M to G1

Surface changes in shape and texture are characteristic of hamster ovary cells grown in the laboratory. Daughter cells (top micrograph), just entering the G1 phase, still have “hairy” surfaces; the projections may be remnants of the longer strands that are abundant in the M phase. The newborn cells are just beginning to spread out on the substrate; the smaller smooth sphere is a fragment of cell cytoplasm and not a living cell.

G1

The flattened cells (bottom micrograph, left of center) have reached a late stage in the G1 phase; their surfaces are covered with microvilli (small hairs) and blebs (blister-like spheres). Several have established contact with adjacent cells.

S

The even flatter cells (top right micrograph) have entered the S phase. Their surfaces are free of blebs and the microvilli are fewer and less prominent. On some parts of the cells’ edges vertical “ruffles” (light areas) have appeared.

G2

Next the cells have entered the G2 phase. Ruffles are still present; the microvilli are more prominent and one cell (foreground) shows an array of blebs. The significance of the surface changes is not yet wholly understood, but their coincidence with the four cell-cycle phases suggests that cell membrane is active in the cycle. All of these electron micrographs are by Keith R. Porter and his colleagues.