Histological Specimen Features
In educational practice, the histology of the inner ear is most commonly studied using a classic section referred to as the "cochlea" or "spiral organ." However, this section actually encompasses a wide area of the embryonic mouse head.
Due to this broad scope, an axial section reveals not only auditory elements but also several adjacent diagnostic structures:
- Epithelial tissues: Stratified surface epithelium of the developing oral cavity and glandular epithelium (rudiments of lacrimal or salivary glands).
- Musculoskeletal system: Muscle tissue (striated skeletal musculature of the head and tongue), as well as bony trabeculae of flat skull bones undergoing direct osteogenesis. Red bone marrow is located between the trabeculae.
- Additional structures: Tooth buds, fragments of the brain, the eyeball with its lens and retina, large blood vessels, cranial nerves, and nasal passages.
Actual inner ear structures 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 known as the modiolus. Around this axis, the bony cochlear canal makes 2.5 spiral turns.
A specialized bony shelf, the spiral lamina (lamina spiralis ossea), projects from the central axis into the canal lumen. It is positioned roughly at the midpoint of the lumen and extends along the entire spiral course, almost reaching the geometric center of the canal without completely blocking it. Enclosed within this bony structure is an important cluster of neurons—the spiral ganglion.
On the upper surface of the bony lamina, the periosteum forms a sharp thickening called the spiral limbus (limbus spiralis). Its edge morphologically divides into two "lips": upper (vestibular) and lower (tympanic). The space between these lips forms a depression known as the spiral sulcus.
The Three Scalae and Hydrodynamics
Due to the attachment of the membranous cochlear duct (cochlear duct) to the vestibular lip of the limbus and other structures, the total lumen of the bony canal is strictly divided throughout its length into three isolated compartments (scalae):
- Scala vestibuli — the upper tier, filled with perilymph. It originates from the oval window (fenestra vestibuli), into which the footplate of the stapes is tightly fitted.
- Cochlear duct (scala media) — the middle tier, filled with endolymph.
- Scala tympani — the lower tier, also containing perilymph.
Fluid Motion Physiology: Sound waves are transmitted via the stapes footplate to the oval window. Because fluid is incompressible, these vibrations initiate movement of the perilymph in the scala vestibuli. The wave travels up to the apex of the cochlea (to an area called the helicotrema), where it passes into the scala tympani. The fluid then descends toward the base of the cochlea. Pressure is compensated by the bulging of the secondary tympanic membrane, which seals the round window at the end of the scala tympani.
Walls of the Cochlear Duct (Scala media)
In cross section, the middle compartment (cochlear duct) has a distinct triangular shape, formed by three walls and one internal angle.
Upper-medial wall (Vestibular or Reissner's membrane): Stretched from the limbus to the outer wall. It consists of three extremely thin 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 and endolymph).
Outer wall: Fused tightly with the periosteum of the bony canal. It includes the spiral ligament (a periosteal thickening of dense fibrous tissue) and the stria vascularis. The stria vascularis faces the endolymph and consists of a stratified epithelium containing intraepithelial blood capillaries—a unique histological phenomenon. Its primary function is the continuous production of endolymph.
Lower wall (Basilar membrane): Stretched between the spiral lamina and the spiral ligament. On the scala tympani side, it is covered by endothelium. Its core consists of collagen fibers arranged obliquely (rather than strictly perpendicularly) within a specialized matrix. These fibers act as "strings" that resonate at specific vibration frequencies. Their length increases from the cochlear base to its apex. On the upper surface of the membrane, bathed in endolymph, sits the spiral (cochlear) organ.
Internal angle: Formed by the limbus (the junction of the vestibular and basilar membranes) and the tectorial membrane extending from its vestibular lip. It has a gelatinous consistency (composed of collagen fibers and a large volume of amorphous ground substance) and overhanging the receptor cells of the spiral organ.