139 Endocrine System the Endocrine System Interacts with the Nervous System to Maintain Homeostasis, Regulate Growth and Development, and Modulate Behavior

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contents Principles of Biology 139 Endocrine System The endocrine system interacts with the nervous system to maintain homeostasis, regulate growth and development, and modulate behavior.

A mother spotted hyena (Crocuta crocuta) and cubs. This mother hyena cares for her cubs, a behavior that is modulated by the endocrine system. © 2010 Nature Publishing Group Höner, O. et al. The fitness of dispersing spotted hyaena sons is influenced by maternal social status. Nature Communications 1, (2010) doi:10.1038/ncomms1059. Used with permission.

Topics Covered in this Module

Endocrine System Function Major Organs of the Endocrine System The Hypothalamus and Pituitary Glands The Thyroid and Parathyroid Glands The Gonads The Adrenal Gland The Pineal Gland The Pancreas

Major Objectives of this Module

Give examples of how the endocrine system uses hormones to regulate homeostasis. Explain how certain behaviors are triggered by hormones. Describe how hormones regulate reproduction and the development of sex characteristics. Describe the major glands of the endocrine system.

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139 Endocrine System

Endocrine System Function The endocrine system produces a variety of complex responses to stimuli. Some responses are immediate and temporary, such as the increases in heart, respiratory, and metabolic rates associated with an "adrenaline rush," while others are long term, such as the testosterone and estrogenassociated development of secondary sexual characteristics during puberty. Organs of the endocrine system are ductless glands called endocrine glands. In addition to dedicated glands, specialized hormoneproducing cells within nonendocrine organs such as the intestines, heart, and kidney are also part of the endocrine system. In response to certain stimuli, endocrine cells secrete hormones into the bloodstream. From there, hormones travel to their target cells in tissues throughout the body.

Major Organs of the Endocrine System The location of major endocrine glands in the human body is summarized in Figure 1. Three of these glands, the hypothalamus, the pituitary gland, and the pineal gland, are found in the brain. The thyroid and parathyroid are located in the neck. The pancreas is located beneath the stomach. The adrenal glands sit on top of the kidneys. The ovaries are located on either side of the uterus, and the testes are located in the scrotum. Endocrine cells are also associated with the thymus (which is part of the immune system), heart, liver, stomach, kidneys, and small intestine.

Some hormones, called tropic hormones, regulate the secretion of hormones by another endocrine gland or cell. The hypothalamus, which is the regulatory center of the endocrine system, releases two types of tropic hormones that regulate hormone production by the anterior pituitary: releasing hormones that stimulate hormone production and inhibiting hormones that repress hormone production. Nontropic hormones are hormones that directly produce an effect on target cells. The hormones produced by the major endocrine glands are summarized in Table 1.

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IN THIS MODULE

Endocrine System Function Major Organs of the Endocrine System http://www.nature.com/principles/ebooks/principlesofbiology104015/29145735/1 2/3 1/29/2015 Endocrine System | Principles of Biology from Nature Education The Hypothalamus and Pituitary Glands The Thyroid and Parathyroid Glands The Gonads The Adrenal Gland The Pineal Gland The Pancreas Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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139 Endocrine System

The Hypothalamus and Pituitary Glands Most activities of the endocrine system are directly or indirectly regulated by the hypothalamus, a region in the brain critical for maintaining homeostasis. Importantly, the hypothalamus also serves as the primary link between the nervous system and the endocrine system. The pituitary gland, which hangs beneath the hypothalamus, has anterior and posterior lobes.

The posterior pituitary is an extension of the hypothalamus. Specialized neurons, called neurosecretory neurons, extend from the hypothalamus into the posterior pituitary. These neurons produce two hormones, oxytocin and vasopressin (also called antidiuretic hormone, or ADH). Oxytocin promotes maternal bonding and milk production and stimulates contractions during labor. Vasopressin regulates water and salt balance.

