AGE-RELATED-CHANGES IN THE MEDULLARY RETICULAR EPITHELIAL-CELLS OF THE PIGEON BURSA OF FABRICIUS

Background: The medulla of the avian bursal lymphoid follicles contain s heterogeneous cell populations, including the so called medullary re ticular epithelial cells (REC). These cells may contribute to the burs al microenvironment for B-lymphocyte differentiation and maturation. T he bursa of Fabricius undergoes well-characterized posthatching develo pmental changes, but the age-related changes of the medullary REC have not been studied. The present study approaches this topic by analyzin g hallmarks of epithelial cells: the occurrence of cytokeratin-type in termediate filaments and of desmosomes and desmoplakins in pigeon medu llary REC. Methods: The bursae of Fabricius of male king pigeons (Colu mba livia L.) Morini's strain were examined at different ages (from ha tching to 120 days after hatching) by light microscopic immunohistoche mistry for pan-cytokeratins and desmoplakins and by transmission elect ron microscopy. The area occupied by medullary cytokeratin-immunoreact ive cells was evaluated with quantitative image analysis. Results: At hatching, cytokeratin immunoreactivity was not detected in the bursal lymphoid follicles. During the posthatching growth period of the organ (7-75 days), there was a progressive and significant increase in the area occupied by cytokeratin-immunoreactive medullary REC, in the inte rmediate filaments filling the cytoplasm of REC, and in the number of desmosomes. Conversely, during the regressive period analyzed (90-120 days), the density of cytokeratin-positive cells progressively decreas ed, although they retained their ultrastructural characteristics. The evaluation of desmoplakin immunoreactivity paralleled that of cytokera tin. Conclusion: The present results demonstrate that the medullary RE C of the pigeon bursa of Fabricius undergoes age dependent changes par allel with that involving the whole organ. The possible contribution o f medullary REC to the bursal microenvironment is discussed.