Murine intracisternal A type particles fail to separate from the membrane of the endoplasmic reticulum

Analysis of serial sections of murine cells containing intracisternal A particles revealed that over 99% of all A particles remain in a budding configuration. This indicates that these particles fail to detach from the membrane of the endoplasmic reticulum. This observation explains how, despite their intracellular abundance in certain murine tumors, no extracellular A-type particles can be found.