The anterior pituitary, which is regulated by releasing and inhibiting hormones secreted from the hypothalamus, produces both tropic and nontropic hormones. Thyroidstimulating hormone (TSH) is considered a tropic hormone because it stimulates the production of thyroid hormones by the thyroid gland. Thyrotropinreleasing hormone (TRH), a releasing hormone produced by the hypothalamus, stimulates TSH secretion. Growth hormone (GH), which directly stimulates growth and cell division in a wide variety of cells, is an example of a nontropic hormone.

Pituitary hormones and behavior. Some hormones secreted by the pituitary affect behavior. For example, oxytocin, which is secreted by the posterior pituitary, and prolactin, which is secreted by the anterior pituitary, are both involved in parental bonding. Recent studies indicate that prolactin may even be involved in parental bonding in nonmammals. For example, a study of the male North American bluegill fish (Lepomis macrochirus) indicates that disruption of prolactin signaling alters behavior associated with paternal care. Male bluegills take exclusive responsibility for care of eggs. When a prolactin antagonist (a drug that prevents prolactin from binding its receptor) was administered to male fish caring for eggs, behavior associated with paternal care, such as fanning eggs, diminished.

IN THIS MODULE

Endocrine System Function Major Organs of the Endocrine System The Hypothalamus and Pituitary Glands The Thyroid and Parathyroid Glands The Gonads The Adrenal Gland The Pineal Gland The Pancreas Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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contents Principles of Biology

139 Endocrine System

The Thyroid and Parathyroid Glands The thyroid gland, located in the neck in humans, plays an important role in metabolism. The thyroid secretes two thyroid hormones, triiodothyronine (T3) and thyroxine (T4), which increase the metabolic rate of most cells in the body. The thyroid also produces calcitonin, a hormone that reduces the concentration of calcium ions in the blood. Calcitonin inhibits calcium ion absorption in the intestines and calcium ion reabsorption by the kidneys. Calcitonin also inhibits the action of osteoclasts, cells that reabsorb (break down) bone tissue. However, in humans, calcitonin appears to play a relatively minor role in calcium ion homeostasis.

Parathyroid hormone (PTH), which is produced by four small glands, called parathyroid glands, that are located on the surface of the thyroid, increases serum calcium ion levels. PTH promotes calcium ion absorption by the intestines and reabsorption by the kidneys and stimulates bone reabsorption by osteoclasts.

Thyroid hormones play critical roles in the development of many animals. In particular, thyroid hormones direct the metamorphosis of animals such as amphibians. For example, thyroid hormones induce metamorphosis of tadpoles into frogs. Test Yourself

What are the three ways that PTH increases blood calcium ion levels?

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IN THIS MODULE

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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139 Endocrine System

The Gonads The gonads produce steroid sex hormones that are responsible for development of both primary sexual characteristics, which are formed during embryonic development, and secondary sexual characteristics, which develop during puberty. The role of gonads in determination of primary sex characteristics was established in the 1940s by Alfred Jost, a French endocrinologist. In rabbits as in humans, animals with two X chromosomes (XX) normally develop into females, and animals with an X chromosome and a Y chromosome (XY) normally develop into males. Jost castrated male (XY) rabbits during embryonic development and found that they developed female sexual characteristics. From these observations, Jost hypothesized that the testes secreted a substance that "masculinized" the embryos. The substance responsible for masculinization was later found to contain hormones from a group of male sex steroid hormones collectively known as androgens. Androgens, which are produced in the testes, promote sperm development as well as development of male sexual characteristics. In humans, the primary androgen is testosterone. Androgens are one of three classes of steroid sex hormones. The other two are estrogens, which promote development of female sexual characteristics, and progestins. In humans, an estrogen called estradiol and a progestin called progesterone regulate the menstrual cycle. Test Yourself

Based on the results of Jost's experiments, what outcome would you expect if the testes of an XY (male) fetus failed to develop normally?

