Fig. 5. The tubular plastid shows tight association with centrosomes. (A) Infected cultures were double labeled with antibodies to ACP (green) and centrin (red). Centrosomes (arrowheads) can be seen in association with the tubular plastid. Cultures were fixed in situ, embedded in Epon (B-D) and ultrathin sections were cut parallel to the culture surface and analyzed by electron microscopy. (B,C) Two consecutive serial sections through a developing schizont with its host cell (HC). The nucleus (N) is located in the center of the cell and as seen in a higher magnification (C), four spindle poles (large arrowheads indicate the centrioles) are present. Several intranuclear microtubules (small arrowheads) indicate that this cell is in the early stages of mitosis. Wrapped around the upper right corner of the nucleus is the plastid (P) as a continuous tubular organelle, which is surrounded by four membranes and closely apposed to the centrioles of the centrosome (D, white arrowheads indicate the four plastid membranes; black arrowheads, the two membranes of the nuclear envelope). This organelle is devoid of tubular cristae, which are clearly identifiable on the multiple sections through mitochondria (M). Ultrathin sections were also cut from material that was fixed with formaldehyde and embedded in LR-White resin to preserve antigenicity. Sections were reacted with the ACP antibody followed by an anti-immunoglobulin labeled with 10 nm gold particles. Again a tubular organelle in close proximity to the nucleus was observed (E and enlarged in F) which under this conditions showed heavy and specific gold labeling (arrowheads).