VAN 121: Veterinary Gross Anatomy-II (2+2)

Microscopic structure of the membranous labyrinth-I

MICROSCOPIC STRUCTURE OF THE MEMBRANOUS LABYRINTH

• 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.