Calcitonin has not yet been identified in the gut but its identification in the brain has considerably modified the view of its functions as a neurotransmitter. It is present in various cyclostomes and has been shown to be present in molluscs. C-cells of the thyroid gland in mammals originate from the rhombencephalic neural crest to populate the thyroid in mammals and the ultimobranchial gland in other vertebrates ontogenetically. It is demonstrated that calcitonin immunoreactivity is present in the K-cells of the lungs of lizards. Thus it may seem highly probable that K-cells whether human or other vertebrates are just C-cells that migrated from the neural crest to populate various structures like the thyroid, ultimobranchial gland, thymus, the parathyroid glands and the lungs. The role of calcitonin was originally thought to be continuous physiological hypocalcaemia but it has now been suggested that it minimizes skeletal calcium loss in vertebrates during growth and reproduction only. Before now, it was known that removal of the gland causes no harm to adult animal and its hypersecretion in medullary carcinoma of the thyroid does not necessarily cause hypocalcaemia. Calcitonin is hereby suggested to be as an enigmatic hormone whose role in vertebrate evolution is very old probably as old as that of acetylcholine, catecholamines and serotonin which have been demonstrated in cells of coelenterates. There is also evidence that specific neuropeptides exist in hydra neurons. If the case is proven for the existence of calcitonin in vertebrate neural elements, the cells containing them may well represent the lowly APUD cell with polypeptide hormonal secretion. In the light of the new findings on calcitonin the following can be postulated for its functions.

1)                                                      Calcitonin may not really be a hormonal principle evolved for the regulation of blood calcium as was first suggested. Its phylogenetic history is older than that of parathormones.

2)                                                      It may have evolved earlier on in invertebrates as a neurotransmitter like acetylcholine etc.

3)                                                      It may have aside effects like decrease in blood calcium levels as opposed to its main physiological role.

4)                                                      C-cells being a member of the DNES, it is possible that like adrenalin, cacitonin is a potent behavioral modulator as has been shown for other neuropeptides.

The problem of function of  C-cells and calcitonin in invertebrate biology still remains a mystery.


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