In situ arrangement of nor-bearing chromosomes in the interphase nucleus of Aotus trivirgatus.

The spatial organization of the two nucleolus-organizing region (NOR)-bearing chromosomes during interphase was studied in Aotus trivirgatus fibroblasts, using nucleoli as ultrastructural markers. Their distribution was examined by measuring the distances between them in 30 reconstructed nuclei, and comparing these experimental values with the theoretical ones obtained by simulation. Results were as follows: (1) the nucleoli are arranged in a polarized manner inside the nucleus; (2) the nucleoli are tightly bound to the nuclear envelope at two opposites sites; (3) the distance between the two nucleoli is variable, and is shorter than it would be if the two nucleoli were distributed at random. These findings indicate that the NOR-bearing chromosomes are fixed at the nuclear envelope in two opposite areas. They are also consistent with the hypothesis that each chromosome occupies a separate domain inside the nucleus. They can be interpreted according to the model in which chromosome arrangement within the interphase nucleus is based on the separation of the diploid complement into two independent haploid sets.