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Starting at puberty, the hypothalamus begins secreting gonadotropinreleasing hormone (GnRH), which mediates development of secondary sex characteristics in both males and females (Figure 2). GnRH stimulates the anterior pituitary to release two gonadotropins, luteinizing hormone (LH) and folliclestimulating hormone (FSH). Gonadotropins are peptide hormones that target the gonads. In males, these hormones act on two different cell types found in the testes. FSH stimulates Sertoli cells, which nourish sperm cells. LH causes the Leydig cells to secrete testosterone. The appearance of male secondary sex characteristics, such as facial hair, increased muscle mass, and deepening of the voice, is primarily due to the increase in testosterone levels. Testosterone also promotes sperm cell development, or spermatogenesis.

Sex steroid hormone production is regulated by a negative feedback loop. Testosterone inhibits production of GnRH by the hypothalamus and of LH and FSH by the anterior pituitary. Sertoli cells also produce the hormone called inhibin that inhibits anterior pituitary secretion of FSH and LH.

In females, LH and FSH regulate the menstrual cycle. In the ovaries, FSH stimulates the maturation of follicles, which consist of an oocyte (egg) and support cells. Maturing follicles produce the estrogen estradiol. Estrogen causes the uterine lining, called the endometrium, to grow. As the follicle matures, it produces more estradiol. Low levels of estradiol inhibit FSH and LH production, but high levels stimulate production of these hormones. Thus, as the follicle matures, increased estradiol levels begin to stimulate FSH and LH production. A surge in FSH and LH coincides with ovulation, the rupture of the follicle that releases the egg into the reproductive tract. The remnants of the ruptured follicle differentiate into the corpus luteum, an endocrine structure that secretes estrogen and progesterone, another female sex steroid hormone. Together, estrogen and progesterone inhibit the secretion of FSH and LH from the anterior pituitary, preventing the maturation of another follicle.

If the egg is fertilized, the developing embryo secretes human chorionic gonadotropin (hCG), a hormone that causes the corpus luteum to continue secreting progesterone, allowing the body to maintain the pregnancy. If the egg remains unfertilized, the corpus luteum eventually degenerates, and estrogen and progesterone levels fall. Without these hormones, the uterine lining sloughs off during the process of menstruation. At this point LH and FSH secretion is no longer inhibited and the next round of ovarian follicle development begins.

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Figure 2: Starting at puberty, the hypothalamus stimulates production of steroid sex hormones in the gonads. At puberty, the hypothalamus begins synthesizing GnRH. GnRH stimulates the anterior pituitary to secrete LH and FSH. In males, LH and FSH stimulate the production of testosterone in testes. In females, LH and FSH stimulate the production of estradiol in ovaries. © 2014 Nature Education All rights reserved.

Anabolic steroids. Anabolic steroids are synthetic steroid hormones that mimic testosterone. Like natural testosterone, anabolic steroids build muscle mass and enhance endurance, and for this reason, they have become popular among athletes. However, to have the desired effect, anabolic steroids are often taken in very large doses, which can cause adverse side effects, such as increased aggression. In women, use of anabolic steroids, which are androgens, may disrupt the menstrual cycle and disrupt follicle maturation, resulting in infertility. In men, anabolic steroid use may result in testicular atrophy (shrinking of the testes), erectile dysfunction (inability to initiate or maintain a penile erection), and reduced sperm count.

Because anabolic steroids are androgens, it is not surprising that they impair the female reproductive cycle, but why do they impair the male reproductive cycle? Like testosterone, anabolic steroids inhibit the production of GnRH. Without the stimulating effects of LH and FSH, testicular tissue atrophies, and both spermatogenesis and testosterone synthesis in the testes are reduced. Usually, the negative effects of anabolic steroids are reversed a few months after drug use is stopped.

IN THIS MODULE

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can http://www.nature.com/principles/ebooks/principlesofbiology104015/29145735/4 2/3 1/29/2015 Endocrine System | Principles of Biology from Nature Education cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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139 Endocrine System

The Adrenal Gland The adrenal glands, which sit above the kidneys, mediate the response to stress. The adrenal gland has two parts: an inner adrenal medulla that mediates the response to acute stress and an outer adrenal cortex that mediates the response to longterm stress.

