Neurosecretory innervation of the pituitary of the eels anguilla and conger II. The structure and innervation of the pars distalis at different stages of the life-cycle

A number of distinct cell types may be recognized in the pituitary of the eel at the level of ultra-­structure by reason of the specificity of the size and electron-density of the granules they contain. The size of the granules and changes in the different cell types at different stages of the life-cycle permit a tentative identification in terms of function. The pars distalis of the eel pituitary receives the greater part of its innervation from the nucleus lateralis tuberis by Type B neurosecretory fibres (Knowles 1965 a), which do not stain with the so-called neurosecretory stains, but which nevertheless contain elementary neurosecretory vesicles. Type A, or classical, neurosecretory innervation is also present and seems to be of special importance at certain stages of the life-cycle. The possible function of these two forms of neurosecretory inner­- vation is discussed. The relationship between the intrinsic endocrine cells of the pars distalis and their neurosecretory innervation is fundamentally similar, at the level of ultrastructure, to that of the neuro-intermediate lobe. There are no direct contacts between the neurosecretory fibres and the intrinsic endocrine cells, but the proximity of the fibre terminals (ca. 2000 to 4000 A) to endocrine cells indicates a functional relationship between these two elements of the pituitary of the eel.