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In differentiating B cells, proliferation of endoplasmic reticulum appears as tightly clustered juxtanuclear spherical structures which expand to fill the cytosol without synchronizing to cell expansion.

The differentiation of B cells into Ig-secreting plasma cells requires the expansion of secretory organelles to cope with the increased cargo load. To evaluate the timeline of this process, we have quantitated the kinetics of secretory organelle expansion relative to Ig secretion and examined regulatory components of secretory transport following in vitro activation of human B lymphocytes. Unstimulated B cells contain minimal endomembranes. After activation, ER membrane induction appears as tightly packed spherical structures of 0.5–1 μm diameter concentrated in a juxtanuclear position. When the cells differentiate into plasmablasts, there is dramatic cell-size increase, but the ER remains concentrated close to …