The endolymphatic sac, a potential endocrine gland?

“Reversed polarization” of Na/K-ATPase—a sign of inverted transport in the human endolymphatic sac: a super-resolution structured illumination microscopy (SR-SIM) study

The human endolymphatic sac (ES) is believed to regulate inner ear fluid homeostasis and to be associated with Meniere’s disease (MD). We analyzed the ion transport protein sodium/potassium-ATPase (Na/K-ATPase) and its isoforms in the human ES using super-resolution structured illumination microscopy (SR-SIM). Human vestibular aqueducts were collected during trans-labyrinthine vestibular schwannoma surgery after obtaining ethical permission. Antibodies against various isoforms of Na/K-ATPase and additional solute-transporting proteins, believed to be essential for ion and fluid transport, were used for immunohistochemistry. A population of epithelial cells of the human ES strongly expressed Na/K-ATPase α1, β1, and β3 subunit isoforms in either the lateral/basolateral or apical plasma membrane domains. The β1 isoform was expressed in the lateral/basolateral plasma membranes in mostly large cylindrical cells, while β3 and α1 both were expressed with “reversed polarity” in the apical cell membrane in lower epithelial cells. The heterogeneous expression of Na/K-ATPase subunits substantiates earlier notions that the ES is a dynamic structure where epithelial cells show inverted epithelial transport. Dual absorption and secretion processes may regulate and maintain inner ear fluid homeostasis. These findings may shed new light on the etiology of endolymphatic hydrops and MD.

Lamellar projections in the endolymphatic sac act as a relief valve to regulate inner ear pressure

The inner ear is a fluid-filled closed-epithelial structure whose function requires maintenance of an internal hydrostatic pressure and fluid composition. The endolymphatic sac (ES) is a dead-end epithelial tube connected to the inner ear whose function is unclear. ES defects can cause distended ear tissue, a pathology often seen in hearing and balance disorders. Using live imaging of zebrafish larvae, we reveal that the ES undergoes cycles of slow pressure-driven inflation followed by rapid deflation. Absence of these cycles in lmx1bb mutants leads to distended ear tissue. Using serial-section electron microscopy and adaptive optics lattice light-sheet microscopy, we find a pressure relief valve in the ES comprised of partially separated apical junctions and dynamic overlapping basal lamellae that separate under pressure to release fluid. We propose that this lmx1-dependent pressure relief valve is required to maintain fluid homeostasis in the inner ear and other fluid-filled cavities.

The most internal part of the human ear, the inner ear, is essential for us to hear and have a sense of balance. It is formed by a complex series of connected cavities filled by a liquid. When sound waves and changes in the position of the body make this liquid move, specialized ‘hair’ cells can detect these subtle movements; neurons then relay this information to the brain where it is decoded and interpreted.

For the inner ear to work properly, the body needs to finely regulate the pressure created by the liquid inside the cavities. For example, people with unstable pressure in their ears can experience deafness or problems with balance. A structure known as the endolymphatic sac, which is a balloon-like chamber connected to the rest of the inner ear by a thin tube, helps with this regulation. However, scientists are still unsure about how exactly the sac performs its role. One problem is that the inner ear is difficult to study because it is encased in one of the densest bones in the body.

Many other animals also have inner ears, from fish to birds and mammals. Here, Swinburne et al. examine the inner ear of zebrafish embryos because, in this fish, the ear starts working before the bones around it form; the structure is therefore accessible for injections and microscopy. Experiments show that when the pressure in the inner ear rises, the endolymphatic sac slowly fills up with the ear liquid, and then it rapidly deflates. Fish with mutations that stop the sac from deflating have overinflated sacs, which is a symptom also found in certain patients with hearing and balance disorders.

Looking into the details of these inflation-deflation cycles, Swinburne et al. found that the cells that form the sac have gaps between them, unlike a normal sheet of cells. A flap covers these gaps to keep the liquid in, but under pressure, the flap opens and the liquid can escape. These results show that the endolymphatic sac works as a pressure relief valve for the inner ear.

