Structure and Development of Bronchus-Associated Lymphoid Tissue in Conventionally Reared Broiler Chickens

The development of bronchus-associated lymphoid tissue (BALT) in conventionally reared broiler chickens of 1 day and 1, 2, 3, 4, 6, and 8 weeks of age was studied using light and electron microscopy (scanning and transmission). BALT in these chickens resembled other mucosa-associated lymphoid tissues (MALT) in that it was composed of an altered epithelium overlying a population of lymphocytes and contained potential antigen-presenting cells, such as macrophages and dendritic cells; high endothelial venules were also present. In contrast to other MALT, epithelial cells in chicken BALT were not of the M-cell type; i.e., they lacked large numbers of apical tubules and vesicles for specialized uptake of luminal antigens. There were age-related differences in size, number, and cellular composition of BALT nodules. Lymphoid nodules were progressively larger and more numerous with increasing age. Germinal centers were present in birds 2 weeks of age and older. BALT epithelium was primarily squamous and non-ciliated in 1-day and 1-week-old chicks, becoming progressively more columnar and ciliated in older chickens. Lymphocyte infiltration of the epithelium was extensive at 1 to 4 weeks of age; in older chickens, distinct epithelial and lymphocytic compartments were separated by connective tissue.