Immunolocalization of the HNK-1 epitope in the autonomic innervation to the liver and upper digestive tract of the developing rat embryo.

The immunohistochemical analysis of the HNK-1 epitope presence in the liver and upper digestive tract nerves was carried out in 12- to 18-day-old rat embryos embedded in acrylamide-agarose and observed with laser scanning confocal microscopy. The vagus and sympathetic trunk were intensely immunostained at all ages; branches of both structures were also HNK-1 positive, and ramified ventrocaudally following the course of the thoracic and abdominal aorta, caval vein, portal vein and ductus venosus. As early as day 12, some immunostained cells were seen in the mesentery that formed the enteric nervous system. Clearly immunostained HNK-1-immunoreactive fibres were detected innervating the digestive wall after day 14, forming both myenteric and submucosal plexuses. After day 16, the Glisson sheath showed streams of HNK-1-positive fibres coming from dorsal areas, lining the peritoneal surface of the diaphragm, invading the capsule, and ramifying superficially around the lobes of the liver. We saw no immunoreactive structures pervading the hepatic lobes at all ages studied, with the exception of occasional HNK-l-positive cells in the superficial parenchyma, which were visualized after 16 days of gestation. Our findings can help to understand the development of the gastrointestinal and liver innervation in the rat.