First, it was identified in intestinal cells
Second, it’s a tiny molecule
Third, only 5% of our body store of serotonin exists in the brain
Fourth, ....at about 8 weeks of gestation, the mother’s placenta turns huge stores of tryptophan into serotonin and ships them ..to... the (baby's) brain... instigates rapid growth of new neural connections.
Fifth, we have a long way to go to really understand serotonin. Eg:, adult healthy bones, ... rely on the balancing forces of brain serotonin induced bone cell growth and gut serotonin induced bone cell destruction. Which is to say, when it comes to serotonin there is more then meets the eye, and it’s not as simple as popping a(n) (SSRI) pill.