Morphological adaptations of the eyes of vertebrates: retinal trophism and the response to environmental stimuli

The results of many investigations on the morphology of the eyes of Vertebrates are proposed, comparing our personal data with the literature available on this argument. It is firstly taken into account the retinal nourishment: it takes place according to direct and indirect mechanisms (Scheme 1). The former (Scheme 2) consist of intraretinal blood vessels and are particularly rare among the Vertebrates; the latter are more numerous and can be classified as constant (choriocapillaris) (Schemes 3A, B), if they are present in all the species up today investigated, or unconstant [Müller cells increased in their number and size (Scheme 5A), papillary cone (Schemes 5B, 8B), membrana vascularis retinae (Schemes 6A, C), falciform process (Schemes 6B, 7A, B, 8B), pecten oculi (Schemes 8A, B)], if they can be demonstrated only in some species, even if belonging to different classes. Their structural and ultrastructural organization, likewise their embryological processes, are examined in detail. The effects of the cyclic changes of light and darkness during a 24 hrs period on the eyeball are then examined; quantitative and qualitative modifications of the mitochondria (Schemes 9A, B) and of the synaptic ribbons (Schemes 10A, B) in the outer plexiform layer, of the photosensitive disks of rods and cones, and of the secretory cells and of the excretory ducts of the Meibomian tarsal glands (Schemes 11A, B, 12A, B) are described. If the animals are exposed to prolonged darkness, no structural changes can be demonstrated; nevertheless, a dark environment can induce the differentiation of peculiar structural specializations, such as the retinal or choroidal tapetum lucidum (Schemes 13, 14, 15). On the contrary, the light, if prolonged or of high intensity, is able to induce irreversible lesions on the photoreceptors and on the pigment epithelium, according to similar mechanisms in all the animals up to now investigated (Scheme 16). In conclusion, it is confirmed the peculiar structural complexity of the eyeball in all the classes of Vertebrates, due to adaptation to the various environmental requests and developed either during the evolutionary processes or during the different steps of the life of a single animal.