Rearrangement of chicken immunoglobulin genes is not an ongoing process in the embryonic bursa of Fabricius.

We report a molecular analysis of the chicken Ig loci in single bursal follicles from 3- to 7-week-old chickens. Each follicle contained between 10(5) and 3 X 10(5) cells. The Ig gene rearrangement patterns obtained were compared to the pattern observed with the corresponding total bursal DNA. The results obtained for the light chain locus imply that a very small number (two on average) of rearrangement events takes place in each follicle. For the heavy chain locus similar results were obtained, each follicle showing a more restricted pattern than the total bursa. These data favor a model in which each follicle is colonized by a very few prebursal stem cells that are committed to a particular Ig gene rearrangement at the very beginning of the development of the embryonic bursa. The role of the bursa as the organ in which such a committed stem cell population for the B-cell lineage arises is discussed.