A ganglion is a sac-like swelling or cyst formed from the tissue that lines a joint or tendon. The tissue, called synovium, normally functions to produce lubricating fluid for these areas. A ganglion is a cyst formed by the synovium that is filled with a thick jelly-like fluid.While ganglia can follow local trauma to the tendon or joint, they usually form for unknown reasons. ganglion is a cyst formed by the synovium that is filled with a thick jelly-like fluid. While ganglia can follow local trauma to the tendon or joint, they usually form for unknown reasons.