Types of Secretory Neurones in the Pre-optic Nucleus of the European Eel, Anguilla anguilla L.

Abstract

Knowles and Vollrath¹, working on the ultrastructure of the neurohypophysis of the eel, Anguilla anguilla, have shown that it contains three distinct types of neurosecretory axons separable by the size and appearance of their elementary granules, and termed types A1, A2 and B respectively. A type fibres contain elementary granules of diameter greater than 1000 Å, whereas in the B type fibres they are less than 1000 Å. There is some evidence that only the A type fibres are stained by so-called neurosecretory dyes. For this reason and because the neurones of the pre-optic nucleus are the only ones in the hypothalamus of fishes that stain with these dyes, it seems likely that the A type fibres originate from these neurones. The detailed morphology of the entire hypothalamo-hypophysial complex of the eel has been described² and a good deal is known of the histology of the pre-optic nucleus from optical microscopy (see ref. 2 for references) although these studies have so far failed to identify two cell types that might give rise to the two A type fibres of the neurohypophysis.