New ultrastructural features on the cream hamster thyroid with special reference to the second kind of follicle.

The thyroid cells of the cream hamster, characterized by abundance of microtubules and stratification of the organelles, undergo a particular evolution when the animals grow older. These changes are characterized by an increase of the number of lysosomes which in extreme cases become so prominent that they occupy the whole cytoplasm of the cell which thus loses its organelle stratification. As in other species, cream hamster thyroid contains so-called ultimobranchial follicles made up of at least six cell types: fibrillar dark and light cells, parafollicular cells, ciliated cells, vesicular cells, and cells with myelinic inclusions. The ultrastructure of these follicles in the cream hamster represents a mixture of the ultrastructural characteristics of the same follicles encountered in the rat and the mouse thyroid. Here also mixed follicles are seen. Nevertheless vesicular cells present such abundant "secretion granules" that the question arises as to whether these follicles produce a special secretion and perhaps a new hormone. Incubation of cream hamster thyroids in the prescence of vincristine induces vanishing of microtubules, formation of paracrystalline structures, and loss of stratification of the organelles. Although these last effects might be due to some specific toxic effect of the drug, it is suggested that the disappearing of the organelle stratification might result from a specific vincristine-induced disaggregation of the microtubules acting as a cytoskeleton.