Storage vesicles accumulate in MPSIIIB mouse cortical neuron cultures. A–C: MPSIIIB embryonic cortical neuron cultures were fixed at day 7 to 16 and processed for electron microscopy. Many polymorphic vesicles are visible in the cell soma (A and B) and prolongations (C). Scale bars, 5 μm (A); 0.3 μm (B and C). D and E: Cultures of wild-type or MPSIIIB embryonic cortical neurons were fixed at day 12 and immunolabeled with anti-LAMP1 antibodies (green). Nuclei were counterstained in blue. Abundant and often distended LAMP1-positive vesicles are visible in MPSIIIB neuronal processes. Scale bars, 10 μm. F and G: Cultures of wild-type or MPSIIIB embryonic cortical neurons were exposed to a lentivirus vector coding for the fluorescent lysosomal hydrolase IDUA-GFP at day 7 and examined at day 12 by fluorescent time-lapse microscopy (see Supplemental Videos 1, 2, and 3 at http://ajp.amjpathol.org). A cluster of IDUA-GFP–positive vesicles is visible in a MPSIIIB neuronal process (arrow). Scale bars, 10 μm.