A light and electron microscope study of the basal optic system in the reptile Vipera aspis.

The basal optic system of the reptile Vipera aspis was investigated under both light and electron microscopy. The basal optic root (BOR), mainly composed of large diameter myelinated fibers (congruent to 2 micrometers) terminates in he nucleus of the basal optic root (nBOR). The different histological techniques employed disclose a neuronal population distributed in the following three categories: 1) small neurons, of low proportion (21%), have cytological features which allow their classification as Golgi type II neurons, 2) medium-sized neurons (35%) and 3) large multipolar neurons (44%). Both of the latter types of neurons are contacted by optic terminals. Neuroglial elements, astrocytes, oligodendrocytes, and microgliacytes were observed either in an intermediate fascicular or perineuronal satellite position. The neuropil of nBOR is made up of both axonal and dendritic profiles. The dendrites which contain synaptic vesicles (DCSVs) are not numerous (15% of the total population of profiles containing synaptic vesicles, PCSVs) and have the peculiarity of participating in serial and triadic arrangements within glomerulus-like structures where they are postsynaptic to optic terminals. Six types of axon terminals have been identified. Those containing a more or less spheroidal synaptic vesicle population are subdivided in two classes: S 1, the optic terminals being the most numerous (48,5% of the total PCSVs) and S 2 (19% of the total PCSVs). On the basis of density of flattened synaptic vesicles, F type terminals are classified either as F 1 (9%) or F 2 (8%), the first being more densely packed with synaptic vesicles. A very low proportion of terminals (0,5%) comprises a mixed population of large granular vesicles (G) associated with rounded or ellipsoidal synaptic vesicles. Occasionally, gap junctions are observed between an axon terminal and a dendritic profile. The present results show a similarity of synaptic circuitry when compared to bird and mammal nBOR.