Embedding in a collagen gel stabilizes the polarity of epithelial cells in thyroid follicles in suspension culture.

Separated thyroid follicles are stable in suspension culture in Coon's modified Ham's F12 medium containing 0.5% calf serum. They resemble follicles in vivo except for the absence of a basal lamina. However, the epithelial cells reverse polarity and the follicles invert when the serum concentration is raised to 5%. A number of substances, especially components of extracellular matrix, were added to the medium to ascertain if they could stabilize the follicles against inversion in 5% serum. Cellular and plasma fibronectin, gelatin, heat-denatured collagen, methylcellulose and laminin did not stabilize. The addition to the medium of as little as 50 micrograms/ml of acid-soluble collagen prepared from calf skin or rat tail tendons resulted in the formation of small clouds of gel. Follicles embedded within the gel were stabilized. Follicles in the same dish but not embedded in the gel inverted. Stabilization was not specific for collagen, since follicles embedded in a plasma clot were also stabilized. A gel was not sufficient for stabilization, since embedding in an agarose gel did not stabilize. Ultrastructural studies indicate that adherence to a limited number of gelled fibers of collagen covering only a small fraction of the basal plasma membrane may be sufficient to stabilize and that a basal lamina formed in the presence of laminin but without added collagen does not stabilize.