Ribbon synapses of the mammalian retina contain two types of synaptic bodies-ribbons and spheres

Summary

The present paper reports that the synaptic bodies of the retinal ribbon synapses in rat, guinea pig, golden hamster and mouse are a heterogeneous population of organelles. In addition to the well-known synaptic ribbonssensu stricto which consist of a platelike electron-dense central structure surrounded by electron-lucent synaptic vesicles, there are what is termed synaptic spheres, in which the core is not platelike, but round to oval. In rat retinae procured at day, ribbons outnumbered spheres by a factor of 4. At night spheres were not seen in photoreceptor cells. Spheres, like ribbons, may lie some distance from the synaptic site, perhaps indicating transit from their site of origin to the synapse. At night ribbons are longer than at daytime. In addition to the previously described connecting stalks between synaptic vesicles and the electron-dense ribbons, the presence of filamentous stalks between adjacent synaptic vesicles is described. The latter stalks, depending on their presence or absence, may influence the position of the synaptic vesicles in relation to the synaptic body and/or the presynaptic membrane. It is concluded that the plasticity of retinal synapses cannot be fully appreciated unless the temporal changes of ribbons, spheres and the connecting stalks are taken into consideration.