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B cell precursors in chick embryos surgically bursectomized at 72 h of incubation

To determine the presence of precursor B cells in chick embryos surgically bursectomized at 72 h of incubation (E‐Bx) we studied chick chimeras that were produced by establishing parabiotic connections between blood vessels of chorioallantoic membranes of normal and surgically bursectomized chick embryos. Using sex chromosomes and a B cell alloantigen (Bu‐1a) as markers we showed that chick embryos bursectomized at 72 h of incubation contain B cell precursors capable of colonizing the bursa of Fabricius and developing into B lymphocytes. The repopulation capacity of 14‐day‐old embryonic spleen cells from E‐Bx recipients was tested by transferring them into age‐matched X‐irradiated Bu‐1‐disparate embryos. The results show that B cell precursors are present in 14‐day spleen of chick embryos bursectomized at 72 h of incubation. These precursors carry the Bu‐1 B cell alloantigen, suggesting that commitment to the B cell lineage can take place in the absence of bursa.