Until embryonic day (ED) 19, it is mainly the gastrointestinal content that is transported to the yolk sac. 4 There is a change in direction after ED19, when the yolk content is transported towards the intestinal lumen until the first few days after hatching.2,3 The production of lymphocytes occurs in the specialized microenvironments of the primary lymphoid organs, where the repertoire of antigen-recognizing molecules [immunoglobulins and T-cell receptors (TCRs)] develops independently of external antigens. direct (intestinal) transport of yolk substances into the intestine, but left the vitelline circulation intact. Vitelline duct ligation performed on embryonic day 17 resulted in serious but transient bursal underdevelopment during the first week of life: (1) IgG and the follicular dendritic cell marker 743 were not detectable around the bursal secretory dendritic cells, in spite of a normal serum IgG level and free communication with the cloacal lumen; (2) the number of B cells in the follicles was greatly reduced and they showed an Rabbit Polyclonal to AF4 altered phenotype, resembling that of the prebursal B cells. The intracloacal administration of different proteins effectively restored the bursal phenotype. These data suggest that maternal antigens indirectly help the maturation of bursal secretory dendritic cells and concomitantly that of B cells during the first week of life. Keywords: chicken, bursa, vitelline duct, dendritic cells, B cells Introduction The vitelline duct (VD) in the yolk stalk connects the lumen of the yolk sac with that of the midgut. The VD is usually a simple, enteral tube-like structure (at hatch its lengths is usually 4C5 mm and its diameter 1 mm) lined with cylindrical microvillous epithelium and surrounded by a well-developed muscular layer. The yolk content is mainly assimilated by the yolk sac entoderm and transported by the vitelline circulation. However, the presence of yolk in the intestinal lumen of late avian embryos was shown as early as 1753 (reviewed in reference1), suggesting an intestinal route of yolk absorption via the VD. Non-absorbable tracers [such as blue dextran or radioactively labelled polyethylene glycol (PEG)-4000] injected into the MC-Val-Cit-PAB-tubulysin5a yolk sac of the chicken embryo appear in the intestinal contents and the faeces after hatching.2C4 Through the VD, there is bidirectional transport in the late chicken embryo. Until embryonic day (ED) 19, it is mainly the gastrointestinal content that is transported to the yolk sac.4 There is a change in direction after ED19, when the yolk content is transported towards the intestinal lumen until the first few days after hatching.2,3 The production of lymphocytes occurs in the specialized microenvironments of the primary lymphoid organs, where the repertoire of antigen-recognizing molecules [immunoglobulins and T-cell receptors (TCRs)] develops independently of external antigens. The subsequent selection procedure using internal (self) antigens guarantees the elimination of (strongly) autoreactive or abortive clones. These microenvironments (avian and mammalian thymus MC-Val-Cit-PAB-tubulysin5a and mammalian bone marrow) do not allow the entry of foreign antigens. However, the ruminant Peyer MC-Val-Cit-PAB-tubulysin5a plaques and the avian bursa of Fabricius represent primary lymphoid organs for B-cell development where the cells are exposed to external (intestinal) antigens.5 The scarcity of experimental data suggests that the development of antibody diversity is independent of external antigens, but they are probably used in subsequent selection MC-Val-Cit-PAB-tubulysin5a and maturation processes.5 The bursa of Fabricius has a direct connection with the cloacal lumen through the bursal duct (BD). Although bacteria are normally not present in the bursal lumen,6 material from the cloaca reaches the bursal cavity. Under experimental conditions, proteins (enzymes or fluorochrome-labelled proteins) or particles (latex beads or Indian ink particles) introduced into the cloacal lumen or decreased onto the anal lips of adult chickens are rapidly transported into the bursal lumen.7C9 From the bursal lumen, they are taken up by specialized cells of the surface epithelium, the follicle-associated epithelial (FAE) cells,10 and transported towards the follicular medulla. Fifteen minutes after cloacal administration, the label is present in the FAE cells, and 24 hr later it can be detected throughout the follicle.8 The external antigens taken up by the FAE cells are thought to be transported to the dendritic cells of the bursal follicles. According to this theory, the antigens combine with serum-derived immunoglobulin G (IgG),.