Folliculogenesis in the neonatal thyroid gland of the rat

The ultrastructure of the thyroid gland of the neonatal rat was studied from birth to 4 weeks postnatally. Epithelial cell nests in stages of differentiation were found to be present in the gland of the newborn rat. Folliculogenesis proceeded with junctional complexes or contact zones between cells, indicating the site for follicle development. This was followed by intracellular lumina or cavity formation in cells adjacent to the junctional complex. Subsequently, a membrane adherens of lumina and the junctional complex took place. Rudimentary follicular lumina are formed by the fusion of intracellular lumina and resolvement of the junctional complex. Folliculogenesis continued to be present up to 3 weeks after birth.