Studies on the intranuclear distribution of human and mouse genomes and formation of human-mouse hybrid cells.

Autoradiographic examination of early hybrid cells formed by the fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells revealed that human and mouse chromosomes are often separated within metaphase and interphase nuclei. Although the marked separation of human and mouse chromosomes progressively disappeared with succeeding cell division, the occurrence of sectored nuclei in 16-cell hybrid colonies and the labeling pattern of human chromosomes within separated metaphases indicate that separation of human and mouse chromosomes may persist through several mitoses. Fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells was coupled with aminopterin selection to study other aspects of hybrid cell formation. Hybrid cells arose from heterokaryons by cell division. Nuclear fusion may have occurred in two out of several thousand heterokaryons examined. However, these could reflect the close apposition of adjacent nuclei. A large fraction (greater than 0.5) of D98/3T3 heterokaryons underwent at least one cell division. However, the number of hybrid colonies containing more than eight cells at ten days following fusion was about 0.03 of the total number of heterokaryons. Many hybrid colonies arrested growth before the 8-cell stage, and the cells in such colonies exhibited nuclear abnormalities.