Bouton ultrastructure and synaptic growth in a frog autonomic ganglion.

Postmetamorphic growth in the frog, Xenopus laevis, is accompanied by an increase both in the size of autonomic neurons in the heart and in the number of synaptic boutons that contact their surface. To determine whether the properties of individual boutons change as their number increases, serial-section electron microscopy was used to examine bouton ultrastructure at the end of metamorphosis and in the adult. The area of bouton contact, number of active zones per bouton, active zone size, percent of bouton area occupied by active zone, and vesicle density were examined. No differences were found between the two bouton populations for any of the parameters examined. These results support the hypothesis that boutons are structural units of synaptic growth, whereby the total area of synaptic contact increases through the addition of boutons without a change in their morphological properties.