Ultimately, understanding how pressure is regulated in the ear could help patients with inner ear disorders. It could also serve as a template to investigate how eyes, kidneys and the brain, which all have liquid-filled cavities, control their internal pressure.

Experimental hyperactivity of the endolymphatic sac

Injury to the endolymphatic sac may play an important role in the pathogenesis of Ménière's disease, an inner ear disorder characterized by hearing loss, tinnitus and attacks of vertigo. Isoimmunization of 16 inbred Lewis rats with a crude endolymphatic sac extract and complete Freund's adjuvant induced hyperactivity of the endolymphatic sac. One group of rats was immunized by a single dose whereas a second group was immunized twice. Control animals were injected with Freund's adjuvant in saline only. Serum was collected from all rats by the end of the study and harvested autoantibodies were tested by immunohistochemistry. The endolymphatic sacs were investigated by transmission electron microscopy. Endolymphatic sac stimulation was observed in all immunized rats. Based on detailed ultrastructural observations, the degree of reactivity seemed proportional to the number of injections and the extent of immunization. Moreover, the ribosome-rich cells seemed hyperactive with an extravagant content of intracellular components: numerous rough endoplasmic reticulum and free ribosomes, morphological signs of extensive endo- and exocytosis, vesicles of material with a density similar to the homogeneous substance of which many were observed to fuse with primary lysozymes. Basolateral foldings were numerous and in the subepithelial capillaries formation of multiple and apposing fenestrations were observed. No endolymphatic sac stimulation was observed in the control animals. Specific ribosome-rich cell alterations identical to those present in the endolymphatic sac of Ménière's disease were observed 21 days after the first immunization. The observations suggest that either an autoantigen or a trophic factor, capable of inducing a hyperactivity of the ribosome-rich cells and an imbalance of the homogeneous substance metabolism, exists in the endolymphatic sac of the rat.

The H723R mutation in the PDS/SLC26A4 gene is associated with typical Pendred syndrome in Korean patients

Inherited as an autosomal recessive trait, Pendred syndrome is a disease that shows congenital sensorineural hearing loss and goiter, with a positive finding in the perchlorate discharge test. Pendred syndrome results from various mutations in the PDS/SLC26A4 gene that cause production of an abnormal pendrin protein. More than 90 mutations in the PDS/SLC26A4 gene have been reported throughout the world. A recent study of 26 Korean patients with a relatively high frequency (65%) of a mutated PDS/SLC26A4 gene exhibited nonsyndromic deafness and an enlarged vestibular aqueduct. We report two patients with characteristics of typical Pendred syndrome, a 26-yr-old female and a 61-yr-old male, who were both homozygous for a previously reported missense mutation, H723R (Histidine 723Arginine) in the PDS/SLC26A4 gene.

Morphology of the endolymphatic sac in the guinea pig after an acute endolymphatic hydrops

The role of the endolymphatic sac (ES) in endolymph volume homeostasis is speculative. The present study investigates changes of the ES's epithelia and luminal filling after induction of an acute endolymphatic hydrops. After microinjection of 1.1 mul artificial endolymph into scala media of the cochlea, guinea pigs were terminated immediately (n = 6) or after different time intervals ; 1/2 h (n = 3), 1 h (n = 4) and 2 h (n = 4). Inner ear specimens were processed for light and/or transmission electron microscopy. The non-injected contralateral ear served as a histological control. Correct injection was confirmed by detection of microspheres in the endolymphatic compartment after the same microinjection procedure. In all specimens, ribosome rich cells and intraluminal macrophages appeared to be actively involved in degradation of homogeneous substance (HS) by secreting lytic enzymes and digestion, respectively. Amazingly, in our study no ES differences were found between injected and non-injected ears and no distinct changes were observed in guinea pigs terminated after different time intervals. The ES's luminal HS was always present and often to a large extent. This is in contrast with [Hear. Res. 138, 81] dramatic changes were observed. Endolymph volume homeostasis is a complex mechanism, in which the role of HS remains obscure.

