Equine Endocrine System - Horse Anatomy

Pituitary Gland

The pituitary gland, or hypophysis is an elongated appendage of the brain lying within a bony cavity of the sphenoid bone in the base of the skull - the Sella Turcica. The hypophysis is suspended ventral to the the hypothalamus by a thin infundibular stalk. It lies between the more rostral optic chiasma, and the more caudal mammillary bodies, separated from the brain by a fold of dura mater known as the diaphragma sellae.

The gland is divided into anterior and posterior, with different embryological origins reflecting a difference in function:

• Posterior pituitary: (Neurohypophysis), consisting of the Pars Nervosa, this section is derived from a downgrowth of neural tissue from the hypothalamus. As such, it is a collection of axons and nerve terminals that originate in the paraventricular and superoptic nuclei of the hypothalamus. The pars nervosa stores and releases oxytocin and antidiuretic hormone (ADH).
• Anterior pituitary: (Adenohypophysis), consisting of the Pars Tuberalis and Pars Distalis this part has an embryological origin as an epithelial upgrowth from the foetal oral cavity called Rathke's Pouch. The pars tuberalis is a thin band of endocrine cells around the infundibular stalk. It is dense in melatonin receptors, and functions to regulate release of reproductive hormones according to season. The endocrine cells within the pars distalis synthesize, store and release hormones in response releasing and inhibiting factors from the hypothalamus. These factors reach the pars distalis in the hypophyseal portal system, which connects capillaries of the median eminence to the capillaries of the pars distalis.
• Pars Intermedia: This is the residual lumen of Rathke's Pouch and consists of a series of small cystic cavities (follicles filled with colloid), with both basophillic and chromatophobic (poorly staining) cell types. These extend into the pars nervosa.T The pars intermedia separates the anterior and posterior lobes. Endocrine cells in this region are known as melanotropes. These convert the prohormone pro-opiomelanocorticotropin (POMC) to melanocyte stimulating hormone (a-MSH) and corticotropin-like intermediate lobe peptide (CLIP). The pars intermedia is innervated by dopaminergic neurones originating in the periventricular nucleus of the hypothalamus.