Development and structure

The bursal anlage is first observed in the chicken embryo at 4 days of incubation as an epithelial bud in the cloacal region. While enlarging, the bud becomes increasingly vacuolized, the coalescence of the vacuoles giving rise to the bursal lumen. Epithelial longitudinal folds are formed on the inner sur face of the sac, and the columnal epithelium, covering the plicae, proliferates and forms outgrowths into the underlying lamina propria. Formation of these epithelial buds is dependent on interaction between the mesenchymal and epithelial tissues. Each epithelial bud is colonized by one or two prebursal stem cells. They proliferate vigorously, leading to formation of the medulla of the bursal follicles, which start to appear on day 12 of incubation in the chicken embryo. Prebursal stem cells originate in the intra-embryonic mesenchyme. On arrival at the bursa they have the germ-line immunogloublin (Ig) genes rearranged, being already committed to development along the B cell lineage. The period of prebursal stem cell arrival in the chicken bursa extends from day 8 to day 14 of incubation. Cells bearing IgM on the surface appear in the bursa on day 12 of embryogen-esis. At the time of hatching, about 90% of the bursal cells carry surface IgM and <1% IgG or IgA.

The epithelial anlage of the bursa of Fabricius is decisive in providing differentiation signals to the arriving prebursal stem cells and to their descendants, the bursal stem cells. How the signals are mediated remains unknown, as no definitive evidence, e.g. for existence of a specific bursal hormone, is available.

Epithelial cells within the medulla form a loose cellular network with desmosome connections, and

Figure 1 (A) Location of bursa of Fabricius (F) and thymus (T). (B) Cross-section of bursa of Fabricius at the level indicated by a broken line in Figure 1A. The plicae are longitudinal in dorsoventral direction, with a bursal lumen opening ventrally to the cloaca; 4-day-old chick. H & E; magnification x 10. (C) A bursal follicle from the area marked with a square in Figure 1B. The cortex and medulla are separated by a basement membrane (double arrows). The interfollicular epithelium against the bursal lumen is broken by the follicle-associated epithelium (arrow). H & E; magnification x 250. (D) Scanning electron micrograph of plical surface against the bursal lumen. The phagocytosing follicle-associated epithelium appears as clearly distinguishable areas (arrow). Magnification x 200.

they are secretory in morphological structure. In addition to epithelial cells, the fully developed bursal medulla contains small lymphocytes, plasma cells and phagocytic histiocytic cells. Less than 1 % of the bursal cells are T cells. They are mostly located in a diffusely infiltrated area of lymphoid cells, dorsal to the bursal duct opening to the cloaca. At the time of hatching lymphoid cells also appear in the mesenchymal tissue surrounding each follicular medulla, and a follicular cortex develops. The medulla and cortex are separated by a basement membrane.

At the plical surface against the bursal lumen the epithelial cells differentiate into a follicle-associated epithelium, with a specific function to phagocytose and transport antigenic material from the bursal lumen into the follicular medulla. The epithelial cells in the follicle-associated epithelium and within the bursal medulla are different in their phagocytic capacity and enzyme activities. Structural organization of the bursa is presented in Figure 1.

At the time of hatching the chick bursa of Fab-ricius weight 30-40 mg and reaches a maximum of 3—4 g at the age of 2-4 months. Thereafter, with the beginning of sexual maturation, it involutes, and both the lymphoid and epithelial structures have completely disappeared at the age of 6-12 months.

Figure 1 (A) Location of bursa of Fabricius (F) and thymus (T). (B) Cross-section of bursa of Fabricius at the level indicated by a broken line in Figure 1A. The plicae are longitudinal in dorsoventral direction, with a bursal lumen opening ventrally to the cloaca; 4-day-old chick. H & E; magnification x 10. (C) A bursal follicle from the area marked with a square in Figure 1B. The cortex and medulla are separated by a basement membrane (double arrows). The interfollicular epithelium against the bursal lumen is broken by the follicle-associated epithelium (arrow). H & E; magnification x 250. (D) Scanning electron micrograph of plical surface against the bursal lumen. The phagocytosing follicle-associated epithelium appears as clearly distinguishable areas (arrow). Magnification x 200.

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