[Ultrastructure of the proximal neurosecretory contact region in a sturgeon].

The proximal neurosecretory contact region (PCR), a homologue of the tetrapoda median eminence occupies the rostral and chiefly the ventral surfaces of the hypothalamic tuber cinereum. Three types of granule-containing neurosecretory fibres and their terminals have been found in the PCR with the electron microscope. Peptidergic, A1 and A2 type fibres contain granules of 120--300 nm and 100--170 nm in diameter, correspondingly, and monoaminergic B fibres have granules of 80--100 nm in diameter. Neurosecretory terminals and tanycyte vascular "endfeet" make contacts with a 70 nm thick outer basement membrane of the primary portal capillaries. Some thin horizontally oriented tanycyte branches form a multiserial layer bordering the external zone of the PCR from the preoptico-hypophyseal tract. Few neuroglial cells with pale cytoplasm and numerous lysosomes and lipofuscin granules are seen there. It is hypothesized that, like in other vertebrates, in the PCR of Acipenseridae both peptide and monoamine hypophysiotropic neurohormones may be discharged in the portal circulation to affect the functional activity of glandular cells of the pars distalis.