ABSTRACT
The bursa of Fabricius (Figure 6.1) is located near the terminal portion of the cloaca and, like the thymus, is a lympho-epithelial organ. The bursa begins to develop after the 5th day of incubation and becomes functional around the 10th to 12th day. It has an asymmetric sac-like shape and a star-like lumen, which is continuous with the cloacal cavity. The epithelium of the intestine covers the bursal lumen but lacks mucous cells. The bursa contains abundant lymphoid tissue, forming nodules beneath the epithelium. The nodules show a central medullary region containing epithelial cells and project into the epithelial coating. The center of the medullary region is less structured and also contains macrophages, large lymphocytes, plasma cells, and granulocytes. A basement membrane separates the medulla from the cortex; the latter comprises mostly small lymphocytes and plasma cells. The bursa is well developed at birth but begins to involute around the 4th month; it is vestigial at the end of the first year. There is a direct relationship between the hormonal status of the bird and involution of the bursa. Injections of testosterone may lead to premature regression or even lack of development, depending on the time of hormone administration. The lymphocytes in the bursa originate from the yolk sac and migrate there via the bloodstream. They comprise B cells which undergo maturation to immunocompetent cells capable of antibody synthesis. Bursectomy at the 17th day of incubation induces agammaglobulinemia, with the absence of germinal centers and plasma cells in peripheral lymphoid organs.