The hypothalamus regulates both parts of the adrenal glands, but the mechanism of activation differs. During an acutely stressful situation (for example, slipping at the edge of a steep cliff), the hypothalamus sends a nerve signal to the adrenal medulla via the spinal cord. In response, chromaffin cells of the adrenal medulla secrete epinephrine (also known as adrenaline) and norepinephrine (also known as noradrenaline). Epinephrine and norepinephrine activate the "fightorflight" response, which increases heart and respiratory rate and causes the breakdown of glycogen and fats (Figure 3a).

During longterm stress, such as the loss of a job, the hypothalamus secretes corticotropinreleasing hormone (CRH), which in turn stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary. ACTH travels to the adrenal cortex, where it stimulates the secretion of glucocorticoids. The glucocorticoids, the most significant of which is the steroid hormone cortisol, suppress some immune functions and increase blood glucose levels (Figure 3b). Increased levels of glucocorticoids are associated with improved memory and vigilance, which are presumably needed to get an animal through a stressful situation.

Figure 3: The adrenal gland mediates acute and longterm stress response. During acute stress, neural signals from the hypothalamus stimulate the adrenal medulla to secrete epinephrine and norepinephrine. During longterm stress, the hypothalamus causes the anterior pituitary to secrete a hormonal message, which stimulates the adrenal cortex to secrete glucocorticoids. © 2014 Nature Education All rights reserved.

Glucocorticoids are members of a larger class of steroid hormones known as the corticosteroids, so named because they are produced primarily by the adrenal cortex. Corticosteroids also include the mineralocorticoids, which regulate water balance and concentration of sodium and potassium "mineral" ions. A mineralocorticoid called aldosterone is secreted in response to increased serum potassium levels or the presence of a hormone called angiotensin II. Angiotensin II is produced in the kidneys in response to a decrease in blood pressure or blood volume. The adrenal gland also produces small amounts of androgens.

Glucocorticoids and behavior. A 2004 study led by Menno Kruk of the Leiden/Amsterdam Center for Drug Research in the Netherlands indicates that the adrenal gland may be involved in regulation of aggressive behavior. Kruk found that electrical stimulation of a region of the rat hypothalamus associated with aggression caused aggressive behavior. After the electrical stimulus was applied, serum levels of corticosterone, which is produced by the adrenal cortex, increased. Next, Kruk removed the adrenal glands so that the rats were unable to produce corticosterone. In rats whose adrenal glands were removed, electrical stimulation of the hypothalamus did not result in aggressive behavior. However, aggressive behavior in these rats could be induced by corticosterone injections. Taken together, these results suggest that corticosterones are involved in aggressive behavior. Stress is associated with aggression in humans, and the results of this study provide a http://www.nature.com/principles/ebooks/principlesofbiology104015/29145735/5 1/3 1/29/2015 Endocrine System | Principles of Biology from Nature Education

valuable insight into the link between stress and aggression.

BIOSKILL Addison's Disease Is Caused by Adrenal Insufficiency A woman is rushed to the emergency room suffering from extreme fatigue, severe vomiting and diarrhea, low blood pressure, and low blood glucose. A blood test reveals that the woman has abnormally elevated ACTH, extremely low cortisol, and an increase in the number of inflammatory immune cells. These symptoms support a diagnosis of Addison's disease, a disorder that is typically caused by autoimmune destruction of cells of the adrenal cortex. The adrenal cortex produces cortisol and other glucocorticoids, which regulate glucose metabolism and suppress the immune system, and mineralocorticoids, which regulate salt and water balance. Thus, the observed symptoms of low blood pressure, low blood glucose, and increased inflammatory cells result from reduced levels of glucocorticoids. The anterior pituitary produces more ACTH in an attempt to increase cortisol production, resulting in the observed high serum ACTH levels. Addison's disease progresses slowly with symptoms generally not appearing until approximately 90% of the adrenal cortex has been destroyed. Patients treated with steroid supplements can live a reasonably normal life. Test Yourself

Addison's disease is a form of adrenal insufficiency, or inability of the adrenal cortex to produce steroid hormones. Given your understanding of the triggers for cortisol secretion, can you predict another cause of adrenal insufficiency that does NOT involve destruction of the adrenal cortex?

