Degeneration and regeneration of hypothalamic nerve fibers in the neurohypophysis after pituitary stalk section in the ferret

Transection of the pituitary stalk caused rapid and severe degeneration of the hypothalamo‐hypophysial nerve tract distal to the operation site. When a barrier had been inserted to prevent reunion of the stalk this degeneration was permanent. All nerve fibers and stainable neurosecretory material (NSM) disappeared. When a barrier was not used the initial degeneration was followed by regeneration. From two weeks after operation new nerve fibers from the hypothalamus, carrying NSM, were found growing through the scar and down the degenerated lower infundibular stem towards the infundibular process, which was fully re‐innervated by the end of a year. The re‐innervated lower infundibular stem contained an abnormally large amount of NSM. In many cases an outgrowth of nerve fibers from the regenerating tract had invaded the rostral end of pars distalis; these also were surrounded by NSM. Another unusual feature often seen was an ectopic infundibular process which had developed not far below the site of operation. In its capacity for vigorous regeneration the hypothalamo‐hypophysial nerve tract appears to be unique among tracts of the central nervous system. It is suggested that this regenerative capacity may be related to the fact that this tract is composed of non‐myelinated nerve fibers.