Ontogenesis of the myenteric plexus in the cat gastrointestinal sphincters. III. Synaptogenesis.

Synaptogenesis in the cat myenteric ganglia of the lower esophageal, pyloric and ileocecal sphincters in the fetal life and at different postnatal stages of development was examined. The synaptic formation began in the early fetal period. The first immature synapses were of axosomatic and axodendritic types. The new formation and maturation of the synaptic contacts was an active process in the fetal period and in the first postnatal weeks, but it continued to the adult level. It was expressed quantitatively in the increase of the surface area of the synaptic contact zones, the number of the synapses, the length of the synaptic contact zones and the number of the synaptic vesicles. Fine structural features of the synaptic maturation were the increased pleomorphism of the synaptic vesicles, widing and lengthening of the thickening of the opposed membranes, the appearance of the coated evaginations and vesicles. Synaptic morphology became more complex because of the including of different in size and structure pre- and postsynaptic components. The frequency of the synapses on dendritic spines and synapses "en passant" increased. The synaptogenesis in the three sphincters studied exhibited a different dynamics. The parallel advance in the neuronal and synaptic differentiation and the relationship between the desmosomal junctions and synapses were discussed.