Changes in ribbon synapses and rough endoplasmic reticulum of rat utricular macular hair cells in weightlessness

This study combined ultrastructural and statistical methods to learn the effects of weightlessness on rat utricular maculae. A principle aim was to determine whether weightlessness chiefly affects ribbon synapses of type II cells, since the cells communicate predominantly with branches of primary vestibular afferent endings. Maculae were microdissected from Right and ground control rat inner ears collected on day 13 of a 14-day spaceflight (F13), landing day (R0) and day 14 postflight (R14) and were prepared for ultrastructural study. Ribbon synapses were counted in hair cells examined in a Zeiss 902 transmission electron microscope. Significance of synaptic mean differences was determined for all hair cells contained within 100 section series, and for a subset of complete hair cells, using SuperANOVA(TM) software. The synaptic mean for all type II hair cells of F13 flight rats increased by 100% and that for complete cells by 200%. Type I cells were less affected, with synaptic mean differences statistically insignificant in complete cells. Synapse deletion began within 8 h upon return to Earth. Additionally, hair cell laminated rough endoplasmic reticulum of flight rats was reversibly disorganized on RO. Results support the thesis that synapses in type II hair cells are uniquely affected by altered gravity. Type II hair cells may be chiefly sensors of gravitational and type I cells of translational linear accelerations.