Posterior Pituitary Created by: Kristina Eriksen, Miles Johnson, McKenzie McPherson, and Courteny Cox
Structure and Function of the Posterior Pituitary
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Structure
During embryonic development, the posterior pituitary, also known as the neurohypophysis, is derived from the hypothalamus and differentiated by the neural ectoderm. The neurohypophysis contains three structures called the median eminence, the pituitary stalk, and the pars nervosa. The median eminence is composed of nerve endings of axons from the hypothalamus. It is known to store 10 hypothalamic-releasing hormones and also several neurotransmitters like dopamine, norepinephrine, acetylcholine, and among others. The pituitary stalk is comprised of the axons of neurons that originate in the hypothalamus and also connects the posterior pituitary to the hypothalamus and brain. Finally, the axons that originate in the hypothalamus cease in the pars nervosa. The pars nervosa is the structure of the neurohypophysis that actually secretes the hormones Antidiuretic Hormone and Oxytocin.
Function
The main function of the posterior pituitary is to regulate homeostasis in the body. It secretes the hormones ADH and Oxytocin, but ADH is the main hormone regulating homeostasis. Antidiuretic hormone is secreted to regulate the control of plasma osmolality in blood vessels. Increased levels of ADH cause the permeability of the distal renal tubules and collecting ducts to increase. The increase leads to increased water reabsorption into the blood, causing the urine to be more concentrated. Antidiuretic hormone was originally named vasopressin because at very high levels it causes the vessels to constrict, which then causes blood pressure to increase. This may be done with medication to prevent someone with a drop in blood pressure from going into shock. Antidiuretic hormone is regulated by the osmoreceptors in the hypothalamus. The receptors may be stimulated by many things such as stress, volume loss, trauma, pain, exercise, nicotine, and drugs. ADH will decrease in secretion in response to hypertension, decreased plasma osmolality, and alcohol ingestion.
Function Continued
The posterior pituitary also secretes Oxytocin. This hormone has its main effect on uterus and mammary glands. When secreted, oxytocin will bind to receptors on the uterine wall and cause contractions during childbirth. Oxytocin enhances the effectiveness of contractions, as well as, promoting the delivery of the placenta, and stimulating postpartum uterine contractions. Postpartum uterine contractions are important to prevent excess hemorrhage. The other function of Oxytocin is to target acini cells of the mammary gland to produce milk let-down for nursing women. Oxytocin may have an effect on sperm motility in males.
Causes and Risk Factors of Posterior Pituitary Dysfunction
Dysfunction of the posterior pituitary gland is largely due to damage of the gland, such as with an injury or a tumor, or damage of the nervous system. Any damage to the posterior pituitary gland can potentially alter the storage and release of antidiuretic hormone. Damage can come from a variety of causes, including tumors in or near the pituitary gland, head injury, surgery, inflammation, or even illness such as meningitis (Mayo Clinic, 2013). The damage leads to either the compression or destruction of the tissue inside the posterior pituitary gland itself; the biggest issue as a result of this damage is often decreased release of ADH which can cause central diabetes insipidus. However, tumors of the posterior pituitary gland can also secrete hormones themselves (Mayo Clinic, 2015), which can lead to an increased production of ADH which in turn can cause SIADH.
Stroke, trauma, and hemorrhage in the brain can impact the central nervous system in a way that can also affect the function of the posterior pituitary gland (Unnikrishnan, 2011). Because ADH is produced by cell bodies in the hypothalamus, and is transported by axons to the posterior pituitary gland to be stored and secreted, damage to these neurons in either the hypothalamus or posterior pituitary can lead to either increased or decreased secretion of ADH.
The direct causes of pituitary tumors that can lead to posterior pituitary gland dysfunction are unknown, but scientists believe genetic alterations are a strong contributing factor in their development (Mayo Clinic, 2015). People with a family history of multiple endocrine neoplasia type I also have a higher risk of developing a pituitary tumor in their lifetime (Mayo Clinic, 2015) and therefore are at higher risk for posterior pituitary gland dysfunction.
