[Non random position of metaphasic chromosomes. II. Effect of condensation on the position of X chromosomes].

A study is reported on the location of X chromosomes in human female lymphocytes and fibroblasts at mitosis. By the use of BrdU, incorporated during either early or late S-phase, the late and the early replicating Xs were identified, so that it could be determined which was more peripheral in the metaphase plate. In lymphocytes, when BrdU is incorporated during late S-phase, the late-replicating X, which is the less condensed, is the more peripheral. On the contrary, when BrdU is incorporated during early S-phase, the early replicating X, which is the less condensed, is the more peripheral. On the other hand, in fibroblasts grown on coverslips, the late-replicating X is always peripheral, regardless of the time of BrdU treatment. These results confirm the peripheral location of the late-replicating X and demonstrate the presence of artifacts, depending on chromosome condensation, occurring during the procedures of slide preparation.