The fine structures of the suprachiasmatic nucleus of the golden hamster.

The fine structure of the suprachiasmatic nucleus of the golden hamster was studied with special reference to the synaptic endings. The somata of SCN neurons contained well developed cytoplasmic organelles including the Golgi complex, mitochondria and polysomes. The nuclei had deeply invaginated nuclear membrane. Some neurons were characterized by the presence of a large number of granulated vesicles and an abundance of cytoplasmic organelles. Several kinds of synapses and gap junctions were also observed. Axo-somatic synapses were identified and could be differentiated based upon possessing two types of presynaptic elements. The first contained clear round synaptic vesicles (40-60 nm in diameter), the second contained clear round vesicles (40-60 nm in diameter) and dense cored vesicles (70-120 nm in diameter). Asymmetrical synaptic membrane thickening were observed on both the pre- and postsynaptic sides of most of the axosomatic synapses. Axo-dendritic synapses could also be divided into two sub-types according to their membrane specializations. In Type 1, the axon terminals contained both clear round vesicles and dense cored vesicles and formed asymmetrical synapses. Terminals in the second group (type 2) were characterized by symmetrical synapses that contained clear, round vesicles as well as dense cored vesicles. The type 1 terminals were evenly distributed throughout the SCN, but the type 2 terminals were encountered more frequently in the ventral SCN. These observations indicate that the ventral and dorsal components of the SCN may possess different functional roles.