Ultrastructure of the Dorsal Lateral Geniculate Complex in Turtles of the Genera <i>Pseudemys </i>and <i>Chrysemys</i> (Part 1 of 2)

Abstract

The ultrastructure of the dorsal lateral geniculate complex in the turtles Pseudemys scripta elegans and Chrysemys picta belli has been studied. The majority of neurons have somata situated in a cell plate that forms the medial face of the complex. Relatively few synaptic contacts occur on the somata of cell plate neurons, but three types of axon terminals contact their proximal dendrites as they course through the cell plate. The most frequent are terminals with clear, round synaptic vesicles that form asymmetric junctional complexes. A second type has both clear and round, and dense-cored vesicles and forms asymmetric junctional complexes. The third type has clear, pleomorphic vesicles and forms symmetric junctional complexes. The same three types of terminals are presynaptic to the dendrites of cell plate neurons as they proceed through the inner half of the geniculate neuropile that is interposed between the cell plate and optic tract. These dendrites branch into varicose, finger-like distal dendrites within the outer half of the neuropile. The finger-like processes contain clear, pleomorphic vesicles and are linked with each other via dendrodendritic synapses with symmetric junctional complexes. The distal dendrites are postsynaptic to axon terminals with clear, round synaptic vesicles and asymmetric junctional complexes.Cell plate neurons thus comprise three regions, each with a distinct synaptic organization. The first region is formed by the somata of cell plate neurons and receives few synaptic contacts. The second region is formed by the proximal dendrites both in the cell plate and the inner half of the neuropile. It receives a moderate contingent of axodendritic terminals. The last region is formed by the distal dendrites of the cell plate neurons. They participate in dendrodendritic synapses and are postsynaptic to axon terminals with clear, round synaptic vesicles.

Fine structure of the dorsal latreral geniculate nucleus of the turtle, Emys orbicularis: A Golgi, combined hrp tracing and GABA immunocytochemical study

“…In contrast to previous theories concerning the reptilian GLD (Rainey and Ulinski, 1986;Ulinski, 1986aUlinski, , 1987Stritzel, 1988, 1990), our findings in Emys orbicularis show that the organization of this nucleus shares many points in common with its mammalian counterpart. In both vertebrate groups, the GLD contains GABA-immunoreactive interneurons and a population of GABA-positive axon terminals, a considerable proportion of which are of extrinsic origin.…”

“…While Ulinski (1986a) describes dendrites containing synaptic vesicles in the GLDn, postsynaptic to visual fibers, his interpretation of the origin of these dendrites differs considerably from ours. Starting with the hypothesis that the GLD is devoid of interneurons (Rainey and Ulinski, 1986), he concludes that these dendrites belong to efferent neurons situated in the cell plate (type 1 neurons in our terminology).…”

“…It has been further shown that this projection involves primarily the GLDn and, to a considerably lesser extent, the GLDc (Hall et al, 1977;Sjostrom and Ulinski, 1985;Ulinski and Nautiyal, 1988;Hergueta et al, 1992af. Our observations under the electron microscope confirm these findings. The previous ultrastructural investigations of the chelonian GLD are those of Manohar (1973), Ulinski (1986a) and Ulinski and Nautiyal (1988), carried out in Pseudemys scripta and Chrysemys pictu helli. These authors describe two types of axon terminals containing clear round synaptic vesicles which establish asymmetrical synaptic contacts; some of these are of retinal origin since they degenerate after retinal ablation.…”

“…concur as to the topographical distribution of projections to this nucleus, the retinal terminals and the nature of their postsynaptic elements have not been clearly identified by electron microscopy (Mano har, 1973;Ulinski, 1986a;Ulinski and Nautiyal, 1988). Contradictory results also have been presented concerning the localization and nature of the efferent corticopetal neurons.…”

Retinal and cortical afferents to the dorsal lateral geniculate nucleus of the turtle,Emys orbicularis: A combined axonal tracing, glutamate, and GABA immunocytochemical electron microscopic study

“…The first data about the ultrastructural organization of the turtle GLd have been obtained in normal (Manohar 1973;Ulinski, 1986a) and enucleated (Ulinski and Nautiyal, 1988) animals. In a previous study (Kenigfest et al, 1995), we have described the cytologic and synaptologic characteristics of projection cells, GABAergic interneurons, and retinal terminals.…”