Differences in Endolymphatic Sac Mitochondria‐Rich Cells Indicate Specific Functions

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Abstract
Objective/Hypothesis The purpose of the study was to examine the specific involvement of endolymphatic sac mitochondria‐rich cells in endolymph homeostasis.Study Design Transmission electron microscopy and immunohistochemistry were performed on the endolymphatic sac of young adult rats, and two important developmental stages were also investigated.Methods Ultrastructural characteristics of endolymphatic sac mitochondria‐rich cells were studied more concisely and compared with renal mitochondria‐rich cells (i.e., the intercalated cells). In addition, expression of cytokeratins 7 and 19 was determined.Results Until birth, only one type of mitochondria‐rich cell is observed in the rat endolymphatic sac. In young adult animals, distinct differences in mitochondria‐rich cell ultrastructure in the endolymphatic sac enables classification into subtypes or configurations. Comparison of endolymphatic sac mitochondria‐rich cells with renal intercalated cells reveals striking similarities and provides additional information on their specific function in endolymph homeostasis. Furthermore, differences in cytokeratin expression are determined in endolymphatic sac mitochondria‐rich cells.Conclusions Differences in morphology of endolymphatic sac mitochondria‐rich cells develop after birth and may reflect a distinct functional or physiological state of the cell. In analogy to renal intercalated cells, the distribution patterns of H+‐adenosine triphosphatase and Cl−/HCO3− exchanger may differ between subtypes. We propose that subtype A mitochondria‐rich cells, from which protruding A mitochondria‐rich cells are the activated state, are involved in proton secretion (apical H+‐adenosine triphosphatase) and thus are potential candidates for hearing loss accompanying renal tubular acidosis. Subtype B mitochondria‐rich cells are the most likely candidates to be affected in Pendred syndrome because of the assumed function of pendrin as apical Cl−/HCO3− exchanger.