Light and electron microscopic studies on the medial forebrain bundle in rat: III. Degenerated nerve elements in the medial hypothalamic nuclei following surgical transections of the medial forebrain bundle.

Transections of the rat medial forebrain bundle at various levels and the separation of the medial and lateral hypothalamus were performed to study with light and electron microscopy axonal and terminal degeneration in the arcuate, ventromedial and dorsomedial nuclei and in the median eminence. Fibres entering the MFB from various directions participate in the innervation of the arcuate nucleus. The bulk of extrahypothalamic fibres terminating in the median eminence derive from the lower brain stem and reach their target through the MFB. Preoptic originating and transient fibres also terminate in the median eminence. Following parasagittal separation of the medial and lateral hypothalamus massive degeneration was found in both layers of the median eminence. This is likely to be due to interruption of the supraoptic-hypophyseal tract but axons of intrinsic MFB-neurons might also contribute to the innervation of the median eminence.