Ultrastructure of the nuclear matrix from Physarum polycephalum during the mitotic cycle

Abstract

We have investigated the ultrastructural changes of the nuclear matrix in nuclei of Physarum polycephalum during the mitotic cycle. Thin sections of high-salt and DNase-extracted nuclei reveal a residual nuclear structure consisting of the lamina and nuclear pore complexes, an internal fibrillogranular matrix, and the residual nucleolus. The lamina remains present until anaphase. The internal matrix is attached to the lamina in interphase but becomes separated from it in prophase. As prophase progresses, the residual nucleolus moves to the periphery of the structure and disintegrates. The internal matrix condenses and takes a more central position in the structure. Its distribution during mitosis follows the known movements of the chromosomes of nuclei in fixed plasmodia. DNase labile threads are continuously attached to the matrix. The results suggest that the internal matrix serves as a carrier of the chromosomal DNA comparable to the function of the chromosomal scaffold of higher eucaryotes.