[Electron microscopic study of the minichromosomes in murine fibroblast cells fixed in situ].

The ultrastructure and differential staining of minute chromosomes (MC) from mouse fibroblasts line C3H10T1/2 (LS9/0.1) was studied. MC in mitotic cells of this line contain R-positive material and are not stained according to the G- and C-methods. Reconstruction of ultrathin serial sections made it possible to identify MC in situ. It is shown that the basic structural unit of MC is a DNP fibril 20-30 nm in diameter. The density of packing of the MC material corresponds to that of normal chromosomal arms. In some cases the fibrillar material is present between MC and normal chromosomes. It is suggested therefore that some MC may be structurally linked to a normal chromosome arm. The MC structure is discussed in terms of the ultrastructural properties of R- and C-positive segments.