The neurons of the myenteric plexus of the entire small intestine were stained in young adult (3-4 mo old) and aging guinea pigs (26-30 mo old). Total length and circumference of the intestine were measured in the same experiments. The small intestine of the aging guinea pigs was longer, and (in the conditions of distention used) had a total serosal surface approximately 70% greater than in young adult animals. The spatial density of myenteric neurons per unit of serosal surface fell dramatically in aging animals, and the total number of myenteric neurons in the small intestine ranged between 1.1 and 1.6 million, i.e., it was only 40%-60% of the value obtained in young adult guinea pigs (2.75 million). The light microscope appearance of the neurons of the two groups of animals was markedly different and the suggestion is put forward that in aging guinea pigs the substantial reduction in neuron number is accompanied by structural changes and reorganization of the neurons that are left.
