[Ultrastructure of the neurohypophysis in the lamprey, Lampetra fluviatilis L].

Three distinct zones are observed in the neurohypophysis (Nh) of adult lampreys: ependymal (tanycyte), fiber and neurovascular. The ependymal zone consists of a single layer of tanycyte perikarya. Their basal processes pass via the fiber zone and form vascular "endfeet" on the connective tissue layer, separating Nh and the meta-adenohypophysis. Few funnel-shaped protrusions of the recessus infundibuli lined with tanycytes penetrates deeply in Nh. Pituicytes are rare. Peptidergic A1 and A2 fibers and their terminals contain granules of 1600-3400 and 1200-2200 A in diameter respectively. Monoaminergic B type fibers have granules of 800-1000 A in diameter. Percentage of the A1 fibers is the lowest. Nerve terminals and tanycyte vascular "endfeet" make contact with the unusually thick (up to 3000 A) outer basement membrane. The latter is about 1 mu thick in sites where in invaginates deeply in Nh. Single short fragments of the inner basement membrane are seen. Sometimes peptidergic A1 and A2 nerve terminals are separated from the third ventricle or its protrusions only by a thin layer of tanycyte cytoplasm. Peptide neurohormones are probably released in the cerebro-spinal fluid in these sites. The possibility of peptide neurohormone and monoamine be discharged into the common circulation and their diffusion through the the connective tissue layer into the meta-adenohypophysis is discussed.