Morphology of the Second Kind of Follicle in the Guinea Pig Thyroid Gland

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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.