The pituitary gland (hypophysis) is composed of the adenohypophysis (anterior lobe) and the neurohypophysis (posterior lobe).
The Adenohypophysis in Animals
The adenohypophysis, which surrounds the pars nervosa of the neurohypophyseal system to varying extents in different species, consists of the pars distalis, the pars tuberalis, and the pars intermedia.
The pars distalis is the largest part and contains multiple populations of endocrine cells.
The pars tuberalis functions primarily as a scaffold for the capillary network of the hypophyseal portal system.
The pars intermedia forms the junction between the pars distalis and the pars nervosa. In dogs, it contains two populations of cells, one of which synthesizes ACTH.
A specific population of endocrine cells in the pars distalis (and in the pars intermedia for ACTH in dogs) synthesizes and secretes each of the pituitary tropic hormones. Pituitary cells have a secretory cycle and enter an actively synthesizing phase in response to increased demand for a particular hormone.
Secretory cells in the adenohypophysis are often subdivided into chromophils (acidophils or basophils) and chromophobes according to interaction of the secretory granules with pH-dependent histochemical stains (see the table Endocrine Cells and Hormones of the Adenohypophysis). Chromophils stain readily; chromophobes do not.
Endocrine Cells and Hormones of the Adenohypophysis
Chromophils | Chromophobes |
|---|---|
Acidophils
Basophils
| ACTH-producing cells Melanotropin (melanocyte-stimulating hormone [MSH])–producing cells Nonsecretory follicular cells Undifferentiated stem cells |
Endocrine cells in the adenohypophysis are under the control of corresponding hypothalamic-releasing hormones. These releasing hormones are conveyed by the hypophyseal portal system to specific cells in the adenohypophysis, where they stimulate the rapid release of preformed tropic hormones.
Separate hypothalamic-releasing hormones regulate the rate of secretion of each tropic hormone from the adenohypophysis. For most pituitary tropic hormones, negative feedback control is accomplished by a feedback loop dependent on the blood concentration of the hormone produced by the target endocrine gland (eg, thyroid gland, adrenal cortex, ovary, or testis).
Hormones such as prolactin, growth hormone (GH), and melanocyte-stimulating hormone (MSH) have more complex feedback mechanisms. For example, prolactin affects primarily mammary glands, and GH affects principally the liver—both nonendocrine tissues. The negative feedback in such cases includes metabolites and other messengers (eg, insulin-like growth factor 1 [IGF-1] produced by the liver). GH has both an inhibitory (somatostatin) and a stimulatory (GH-releasing hormone) hypothalamic regulator.
The Neurohypophysis in Animals
In dogs, the neurohypophysis (pars nervosa, posterior lobe) has three anatomical subdivisions: the infundibulum, pars cava, and lobus nervosa.
Secretion granules that contain the neurohypophyseal hormones—ie, antidiuretic hormone (ADH, or vasopressin) and oxytocin—are synthesized in the hypothalamus but released into the bloodstream in the pars nervosa. The infundibular stalk joins the pars nervosa to the overlying hypothalamus.
ADH, an octapeptide synthesized in the hypothalamus, is packaged into membrane-limited granules with a corresponding binding protein (neurophysin) and transported to the pars nervosa, where it is released into the circulation. In the kidney, ADH binds to specific receptors in the distal part of the nephron and collecting ducts; it increases renal tubular reabsorption of water from the glomerular filtrate.
The output of ADH is directly related to the body's hydration status. When the body is well hydrated, ADH release is inhibited; conversely, dehydration or injection of hypertonic electrolyte solutions favors ADH release, which in turn leads to increased water resorption from the glomerular filtrate, resulting in dilution and decreased osmolarity of body fluids.
Barbiturates, ether, chloroform, morphine, acetylcholine, nicotine, and pain increase ADH release, thereby decreasing urine formation. Ethanol inhibits ADH release, leading to diuresis.
The pressor effect of ADH is less pronounced than the antidiuretic effect. However, at dosages several hundred times larger than the antidiuretic dosage, ADH has a pronounced pressor effect, which can also lead to coronary constriction. The contractile mechanism of capillaries, as well as of GI and uterine muscle, is stimulated, and a prolonged increase in blood pressure follows.
Oxytocin has specific effects on uterine smooth muscle and on the myoepithelial cells of mammary glands. In males it has no established physiological function; however, an effect on sperm transport has been suggested.
Key Points
The neurohypophysis secretes primarily vasopressin, the main hormone that regulates extracellular fluid homeostasis.
Hydration is the physiological parameter that controls vasopressin. Disorders of the neurohypophysis include diabetes insipidus from vasopressin deficiency and hypertension.
Chronic dehydration can lead to excessive release of vasopressin, resulting in hypertension and kidney disease.
For More Information
Reece WO, Erickson HH, Goff JP, Uemura EE. Dukes' Physiology of Domestic Animals. 13th ed. Wiley Blackwell; 2015.
Pineda M, Dooley MP, eds. McDonald's Veterinary Endocrinology and Reproduction. 5th ed. Wiley-Blackwell; 2003.
Feldman EC, Nelson RW. Canine and Feline Endocrinology and Reproduction. 3rd ed. Saunders; 2004.
Constanzo LS. Physiology. 7th ed. Elsevier; 2022.
Also see pet owner content regarding disorders of the pituitary gland in dogs and cats.
