Changes in endonuclease activity and in chromatin structure of rat hepatocytes during fetal and neonatal life

A developmental study of rat hepatic endonuclease has been performed. Nuclei, from different stages of hepatocyte maturation, were analyzed for endogenous endonuclease activity. The chromatin extracted from these nuclei does not show any fragmentation during the first 17 days of fetal development. On the 18th day of fetal life there is a massive increase in specific endonuclease activity. At birth this activity reaches a maximum level (3.5 units/mg DNA); thereafter it undergoes a gradual decrease. The size of the basic DNA repeats produced by the endonuclease action is 218.9 +/- 1.6 in 18-day-old fetuses and decreases to 204.9 +/- 2.5 in 19-day-old fetuses, a value which remains constant in the following fetal and postnatal life. This difference in monomer size is due to changes in the chromatin structure. Micrococcal nuclease digests show that the "nucleosome core" does not change during hepatocyte development. Therefore, the difference in size of the endonuclease DNA fragments must be due to the linker regions.