Chromosome organisation in polyploid mouse trophoblast nuclei

At least one-third of mouse trophoblast cells undergo endoreduplication during the first half of gestation. It has been suggested that the endoreduplicated chromosomes may be polytenised. Here it is shown, using in situ hybridisation to the -1 antitrypsin genes, which map at a unique site, that while there is a tendency for duplicated chromosomes to cluster, this does not involve the complete fusion of replicated chromatids found in fully polytene chromosomes, and in a substantial proportion of homologues the sites on the chromosome arms corresponding to these genes are widely separated. The centromeres do not fuse into a single chromocentre but the possibility is not ruled out that individual chromosomes may be polytenised in the centromeric region. Evidence is also presented showing that endoreduplication in trophoblast nuclei is not accompanied by the formation of new prekinetochore structures, in contrast to the situation in polyploid mouse liver and C127 cells.