The initial development and distribution of lymphocytes expressing surface IgM (sIgM) and of specific antigen-binding cells (ABC) were studied in the chicken in an attempt to gain information on the process by which B-cell diversity is generated. The antigens used were sheep erythrocytes (SE), keyhole limpet hemocyanin (KLH), and poly-L(Tyr, Glu)poly-D,L-Ala-poly-L-Lys (TGAL). The results indicate that generation of the total sIgM-positive population begins in the bursa and that specific clones of ABC develop in a fixed sequential pattern which is not influenced by either deprivation of or deliberate exposure to exogenous antigens. Cells bearing sIgM by immunofluorescence (IgM-positive cells) were detected first in the bursa on the 12th day of incubation, KLH-ABC and TGAL-ABC by the 16th day, and SE-ABC on the 18th day. The doubling time of the sIgM-positive population of bursal cells was determined to be approximately 10 h before significant antigen-independent seeding to the spleen began a few days before hatching. Clonal expansion of SE-ABC in the bursa also appeared to be antigen independent as was the initial development of SE-ABC in the blood and spleen which ceased abruptly after bursectomy at hatching. Specific ABC were observed to develop in multiple bursal follicles as small foci of ABC among the much larger total population of sIgM-positive cells within an individual follicle. Intravenously infused SE-ABC homed to the embryonic spleen but not to the bursa. The results are interpreted as favoring a hypothetical model in which individual stem cells give rise to multiple clones of B cells by a predetermined pattern of sequential expression of variable region genes.

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