Two stromal elements, follicle‐associated epithelium and secretory dendritic cells of the bursa of Fabricius were studied by light microscopy and two novel MAbs, that were produced against splenic cell suspensions of guinea fowls. Both antigens recognized by these MAbs, designated GIIF3 and NIC2, are localized in the cytoplasm of the stromal cells, and their molecular weights are 50 and 30 kD, respectively. During embryogenesis the GIIF3 and NIC2 cells emerge in the mesenchyme of the folds before follicle formation. The GIIF3 and the NIC2‐positive cells accumulate under the surface epithelium of the plicae and migrate into the epithelium, that precedes the bud‐formation. From the bud, the GIIF3‐positive cells migrate up to the luminal surface, and they transform to distinct, highly polarized follicle‐associated epithelial cells. Single GIIF3‐positive cells are also present in the interfollicular epithelium. The NIC2 MAb recognized mesenchymal cells harbor in the lymphoepithelial compartment of the folliculus, and they elaborate cytoplasmic granules. Around Day 20 of embryogenesis large amount of NIC2‐positive substance appear extracellularly in the medulla and around it. This period well correlates with the starting up of the bursal functions; clonal expansion of B cells, and generation of immune repertoire. After hatching the NIC2 stainability diminishes, and it is restricted to the medullary bursal secretory dendritic cells. The NIC2‐positive, possibly elderly bursal secretory dendritic cells, are capable for migration into the follicle‐associated epithelium. In eight‐day old birds some cells of the follicle‐associated epithelium reveals temporary NIC2 positivity, that may prove the transport of the follicle‐associated epithelial cells into luminal direction. By 12 weeks of age the presence of NIC2‐positive substance in the intercellular space of the FAE, rather than in the cells of FAE may indicate the termination of the transport of secretory substance. In conclusion, two types of mesenchymal cells enter the surface epithelium of the bursal folds. The GIIF3‐positive cells appear on the luminal surface of the follicles and occupy the place of the follicle‐associated epithelial cells. The NIC2‐positive cells become secretory in nature and differentiate to bursal secretory dendritic cells. The follicle formation possibly, requires the joint presence of both GIIF3 and NIC2 cells in the epithelium. Anat Rec 262:279–292, 2001. © 2001 Wiley‐Liss, Inc.

The extracellular matrix (ECM) of the spleen of the guinea fowl has been studied by immunohistochemistry. Each splenic compartment contains a different composition of ECM. Reticular-fiber-specific collagen III is expressed by the red pulp and thymus-dependent periarterial lymphatic sheath, but silver impregnation reveals a reticular-fiber-like structure in the periellipsoidal white pulp (PWP) where collagen III is absent. The penicillar capillaries of one central artery are enveloped by a single branching sheath or sleeve: the ellipsoid or Schweigger-Seidel sheath. The shape of the sleeve shows more resemblance to a deer antler than to an ellipsoid; it emerges at the beginning of the penicillar capillaries and ends at the edge of the PWP. It is wrapped by a novel discontinuous basement-membrane-like structure that expresses tenascin and that is named the capsule of the Schweigger-Seidel sheath (CSS). The cuboidal-shaped inner cell layer of the ellipsoid can be identified by a novel monoclonal antibody: BID3. BID3-positive stellate-shaped cells also occur in the PWP, suggesting that this cell population has migratory capability. Monoclonal antibody KIF8 recognizes an ECM component in the ellipsoid not only of guinea fowl, but also of chicken and quail, and may thus identify an ellipsoid-specific antigen. Collagen I is associated with both the basement membrane of the penicillar capillary and the CSS, whereas collagen III is present only in the CSS. Laminin is expressed in the red pulp, but its staining pattern does not indicate the presence of the "ring fibers", which suggests the absence of sinuses. Fibronectin is the only ECM molecule studied that occurs in every splenic compartment, indicating extensive intrasplenic cell migration.

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