Embryologic origin of the various epithelial cell types in the second kind of thyroid follicle in the C3H mouse.

The thyroid gland of the C3H mouse is composed largely of the usual follicle but it also contains a second kind of follicle. To ascertain the embryologic origin of the cell types in each of these follicles, ventral pharyngeal outpocketings and ultimobranchial outpocketings were isolated (before they fused to form the thyroid gland) from the 12-day-old fetus of the C3H mouse. The outpocketings were implanted into different kidney capsules of adult C3H mice and were allowed to grow for several months. Transplants were then excised and examined by electron microscopy. The ventral contribution formed large aggregates of follicles and was recognized as a distinct bump on the surface of the kidney. It formed only typical thyroid ultimobranchial contribution usually formed only a small number of follicles, generally of small size. It was readily located because the follicles occurred next to a white plaque of bone or cartilage at the site of implantation. The ultimobranchial contribution formed follicles containing four cell types: a ciliated cell, a cell with abundant agranular reticulum, a cell with many free ribosomes and fiber and occasional hemidesmosomes, and the C cell which was the most frequent cell type. No typical thyroid epithelium was observed in the ultimobranchial transplant. These observations suggest that the C cell in the usual follicle is derived from the ultimobranchial contribution, and that the second kind of follicle is largely an ultimobranchial contribution but the typical thyroid epithelium in it is largely or entirely a ventral contribution.