane, consisting of a gelatinous substance containing a number of small bodies, the otoconia or otoliths. The otoconia are made of calcium carbonate.
- The hair cells are intimately related to the fibres of vestibular nerve. The fibres enter the tunica propria of the macula, lose their myelin sheath and the naked axis cylinders pass through the basement membrane and terminate around the hair cells. The maculae are concerned with static equilibrium i.e., the position of head in space in relation to gravity.
- Crista ampullaris: Each crista ampullaris is thickened to form a ridge, placed transversely to the long axis of the duct. The ridge consists of connective tissue propria containing many nerves and blood vessels and surmounted by a specialized columnar epithelium. The columnar epithelium is similar to that of macula consisting of sustentacular cells and hair cells. In certain fixed preparations, the epithelium is surmounted by a tall rounded longitudinally striated mass, the cupola. The fibres of the vestibular nerve terminate in the epithelium of the crista in the same manner as in the macula. The cristae ampullaris are supposed to be concerned in dynamic equilibrium. They are stimulated by movements, which involve any amount of rotation.
- Cochlear duct: The wall of the bony cochlea is lined by a thin periosteal connective tissue covered by mesenchymal epithelium in the two scalae. The vestibular membrane is thin and homogeneous and is covered on its two surfaces by a layer of flattened epithelium. The periosteum forming the outer wall of the cochlear duct called the spiral ligament is greatly thickened and projects inwards as a triangular prominence termed the crista basilaris to which the outer edge of the basilaris membrane is fixed. Immediately above this there is concavity termed the sulcus spiralis externus, above which the periosteum contains numerous blood vessels and is termed the stria vascularis. It is believed that the stria vascularis secretes the endolymph of the cochlear duct.
- The osseous spiral lamina consists of two plates of bone and between these are the bipolar cells of the spiral ganglion. Minute canals pass outwards between the two plates leading to openings in the tympanic lip of the spiral lamina, through which the peripheral processes of the nerve cells from spiral ganglion pass to the organ of Corti on the basilar membrane.
- The basilar membrane stretches from the tympanic lip of the osseous spiral lamina to the crista basilaris. It supports the spiral organ of Corti. The basilar membrane consists of fine straight, unbranched fibres and the basilar fibres or auditory strings embedded in a sparse, homogeneous ground substance. The breadth of basilar membrane varies in the different turns of the cochlea. It is greatest at the apex, gradually diminishing towards the base until its narrowest extent is reached in the proximal end of the basal coil.
- The spiral organ of Corti: It is composed of a series of epithelial cells placed upon the basilar membrane projecting into the endolymph of cochlear duct. It extends through the entire length of the cochlear duct with the exception of a short distance at either end. Named in their order from within outwards the following specialized cell types comprise the organ of corti.
- The border cells.
- The inner hair cells.
- The inner phalangeal cells.
- The inner and outer pillar cells (inner and outer rods)
- Outer phalangeal cells (cells of Dieters).
- Outer hair cells.
- Cells of Hensen.
- The Hensen cells are continuous with the cells of Claudius, which are cubical and extend over the remainder of the basilar membrane to the spiral ligament. All except the hair cells may be considered as sustentacular or supporting cells. The hair cells are intimately related to the endings of the cochlear nerve and are neuroepithelial cells.
- 1. The border cells and cells of Hensen are slender columnar cells which rest on the basilar membrane and are arranged in the inner and outer aspects of the hair cells.
- 2. The hair cells are columnar cells. The free surface of each has short stiff hairs, which are in contact with the tectorial membrane. Their rounded basal ends rest on the phalangeal cells. The inner hair cells form a single row while the outer hair cells are arranged from 3 rows in the basal coil to 5 rows in the apical coil. The hair cell is arranged on either side of the inner and outer pillar cells or rods of corti.
- 3. The phalangeal cells are arranged below the hair cells in corresponding number of rows. Their bases rest on the basilar membrane and their upper borders are concave and on these rest the lower ends of hair cells. The nucleus is located in the basal part of the cells and from the upper part of the cell a slender process passes between the hair cells to the surface and ends in a cuticular plate.
- 4. The pillar cells or rods of Corti: These are inner and outer rods or pillar cells and are modified columnar cells with a broad basal part with nucleus rests on the basilar membrane and an elongated body or pillar, which ends at the free upper extremity in an enlarged head. The bases of the inner and outer pillar cells are farther apart but their heads converge and articulate with each other. Thus a triangular tunnel of Corti is formed between the two pillar cells and the basilar membrane.
- Tectorial membrane: Above the cells of the organ of corti and lying in contact with the hair cells in life is a thick, elongated gelatinous structure called the tectorial membrane. It is usually distorted during fixation. It begins at the inner angle of the cochlear duct as a thin layer attached to the epithelial surface on the thickened periosteum over the osseous spiral lamina (limbus spiralis). Farther outward, the membrane becomes thicker with a pronounced convexity on its upper surface and its lower surface is in contact with hair cells of the organ of corti. Beyond this, it becomes thinner and ends in a rounded tip.
- Nerve fibres : The peripheral processes of the bipolar cells of the spiral ganglion, pass through the tympanic lip of the spiral lamina and reach the organ of corti. Some of the fibres end on the inner hair cells, while others pass across the tunnel of corti and end on the outer hair cells. The nerve fibres form fine plexuses around the bases of the hair cells and terminate on button like endings in contact with the surfaces of hair cells.
- Functions
- Cochlea: The organ of corti in the cochlea is the receptor for the sense of hearing. Vibrations transmitted from the tympanic membrane through the chain of auditory ossicles reach the fenestra ovalis and perilymph. It is believed that the vibrations of the basilar membrane caused by those in perilymph stimulate the hair cells. The vibrating hair cells are in contact with the tectorial membrane through their hairs. The stimuli are conveyed through the spiral ganglion and the eighth cranial nerve to the brain. (For further pathways see neurology)
- Vestibule : The receptors in the macula utriculi and crista ampullaris are stimulated by alterations of the position of head and the impulses (equilibrator or vestibular) are concerned with the maintenance of equilibrium of the body under various conditions.