In mitotic cells, some proteins are known to specifically localize to the periphery of the condensed mitotic chromosomes. This region, also known as the perichromosomal layer (PCL), contains proteins and RNA, many of which originate from the nucleolus. The most well studied component, Ki67, is required for recruitment of several proteins and RNAs to the PCL, and is thus believed to act as a scaffolding protein for the organization of this compartment. The PCL has been suggested to act as a barrier between mitotic chromosomes and the cytosol, as well as serving as a surfactant that facilitates the separation of the sister chromatids during mitosis. In addition, the PCL may help to distribute certain components equally between daughter cells.
The perichromosomal layer, annotated as mitotic chromosomes in the Cell Atlas, is present from prophase to telophase in mitosis. As the mitotic chromosome proteins are coating the surface of the chromosomes, the staining is visible as thin lines around the chromosomes of dividing cells.
Read more about the proteome of mitotic chromosomes as a substructure of the nucleoplasm.