There are no known genetic risk factors affecting the function of the posterior pituitary gland that leads to increased risk of developing either diabetes insipidus or syndrome of inappropriate anti-diuretic hormone (SIADH). According to the Mayo Clinic, genetic factors for developing diabetes insipidus are related to a child being born with a permanent alteration to their kidneys which prevents them from concentrating urine correctly, causing nephrogenic diabetes insipidus (2013). Nephrogenic diabetes insipidus is not related to the storage or release of ADH by the posterior pituitary which causes central diabetes insipidus. Similarly, any genetic predisposition for SIADH is related to dysfunction of renal receptors and the hypothalamus and also is not related to dysfunction of the posterior pituitary (Unnikrishnan, 2011).
Hyposecretion of the Posterior Pituitary
Hyposecretion of antidiuretic hormone is the underlying cause of diabetes insipidus. Diabetes insipidus causes the body to lose its ability to concentrate urine. This results in intense thirst, polydipsia, and excreting large amounts of urine, or polyuria. There are two main reasons why ADH may be hyposecreted. First, the kidney receptors may become insensitive to the hormone. This would be called nephrogenic DI. There are several disease processes that may lead to nephrogenic DI. Pyelonephritis, polycystic kidney disease, and amyloidosis are three diseases that cause the kidneys to become insensitive to ADH. Second, hypothalamic-neurohypophyseal lesions may prevent the hormone from being properly stored or secreted. This form is called neurogenic DI. Some disease processes that cause these types of lesions are brain tumors, aneurysm, thrombosis, and infections.
Hypersecretion of the Posterior Pituitary
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Hypersecretion of antidiuretic hormone causes syndrome of inappropriate antidiuretic hormone, or SIADH. SIADH is caused by renal water retention which causes blood volume to increase, also called hypervolemia. Hypervolemia causes the body fluids to become diluted, which means less sodium is crossing membranes. This is called hyponatremia, which may cause several harmful disease processes such as heart failure or kidney failure. The normal range for sodium in the body is 135-145. If sodium drops below 130, symptoms such as fatigue, intense thirst, anorexia, and dyspnea may occur. If dropped below 120, vomiting and severe GI symptoms appear. Once sodium levels reach 110 or lower, convulsions and irreversible damage to the brain may occur.
Associated Diseases
There are no known disease associated with the hypoactivity or hyperactivity of oxytocin. However, hypoactivity during childbirth may promote excessive bleeding postpartum. Hyperactivity of oxytocin may cause stronger contractions of the uterus during childbirth and may cause stress on the fetus.
Diagnostics
Tumors within the pituitary can go undiagnosed because many symptoms associated with the tumor resemble symptoms associated with other conditions. Pituitary tumors can be found while medical tests are being performed for other conditions. According to the Mayo Clinic, a doctor will perform a physical exam with a detailed history along with the following tests; blood and urine test, CT scan or MRI, and a vision test. The blood and urine test determine an overproduction or deficiency of any hormones. The CT scan and MRI allow for the size and shape of the tumor to be examined, while the vision test is to determine if the pressure from the pituitary tumor is causing vision problems.
Tumors and lesions of the posterior pituitary gland can cause SIADH (Syndrome of Inappropriate Antidieruetic Hormone Secretion) and DI (Diabetes Insipidus). Diagnosis for SIADH includes a complete medical history, physical examination and a blood and urine test to measure the osmolality of the blood and urine by measuring sodium and potassium concentration. Diagnosis of DI includes complete medical history, physical examination, daily fluid intake, dietary intake, voiding patters, urine tests, blood tests, water deprivation test and MRI is used to determine problems within the pituitary.
