Bursa of Fabricius
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Also called the cloacal bursa
It is a primary lymphoid organ found in birds. The bursa was the first place that a certain subset of lymphocytes was observed and consequently they were named B lymphocytes (bursa of Fabricius or bursa equivalent organs). The bursa is involved in the differentiation of B lymphocytes.
Development
Lymphoid precursors migrate into the developing bursa during the first few weeks of embryo development. The bursa continues to grow through development and in chickens the bursa reaches its maximum size by six weeks. After this it slowly regresses (involutes) until only a small remnant is present in the adult.
Structure
The bursa is a round out pouching of the cloaca. It is found on the caudodorsal surface of the cloaca (in the proctodeum) cranial to the dorsal proctodeal gland. The bursa consists of a number of lymphoid lobules and crypt-like folds surrounding a lumen, which enclosed in a thin layer of stratified squamous epithelium. The lumen opens into the proctodeum.
Like the thymus the lobules have a cortex and medulla and the lymphocytes are supported by epithelial cells. B lymphocytes developing from the lymphoid precursors gather in the cortex, hence the cortex stains stronger than the medulla.
Histology
Low power magnification
©Nottingham Uni 2008
High power magnification
©Nottingham Uni 2008
Functions
The bursa’s primary function is maturating and causing the differentiation of B lymphocytes, it also produces the hormone bursin which activates B lymphocytes.
The bursa has some T lymphocytes present near to its opening into the proctodeum where it surveys antigens.