Abstract

To evaluate the morphogenesis of the human thyroid, a histologic study was made based on 100 normal thyroids of human embryos and fetuses ranging in age from 4 to 42 weeks of gestation. The embryos were serially sectioned and fetuses were examinated as an individual organ. 1) The first sign of thyroid primordium was the spherical proliferation of median ventral pharyngeal wall at the 4th week of development. 2) At the 6th week of gestation, the thyroid differentiated into two lobes that were connected by an isthmus, and was on the way of migration to the definite position from the foramen cecum. 3) The developing thyroid consisted of two cell cords, solid nests or interconnecting complex pattern until 14th week of gestation, when the entire portion of thyroid was replaced by follicles of variable size. 4) At the 9th week, the first follicle was recognizable at the periphery of the gland. 5) At the 14th week, follicles were partly filled with faintly eosinophilic colloid. 6) After the 18th week of gestation, lobulation of the thyroid parenchyme was a prominent feature. 7) After the 24th week, large follicles with rich colloid content are distributed through both superificial and deep portions. And after the 34th week, maturation reached the general pattern of adult thyroid. 8) The ability of thyroglobulin synthesis which was confirmed by PAP method, was first recognized at the 10th week of gestation.

• Keywords: Development of thyroid;