Ultimobranchial Follicles in the Thyroid Glands of Rats and Mice

Abstract

This chapter discusses ultimobranchial follicles in the thyroid glands of rats and mice. The mature ultimobranchial follicle in the rat thyroid has two or more layers of cells bounding a lumen containing amorphous material and cell debris. Although mature ultimobranchial follicles were found in almost all thyroid lobes in adult rats, follicles with this structure were not present in the thyroids of newborn rats. Mixed follicles containing both typical thyroid epithelium and U-cells occur at all ages. Thyroid glands of many strains of mice contain numerous follicles that differ from the usual thyroid follicle in size, shape, cell complement, and luminal contents. The relative abundance of the five cell types varies markedly from follicle to follicle. The striking differences between the cell complement and morphology of ultimobranchial follicles in the Fischer rat and the C 3 H mouse may be due to differences in species or in strain. The thyroid glands of mice and rats contain one or more unusual types of cells that form follicles by themselves, and/or mixed follicles with each other along with typical thyroid epithelium and parafollicular cells.