Chapter 9

Abstract

Parathyroid glands, two pairs usually being present in humans, are endodermal pharyngeal derivatives. A primordial stage, a separation stage and a definitive stage were distinguished during their development (Norris, 1937). Primordia of the parathyroid glands are recognizable in 9–12 mm embryos (S. H. XVI, 38–42 days old) in the dorsal area of the IIIrd and IVth endodermal pharyngeal pouch. They are formed by cylindrical glycogen-rich endodermal cells. The proliferating cells fill the branchial pouch they originate from, forming a solid cord or nodule. During this stage, the parathyroidal nodules are invaded by capillaries (S. H. XVIII, XIX, 45–50 days), and are detached from the pharyngeal endoderm (separation stage) (Fig. 100). During detachment, the cellular cords connecting the parathyroids with the pharynx are known as the pharyngobranchial ducts. Fragmentation of the parathyroid epithelium may occur during the stage of separation resulting in the formation of accessory parathyroids. The parathyroids, III derived from the IIIrd pouch, become detached in 17–22 mm embryos (S. H., XIX, 48–51 days), the parathyroid IV (from the IVth pouch) become detached from the thymic primordium in 25 to 30 mm embryos (54–60 days, S. H. XXII, XXIII).