The pituitary gland (hypophysis) is situated at the base of the brain caudal to the optic chiasm, between the piriform areas and anterior to the mammillary bodies. The gland consists of the adenohypophysis and the neurohypophysis connected to the overlying hypothalamus by the infundibular stalk. It rests in the hypophyseal fossa formed by the basisphenoid bone. The neurohypophysis is derived from a down growth of diencephalic neuroectoderm, while the adenohypophysis comes from a dorsal evagination of oropharyngeal ectoderm called the craniopharyngeal duct (Rathke’s pouch). The pars intermedia forms where these two structures come in contact. The developing basisphenoid bone eventually separates the craniopharyngeal duct from the oropharynx. The rest of the cells in Rathke’s pouch proliferate to form the pars distalis, the largest part of the adenohypophysis, which surrounds the developing pars intermedia and pars nervosa. As the pars nervosa enlarges, the lumen of Rathke’s pouch collapses resulting in a residual lumen called Rathke’s cleft. In some species (e.g. horse), Rathke’s cleft is obliterated. A dorsal projection of cells along the infundibular stalk forms the pars tuberalis, which serves as a scaffold for the hypophyseal portal system as it courses from the median eminence to the pars distalis.

The thyroid is a single gland with two or three lobes that are located adjacent to the cranial trachea (Figure 13.1). The lobes may be connected by an isthmus, which can be fibrous or glandular, depending on the species (see Table 13.1). The prominence of the isthmus in herbivores primarily reflects the amount of iodine in the forage. Since the great lakes are in the “goiter belt,” the isthmus is often prominent, while those animals in areas replete with iodine may have complete atrophy of the isthmus. The cranial aspect of the thyroid gland is typically at the level of the first tracheal cartilage, although there can be some individual variation.

The parathyroid glands are associated with the thyroid gland, but they are distinct in function and origin (Figure 13.2). The parathyroid glands are composed of chief cells that secrete parathyroid hormone, which is crucial to calcium homeostasis. Most species of mammals have two pairs of parathyroid glands: the external or cranial parathyroid glands are derived from the third pharyngeal pouch (parathyroid III), while the internal or caudal parathyroids come from the fourth pharyngeal pouch (ultimobranchial body; parathyroid IV). However, pigs and rodents only have one pair of parathyroid glands (see Table 13.1).

Stay updated, free articles. Join our Telegram channel

Join