Transitional cells in the postnatal thyroid gland of the rat.

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AbstractIn two and four‐week old rats three types of follicles were observed: thyroid follicles made of follicular cells and C cells; ultimobranchial follicles (or cysts) and ducts composed of lumen‐bordering cells and subjacent cells; mixed follicles lined by follicular cells on one side and by lumen‐bordering cells on the other. Occasionally C cells were seen in the wall of mixed follicles. On their basal side the lumen‐bordering cells of mixed follicles were in direct contact with the wall of ultimobranchial ducts or cysts. In the wall of these ducts transitional cells were observed. One type had ultrastructural features of subjacent cells (i.e., hemidesmosomes) and of lumen‐bordering cells (i.e., pale cytoplasm and dispersed tonofilaments). Another type of transitional cell had hemidesmosomes and a deeply indented nucleus, like subjacent cells, but had accumulated slightly distended cisternae of rough‐surfaced endoplasmic reticulum and was displaying long slender microvilli, which is typical of follicular cells. A third type of cells were called “young follicular cells.” They contained slightly distended cisternal profiles of rough‐surfaced endoplasmic reticulum collected in one corner of the cells and few apical vesicles. However, their cisternae were less abundant, less distended and had a more restricted distribution than those found in mature follicular cells. Hence the subjacent cells of ultimobran‐chial ducts might possibly give rise to lumen‐bordering cells and to some follicular cells. The latter are associated with the lumen‐bordering cells in the formation of mixed follicles; at the beginning these new follicular cells seem less active than the mature follicular cells. The origin of C cells after birth is discussed.