Structure of the pars flaccida after occlusion of the Eustachian tube or blockade of the tympanic isthmus.

In this experimental study performed on the rat middle ear either the Eustachian tube or the tympanic isthmus was blocked via the tympanic bulla with a polyethylene plug or a piece of Gelfoam, respectively. Effusion material was immediately observed in the attic space and subsequently the pars flaccida was drawn in a medial direction to form a retraction pocket. The initial changes in the pars flaccida structure were a degranulation of the mast cells and concomitantly metaplasia of the mucosal epithelium into cells displaying numerous osmiophilic inclusions, vacuoles and multivesicular bodies. It cannot be excluded that the initial cause of the effusion production is the histamine released from mast cell granules.