a–e FIB-SEM datasets of basolateral membrane contacts between IHCs. a P15 IHCs are tightly wrapped around by SCs and are separated by a distance ranging from 100 to 648 nm (first section below the nucleus). c P34 IHCs share prominent flat membrane contacts (blue–red contacts). For each FIB-run we numbered the IHCs, whereby IHC 2 is the full IHC in the centre of the region of interest (highlighted in blue), making contacts to the neighboring IHCs (shown in red). b–e Representative micrographs of IHC–IHC contacts. P15 IHCs frequently show filopodia reaching neighboring IHCs (b). d At P34, some flat membrane contacts appear to include fusion sites with membrane continuity and cytoplasmic bridge (white arrowheads). e Depicted are the observed putative IHC fusion sites in the P16 and the P34/37 animals. n FIB-SEM run (P34/37) = 3, Nanimals = 2; n FIB-SEM run (P15/16) = 3, Nanimals = 2; 2 independent embeddings for each age group. FIB-SEM focused ion beam-scanning electron microscopy, IHC inner hair cell, SC supporting cells. Scale bar: a, c = 5 µm; b, d = 500 nm; e = 200 nm.