Temporal variation in nucleolar organizer region expression in bone marrow cells of individuals with leukemia

Silver staining was used to study nucleolar organizer region (NOR) expression in bone marrow cells obtained at two or, in one case, three time points from each of six leukemia patients. Using three measures of silver positivity, we observed that NOR expression was influenced by both metaphase stage and time. Silver positivity decreased significantly from one metaphase stage to the next, from prometaphase through late metaphase. When this variable was controlled for, significant changes in NOR activity were documented in comparisons between disease stages in the patients examined. However, patterns of NOR expression were not consistently associated with disease stage. These results indicate that in previous reports both the metaphase stage effect and the temporally changing nature of NOR activity have, as unrecognized variables, influenced observations of heterogeneity in NOR expression.