Organellar relationships in the Golgi region of the pancreatic beta cell line, HIT-T15, visualized by high resolution electron tomography

The positional relationships among all of the visible organelles in a dense ly packed region of cytoplasm from an insulin secreting, cultured mammalian cell have been analyzed in three dimensions (3-D) at approximate to6 nm re solution. Pa rt of a fast frozen/freeze-substituted HIT-T15 cell that inclu ded a large portion of the Golgi ribbon was reconstructed in 3-D by electro n tomography. The reconstructed Volume(3.1 x 3.2 x 1.2 mum(3)) allowed site s of interaction between organelles, and between microtubules and organella r membranes, to be accurately defined in 3-D and quantitatively analyzed by spatial density analyses. Our data confirm that the Golgi in an interphase mammalian cell is a single, ribbon-like organelle composed of stacks of fl attened cisternae punctuated by openings of Various sizes [Rambourg, A., Cl ermont, Y., & Hermo, L. (1979) Am. J. Anat 154, 455-476]. The data also sho w that the endoplasmic reticulum (ER) is a single continuous compartment th at forms close contacts with mitochondria, multiple trans Golgi cisternae, and compartments of the endo-lysosomal system. This ER traverses the Golgi ribbon from one side to the other via cisternal openings. Microtubules form close, non-random associations with the cis Golgi, the ER, and endo-lysoso mal compartments. Despite the dense packing of organelles in this Golgi reg ion, approximate to 66% of the reconstructed Volume is calculated to repres ent cytoplasmic matrix. We relate the intimacy of structural associations b etween organelles in the Golgi region, as quantified by spatial density ana lyses, to biochemical mechanisms for membrane trafficking and organellar co mmunication in mammalian cells.