Cytological evidence of transient sex chromatin decondensation during compensatory hypertrophy in rat livers.

The left lateral lobes were surgically removed from livers of female rats. The number of cells containing sex chromatin bodies was counted in the surgically removed lobes and compared with counts from the remaining lobes removed at various intervals after the operation. The proportion of cells showing positive sex chromatin was found to decrease to nearly one-half the initial preoperative value by four days after partial hepatectomy. Sex chromatin frequency returned to near preoperative levels by 21 days. A 3H-thymidine autoradiography showed that the number of cells in the S-phase was less than 5 percent at the postoperative time when sex chromatin frequency was lowest, thus ruling out the possibility that the decreased numbers of sex chromatin positive cells was related to genome replication. These data show that condensation of the late-replicating, and, presumably, inactive X-chromosome is not permanent, a fact that may relate to X-inactivation observed in embryogenesis.