![]() The tissue was fixed in 4% paraformaldehyde diluted with 0.1 M phosphate buffered saline (PBS) at pH 7.4 in the operating room. The endolymphatic duct ED and the ES overlying the sigmoid sinus (SS) were not analyzed (Fig. The ES was drilled out with a thin rim of surrounding bone to preserve the epithelium. After obtaining ethical permission, human ESs were collected during trans-labyrinthine vestibular schwannoma removal. The technique used to collect the ES was recently described by the authors (Kämpfe Nordström et al. An SR-SIM study of Na/K-ATPase activity in the human cochlea was recently presented (Liu et al. ![]() The goal was to expand our knowledge of endolymph resorption in the human ES and its role in MD. The technique allows immune histochemical investigation of proteins with high resolution beyond the diffraction (Hell and Wichmann 1994 Gustafsson et al. We used super-resolution structured illumination microscopy (SR-SIM) to analyze the Na/K-ATPase protein and its isoforms in the human ES. Recently, several reports on ion transporters in the human ES were presented using cDNA micro-array, reverse transcription polymerase chain reaction (RT-PCR), and immunohistochemistry (Kumagami et al. Studies of the human ES are motivated because of species differences. Sodium absorption may be driven by the Na/K-ATPase pump in combination with selective ion channels and a positive ES potential (ESP) in the lumen (Amano et al. Na/K-ATPase is localized in the basolateral cell membrane of the MCRs and is thought to expel sodium ions into the extracellular space along with fluid. MRCs express SLC26A4, Na +-permeable channels, and Na/K-ATPase (Miyashita et al. The MRCs are believed to absorb sodium and fluid from the ES lumen, while the RRCs may be involved in secretion and immune activity (Erwall et al. 1964) and in man by the cylindrical cells (Friberg et al. In the guinea pig, these cells are represented by the “light” cells (Lundquist et al. 1991 Dahlmann and von Düring 1995 Qvortrup et al. Human ES contains a heterogeneous population of epithelial cells, while the rodent ES contains two distinguishable cell types, namely, the mitochondria-rich cells (MRCs) and ribosome-rich cells (RRCs) (Friberg et al. A lack of pendrin reduces endolymph resorption and leads to congenital enlargement of the ES and vestibular aqueduct (EVA) which is associated with childhood deafness (Park 2003). Pendrin, an anion exchanger encoded by the SLC26A4 gene, may play a crucial role in the development of the inner ear fluid spaces (Honda et al. The ES is linked to Meniere’s disease (MD), as interruption from the rest of the inner ear leads to endolymphatic hydrops, an equivalent of MD (Kimura and Schuknecht 1965 Kimura 1967). 2014), suggesting a role in the maturation of the inner ear fluid compartments. This intriguing part of the inner ear is believed to be involved in several activities, such as endolymph resorption, pressure regulation, and immune reactions (Lundquist et al. The function of the human endolymphatic sac (ES) remains an enigma. These findings may shed new light on the etiology of endolymphatic hydrops and MD. Dual absorption and secretion processes may regulate and maintain inner ear fluid homeostasis. 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. 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. 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. 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. Human vestibular aqueducts were collected during trans-labyrinthine vestibular schwannoma surgery after obtaining ethical permission. 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). The human endolymphatic sac (ES) is believed to regulate inner ear fluid homeostasis and to be associated with Meniere’s disease (MD).
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