Change of inverted thyroid follicle into a spheroid after embedding in a collagen gel.

When inverted thyroid follicles in suspension culture are embedded in a collagen gel, there is extensive reorganization of the follicle. To identify intermediate stages in the reorganization, a suspension of inverted follicles was mixed with a cold solution of collagen (0.1 mg/ml) in culture medium and the resultant was warmed and allowed to gel. Prior to embedding, the epithelial cells bounding the lumens formed a monolayer of attenuated cells with their microvilli-bearing surface in contact with the medium. The first change noted was a shrinkage of the lumen in many follicles by 18 h. The cells became cuboidal to columnar. Some of the cells had long sheet-like processes extending into the lumen in contact with those of other cells. In late stages of the reorganization, 48 h, the cells were arranged in a compact spheroid. The spheroids contained two different kinds of colloid-filled lumens, possibly of different origins, one a spherical microlumen, the other very long and narrow in section. The peripheral cells of the spheroid had a smooth plasma membrane (without microvilli) in contact with collagen. Although most of the cells in a section had a microvilli-bearing surface forming part of the boundary of a lumen, it is not certain that all cells were in contact with a lumen.