Microtubules and microfilaments during cell spreading and colony formation in PK 15 epithelial cells.

We have studied the distribution of microtubules and microfilaments during the cell spreading and subsequent colony formation in PK 15 pig kidney epithelial cells using indirect immunofluorescence. During the cell spreading on a solid substratum, microtubules grew out from the region around the nucleus, and a collar of microfilament bundles formed around the cell periphery. Although virtually all well-spread cells showed a complex microtubular network, distinctly different patterns of stress fibers were observed. In small colonies, the most commonly observed pattern was a ring of microfilament bundles that appeared to be in register between adjacent cells and encircled the entire colony in a fashion similar to that seen in single cells. In large colonies (more than 50 cells), approximately 60% of the cells displayed clearly stained microfilament bundles, either at the cell periphery or throughout their cytoplasm, whereas in the remaining 40%, no microfilament bundles were observed and only the outline of the cells was delineated by interaction with anti-actin. Such "negative" cells were seen in groups alongside "positive" cells (i.e., cells possessing extensive stress fiber networks) within the same colony. Independent of their stress fiber phenotype, all cells maintained a flattened shape and an extensive network of microtubules. We suggest that dense microfilament bundles are not a uniform feature of well-spread PI 15 cells in culture and that a loss of microfilament bundle occurs in some cells.