Freeze Fracturing of the Human Stria Vascularis

Abstract

The stria vascularis is an important functional element in the mammalian cochlea. This special tissue is considered to be the source of the endocochlear potential and thus the driving force for the production of a receptor response to the auditory stimulus. In order to maintain its function, the stria vascularis needs to be separated from the endolymphatic space by a tight seal. This seal is comprised of tight junctions in the marginal cell layer. The junctional arrangement in the stria vascularis is described, utilizing the freeze-fracturing technique which allows the visualization of large expansions of plasma membrane. The marginal cells are generally separated by tight junctions of the moderately tight to tight type. In places, however, even so-called leaky junctions with only a few sealing strands are present. Whereas the intermediate cell layer seems to lack tight junctions, the basal cells are connected by extensive tight junctions more or less covering the entire cell. These junctions seem to form an extremely tight barrier against the spiral ligament. Gap junctions are also present in the tissue. Intermediate cells as well as the basal cells are coupled by gap junctions. In the basal cell layer, gap junctional elements may also be found inside the large tight junctions comprising so-called mixed junctions.

Relationship of Sertoli-Sertoli Tight Junctions to Ectoplasmic Specialization in Conventional and En Face Views

“…It is difficult to understand the necessity for the extensive cytoskeletal actin complex in this region. Other regions where tight junctions are present do not have highly organized actin like that seen at Sertoli-Sertoli junctions, yet they do have actin that is apparently important for tight junction integrity [30]. It has been proposed that the ectoplasmic specialization is important for stiffening this region of the Sertoli-Sertoli interface such that the interface could be maintained in relationship to basal compartment germ cells that would be moving at a specific time through such junctions to the adluminal compartment of the testis [1].…”

Ion flow in stria vascularis and the production and regulation of cochlear endolymph and the endolymphatic potential

“…In 'viable dominant spotting mutant' mice, ICs fail to migrate into the stria, and the EP is around 0 mV (see Cable et al, 1994), indicating that they are essential for normal strial function. They are also linked to each other by gap junctions (Bagger-Sjoback et al, 1987;Kikuchi et al, 1995Kikuchi et al, , 2000, which explains why no sharp boundaries between ICs can be distinguished with dye marking (Salt et al,1987). They are also less densely packed with organelles (including mitochondria) than MCs, suggesting less metabolic activity.…”

“…Oxygen and nutrients are clearly supplied by the rich capillary network, predominantly within the IC layer or between the ICs and MCs, with freeze fracture suggesting that the capillaries are surrounded by endothelial cells connected to each other by leaky junctions (Bagger-Sjoback et al, 1987;Kimura and Schuknecht, 1970;Takeuchi and Ando, 1998a,b). This leakiness seems inconsistent with the high þ110 mV potential in the intrastrial space, which would seem to require a capillary network electrically insulated from the intrastrial space (clearly it is permeable to O 2 and CO 2 , and to various pharmaceuticals, including the loop diuretics).…”

“…A basement membrane temporarily separates the MCs from the underlying ICs (Kikuchi and Hilding,1966), but as they mature and the membrane partly degenerates, they change shape and grow basal processes rich in mitochondria (Bagger-Sjoback et al, 1987;Duvall et al, 1981) that supply energy (ATP) to their basolateral NaKATPase pumps (Ikeda and Morizono, 1989a,b;Mees, 1983;Kerr et al, 1982), and it has been suggested that they form contacts through gap junctions (Kumar and Gilula, 1996), at least early in development (Bagger-Sjoback et al, 1987;Hama and Saito, 1977;Lautermann et al, 1998). The residual basement membrane is also sometimes found encapsulating capillaries, and the lateral surfaces of adjacent MCs also interdigitate, and are connected by tight junctions mainly near their apex (Bagge-Sjoback et al, 1987;Kimura and Schuknecht, 1970).…”

“…The single layer of MCs (Bagger-Sjoback et al, 1987) is adjacent to the K þ rich endolymph, and at birth the MCs are cuboidal in shape, with numerous microvilli and a single kinocilium on their luminal surface (Kikuchi and Hilding, 1966;Kimura and Schuknecht, 1970). A basement membrane temporarily separates the MCs from the underlying ICs (Kikuchi and Hilding,1966), but as they mature and the membrane partly degenerates, they change shape and grow basal processes rich in mitochondria (Bagger-Sjoback et al, 1987;Duvall et al, 1981) that supply energy (ATP) to their basolateral NaKATPase pumps (Ikeda and Morizono, 1989a,b;Mees, 1983;Kerr et al, 1982), and it has been suggested that they form contacts through gap junctions (Kumar and Gilula, 1996), at least early in development (Bagger-Sjoback et al, 1987;Hama and Saito, 1977;Lautermann et al, 1998).…”