Bursa of Fabricius
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Cross section of the bursa of Fabricius of a chicken (HE): overview 1
The bursa of Fabricius (bursa cloacalis) is present in birds only. It is a blind sac-like dorsal diverticulum of the wall of the proctodeum of the cloaca in which maturation of B lymphocytes takes place.
The bursa regresses with the onset of age or when reproductive activity (laying of eggs in females) starts.
The overall shape varies according to species – the bursa of chickens and pigeons is oval shaped, ducks have a more elongated bursa. In most birds, the bursa is connected to the proctodeum with a small stalk. In ratites (Struthioniformes), the bursa is an integral part of the mucosa of the proctodeum.