Phylogenetic and ontogenetic study of neuropeptide Y-containing nerves in the liver

The distribution of neuropeptide Y was investigated by light and electron microscopic immunohistochemistry in the liver of various vertebrates including the eel, carp, bullfrog, turtle, chicken, mouse, rat, guinea-pig, dog, monkey and human. The ontogenetic development of neuropeptide Y was also studied in the mouse liver. In all species examined except the eel, neuropeptide Y-like immunoreactivity was detected in nerve fibres. In the carp, bullfrog, turtle, chicken, mouse and rat, positive fibres were distributed around the wall of hepatic vessels and the bile duct of the Glisson's sheath. The density of the positive fibres increased with evolution. On the other hand, in the guinea-pig, dog monkey and human, numerous neuropeptide Y-positive fibres were observed not only in the Glisson's sheath but also in the liver parenchyma. Positive fibres formed a dense network to surround hepatocytes. The present immunoelectron microscopic study has confirmed that neuropeptide Y-positive terminals are closely apposing to hepatocytes. Ontogenetically, neuropeptide Y-positive fibres were first found in embryonic liver of 19-day-old mice. Positive fibres increased with age and the highest peak was seen one week after birth. This ontogenetic pattern has suggested that neuropeptide Y plays a certain role in developing liver.