Fig. 3. Ultrastructural analysis of PML nuclear bodies in S phase by correlative LM/ESI. (A) Correlative fluorescence and electron microscopy of a G2 cell, fluorescently labelled for PML protein. Elemental maps of nitrogen (N) and phosphorus (P), and the merged maps of a PML nuclear body and its surrounding nucleoplasm reveal protein-based (green) and nucleic-acid-based (red) components. Chromatin appears yellow in the merged image because of its high N and P content. A field is shown at higher magnification. The imaged PML nuclear body (right hand panel, red) makes extensive contacts (white arrowheads) with the surrounding chromatin (yellow) directly and through non-chromosomal protein (blue) contacts. (B) Electron micrographs of PML nuclear bodies in an early S phase cell. PML microbodies are indicated by white asterisks and chromatin contacts by white arrowheads. (C) Electron micrographs of PML nuclear bodies in a mid S phase cell. Chromatin contacts are indicated by white arrowheads. Bars, 500 nm.