Ultrastructure of the dorsal lateral geniculate complex in turtles of the genera Pseudemys and Chrysemys.

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 axondendritic 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.