Investigations on chromatin condensation of hen erythrocyte nuclei in vitro: An ultrastructural and biochemical study

Abstract

Hen erythrocyte nuclei, isolated in non-buffered sucrose, (3 mM Mg 2+, at low ionic strength) have a condensed chromatin to which hemoglobin is bound. Incubation of the isolated nuclei in 0.125 M phosphate-buffer solutions of increasing pH induces a release of the bound hemoglobin and a swelling of the nuclei. Between pH 6.4 and 7.0 both processes reach a plateau and above pH 7.0 a second steep increase in nuclear volume is observable, leading to nuclear disruption above pH 7.4. Titration at low ionic strength shifts the process of hemoglobin release and nuclear swelling to higher pH values. Hemoglobin release and nuclear swelling are fully reversible by backtitration to pH 5.8. The nuclear swelling and shrinking is observed electron-microscopically as decondensation and condensation of the chromatin. The results of these investigations suggest that hemoglobin acts like a cation in the maintenance of nuclear condensation under the conditions used for the isolation of hen erythrocyte nuclei, but that this action is unlikely the physiological mechanism causing chromatin condensation during the maturation of erythroid cells.