Freeze-fracture images on filipin-sterol complexes in the thyroid follicle epithelial cell of mice with special regard to absence of cholesterol at the site of micropinocytosis.

In order to clarify the distribution of cholesterol in the plasma- and cyto-membranes of the thyroid follicle cell, freeze-fracture images of the filipin-treated tissues of normal and TSH-treated mice were observed. The filipin-sterol complexes, 25 to 30 nm protuberances or pits are distributed densely and almost homogeneously on the fractured plasma membrane, though the small depressions showing aggregates of intramembrane particles lack the complexes. Each depression corresponds to the coated pit, which might be an initial site for micropinocytosis of the luminal colloid. The limiting membranes of all the large colloid droplets reabsorbed are generally very rich in the complexes, but some small regions on the limiting membrane of the droplet are less in their density. The membranes of the rough endoplasmic reticulum, of the nucleus and of the Golgi apparatus are almost free from the complexes, though small clusters consisting of 2-5 complexes are rarely scattered. In thin sections, the membranes which are rich in the filipin-sterol complexes become obscure in their fine structure after treatment with filipin for 12-24 h.