STRUCTURE AND POSITION OF HETEROCHROMATIC CHROMOSOMES IN INTERPHASE NUCLEI OF MICROTUS AGRESTIS

Structure and position of chromocenters in interphase nuclei of Microtus agrestis (brain tissue and cultured kidney epithelial cells) were investigated. The chromocenters derived from heterochromatic portions of the very large sex chromosomes appear to be compact in nerve cells and variably condensed in cultured kidney epithelial cells. This variation in condensation is independent of the cell cycle. In smear preparations of brain tissue two thirds of the nuclei contain two chromocenters, whereas one third of the nuclei show one double sized chromocenter, presumably a fusion of two. Suspensions of nerve cells in orcein-acetic acid show that the large fused chromocenters are attached to the nuclear membrane in 88% of the nuclei with one chromocenter. If two isolated chromocenters are present they are always localized adjacent to the nuclear membrane. Measurements of the distance between the two chromocenters show a prevalence of two types of positions: a fusion to a double sized chromocenter and a vis-a-vis position. The frequency distribution of the distances suggest a tendency of somatic pairing of the heterochromatic chromosomes if they come close enough together. In mononucleated sister cells and in sister nuclei of binucleated cells derived from a mononucleated mother cell the position of the chromocenters is identical in the two nuclei. © 1969 Springer-Verlag.