THE SPATIAL LOCALIZATION OF HOMOLOGOUS CHROMOSOMES IN HUMAN FIBROBLASTS AT MITOSIS

Chromosomes from ten human male fibroblast metaphases were completely reconstructed from electron micrographs of serially sectioned material . Chromosome centromere positions were determined by finding the three -dimensional coordinates of the centromere midpoint. The data set show ed the identity of nine chromosome types (chromosomes 1, 2, 3, 6, 9, 1 6, 17, 18 and the Y chromosome) preserved as they are positioned in vi vo. The results indicate that there is (1) no significant association of the homologous chromosomes examined, (2) a significant tendency for a central location of the Y chromosome and of chromosome 18, (3) a si gnificant tendency for a peripheral location of chromosome 6, (4) no s ignificant tendency for homologous chromosomes to reorganize as metaph ase advances and (5) no significant differential condensation across t he metaphase plate. Therefore, the only organization pattern observed for the centromeres of the homologous chromosomes studied is some sort ing by size across the metaphase plate. These results may be typical o f dividing cell types. Different chromosome arrangements are found in some non-dividing cell types (e.g. mammalian brain cells). The differe nt distributions of chromosomes in different cell types can be conside red as forms of ''nuclear differentiation''. It is postulated that nuc lear differentiation may be related to cell differentiation.