Morphology of the second kind of follicle in the guinea pig thyroid gland.

Abstract

The second type of follicle (S follicle) was studied in the thyroid gland of young and mature guinea pigs. A marked activity to non-specific esterase was used as a marker system for the S follicle. Usually a single S follicle was situated in the upper central two-thirds of each lobe occupying 0.5--2.7 mm of the length of the lobe. The S follicle could in both animal groups be divided into three parts, an upper, a middle, and a lower part. The upper tubular part consisted of cuboidal and cylindrical, strong-reacting cells completely separated from the thyroid follicles by connective tissue. The middle part was further subdivided into an irregular follicle without contact to the thyroid follicles, and a distended follicle with several contact areas to the thyroid follicles. An increasing amount of flattened, weak-reacting cells interpreted as thyroid follicle cells were encountered in the S follicle wall adjacent to the contact areas. In young animals the lower part had a tubular appearance like that seen in the upper part, while the lower part of the S follicle in mature animals were more follicle like.