Neurosecretory innervation of the pituitary of the eels Anguilla and Conger I. The structure and ultrastructure of the neuro-intermediate lobe under normal and experimental conditions

Three kinds of neurosecretory fibre (Types A 1 , A 2 and B) are present in the neural component of the neuro-intermediate lobe of the eel pituitary. These fibres do not in the main make any direct contact with the pars intermedia cells, but they are separated by only a narrow extravascular channel, into which both elements discharge their products. Type A neurosecretory fibres do, however, make direct synaptic contact with pituicytes which resemble ependyma and surround finger-like extensions of the infundibular recess. That these contacts are functional is indicated by the fact that their frequency is related to changes in the environment. When eels are placed on an illuminated white background the synaptic junctions between Type A 2 neurosecretory fibres and pituicytes are very frequent. Similar synaptic junctions between A 1 fibres and pituicytes were only found in animals which had been recently transferred from fresh water to sea water. A possibility that the pituicytes play some part in a feed-back from the pituitary to the hypothalamus is discussed.