The occurrence of microvilli during spreading and growth of BHK21/C13 fibroblasts

An electron microscope replica technique has been used to examine the occurrence of microvilli in a normally growing population of BHK21/C13 fibroblasts attached to a glass substrate. Normally, microvilli were observed associated with round, dividing or partially spread cells and were rarely seen on fully-spread cells. Freshly trypsinized cells at 37°C, and trypsinized cells maintained for 24 h at room temperature showed a high incidence of microvilli when plated out on a glass coverslip. As incubation continued at 37°C the proportion of cells with microvilli gradually diminished from the original figure of > 90 % to 20-30 % after 24 h, a figure also given by the normally growing population. We conclude that microvilli are labile structures which appear on these cells at the particular phases in the cell cycle when the cell is rounding up or respreading. We suggest that the microvilli may function as storage organelles for the cell membrane whereby the cell can conserve excess membrane as it rounds up. Thin section studies revealed within each microvillus, several 40-60 Å microfilaments which were indistinguishable from the microfilaments present in a layer just under the cell surface. It is suggested that microvilli may be formed as a result of the activity of this layer during rounding up of a cell or during cell movement.