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BIOSKILL

IN THIS MODULE

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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contents Principles of Biology

139 Endocrine System

The Pineal Gland The pineal gland is a peasized endocrine gland located deep within the brain that produces the hormone melatonin, which is synthesized from serotonin. Light inhibits melatonin production, and melatonin secretion correlates closely with the 24hour diurnal cycle of day and night. Melatonin controls circadian cycles such the sleep cycle and daily changes in body temperature.

The Pancreas The pancreas produces two hormones, insulin and glucagon, that regulate blood sugar levels. Insulin promotes the uptake of glucose by cells and induces the liver to make glycogen, thereby lowering blood sugar levels. Glucagon inhibits the uptake of sugar by cells and induces the liver to break down glycogen, thereby raising blood sugar levels.

IN THIS MODULE

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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139 Endocrine System Summary

OBJECTIVE Describe the major glands of the endocrine system. The hypothalamus, located in the brain, is the regulatory center of the endocrine system and integrates the nervous and endocrine systems. The hypothalamus secretes hormones, including oxytocin and antidiuretic hormone (ADH), into the posterior pituitary. The hypothalamus also secretes releasing and inhibiting hormones that regulate hormone secretion by the anterior pituitary. The anterior pituitary secretes both tropic hormones, which regulate secretion of hormones by other endocrine glands, and nontropic hormones, which directly affect cells of the body. The thyroid, located in the neck in humans, secretes hormones that regulate metabolism, development and calcium levels. The parathyroid glands, located on the thyroid gland, secrete a hormone that regulates blood calcium levels. The pancreas, located beneath the stomach, secretes hormones that regulate blood glucose levels. The adrenal glands secrete hormones that regulate stress, aggression, and salt balance. The gonads, including the testes and ovaries, secrete steroid sex hormones and produce gametes. The pineal gland secretes melatonin, which regulates circadian rhythms.

OBJECTIVE Give examples of how the endocrine system uses hormones to regulate homeostasis. A major function of the endocrine system is to maintain homeostasis, often through negative feedback loops. In coordination with the hypothalamus and anterior pituitary, the thyroid gland secretes thyroid hormones to regulate metabolic rate. The thyroid also secretes calcitonin in response to elevated serum calcium ion levels; the actions of calcitonin are opposed by parathyroid hormone. The production of testosterone operates through a similar negative feedback system; increased testosterone levels inhibit the production of gonadotropinreleasing hormone in the hypothalamus and the production of FSH/LH in the anterior pituitary, which prevents further stimulation of testosterone production in the Leydig cells of the testes.

OBJECTIVE Explain how certain behaviors are triggered by hormones. Environmental and internal stimuli can induce the production of hormones, which in turn can drive behavior. Stress generates two responses in the adrenal gland: a rapid response to acute stress mediated by epinephrine and norepinephrine produced in the adrenal medulla, and a slower, longterm response to chronic stress mediated by corticosteroids produced in the adrenal cortex. Prolactin and oxytocin, which are secreted from the pituitary, are involved in the development of parental behavior.

OBJECTIVE Describe how hormones regulate reproduction and the development of sex characteristics. Androgens, the male sex steroid hormones, and estrogens, the female sex steroids, control the development of both primary and secondary sex characteristics. During embryonic development, testosterone stimulates the development of male sex characteristics. At puberty, sex hormones drive the development of secondary sex characteristics in both males and females. Sex hormone production is regulated by gonadotropinreleasing hormone produced by the hypothalamus, which stimulates production of the gonadotropins FSH and LH from the anterior pituitary. FSH and LH stimulate the production of gametes and sex hormones in the gonads (testes in males and ovaries in females). Cyclic fluctuations in FSH, LH, and female sex steroids are responsible for the menstrual cycle in women.

