Synaptic ribbons of the outer plexiform layer in the retina of albino rats are examined, with particular regard to their ultrastructure and to their chronobiological behaviour. As to the ultrastructure, aside from the typical lamellar and granular morphology, synaptic ribbons, whose central bar is roundish and granular, delaminate or furnished with enlarged extremities, are shown. As to the chronobiological pattern, synaptic ribbons undergo to circadian changes in their absolute number, being particularly numerous during the light phase, and few during darkness. It is also considered the morphology (ratio between granular and lamellar) and the size (long, intermediate and short) during the 24-hour cycle; in both cases, diphasic curves are observed. On the basis of these results, in the junction photoreceptors-bipolar cells, synaptic ribbons are polymorphic, dynamic structures, whose number and morphology strictly depend on environmental light conditions.
Multinucleated cells in the retinal pigment epithelium of the albino rat are shown with the scanning electron microscope beside normal mononucleated cells. We suggest that such elements are formed during foetal life, owing to the absence of any mitotic activity in adult rats.
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