BACKGROUND The medulla of the avian bursal lymphoid follicles contains heterogeneous cell populations, including the so-called medullary reticular epithelial cells (REC). These cells may contribute to the bursal microenvironment for B-lymphocyte differentiation and maturation. The bursa of Fabricius undergoes well-characterized posthatching developmental changes, but the age-related changes of the medullary REC have not been studied. The present study approaches this topic by analyzing hallmarks of epithelial cells: the occurrence of cytokeratin-type intermediate filaments and of desmosomes and desmoplakins in pigeon medullary REC. METHODS The bursae of Fabricius of male king pigeons (Columba livia L.) Morini's strain were examined at different ages (from hatching to 120 days after hatching) by light microscopic immunohistochemistry for pan-cytokeratins and desmoplakins and by transmission electron microscopy. The area occupied by medullary cytokeratin-immunoreactive 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 intermediate 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 decreased, although they retained their ultrastructural characteristics. The evaluation of desmoplakin immunoreactivity paralleled that of cytokeratin. CONCLUSIONS The present results demonstrate that the medullary REC of the pigeon bursa of Fabricius undergoes age-dependent changes parallel with that involving the whole organ. The possible contribution of medullary REC to the bursal microenvironment is discussed.