Microvilli and blebs as sources of reserve surface membrane during cell spreading

Abstract

The increase in surface area that occurs as cells spread from the rounded to the flattened state has been examined in synchronized BHK 21 cells in the scanning electron microscope. Rounded cells, whether in mitosis or dissociated and freshly seeded in culture, are covered with a mixture of folds, blebs, and microvilli. As cells spread, these protuberances disappear, first in the flattening marginal region and progressively submarginally until the entire cell surface is virtually smooth. The estimated surface area of rounded post-mitotic daughter cells, taking microvilli into account, is close to that of fully spread cells 4 h after mitosis. Likewise, rounded early mitotic mother cells, which are also covered with microvilli, have approximately the same surface as fully spread cells just prior to mitosis. These findings suggest that cells possess a membrane reserve in their microvilli and other protuberances which can be utilized for spreading and initiating cell locomotion.