Features of the histological slide
In practical training, histology of the inner ear is most commonly studied using a classic specimen conventionally called the "Cochlea" or "Organ of Corti." However, this section actually captures a large area of the mouse embryo's head.
Due to this scale, an axial section reveals not only elements of the auditory analyzer, but also a number of adjacent diagnostic structures:
- Epithelial tissues: stratified surface epithelium of the developing oral cavity and glandular epithelium (rudiments of the lacrimal or salivary glands).
- Musculoskeletal system: muscle tissue (skeletal striated musculature of the head and tongue), as well as bony trabeculae of the flat cranial bones demonstrating direct osteogenesis. Areas of red bone marrow are located between the bony trabeculae.
- Additional structures: tooth buds, fragments of the brain, the eyeball with the lens and retina, large blood vessels, cranial nerves, and nasal passages.
The structures of the inner ear proper occupy only a limited portion of this extensive histological section.
Bony framework and general structure
The central axis of the cochlea is a massive bony pillar—the modiolus. Around this pillar, the bony cochlear canal makes 2.5 spiral turns.
A specialized bone plate called the osseous spiral lamina (lamina spiralis ossea) extends into the lumen of the canal from the central core. It is located approximately at the level of the middle of the lumen and extends along the entire spiral course. The lamina reaches almost to the geometric center of the canal, but does not completely block it. An important cluster of neurons—the spiral ganglion—is hidden within the thickness of this bony structure.
On the upper surface of the bony shelf, the periosteum forms a sharp thickening known as the limbus (limbus spiralis). Its edge is morphologically divided into two "lips": upper (vestibular) and lower (tympanic). The space between these lips forms a depression—the internal spiral sulcus.
Three scalae and hydrodynamics
Due to the attachment of the cochlear duct (cochlear duct) to the vestibular lip of the limbus and other structures, the overall lumen of the bony canal is strictly divided throughout its length into three isolated compartments (scalae):
- Scala vestibuli (scala vestibuli) is the upper tier, filled with perilymph. It originates from the oval window of the vestibule (fenestra vestibuli), into which the base of the stapes auditory ossicle is tightly fitted.
- Cochlear duct (scala media) is the middle tier, filled with endolymph.
- Scala tympani (scala tympani) is the lower tier, also containing perilymph.
Fluid movement physiology: The sound wave is transmitted via the base of the stapes to the oval window. Since fluid is incompressible, the vibrations trigger the movement of perilymph in the scala vestibuli. The wave travels up to the apex of the cochlea (to the area called the helicotrema), where it passes into the scala tympani. The fluid then descends to the base of the cochlea. Pressure is compensated by the bulging of the secondary tympanic membrane, which closes the round window at the end of the scala tympani.
Walls of the membranous canal (Scala media)
In transverse section, the middle compartment (cochlear duct) has a distinct triangular shape. It is formed by three walls and one internal angle.
Superomedial wall (Vestibular or Reissner's membrane): Stretched from the limbus to the outer wall. Consists of three microscopic layers:
- Endothelium (mesenchymal origin, facing the scala vestibuli).
- A thin layer of dense fibrous connective tissue.
- Simple squamous epithelium of ectodermal origin (facing the lumen of the canal toward the endolymph).
Outer wall: Fuses tightly with the periosteum of the bony canal. Includes the spiral ligament (a thickening of the periosteum made of dense fibrous tissue) and the stria vascularis (stria vascularis). The stria vascularis faces the endolymph and is a stratified epithelium containing blood capillaries—a unique histological phenomenon. The main function of the stria vascularis is the continuous production of endolymph.
Lower wall (Basilar membrane): Stretched between the spiral limbus and the spiral ligament. Covered with endothelium on the side of the scala tympani. Its basis consists of collagen fibers arranged obliquely (rather than strictly transversely) within a special matrix. These fibers act as peculiar "strings" resonating to specific vibration frequencies. The length of these fibers increases from the base of the cochlea to its apex. The spiral (organ of Corti) is located on the upper surface of the membrane, bathed in endolymph.
Internal angle: Represented by the limbus (the junction zone of the vestibular and basilar membranes) and the tectorial membrane extending from its vestibular lip. It has a jelly-like consistency (formed by collagen fibers and a large volume of ground amorphous substance) and overhangs the receptor cells of the organ of Corti.
Briefly: answers to topical questions
Which cells make up the spiral organ (organ of Corti)?
The organ of Corti consists of two main types of cells: sensory hair epithelial cells and supporting epithelial cells.
Sensory hair epithelial cells (receptor apparatus):
- Inner sensory cells—arranged in a single row.
- Outer sensory cells—form 3–5 rows.
Supporting epithelial cells:
- Pillar cells—arranged in two rows, forming the inner tunnel between them.
- Phalangeal cells (inner and outer)—lie on the sides of the pillar cells, forming a bed for the sensory cell.
- Border cells (inner and outer)—located on the sides of the phalangeal cells; the outer ones are also called Hensen's cells.
Neurons of which morphological type form the spiral ganglion of the cochlea?
The spiral ganglion of the cochlea is formed by the cell bodies of bipolar sensory neurons.
Bipolar neurons have two processes—one dendrite and one axon extending from opposite poles of the cell:
- Dendrites approach the basilar membrane; some fibers run to the inner hair cells, while another portion passes through the tunnel of the organ of Corti and innervates the outer hair cells.
- Axons form the cochlear part of the vestibulocochlear nerve (n. vestibulocochlearis, cranial nerve VIII) and head toward the cochlear nuclei.
To avoid confusing the distribution of fluids across the tiers (from top to bottom: vestibular, middle, tympanic), use the P-E-P "sandwich" rule: Perilymph, Endolymph, Perilymph.
Class and exam questions
What is the histological uniqueness of the stria vascularis of the lateral wall?
The stria vascularis is formed by a stratified epithelium containing blood capillaries. Normally, epithelial tissues lack their own blood vessels, but here they are necessary to supply the process of endolymph secretion.
What is the function of the round window of the tympanic scala?
Because the perilymph inside the cochlea is a fluid and virtually incompressible, vibrations from the stapes through the oval window would be impossible in a closed space. The round window membrane bulges out, compensating for the hydraulic pressure and enabling wave motion.
Why does the length of the collagen fibers of the basilar membrane vary in different regions?
The collagen fibers of the basilar membrane function as acoustic resonators. Their length increases from the base of the cochlea to the apex, allowing different parts of the membrane to respond to sound waves of varying frequencies.
What else to study on this topic
- Histological structure of the spiral (cochlear) organ
- Types and functions of receptor hair cells
- Cellular composition of the inner ear supporting elements
- Structure and functions of the spiral nerve ganglion
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