Serotonin. Polarised light micrograph of serotonin, a neurotransmitter. Serotonin is derived from tryptophan, and is found in the vertebrate brain. It is a powerful vasoconstrictor, and assists in the action of blood clotting. In the liver, it activates phosphofructokinase - the enzyme responsible for the transfer of phosphates from ATP to fructose. Magnification x35 at 5x4 inch size.