Effects of vitamin A deficiency on ultimobranchial tissue in the rat thyroid

Ultimobranchial follicles in the thyroid glands of young Wistar rats, 4 to 12 weeks old, are lined by non‐keratinized stratified squamous epithe‐lium and contain desquamated cellular material. Mitoses are observed in the epithelium of these follicles which are variable in size, shape and frequency. Thyroid glands of young vitamin A deficient rats also contain ultimobranchial follicles. These follicles differ from those found in animals on a stock diet in that the stratified squamous epithelium produces keratinized concentric whorls of non‐nucleated luminal debris. After placing vitamin A deficient rats on a stock diet desquamated cellular material is no longer keratinized and resembles that seen in the control animals. Vitamin A deficient diet apparently keratinizes the stratified squamous epithelium of pre‐existing ultimobranchial follicles rather than solely having a metaplastic effect on thyroid‐like follicles of ultimobranchial tissue origin.