The Ultimobranchial Body of the Chick Embryo

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Abstract
The ultimobranchial bodies originate from the sixth pair of pharyngeal pouches, separate from the pharynx as unifollicular structures and remain as such through nine days of incubation, after which the lumen disappears. Parathyroid rests frequently develop in the ultimobranchial body at ten days of incubation. The secretory activity of the ultimobranchial body is most pronounced during metamorphosis. Disappearance of the lumen, secretory material, and basement membrane following metamorphosis suggest a decline in ultimobranchial function. A holocrine type of secretory activity is suggested by the coagulum and nuclei within the lumen and the disrupted free surface of follicular epithelium. Indications of an endocrine role include a rich vascular supply, secretory products in the lumen, absence of a duct, and integrity of the basement membrane. Nuclear degeneration in the ultimobranchial body is partially the result of histiogenetic processes but is mainly the result of morphogenetic events. On the basis of the periodic acid-Schiff, mercuric bromphenol blue, and Feulgen reactions, there is no indication of a functional similarity between the thyroid gland and the ultimobranchial body. The Sudan black B reaction may suggest a parathyroid-like function of the ultimobranchial body. Functional significance of blood eosinophils in the ultimobranchial body is unknown.