The behavior of heterochromatin in mouse and human nuclei.

The arrangement of heterochromatin in various human and mouse nuclei has been analyzed with C banding. In most nuclei of 7-day mouse trophoblast, the heterochromatin consists of twin dots, or bigger clumps, apparently attached to the nuclear membrane. This finding agrees with the observation that most of these nuclei, which range from diploid to highly polyploid, show endomitotic stages. No polarization of heterochromatin in a Rabl orientation is seen in the trophoblast nuclei. Neither is a Rabl orientation found in the interphases of cultured human lymphocytes or fibroblasts. From their telophase arrangement, the chromosomes have obviously spread rapidly around the nuclear membrane. In many of the giant mouse trophoblast cells in vivo and in vitro, heterochromatin is apparently underreplicated. The same is true of giant cells in human hydatidiform moles and cervical cancer. Of the 82 cervical cancers analyzed, 46 showed chromocenters, and each tumor was characterized by its own pattern of heterochromatin.