Quantitative analysis of the synaptogenic period in the trigeminal mesencephalic nucleus

The morphogenesis of chemical synapses in the trigeminal mesencephalic nucleus of the hamster was examined in the electron microscope. The relatively small complement of synapses and the simple geometry of these neurons facilitates quantitative analyses of synaptic development. Emphasis focused on the chronology of synaptic maturation and the length of the synaptogenic period in this nucleus. Synaptic endings first appear on the soma at three or four days after birth. These early terminals form desmosome‐like junctions without any synaptic vesicles. The surface density of terminals (synapses/100 μm of perimeter) increases rapidly from 5 to 15 days when it is approximately equivalent to the adult. However, calculations that also take into account the expansion in cell surface area suggest that the actual number of terminals continues to increase as the cells grow. This indicates that these neurons may retain their synaptic capacity and continue to add new synaptic sites for a longer period than proposed for other systems. Structural characteristics of these synapses in the adult suggest they may be inhibitory in nature and thus regulate the interaction of electronically coupled clusters of neurons.