Membranes of the mitotic apparatus of mammalian cells.

HeLa cells in mitosis are fixed by KMnO4 to obtain a good preservation of membranes. Inside and around the mitotic apparatus a vesicular, a tubular and a cisternal type of membranes are demonstrated. Small spherical or ellipsoidal vesicles are distributed at random in the spindle area. Zones around the poles and in the midbody of telophase cells are almost free from vesicles. Elements of the tubular ER are often aggregated in parallel arrays at the periphery of the spindle region, thus resembling dictyosomes. During telophase tubular ER is most prominent at the cleavage furrow. Layers of concentrically arranged flat cisternae surround the spindle region during the middle stages of mitosis. Radiating from the poles such membranes form large asters, thereby interrupting the concentric arrangement. This pattern might act as a barrier against larger cytoplasmic organelles or as a cisternal system to bring about an oriented transport of molecules from or to the poles. Fragments of the two large indentations of the nuclear envelops in prophase are aggregated in the spindle region and later on contribute to form new envelopes around the daughter nuclei.