Comparative ultrastructural study of the neuro-intermediary lobe in fresh-water teleosts under natural conditions

A comparative study of the hypophyseal neuro-intermediary lobe (NIL)j is 8 species of fresh-water teleosts (Cyprinidae) was performed by means of light and electron microscopy. NIL is well developed in all species and consists of two types of glandular cells, three types of peptidergic nerve fibres, pituitary cells and blood cappillaries. Type A glandular cells (lead-haematoxylin staining cells) are elongated and small (8...12 micrometer/ 3...6 micrometer). They contain a large number of secretory granules with an average diameter of 160 nm. Type B cells (PAS-positive) are larger (13...16 micrometer/ 7...9 micrometer), constituting the NIL major substance. They present two variants: cells with a clear cytoplasm (light cells) and cells with a dark cytoplasm (dark cells). All the B-type cells include numerous vesicles (300...350 nm in diameter) with a slight electrondens content. In some of them a small number of intense electrondens secretory granules also appear. Type I peptidergic nerve fibres (PNF) are the most frequent; they contain numerous granules with an average diameter of 170 nm. Type II PNF are less frequent and contain granules with an average diameter of 170 nm. Type II PNF are less frequent and contain granules with an average diameter of 220 nm. The least frequent are type III PNF. Their secretory granules have an average diameter of 200 nm. The fenestrated aspect of the endothelial wall of the capillaries and the local discontinuities of the perivascular space indicate the existence of an especially tight neuro-cascular relation. The cytological "picture" of NIL is very similar in all species and characteristic for the period of the autumnal sexual repose of these animals.