(a) B cell development in wild-type mice under normal steady state conditions. The pro-B, large pre-B, and small pre-B cells correspond to fractions C, C′, and D (as discussed in ref. 1). Notably, large pre-B cells are enriched for cycling cells expressing μ heavy chains/surrogate light chains (μ/SLC). (b) Potential influence of lowered availability of arginine on B cell development. The ratio of pre- to pro-B cells was decreased in arginase I transgenic mice compared to wild-type. It must be emphasized that expression of μ/SLC in large pre-B cells was not determined. However, the mRNA levels of stromal-cell derived factor-1 (SDF-1) and IL-7, and maturation beyond the pro-B to pre-B cell transition were not affected, or much less affected, by the reduced availability of arginine. Apoptotic events or a reduction in available survival factors produced by BM stromal cells may influence the transition from pro-B to large pre-B cells during reduced arginine availability.