During embryological development, the thymus is formed from the third and fourth pharyngeal pouches. It descends along a pathway from the mandible to its final resting place of the mediastinum. When the thymus tissue fails to descend appropriately or fails to involute, thymus tissue remains in various locations along this pathway. Locations that solid thymus tissue has been reported include near the thyroid (most common), within the thyroid, the base of the skull, and within the pharynx or trachea.
Ectopic thymus
Abscess of thymus
Cervical thymic cyst
Tracheal agenesis
Persistent thyroglossal duct
Salivary gland atresia
Ectopic salivary gland tissue
Salivary gland aplasia