Microvilli of HeLa cells cultured in vitro were preserved for electron microscopic examination at different stages of routine cultivation procedures. By a double-embedding technique, vertical sectioning for electron microscopy was possible. It revealed that, although the microvilli were present on all sides of the cell in the dispersed stage and in the attached stage, they were not present on the bottom of the cell when it was stretched on the surface of the dish. When the cells were grown in dense colonies, they were found on top of each other, and microvilli were present on all sides, except on the bottom surface of those cells in contact with the dish. We achieved a more dramatic demonstration of the microvilli by developing a surface-replica technique which retains their spatial arrangement and permits characterization of the distribution of their number, length, and diameter.

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