Key Terms

adrenal cortex The outer layer of the adrenal gland; produces cortisol, aldosterone, and a small amount of sex hormones; biologically distinct from the adrenal medulla and produces different hormones.

adrenal medulla The inner layer of the adrenal gland; produces epinephrine and norepinephrine; biologically distinct from the adrenal cortex and produces different hormones.

aldosterone A steroid hormone produced in the adrenal cortex that increases the reabsorption of sodium ions by the distal convoluted tubule and collecting duct; indirectly promotes the reuptake of water by the kidney.

androgen Any of the male sex steroid hormones that influence the development of male primary and secondary sex characteristics; examples include testosterone and androsterone.

anterior pituitary One of two lobes of the pituitary gland; secretes various hormones in response to releasing hormones from the hypothalamus.

calcitonin A peptide hormone produced in the thyroid gland that lowers circulating calcium levels by inhibiting osteoclast activity and calcium absorption in the kidneys and intestines.

corticosteroid Any of the steroid hormones produced primarily by the adrenal cortex; includes the glucocorticoids, most notably cortisol, and the mineralocorticoids, most notably aldosterone.

endocrine gland Ductless organ of the endocrine system that secretes hormones.

estrogen A class of steroid sex hormones that promote development of secondary female sexual characteristics and http://www.nature.com/principles/ebooks/principlesofbiology104015/29145735/7 1/3 1/29/2015 Summary of Endocrine System | Principles of Biology from Nature Education

stimulate growth of the uterine lining; estrogens are produced in the ovaries.

folliclestimulating hormone (FSH) A peptide hormone produced in the anterior pituitary that stimulates growth and development of ovarian follicles in females and stimulates maturation of sperm cells in males.

gonad An organ that produces gametes and sex steroid hormones; the male gonads are the testes, and the female gonads are the ovaries.

gonadotropin Any of several peptide hormones that target the gonads; examples include folliclestimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG).

gonadotropinreleasing hormone (GnRH) A peptide hormone produced by the hypothalamus that stimulates the production of the gonadotropins follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary.

hormone A chemical produced by glands or cells of the endocrine system that is transported in blood or hemolymph to distant target cells.

hypothalamus Endocrine gland located in the brain that integrates information from the nervous system and regulates endocrine function.

inhibiting hormone A hormone produced by the hypothalamus that inhibits release of hormones from the anterior pituitary.

Leydig cell A cell type in the testes that synthesizes and secretes testosterone in response to LH stimulation; sometimes known as an interstitial cell.

luteinizing hormone (LH) A peptide hormone produced in the anterior pituitary that stimulates ovulation and development of the corpus luteum in females and stimulates testosterone production in the Leydig cells in males.

nontropic hormone A hormone that directly produces a response from cells of the body.

parathyroid gland Any of four endocrine glands on the posterior surface of the thyroid gland that secrete parathyroid hormone (PTH) in response to low blood calcium levels.

pineal gland An endocrine gland located deep in the brain between the cerebral hemispheres; secretes melatonin, which regulates sleep patterns and circadian rhythm.

pituitary gland A gland that hangs down from the hypothalamus; secretes a variety of tropic and nontropic hormones.

posterior pituitary One of two lobes of the pituitary gland; directly connected to the hypothalamus; directly responsible for secretion of oxytocin and antidiuretic hormone (ADH).

progestin A steroid sex hormone that promotes growth of the uterine lining in women.

prolactin A peptide hormone produced in the anterior pituitary that stimulates the production of milk in the mammary glands of lactating mammals; may also be involved in parental behaviors in males and nonmammalian species.

releasing hormone Any of several hormones produced by the hypothalamus that stimulate the release of specific hormones from the anterior pituitary; examples include corticotropinreleasing hormone (CRH, which induces ACTH secretion) and gonadotropinreleasing hormone (GnRH, which induces FSH and LH secretion).

Sertoli cell A cell type in the testes that supports the growth and maturation of sperm cells; activated by FSH stimulation; also stimulates the hormone inhibin to regulate FSH secretion by the anterior pituitary.

thyroid gland Gland located in the neck in humans that regulates metabolism, growth, and calcium levels.

tropic hormone Any hormone that stimulates the production of hormones in another endocrine gland; examples include the hypothalamic releasing hormones (which stimulate the anterior pituitary) and thyroidstimulating hormone (which stimulates the thyroid).

