Visualization of a repeating subunit organization in rat hepatocyte chromatin fixed in situ

The chromatin structure of rat hepatocytes fixed in situ was investigated in thin sections selectively stained for DNA with the Feulgen-like osmium-ammine reaction. Visualization of the chromatin structure was facilitated by the dispersal that occurred in the chromatin of regenerating rat hepatocytes at 24 h after partial hepatectomy. Our results show that repeating subunits were present both in the clumps and in the extended fibres of chromatin. These subunit particles appeared to be composed of an electron-translucent core delimited by a strongly electron-opaque DNA ring. Depending on the section plane, they displayed a morphology varying from a flat, oval shape to a rough circle. The mean diameter of the repeating subunits was 11 +/- 0.8 nm (S.D.) with a range from 9.5 to 13 nm. The thickness of the DNA ring was about 3 nm. Regarding the association of the subunits in the chromatin fibres, two types of fibres were visualized, 11 nm and 20-25 nm thick, respectively. The thinner fibres were composed of a linear array of adjoining subunits. In the thicker fibres the subunit organization did not clearly form discrete superunit structures.