The hypothalamo-hypophyseal complex: a paradigm of the paraneuron concept.

The neuronal and epithelial parts of the hypothalamo-hypophyseal complex (proximal and distal neurohypophysis, adenohypophysis) have in common the task of message transfer. According to the paraneuron concept, neurons and endocrine cells as paraneurons produce similar or identical substances, store them in vesicles and release them in response to specific stimuli. The released substances reach their targets via the intercellular spaces in a paracrine mode or via the blood in a more or less far-reaching hemocrine way. Special contacts for message transfer are not mandatory, even in the case of neurons. The biological activity of substances released by neurons and paraneurons depends on their affinities to binding sites, most probably localized on the outer target cell membrane. The neurons and paraneurons under discussion are mostly localized close to the mid-sagittal plane. The common ontogenetic roots of some neurons and paraneurons might be a further important point of the paraneuron concept.