Precursor B Lymphocytes — Specific Monoclonal Antibodies and Genes
The pool of mature, surface immunoglobulin (Ig) positive, antigen-sensitive B cells of a mouse has been estimated to contain 5 × 108 to 109 cells (Osmond 1986). Development from pluripotent stem cells and committed progenitors generates daily some 5 × 107 cells, of which 3 × 106 enter the pool of mature B cells. The same number of mature B cells must consequently die daily to keep the pool at a constant size. The immediate precursors of the mature B cells, called pre B cells, can be identified to be at various stages of their development by the genomic context in which their Ig gene loci are found. Thus, rearrangement of a DH to a JH segment on the Ig heavy (H) chain locus precedes that of VH to DHJH. This, in turn, is followed by rearrangements of the Igk light (L) chain and, finally, of the λ L chain locus (Tonegawa, 1983). No more than five divisions are estimated to occur from the earliest (i.e. at the time of DH to JH rearrangement) to the latest (i.e. at the time of surface Ig expression) state of their B lineage development in either fetal liver during embryogenesis or in bone marrow throughout adult life. The daily generation of 5 × 107 cells must, therefore, be fed from more than 106 B-lineage-committed progenitors and from stem cells. Very little is known of the forces that drive this early expansion of cells towards the B lineage, which should be antigen-independent, polyclonal and self-renewing, and might occur in fetal liver and bone marrow in contact with stromal cells.
KeywordsFetal Liver Chain Locus Stern Cell Basel Institute Amino Terminal Portion
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