part of part of organ part bursa of Fabricius bursal plica bursal plicae UBERON:0003904 Follicle containing part of bursa of Fabricius. The luminal (interior) surface of the bursa is plicated with as many as 15 primary and 7 secondary plicae or folds. These plicae have hundreds of bursal follicles containing follicle-associated epithelial cells, lymphocytes, macrophages, and plasma cells. Lymphoid stem cells migrate from the fetal liver to the bursa during ontogeny. In the bursa, these stem cells acquire the characteristics of mature, immunocompetent B cells[WP]. bursal fold