IN THIS MODULE

Endocrine System Function Major Organs of the Endocrine System The Hypothalamus and Pituitary Glands http://www.nature.com/principles/ebooks/principlesofbiology104015/29145735/7 2/3 1/29/2015 Summary of Endocrine System | Principles of Biology from Nature Education

The Thyroid and Parathyroid Glands The Gonads The Adrenal Gland The Pineal Gland The Pancreas Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Cancer: What's Old Is New Again Is cancer ancient, or is it largely a product of modern times? Can cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

PRIMARY LITERATURE

Adaptor proteins regulate cell signaling Structural basis for regulation of the Crk signaling protein by a proline switch. View | Download

Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) Singlechannel currents recorded from membrane of denervated frog muscle fibers. View | Download

How can nematodes help reduce obesity in humans? A wholeorganism screen identifies new regulators of fat storage. View | Download

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contents Principles of Biology

139 Endocrine System

IN THIS MODULE

Endocrine System Function Test Your Knowledge Major Organs of the Endocrine System The Hypothalamus and Pituitary Glands 1. Which two glands regulate the level of calcium in the blood? The Thyroid and Parathyroid Glands

hypothalamus and pituitary The Gonads thyroid and thymus The Adrenal Gland thymus and parathyroid The Pineal Gland thyroid and parathyroid pituitary and adrenal The Pancreas Summary Test Your Knowledge 2. If the adrenal medulla were removed from an animal, which of the following statements would be true?

The animal would be less able to survive sudden attacks and emergencies. WHY DOES THIS TOPIC MATTER? The animal would exhibit less aggressive behavior. Cancer: What's Old Is New Again The animal would be less able to deal with longterm stress. Is cancer ancient, or is it largely a The "adrenaline rush" would be much more pronounced. product of modern times? Can Water balance could not be maintained. cuttingedge research lead to prevention and treatment strategies that could make cancer obsolete?

3. What is the appropriate treatment for a person with Addison's disease? PRIMARY LITERATURE

administration of supplemental corticosteroids on a longterm basis Adaptor proteins regulate cell administration of supplemental PTH (parathyroid hormone) on a longterm basis signaling removal of the thyroid gland Structural basis for regulation of the Crk removal of the adrenal cortex signaling protein by a proline switch. There is no treatment for Addison's disease. View | Download Classic paper: Breakthrough enables tiny measurements of ion channel activity (1976) 4. Which of the following statements about the hypothalamus and pituitary glands are true? Singlechannel currents recorded from membrane of denervated frog muscle The anterior pituitary secretes releasing and inhibiting hormones that regulate hormone production by fibers. the hypothalamus. View | Download The hypothalamus has two lobes, an anterior lobe and a posterior lobe. How can nematodes help reduce The anterior pituitary is an extension of the hypothalamus that contains neurosecretory neurons. obesity in humans? The hypothalamus secretes releasing and inhibiting hormones that regulate hormone production by A wholeorganism screen identifies new the posterior pituitary. regulators of fat storage. The hypothalamus secretes releasing and inhibiting hormones that regulate hormone production by View | Download the anterior pituitary.

5. Which of the following pairs of hormones are released by the adrenal cortex?

epinephrine and norepinephrine triiodothyronine and thyroxine insulin and glucagon cortisol and aldosterone luteinizing hormone and folliclestimulating hormone

6. Which of the following is true of the pituitary hormone FSH?

FSH is stimulated by CRH from the hypothalamus. FSH stimulates production of testosterone in males and estrogen in females. FSH stimulates the development of gametes. FSH is stimulated by LH from the hypothalamus. FSH directly promotes the development of secondary sex characteristics.

7. Alfred Jost removed the testes from rabbit embryos in order to study the role of these gonads in sexual differentiation. Which of the following conclusions can be drawn from his experiment?

In the absence of testes, male (XY) rabbits appear male but are infertile. Ovaries are necessary for the development of female primary sex characteristics in rabbits. Testosterone causes development of male sexual characteristics. Male primary sex characteristics develop in the absence of testes as long as the rabbit has a Y chromosome. Testes produce a factor that stimulates development of male sex characteristics.

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