(a–h) Electron micrographs showing cuticulosomes in the cuticular plate (cp) apically located beneath the stereocilia (sc) in hair cells from the basilar papilla of 1 day (a and f), 8 day (b and g), 16 day (c and h), 30 day (d and i) and 1 year old pigeons (e and j). At hatching cuticulosomes are amorphous aggregations of ferritin nanoparticles, that increase in size, density and organisation as they mature. In some cases, cuticulosomes are organized in paracrystalline arrays (e and inset), surrounded by membranes (c, h and insets) and decorated with vesicles (vs) (a,b, c, and insets). (i) Graph showing the mean diameter of cuticulosomes from the basilar papilla of 1 day (296.4 ± 69.2 nm, n = 3 birds, n = 4 cuticulosomes), 8 day (387.9 ± 29.0 nm, n = 3 birds, n = 26 cuticulosomes), 16 day (580.7 ± 21.7 nm, n = 3 birds, n = 45 cuticulosomes) and 30 day old birds (514.3 ± 38 nm, n = 3 birds, n = 17 cuticulosomes). Dots show measurements for individual cuticulosomes. Error bars show the mean ± SEM. Scale bars represent 0.25 μm in a and f, 1 μm in b, c, d,e, g, h, i and j, 200 nm in insets of b, c, d, e, g, h, i and j and 100 nm in insets of a and f.