Identification of the porosome complex in the hair cell

Porosomes are proposed to be the universal secretory machinery of the cell plasma membrane, where membrane-bound secretory vesicles transiently dock and fuse to expel their contents to the extracellular space during cell secretion. In neurons, porosomes are manifested as cup-shaped lipoprotein structures in the presynaptic membrane, 12–17 nm in diameter and possessing a central plug. Hair cells of hearing and balance secrete transmitter from synaptic vesicles in sensory signal transduction, but it has not previously been demonstrated that these mechanosensory cells possess porosome structures that could participate in the secretory process. In the present study, we provide evidence obtained using transmission electron microscopy that porosome structures exist in the hair cell, suggesting a mechanism of hair-cell transmitter secretion markedly different from that of the classic view of the exocytotic process.