An ultrastructural study of pigeon bursa of Fabricius during involution.

The ultrastructure of the pigeon bursa of Fabricius during involution is described. According to the AA, the involutive process may be divided into 3 stages: an early involutive stage, a late involutive stage and a residual stage. The involutive process starts from epithelium later affecting the follicular medulla and all other structures of the organ. A mucoid degeneration can be observed, followed by an impressive fatty degeneration.