Fine structure of the afferent synapses in the paratympanic organ of the chicken, with special reference to the synaptic bodies.

The afferent synapses of the paratympanic organ in the chicken were studied by TEM. These synapses were formed by non-myelinated fibres which reached the basal part of the hair cells. The fibres contained a number of irregular mitochondria and a few pale vesicles. In the hair cells, near the presynaptic membrane, typical synaptic bodies formed by an electron-dense core surrounded by several small pale vesicles were present. The core was connected with the vesicles by numerous thin filaments, and at same time with the presynaptic membrane by some dense projections. Moreover, we have observed that the connections between the core and the adjacent vesicles also consisted of similar structures to the dense projections. We suggest that this device is involved in the movement of the vesicles towards the presynaptic membrane. Our hypothesis is in agreement with that formulated by some authors who believe that the electron-dense core of the synaptic bodies is able to channel the vesicles to the presynaptic membrane (conveyor-belt hypothesis). Moreover, our work showed that the synaptic bodies of the paratympanic organ in the chicken are variable in density and in shape. These morphological aspects might be linked to regression-reconstitution cycles of the SBs and to the functional level of the afferent synapses.