A light and electron microscopic examination of the vagal hepatic branch of the rat.

The rat's vagal hepatic branch and associated tissues were studied using light and electron microscopy. Whole mounts, serial sections, and vascular endocasts were used to characterize the tissue from the anterior vagal trunk to the porta hepatis. Fiber number and caliber as well as intraneural organization were analyzed from complete cross-sectional electron micrographic montages of the hepatic branch sampled at its point of separation from the anterior vagal trunk. The hepatic branch ramified from the anterior vagus in one (in 47% of the specimens), two (in 37%) or three (in 16%) bundles. The single bundled hepatic branch contained 2887 +/- 287 unmyelinated fibers, and their size distribution, with a mean diameter of 0.66 +/- 0.02 micron, was Gaussian. Myelinated fibers numbered only 21 +/- 4 per branch and had a complex size distribution ranging from 0.5 to 1.8 micron with a mean of 1.2 +/- 0.03 micron. Forty four +/- 6% of the myelinated fibers were found in a single "subfascicle" in the dorso-medial pole of the nerve. Whole mounts at this level revealed that a distinct bundle, here designated an extrinsic "hepato-gastric bundle", occurred within the hepatic branch and linked the omental hepatic branch and the distal anterior gastric branch, apparently without central vagal connections. In the lesser omentum, between the esophagus and the hepatic artery proper, the hepatic branch formed a plexus which was characterized by numerous nerve divisions, anastomoses and large paraganglia (196-463 glomus cells per paraganglion). This plexiform segment ended with the recombining of the hepatic branch into 5-7 bundles which variously ascended in the porta, descended on the hepatic artery proper, or traversed the portal vein. Through its omental course, the hepatic branch traveled in close apposition to the hepato-esophageal artery and the corresponding vein as well as a prominent lymphatic vessel with associated hemolymph nodes.