Development of synaptic structure and function in organotypic cultures of the rat hippocampus

The sequence of development of synapses, as well as the ultrastructure of axonal growth cones, has been investigated electron microscopically in tissue cultures of the newborn rat hippocampus. During differentiation of the tissue cultures, the formation of synapses is preceded by identifiable growth cones. A characteristic feature of axonal growth cones is the presence of numerous large clear vesicles which vary in diameter from ~100 to 150 nm. The first immature synapses were formed on the 5th, 6th or 7th day in vitro on the growth cones of differentiating neuronal processes. Axonal growth cones are occasionally found to be presynaptic to a dendrite. At first axo-dendritic synapses, most of them being en passant, arise, whereas axo-somatic and axo-spinous-dendritic synapses of different complex structures appear later.It is suggested that the earliest signs of synaptogenesis are vesicular structures (‘growth’ vesicles and few synaptic vesicles), which occur in growth cones, axons and presynaptic boutons of immature synaptic contacts even before formation of the specialized pre- and postsynaptic membranes.