Some aspects of the modern concept of neurosecretion

Certain glandular nerve cells communicate with effector cells by means of hormonal mediators (neurohormones). The production of neurohormones is restricted to this specialized group of glandular neurones, which possess glandular attributes as well as attributes of conventional neurons. These glandular neurons, because of their dual nature, are called ‘neurosecretory cells’ and their secretion is termed ‘neurosecretory material’. The identification of neurosecretion based upon the occurrence of apparent secretory inclusions in the neuron is not very reliable. The classical staining techniques of Gomori as well as newer Alcian blue and pseudocyanin methods are only suggestive rather than conclusive. Ultra structurally, the ‘classical’ neurosecretory neurons may be distinguished as having neurosecretory materials as prominent content of membrane-bound granules of varying size ranges and of varying, usually high, electron density. Intermingled with typical proteinaceous granules are small electron-lucent vesicles in dense aggregate. These are considered as packets of acetylcholine, helping in the release of neurohormones. The secretory material contains active principles regulating multiple and diverse target cells via a circular channel exemplified by polypeptides, e.g., vasopressin and oxytocin, bound to carrier proteins (neurophysins). Various physiologic traits, e.g. colour change in crustaceans, tanning of the integument of insects, water balance in vertebrates, growth and regeneration in annelids, liberation of the ‘tropic hormones’ from the adenohypophysis by the so-called ‘releasing factors’ are regulated by the neurohormones from the central nervous system.