Contact between cells alters their behavior, as demonstrated by a culture of rabbit kidney epithelial cells. Notice, during the high speed playback of this time-lapse sequence, lamellipodia are continually extended and contracted at various points along the peripheries of a group of cells, except along margins where neighboring cells overlap. Such quiescent regions are formed whenever migrating cells collide. Fibroblasts and some other cell types often are able to overcome the adhesions formed following contact, allowing them to eventually break apart and continue migrating in different directions. RK13 cells and other epithelial lines, however, tend to remain connected to one another once contact is made, so that many small colonies are formed in a culture chamber.
The colonial behavior of epithelial cells in culture is reflective of their activity in the body, where they characteristically are joined together to form contiguous sheets. Epithelial sheets line the internal and surfaces of the body and its organs. Their integrity in vivo is crucial since they primarily serve as barriers against diffusion. Colonies of cultured epithelial cells form similar sheets over time, the lamellipodia of the cells along their peripheries stretching out across the chamber surface to bridge any extracellular gaps.
Each time a lamellipodium or other protrusion is extended or contracted by a RK13 cell, cytoplasmic streaming occurs. The cytoplasm in the central region of the cell, termed plasmasol, exhibits a fluid-like consistency, whereas the cytoplasm at the periphery of a surface protrusion characteristically displays the properties of a gel, and is consequently commonly known as plasmagel. As a surface structure grows, plasmasol flows into the forming protrusion and transforms into plasmagel around its margins. When the structure is retracted, the plasmagel liquefies and streams back into the center of the cell. Repeated reversals occur in cytoplasmic flow at different times at disparate regions of a cell, a phenomenon referred to as shuttle streaming.
