The development of pharyngeal endocrine organs in mouse and chick embryos

Some interesting differences exist between development of the pharyngeal endocrine organs in mouse and chicken. In both, the thyroid forms as an evagination of the pharyngeal floor in the midline at the level of pharyngeal arch II and moves caudally to the base of the neck. In the chicken, the thyroid divides to form paired organs whereas in the mouse it forms a moustache-shaped bar that later fuses with the parathyroid and ultimobranchial bodies. The thymi, parathyroids and ultimobranchial bodies form as lateral evaginations of the pharyngeal pouches. The chick forms two pairs of thymi and parathyroids from the third and fourth pouches whereas the mouse forms a single pair of each primordium from the third pouch. Ultimobranchial evaginations form from the caudal wall of the sixth pouch in the chick and from the posterior pharynx in the mouse. In the chick all of these evaginations detach from the pharynx and come to lie along the carotid arteries as separate organs that surround the thyroids. In the mouse the parathyroids and ultimobranchial bodies fuse with the lateral thyroid lobes. The thymi move to the ventral midline and fuse to form a single organ. Positioning of these organs appears to be related to three developmental events. Outgrowth of the original evaginations and their ventral movements conform to the curved shape of the pharyngeal arches. Ventral growth of the primordia should result in their convergence near the origin of the ventral aortic roots. The thyroid appears to be attached to the common carotid arteries. Lateral growth of the carotids is consistent with the change in thyroid shape. In the mouse the thymi lie ventral to the other organs. Their movement appears to be related to ventral closure of ventral ectoderm in the neck region.