Ectodermal ablation of the third branchial arch in chick embryos and the morphogenesis of the parathyroid III gland

The parathyroid glands have been classically considered to be derivatives of the third and fourth pharyngeal pouches in most species, including humans. Furthermore, the presence of neural crest-derived cells in the parathyroid glands connective tissue has been apparently established. However, our previous studies have provided a new hypothesis on the origin of these glands in human and chick embryos. To determine the origin of the parathyroid III (P3) gland, ectoderm of the third branchial arch was cauterized in chick embryos at Hamburger and Hamilton's stage 19 (embryonic day 3). Cauterization of the ventral half of the ectoderm was followed by the non-formation, on the same side, of the P3 gland. When the dorsal half of the ectoderm was cauterized, both the right and left P3 glands formed. Our observations suggest that the ectoderm of the ventral half of the third branchial arch is necessary for the organization of the P3 gland.