Histology and fine structure of the pre-optic nucleus and hypothalamic tracts of the European eel Anguilla anguilla L

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Abstract
The pre-optic nucleus and hypothalamic tracts of intact and hypophysectomized specimens of the European eel Anguilla anguilla L. have been studied in situ and by optical and electron microscopy. The in situ technique reveals a hitherto unsuspected degree of segregation of the neurosecretory axons which form up to five discrete tracts having separate origins and following distinct paths before converging, at the level of the anterior margin of the pituitary, to form a median tract. The structure of the pre-optic neurons, as revealed by several different techniques, is described and it is shown that their synthetic poles, identified by a prominent cap of endoplasmic reticulum, are precisely orientated towards the third ventricle and are separated from it by, at most, two or three layers of ependymal cells. Electron microscopy shows that the secretory products lie mainly in the axonal ends of the cells though in Bouin-fixed, wax-embedded material the entire perikaryon stains with neurosecretory dyes and this, and their proximity to the third ventricle, gives the impression that they secrete into the latter, as well as centripetally. This may well be so, but from the work described below it seems more likely that these neurons receive nutrients, or stimuli, or both, from the third ventricle. Two types of pre-optic neurons, separable by structural features as well as by the size of the elementary granules they contain have been identified; these probably give rise to two of the fibre types identified in the neurohypophysis of the eel by Knowles & Vollrath. Aggregations of neurosecretion, common in the fish pre-optic nucleus, and also, much rarer, colloid vesicles, are described and discussed.