The interpeduncular nucleus (ITP). Ultrastructure data indicative of a possible neurosecretory activity.

Special types of cells of the interpeduncular nucleus (ITP) in the fog, Rana esculenta, are described after three typed of fixation: osmium tetroxide, aldehydes and potassium permanganate. Large granules with a sub-unit structure occurring together with bundles of filaments and elongated mitochondria are illustrated in the processes of cells, the perikarya of which lie distant from the ependymal lining of the aqueduct of Sylvius. Such processes cross the ITP in a dorso-ventral direction perpendicular to the fibers which traverse the nucleus in a horizontal and frontal plane. They reach the subpial ventral surface of the nucleus ending in foot-like expansions where granules and mitochondria are assembled. The secretory nature of the granules is assumed. The release of possible peptide hormones into the interpeduncular cistern, where a pool of cerebrospinal fluid and large blood vessels occur, cannot be excluded.