ABSTRACT
Relatively little has been written recently on the development of the two pairs of parathyroid glands that arise in the human and even less on the single pair of parathyroid glands that develop in the mouse. Those in the human are termed the parathyroid IIIs and parathyroid IVs because of their embryological origin from the dorsal part of the third and fourth pharyngeal pouches, respectively, while the single pair that develops in the mouse is exclusively derived from the dorsal part of the third pharyngeal pouch. With regard to the terminology of the glands in the human, because of the eventual location of these two pairs of glands, the parathyroid IIIs are often termed the inferior parathyroid glands whereas the parathyroid IVs are often termed the superior parathyroid glands. This is because of their usual eventual location in the neck region in the adult, as postero-medial relations of the thyroid gland. Because of the differences observed in the parathyroid glands in the human and in the mouse, it is appropriate to draw attention to what is known of their different developmental origin. As most workers in this field are likely to be aware of the situation in the human, the embryological origin in this species is considered first. It is of considerable clinical importance to be aware that as the thymus gland develops from the ventral part of the third pharyngeal pouch, the parathyroid glands that develop from the dorsal part of the third pouch may be located in a number of aberrant sites. This is of considerable importance when it is necessary to surgically remove a parathyroid tumor that has been diagnosed clinically, particularly if it is of the inferior gland and of third pharyngeal arch origin, as its location may be quite variable. It may be necessary initially to explore the neck region in the proximity of the thyroid gland and then progress caudally until finally the superior mediastinum is explored and the tumor is eventually located. The most caudal of these aberrant sites is in the superior mediastinum. Accordingly, if the parathyroid glands that develop from the third pouch retain their association with the thymic rudiment from the same side, they may also be located anywhere along the path of descent of the thymus gland or, for various reasons, elsewhere (see below).
