Fine structural aspects of inverted follicles in cultured porcine thyroids.

Inversion of thyroid follicles takes place constantly when they are isolated by digestion and cultured in suspension in Eagle's medium containing fetal calf serum without TSH. Most inverted follicles are spherical in shape. The polarity of the follicle epithelial cell is inverted. The luminal surface loses its microvilli, while the outer surface toward the cultured medium protrudes numerous microvilli, the zonula occludens consisting of 3-8 strands moving from the luminal end to the outer end (culture medium side) of the lateral margins of the cell, with the Golgi apparatus becoming located in the cytoplasm between the nucleus and outer surface of the follicle. In all reversed follicle epithelial cells, cyto-organelles such as rough endoplasmic reticulum and Golgi apparatus are extremely poor in development and numerous free ribosomes are scattered. Many lysosome-like dense bodies are distributed in the cytoplasm between the nucleus and the outer surface. A single cilium and long rootlets are sometimes seen in and near the outer surface of the follicle. The cytoplasm near the luminal surface of all the follicle cells shows accumulations of microfilaments which run almost parallel to one another and to the basal plasma membrane. This system is a useful model for studying thyroid function in aspects of cell polarity and analyzing the detailed mechanisms of the secretory activity in the thyroid follicle epithelial cell.