We have used high-pressure freezing, freeze-substitution and dual-axis electron microscope (EM) tomography to study the organelles involved in the synthesis, processing and secretion of insulin in three dimensions (3D) at ~6nm resolution in the pancreatic beta cell line HIT-T15. These data have: (1) revealed the complexity of structural relationships among the Golgi, endoplasmic reticulum (ER) and compartments of the endosomal-lysosomal system [1], (2) provided evidence that multiple transport

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... echanisms act in concert in the same region of the Golgi ribbon [2], and (3) suggested a role for the ER in regulating membrane traffic/sorting at the trans Golgi -the presumptive site where proinsulin is sorted and packaged into nascent secretory granules [1, 2] .