Color enhanced transmission electron micrograph of epithelial cilia and microvilli from rat trachea, fixed in aldehyde and osmium tetroxide and exposed to low molecular weight galloyl-glucoses (tannic acid). The ciliated cells of mammals often have microvilli interspersed among the cilia. These transverse sections near the cell surface permit a comparison of the size and internal structure of the two types of cell process. Both are limited by a trilaminar unit membrane and have a cytoplasmic matrix of similar density. The nonmotile microvilli have no resolvable internal structure, whereas the motile cilia have a consistent pattern of nine doublet microtubules uniformly spaced around a central pair of singlet microtubules.