Mucous cysts are typically found in patients with osteoarthritis who are between 50 and 70 years old. Doctors do not know why mucous cysts develop, nor do they understand exactly how these cysts form. One theory suggests that mucous cysts are formed when connective tissue degenerates, or wears away. Collagen is a protein found in connective tissue. The leftover collagen is thought to collect in pools, and the pools form cysts. Fluid seems to move from the joint into the cyst, but not the other way.