Fine structure of the second kind of thyroid follicles in mature guinea pigs

The second kind of follicle (S-follicle) is in mature guinea pigs a morphologically heterogeneous structure located in the central part of the upper two-thirds of the thyroid lobe. An S-follicle may appropriately be divided in an upper, a middle, and a lower part. This paper deals with the ultrastructure of the upper tubular part and the middle part. The latter is further subdivided in an irregular follicle and a distended follicle. In the tubular part a single cell type, termed the S-follicle cell, is dominating. This cell is most often arranged in a single layer and is characterized by an indented nucleus and by presence of pinocytose vesicles and hemi-desmosomes along the basal cytoplasm membrane. In addition, C cells are frequently found. In the irregular follicle five different cell types are noticed: S-follicle cells, dark fibrillar cells, granulated cells, ciliated cells, and C cells. The S-follicle cells display an ultrastructure similar to those in the tubular part but are frequently arranged in two or more layers. Both the tubular part and the irregular follicle are surrounded by connective tissue. In the distended follicle, however, contact areas to thyroid follicles are found. Adjacent to these contact areas, cells with an ultrastructure similar to that of thyroid follicle cells proper are encountered in the wall of the S-follicle. This observation suggests that the distended part of an S-follicle in mature guinea pigs might be regarded as a kind of mixed follicle. The origin of thyroid follicle cells proper in the wall of the S-follicle is briefly discussed.