Three-dimensional ultrastructure of the autonomic nerve terminals in the lamina propria mucosae of the rat large intestine.

The three-dimensional ultrastructure of the autonomic nerve terminals in the lamina propria mucosae of the rat large intestine was studied by scanning electron microscopy using the KOH-collagenase digestion method as well as by transmission electron microscopy. Observations showed unmyelinated nerves in the lamina propria mucosae as well developed just above the muscularis mucosae and forming a two-dimensional irregular network by anastomosing with one another. Each nerve in the network consisted of axons and Schwann cells. The axons were about 0.1 micron in diameter and had local swellings (about 0.2-0.5 microns diameter) along their course. Transmission electron microscopy revealed the presence of synaptic vesicles in these varicosities. Although some of the axons left the Schwann cell processes to run separately, most of them twined around or were embedded in the Schwann cell bodies or processes. Blind ends of axons were only occasionally observed in the nerve network. These findings suggest that the network of the unmyelinated nerves itself represents a terminal apparatus acting upon the muscular and glandular cells.