Parathyroids and ultimobranchial bodies in monotremes

Only scant information is available in the scientific literature on the par athyroids and ultimobranchial bodies in the primitive mammals, the echidna (Tachyglossus aculeatus) and platypus (Ornithorhynchus anatinus). The major aim of this paper is to describe the morphology of the monotreme parathyro id gland and to compare it with parathyroids in mammals and reptiles. The g ross anatomy and light microscopic structure of the ultimobranchial body, t hymus, and thyroid are also given. Animals were dissected and routine light and electron microscopic technique s used to examine the microscopic morphology. The locations of parathyroid hormone, calcitonin and calcitonin gene-related peptide in tissue sections were identified by immunostaining. Monotremes have one pair of parathyroid glands located in the thorax and th ey are often associated with thymic tissue but never with the thyroid which is also present in the mediastinum. Ultimobranchial bodies are ventrolater al to the commencement of the trachea. Thymic lobules with Hassall's corpus cles are scattered in the fibrofatty tissue of the mediastinum and the vent ral surface of the pericardium. Histologically, principal cells, water-clea r cells, and non-secretory cells were identified in the parathyroid glands. Principal cells showed polarity and had microlamellar projections that for med intercellular canaliculi. Non-secretory cells had features similar to t hose of thymic epithelial reticular cells. Immunostaining of parathyroid ho rmone showed a diffuse distribution in parathyroid principal cells and none in ultimobranchial bodies. Identification of the ultimobranchial bodies wa s confirmed by immunostaining. The monotreme parathyroid gland, ultimobranchial bodies and thyroid show re ptilian as well as mammalian features. Anat Rec 254:269-280, 1999. (C) 1999 Wiley-Liss, Inc.