Ultrastructural changes in synapses in the dentate gyrus of the rat during development

Abstract

The structural transformations in synapses and dendrites in the molecular layer of the dentate gyrus of the rat are described. The appearance of the molecular layer of an adult is distinguished by the presence of numerous complex spines which in thin sections form a ‘U’- or ‘W’-shaped contact with a presynaptic terminal or bouton en passage. The great majority of synaptic connections in the adult are made with spines, and most synaptic contacts on spines are of an asymmetric type. In the molecular layer of the 4-day-old rat complex spines are rare and synapses sparse. Synaptic contacts exist on large or small dendritic shafts and are often of teh symmetric type. Complex spines appear to develop at locations closest to the granule cell body and gradually come to occupy the entire dendritic field. At 25 days complex spines become prominent, and by adulthood complex spines and the spine apparatus are more abundant than at earlier stages examined. Thus, in contrast to synaptogenesis, whicb is essentially complete by 25 days, complex spine formation is more prolonged.