Figure 3.

Formation of the pre-B cell receptor and its effect on B cell development. DJ_H-rearranged pre-B I cells (5) in contact with bone marrow stromal cells undergo asymmetric divisions that yield one cell, which retains contact with stroma and, thus, pre-B I differentiation status. The other cell loses contact to stroma, is induced to differentiate to a pre-B II cell in rearranging a V_H to D_HJ_H segment on the H chain loci. Nonproductive (np) rearrangements leads to μH chain-negative cells that die (). Productive rearrangements can lead to μH chain that cannot pair with surrogate L chain (15). Cells with nonpairing μH chains cannot form a pre-B cell receptor and die. On the other hand, pairing μH chain leads to pre-B cell receptor expression on the surface, which induces pre-B II cell proliferation. The higher the avidity, the longer is the proliferative phase (high avidity  = green, low avidity = yellow). In the simplest form of this model, the surrogate L chain assumes a ligand function for the μH chain to induce proliferation, hence, an external ligand binding to V_preB and/or V_H must not necessarily be present to induce clonal expansion of pre-B cell receptor-expressing pre-B II cells.