Locally applied testosterone is a novel method to influence the development of the avian bursa of Fabricius.

Bursa of Fabricius is a primary lymphoid organ in the birds and its microenvironment is responsible for the B cell maturation. The epithelial anlage develops through epithelial-mesenchymal interaction into which -as a third party - hemopoietic cells immigrate. Chemical bursectomy can be made by dipping or injecting eggs by testosterone solution. Both methods of treatments compromise the development of the whole embryo. Heparin beads of 70-150 microm in diameter were soaked in 5% testosterone solution and implanted into the tail bud of 4 day old chicken embryos before the bursal anlage began to develop at embryonic day 5. The locally applied testosterone inhibited the differentiation of the bursal secretory dendritic cell (BSDC) precursors, which were the inducer of the follicular epithelial bud formation. Testosterone treatment did not blocked the entering of B cells and other hemopoietic cells into the bursal mesenchyme, but in the absence of BSDC precursor cells the dendroepithelial microenvironment is not established, subsequently B cell precursors are lodged only in the bursal mesenchyme. This method is suitable for locally introducing other regulatory molecules, involved in bursal development.