An electron microscopic study on substance P-like immunoreactive nerve fibers in the pars distalis of the adenohypophysis in the dog.

The pars distalis of the adenohypophysis has recently been proved to be innervated by substance P-like immunoreactive nerve fibers in the monkey and the dog. The present study investigated immuno-electron microscopically the morphology of these nerve fibers and their relationship to anterior pituitary gland cells in the dog. The nerve fibers were unmyelinated and varicose. Direct contacts between them and every cell type of the gland could be ascertained, including folliculostellate cells. Typical synapses were identified on somatotropes and corticotropes. Most of them were of asymmetrical type with round to oval small clear vesicles and scattered large dense-cored vesicles. Occasional symmetrical type synapses were also seen. The results imply that substance P-like immunoreactive nerve fibers have an effector role in the pars distalis of the dog.