Quantitation of terminal parameters and their inter-relationships in maturing central synapses: a perspective for experimental studies.

The development and maturation of synapses in the molecular layer of the occipital cortex of rats at 15, 20, 28, 75 and 224 days postnatal were examined and quantitative ultrastructural techniques were employed. The parameters evaluated display a diverse set of trends. A steady increase in mean terminal area is noted, the proportions of the smallest terminals declining progressively over this period. The mean length of the postsynaptic thickening fluctuates throughout the developmental series. A close association is found between terminal area and junction lengthlarger junction lengths being consistently associated with larger areas. Synaptic vesicle numbers within each terminal increase markedly between 15 and 28 days. There is an increase in the frequency of flat and positively curved junctions with increasing age, the bimodal distribution of negative and positive curvatures at 15 days being replaced by a normal distribution in the adult. When synaptic length is compared with curvature, flat junctions emerge as the longest and the most highly curved junctions as the shortest. The terminal areas of positive junctions are greater than those of negative ones. The consequences of these results for the flat disc model of synapses are discussed, and it is postulated that negatively curved synaptic junctions may be non-functional.