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Treatments
Treatments for pituitary tumors can be surgery, radiation therapy and different medications. There are two ways that a pituitary tumor can be removed through surgery endoscopic trans nasal trans sphenoidal and craniotomy. When the tumor is removed with the endoscopic trans nasal trans sphenoidal method it is removed through the nasal cavity and no outer incision made, but this method can usually only be preformed with smaller tumors. Larger tumors are removed by craniotomy in which the tumor is removed through the upper part of the skull. Main reasons for the tumor to be removed is if there is increased pressure on the optic nerves and or if the tumor is invading surrounding tissue. Radiation therapy can be used on the tumor but with the radiation therapy there are increased chance of the brain tissue surrounding the tumor will be affected and cause other problems. There are no medications used to treat pituitary tumors themselves but the medications used differentiate between the different hormones affected by the tumor for example like desmopressin which would then take the place of ADH like a replacement hormone medication.
Treatments for SIADH and DI are similar to treatments for pituitary tumors. With SIADH there will be a surgical removal of a tumor secreting ADH. A hypertonic saline solution can be administered to raise the serum saline. Vasopressin can be administered also to inhibit the action of the ADH being secreted. The most important treatment used is fluid and water restriction in which the patient must control their intake to help control their blood and fluid volume. Treatment for DI includes use of antidiuretic hormone medications along with medications that stimulate the production of ADH such as NSAIDs or chlorpropamide. An adequate amount of fluid intake is needed to compensate for the excessive urinary output.
Why the Posterior Pituitary is the Most Important Gland in the Human Body
All glands in the human body are important for daily function but the posterior pituitary is by far the most important. Without the posterior pituitary the human race would cease to exist. As noted earlier in this presentation oxytocin is released by the posterior pituitary which allows for contractions during child birth to be effective. The oxytocin allows for a smooth and successful delivery of the fetus, placenta and prevents hemorrhaging afterwards. Without this hormone secretion, child birth would not be effective and the mothers and babies would not have such a successful survival rate. The posterior pituitary also secretes ADH which acts upon kidneys and blood vessels to maintain our bodies homeostasis. Without this hormone secretion the kidneys would not be stimulated when needed during exercise, stress, trauma, pain, and volume loss. Blood pressure within the body would be unable to be regulated as the ADH is secreted to raise the blood pressure as the kidneys begin to retain water and concentrate the urine. If the human body experienced severe fluid loss the posterior pituitary would secrete ADH which allows for the body to function properly by maintaining fluid levels within the respiratory, nervous, and cardiovascular systems with no excess fluid loss.
References
Diabetes insipidus. (2013, March 14). Retrieved November 21, 2015, from http://www.mayoclinic.org/diseases-conditions/diabetes-insipidus/basics/definition/con-20026841
Foulad, MD, A., & Bhandarkar, MD, N. (2015, July 29). Pituitary Gland Anatomy (A. Meyers, MD, MBA, Ed.). Retrieved November 20, 2015, from http://emedicine.medscape.com/article/1899167-overview
Huether, S., & McCance, K. (2008). Understanding pathophysiology (4th ed.). St. Louis, Mo.: Mosby/Elsevier.
Hyponatremia. (2014, May 28). Retrieved November 21, 2015, from http://www.mayoclinic.org/diseases-conditions/hyponatremia/basics/definition/con-20031445
Pituitary tumors. (2015, November 19). Retrieved November 22, 2015, from http://www.mayoclinic.org/diseases-conditions/pituitary-tumors/symptoms-causes/dxc-20157631
Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH). (2015). Retrieved November 23, 2015, from http://www.stanfordchildrens.org/en/topic/default?id=syndrome-of-inappropriate-antidiuretic-hormone-secretion-siadh-90-P01974&sid=
Unnikrishnan, A., Pillai, B., & Pavithran, P. (2011). Syndrome of inappropriate antidiuretic hormone secretion: Revisiting a classical endocrine disorder. Indian Journal of Endocrinology and Metabolism Indian J Endocr Metab, 15, 208-208. Retrieved November 21, 2015, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3183532/