Appearance and development of lymphoid cells in the chicken (Gallus gallus) caecal tonsil

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Abstract
Background We have analyzed by electron microscopy, immunohistochemistry, and flow cytometry the development of chicken caecal tonsil, the largest lymphoid organ of avian gut—associated lymphoid tissue (GALT). Methods White Leghorn chickens of different ages obtained from a local supplier were routinely processed by transmission electron microscopy. For both immunohistochemistry and flow cytometry, we tested a battery of specific monoclonal antibodies (mAbs) to chicken cell markers on caecal cryosections or cell suspensions, respectively. Results A rudimentary caecal tonsil occurs at the end of incubation. The organ grows just after birth, reaching the adult condition 4 days later. Firstly (4 days to 2 weeks), it contains predominantly T lymphocytes, principally TcRαβ+ and CD4+ cells, which occupy largely the named caecal diffuse lymphoid tissue. In adult tonsils (6‐week‐old chickens) however, B lymphocytes, mainly expressing either IgM or IgA, predominate. They occur in both the subepithelial zone and the germinal centers, in which there are also a few T cells. After 2 weeks the CD8+ lymphocytes gradually become more numerous than CD4+ cells. In the tonsillar epithelium CD8+TcRγδ+ T cells, CD8+TcRγδ−αβ−, presumably NK cells, and a few B lymphocytes are the main cell subpopulations. Conclusions Chicken caecum grows fast after hatching. The diffuse lymphoid tissue largely contains TcRαβ CD4+ or CD8+ cells. CD8+ cells of caecal epithelium represent γδ T cells or NK cells. B lymphocytes which occur in the subepithelial zone, germinal centers, and, in few numbers, the caecal epithelium predominantly express either IgM or IgA. Anat. Rec. 250:182–189, 1998.

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