Ca2+ transients in cochlear interdental cells during maturation of the tectorial membrane

The formation of the tectorial membrane (TM) starts around embryonic day 16 in mice. Interdental cells (IDCs) of the spiral limbus secrete different types of TM proteins into the developing cochlear duct. Proper morphology of the TM including slow turnover of its proteins is essential for normal hearing, which in mice starts at postnatal day 12 (P12). Morphology of the IDCs and the neonatal spiral limbus were studied in acutely explanted cochlear whole mount preparations by confocal live cell imaging using the membrane markers FM4 – 64 and CellMask Deep Red. Spontaneous and ATP/UTP-evoked Ca2+ signals were recorded in IDCs from P1 to P18 using Fluo-8 AM. Relative fluorescence changes in IDCs were analysed with FIJI and key parameters of the Ca2+ transients were extracted using a custom-made routine. Cochlear cryosections were labelled with fluorescent antibodies and studied with a confocal laser-scanning microscope. IDCs showed a variable and complex anatomy. Towards the lateral margin of the spiral limbus they were perfectly aligned in a column-like fashion divided by extracellular matrix. Upon maturation of the cochlea, these interdigitations became more pronounced. Part of the IDCs formed duct-like structures filled with TM-like substance. We also observed large intracellular vesicles moving within the IDCs. Altogether this suggests secretory activity in IDCs. IDCs displayed spontaneous Ca2+ transients at low frequency and ATP/UTP-triggered Ca2+ oscillations. Both signal types changed their characteristics between P1 and P18 suggesting that they were involved in triggering age-dependent exocytosis of vesicles containing TM material. Further physiological experiments are needed to identify exocytotic processes in IDCs of the developing cochlea.

Publication History

Publication Date:
23 April 2019 (online)