Dynamics of the spindle pole body of the pathogenic yeastCryptococcus neoformansexamined by freeze-substitution electron microscopy

Abstract

Cryptococcus neoformans is an opportunistic human pathogen belonging to basidiomycetous fungi and has unique properties in cell cycle progression. In the present study, dynamics of the spindle pole body (SPB) during the cell cycle was examined using freeze-substitution and serial thin-sectioning electron microscopy. The SPB was located on the outer nuclear envelope and appeared either dumbbell- or bar-shaped in G1 through G2 phases. At the beginning of prophase, globular elements of the SPB enlarged, associated with numerous cytoplasmic microtubules, and separated on the nuclear envelope. At prometaphase, the SPBs entered the nuclear region by breaking a part of the nuclear membrane, were located at the isthmus, and were associated with numerous nuclear microtubules. The nuclear division process was carried out in the daughter cell, though the nucleolus remained in the mother cell. At anaphase, one half of the nucleus returned to the mother cell. At telophase, the SPB element was extruded back to the cytoplasm from the nuclear region. By analyzing serial sections of 63 cells, duplication of the SPB was found to take place in the early G1 phase. Thus, the location, structure, and duplication cycle of the C. neoformans SPB are different from those of Saccharomyces cerevisiae, but have similarities to those of Schizosaccharomyces pombe.