Ultrastructure and localization of substance P and met-enkephalin immunoreactivity in the human fetal gastric antrum.

The ultrastructural localization and relations of substance P- and met-enkephalin-labeled neuronal structures were examined in the wall of the human gastric antrum during early fetal life. By 14-16 weeks of gestation, clearly discernable neural plexuses and a well developed external muscle coat were present. In the submucous coat, neural plexuses varied from immature forms consisting of 1-4 neurites partially enveloped by Schwann cell processes to more mature plexuses where neurons were completely enclosed by Schwann cell processes. Neuronal profiles with substance P- and met-enkephalin-like immunoreactivities were observed in the submucous plexus. In the myenteric plexus met-enkephalin-like immunoreactivity was seen within cell bodies and neurites. By contrast, although substance P-like immunoreactivity was observed in neurites in the myenteric plexus, no substance P-labeled somata could be identified. Unlabeled terminals were seen in contact with both unlabeled dendrites and met-enkephalinergic neurons. An increase in electron density was observed at the sites of contact. These structures probably represent early stages in the development of synaptic specializations. In addition, met-enkephalin-labeled varicosities were seen in apposition to smooth muscle cells of the circular muscle coat. This suggests that antral smooth muscle cells are directly innervated by met-enkephalin neurons.