Fig. 4. Ultrastructure of PDMP-induced ER structures in induced HeLa TetOff cells. Conventional EM analysis at low (A) and high (B) magnification. (A) An SER patch is indicated by the arrow. Abundant free ribosomes are visible in the surrounding cytosol. The asterisk indicates the intercellular space. N, nucleus; M, mitochondrion; ER, elongated ER cisterna partly covered with ribosomes and containing a coated protrusion, which probably corresponds to an ER exit site (arrowhead). (B) A portion of a particularly convoluted SER patch consisting in a dense network of frequently branching tubules of small calibre. (C) G-6-Pase EM cytochemistry of a PDMP-treated cell. The arrow indicates an area of packed G-6-Pase-positive tubules that presumably corresponds to an SER patch. N, nucleus. Bars, 0.5 µm (A); 0.1 µm (B); 1 µm (C).