Bursal development in normal and testosterone-treated chick embryos.

The development of the bursa of Fabricius was studied in normal and testosterone-treated embryos. The bursal anlage appears on the 5th day of incubation as an outgrowth of the dorso-caudal wall of the cloaca. By Day 7, the bursal lumen is present and separated from the anal invagination by a thin epithelial membrane. Shortly after this developmental state, the epithelial membrane disappears and the bursal lumen freely communicates with the amniotic cavity. The remnant of the bursal anlage forms the ventral wall of the anal invagination that is actually the bursal duct. Disappearance of the remnant of the bursal anlage, which takes place after the 16th day of incubation, allows the bursa to join the cloaca. Bursal folds appear on the 10th day of incubation. Mesenchymal cells then differentiate into dark and light cells. The dark cells, between 11 and 14 days of incubation, mobilize in the mesenchymal network and assemble in small groups under the epithelial rudiment of the folds. They, then, enter the epithelium where they induce bud formation. During the assemblage of dark cells beneath the epithelium and their emigration into the epithelium, they acquire cytoplasmic granules that resemble avian secretory cells. The light cells do not associate with the epithelium. They may be the precursors of cortical reticular cells. The differentiation of dark cells is inhibited in the presence of testosterone. Therefore, the failure of bud formation and subsequent follicular formation in the bursa of testosterone-treated embryos may be attributed to the elimination of an induction signal supplied by the differentiated dark cells of the mesenchyme.