Evidence for a blood-bursa barrier

Distribution of bursal secretory dendritic cells in the chicken

BACKGROUND

The bursa of Fabricius provided the microenvironment for B-cell differentiation. Continuous contact between lymphoid cells and antigen in the bursa further suggested that antigenic material has an important influence on the maintenance and development of B cells in the bursa. In addition, a dendritic cell, the bursal secretory dendritic cell (BSDC), has been identified in the medulla. The hypothesis that, in the bursal follicles, the contact between the lymphoid cells and the antigen may be mediated by dendritic cells prompted us to identify a bursal dendritic cell that becomes activated after contact with the antigen.

METHODS

A polyclonal antiserum to S-100 protein was used to identify bursal dendritic cells because S-100 protein, a calcium-binding protein, has been shown to be a marker for the identification of chicken dendritic cells following recent contact with antigen.

RESULTS

At every age investigated, S-100-positive cells showed a location and shape identical to those described for BSDCs. Positive cells were found within and under the follicle-associated epithelial cells (FAE), indicating that these cells were strategically placed where they would encounter the antigen. In addition, positive cells were found arranged along the corticomedullary junction, which is a regenerative zone for the BSDC. After 10 weeks of age, the number of positive cells dramatically decreased, suggesting that the endocytic activity of the FAE may become impaired as the bursa regresses.

CONCLUSIONS

The polyclonal antiserum to S-100 protein identified the BSDCs in the bursal follicles. Positive cells may be BSDCs that have undergone a functional activation after contact with the antigen. These cells may have a role as antigen-presenting cells in the bursal follicles. Hence, these cells may be involved in the events that lead to B-cell differentiation.

Dome epithelium and follicle-associated basal lamina pores in the avian bursa of Fabricius

BACKGROUND

The immunological role played by the avian bursa of Fabricius has been well established. Although numerous studies have also reported on the development and general morphology of this organ, some structure-function relationships still have not been fully explained.

METHODS

Bursae from chickens at three developmental stages were removed and examined by scanning electron microscopy. Routine preparation was used as well as sonication (microdissection). Micrographs were used for qualitative morphological study and for quantitative morphometric analyses.

RESULTS

Routine SEM observations were similar to those previously reported in the literature. Sonicated specimens allowed topographical study of various levels of surface erosion. Two types of surface cells were observed: typical absorptive epithelium and follicle-associated epithelial (FAE) cells. Erosion of the dome surface epithelium revealed basal lamina pores in the region over the subepithelial lymphoid follicles. These pores were present at hatching. Morphometric analysis of dome and pore areas revealed that the pore area decreases in relation to dome area with aging.

CONCLUSIONS

Basal lamina pores may provide a communication route between the lymphoid follicles and the external environment via the FAE cells. Also, the close association between the FAE cells of the epithelial domes, the epithelial pores, the capillary complex of the previously described bursal--blood barrier, and the subepithelial lymphoid follicles could represent a morphological "pore complex" that matures early in posthatching development and may be related to the immunological function of the bursa.