Le sac endolymphatique : ses fonctions au sein de l’oreille interne

The endolymphatic sac is a non-sensory organ of the inner ear. It is connected to the endolymphatic compartment that is filled with endolymph, a potassium-rich fluid that bathes the apical side of inner ear sensory cells. The main functions ascribed to the endolymphatic sac are the regulation of the volume and pressure of endolymph, the immune response of the inner ear, and the elimination of endolymphatic waste products by phagocytosis. Functional alteration of these functions, leading to deficient endolymph homeostasis and/or altered inner ear immune response, may participate to the pathophysiology of Ménière's disease, an inner ear pathology that causes episodes of vertigo, sensorineural hearing loss and tinnitus, and is characterized by an increase in volume of the cochleo-vestibular endolymph (endolymphatic hydrops).

Changes in ultrastructural characteristics of endolymphatic sac ribosome-rich cells of the rat during development

It has recently been demonstrated that endolymphatic sac (ES) ribosome-rich (dark) cells respond to induced endolymph changes and are thus likely to be involved in endolymph homeostasis. Therefore, we studied the ultrastructural characteristics of rat ES ribosome-rich cells during development in order to determine the cellular distribution of organelles involved in protein metabolism, secretion and absorption, indicative for their contribution to endolymph homeostasis. During embryonal stages ribosome-rich cells contain a limited number and variety of organelles and are predominantly involved in the production of components for cell growth and differentiation. In the young adult stage (P60) three different states of ribosome-rich cells may be distinguished. State A resembles a cell with only limited metabolic activities whereas state B is characterized by numerous different intracellular organelles and is considered to be involved in production and secretion as well as absorption and degradation of complex proteins. A third cellular state, state C, is filled with phagolysosomes and contains very few other organelles. This is considered to be a final (pre)apoptotic state. Autoradiography data suggest that ES ribosome-rich cells are capable of synthesis and secretion of tyrosine-containing proteins and may thus be involved in regulation of the osmolarity of endolymph based on the capacity to bind cations as well as water molecules. In addition, ES ribosome-rich cells appear to synthesize and secrete fucosylated glycoproteins into the endolymph. In conclusion, the present data suggest that ES ribosome-rich cells are actively involved in endolymph homeostasis through secretion and absorption of complex proteins and it is hypothesized that they are able to adapt their function or activities in response to changes in endolymph composition.

Molecular analysis of the Pendred's syndrome gene and magnetic resonance imaging studies of the inner ear are essential for the diagnosis of true Pendred's syndrome

Pendred's syndrome is a combination of congenital sensorineural hearing loss and iodine organification defect leading to a positive perchlorate test and goiter. Although it is the commonest form of syndromic hearing loss, the variable clinical presentation contributes to the difficulty in securing a diagnosis. The identification of the disease gene (PDS) prompts the need to reevaluate the syndrome to identify possible clues for the diagnosis. To this purpose, in three Italian families presenting with the clinical features of Pendred's syndrome, the molecular analysis was accompanied by full clinical, biochemical, and radiological examination. A correlation between genotype and phenotype was found in the only patient with enlargement of vestibular aqueduct and endolymphatic duct and sac at magnetic resonance imaging. This subject was a compound heterozygote for a deletion in PDS exon 10 (1197delT, FS400) and a novel insertion in exon 19 (2182-2183insG, Y728X). The present study demonstrates for the first time the value of the combination of clinical/radiological and genetic studies in the diagnosis of Pendred's syndrome. The positivity of a perchlorate discharge test and the malformations of membranous labyrinth fit well with the recent achievements on the role of pendrin in thyroid hormonogenesis and the maintenance of endolymph homeostasis.