ABSTRACT
The pituitary gland is formed by the confluence of two primary embryonic rudiments, one of which originates from an outpouching of neural tissue from the brain’s third ventricle (the neurohypophysis), the other from ectoderm of the oral cavity (the adenohypophysis). The anterior lobe comprises about 80% of pituitary weight in most species. The pituitary stalk extends to the hypothalamus through a dural reflection, and an intermediate lobe located between the anterior and posterior lobes is present in certain species and during fetal development, but is vestigial in primates. This lobe (the pars intermedia) is formed embryologically from the dorsal half of Rathke’s pouch, an evagination of the roof of the pharynx, but is closely adherent to the posterior lobe in adult animals. It is separated from the anterior lobe by the remains of the cavity in Rathke’s pouch, the residual cleft. The pars intermedia of the dog and horse is a significant source of ACTH, with tumors therein leading to spontaneous pituitary-dependent hyperadrenocorticism (PDH; Ch. 25). Because surrounding structures are vital, expansion of the pituitary gland due to tumor formation can result in superior extension with compression of the optic chiasm, and loss of vision.
