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Bursa of Fabricius

lumen
bursal fold (plica bursalis)
bursal capsule (capsula bursalis)
bursal follicle (folliculus bursalis)
interfollicular connective tissue

Cross section of the bursa of Fabricius of a chicken (HE): overview 2

The wall of the bursa of Fabricius consists of a capsule (capsula bursalis) and a mucosal layer. The mucosa forms folds (plicae bursales), the number and length of which varies with species (chicken: 12 folds mostly).

In most birds, the lymphoid tissue in the folds is organized into follicles (folliculi bursales), separated by sparse connective tissue.