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15.3: Pituitary Gland and Hypothalamus

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    22356
  • By the end of this section, you will be able to:

    • Explain the interrelationships of the anatomy and functions of the hypothalamus and the posterior and anterior lobes of the pituitary gland
    • Identify the two hormones released from the posterior pituitary, their target cells, and their principal actions
    • Identify the six hormones produced by the anterior lobe of the pituitary gland, their target cells, their principal actions, and their regulation by the hypothalamus

    The hypothalamus–pituitary complex can be thought of as the “command center” of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus–pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamus–pituitary complex to be translated into hormones that can initiate a response.

    The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus (Figure \(\PageIndex{1}\)). It has both neural and endocrine functions, producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally related to the pituitary gland (or hypophysis), a bean-sized organ suspended from it by a stem called the infundibulum (or pituitary stalk). The pituitary gland is cradled within the sella turcica of the sphenoid bone of the skull. It consists of two lobes that arise from distinct parts of embryonic tissue: the posterior pituitary (neurohypophysis) is neural tissue, whereas the anterior pituitary (also known as the adenohypophysis) is glandular tissue that develops from the primitive digestive tract. The hormones secreted by the posterior and anterior pituitary, and the intermediate zone between the lobes are summarized in Table \(\PageIndex{1}\).

    Diagram of Hypothalamus-Pituitary Complex. The pituitary gland is connected to the hypothalamus via the infundibulum.
    Figure \(\PageIndex{1}\): Hypothalamus–Pituitary Complex. The hypothalamus region of the brain lies inferior and anterior to the thalamus. It connects to the pituitary gland by way of the stalk-like infundibulum. The pituitary gland consists of an anterior and posterior lobe, with each lobe releasing different hormones in response to signals from the hypothalamus. (Image credit: "Hypothalamus-Pituitary Complex" by OpenStax is licensed under CC BY 3.0)

    Table \(\PageIndex{1}\): Pituitary Hormones

    Pituitary lobe Associated hormones Effect
    Anterior Growth hormone (GH) Promotes growth of body tissues
    Anterior Prolactin (PRL) Promotes milk production from mammary glands
    Anterior Thyroid-stimulating hormone (TSH) Stimulates thyroid hormone release from thyroid
    Anterior Adrenocorticotropic hormone (ACTH) Stimulates hormone release by adrenal cortex
    Anterior Follicle-stimulating hormone (FSH) Stimulates gamete production in gonads
    Anterior Luteinizing hormone (LH) Stimulates androgen production by gonads
    Posterior Antidiuretic hormone (ADH) Stimulates water reabsorption by kidneys
    Posterior Oxytocin Stimulates uterine contractions during childbirth
    Intermediate zone Melanocyte-stimulating hormone Stimulates melanin formation in melanocytes

    Posterior Pituitary

    The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum and end in axon terminals that comprise the posterior pituitary (Figure \(\PageIndex{2}\)).

    Diagram of hypothalamus to posterior pituitary pathways via neurosecretory cellsFigure \(\PageIndex{2}\): Posterior Pituitary. Neurosecretory cells in the supraoptic nucleus of the hypothalamus release antidiuretic hormone (ADH) and neurosecretory cells in the paraventricular nucleus of the hypothalamus release oxytocin (OT). The hormones move through axons of separate neurons in the hypothalamo-hypophyseal tract through the infundibulum to axon terminals located in the posterior lobe of the pituitary gland. These hormones are stored or released into the bloodstream via the capillary plexus in the posterior pituitary gland. (Image credit: "Posterior Pituitary Complex" by Julie Jenks is licensed under CC BY 4.0 / A derivative from the original work)

    The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nucleus produces the hormone oxytocin, whereas the supraoptic nucleus produces antidiuretic hormone (ADH). These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.

    Oxytocin

    When fetal development is complete, the peptide-derived hormone oxytocin (tocia- = “childbirth”) stimulates uterine contractions and dilation of the cervix. Throughout most of pregnancy, oxytocin hormone receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the synthesis of oxytocin receptors in the uterus increases, and the smooth muscle cells of the uterus become more sensitive to its effects. Oxytocin is continually released throughout childbirth through a positive feedback mechanism. As noted earlier, oxytocin prompts uterine contractions that push the fetal head toward the cervix. In response, cervical stretching stimulates additional oxytocin to be synthesized by the hypothalamus and released from the pituitary. This increases the intensity and effectiveness of uterine contractions and prompts additional dilation of the cervix. The feedback loop continues until birth.

    Although the mother’s high blood levels of oxytocin begin to decrease immediately following birth, oxytocin continues to play a role in maternal and newborn health. First, oxytocin is necessary for the milk ejection reflex (commonly referred to as “let-down”) in breastfeeding women. As the newborn begins suckling, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is secreted and released into the bloodstream. Within seconds, cells in the mother’s milk ducts contract, ejecting milk into the infant’s mouth. Secondly, in both males and females, oxytocin is thought to contribute to parent–newborn bonding, known as attachment. Oxytocin is also thought to be involved in feelings of love and closeness, as well as in the sexual response.

    Antidiuretic Hormone (ADH)

    The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors—specialized cells within the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes.

    In response to high blood osmolarity, which can occur during dehydration or following a very salty meal, the osmoreceptors signal the posterior pituitary to release antidiuretic hormone (ADH), also known as vasopressin. The target cells of ADH are located in the tubular cells of the kidneys, but it also targets sweat glands and smooth muscle of blood vessels. Its effect is to increase epithelial permeability to water, allowing increased water reabsorption. The more water reabsorbed from the filtrate, the greater the amount of water that is returned to the blood and the less that is excreted in the urine. A greater concentration of water results in a reduced concentration of solutes in the blood. ADH is also known as vasopressin because, in very high concentrations, it causes constriction of blood vessels, which increases blood pressure. The release of ADH is controlled by a negative feedback loop. As blood osmolarity decreases, the hypothalamic osmoreceptors sense the change and prompt a corresponding decrease in the secretion of ADH. As a result, less water is reabsorbed from the urine filtrate.

    Interestingly, drugs can affect the secretion of ADH. For example, alcohol consumption inhibits the release of ADH, resulting in increased urine production that can eventually lead to dehydration and a hangover. A disease called diabetes insipidus is characterized by chronic underproduction of ADH that causes chronic dehydration. Because little ADH is produced and secreted, not enough water is reabsorbed by the kidneys. Although patients feel thirsty, and increase their fluid consumption, this doesn’t effectively decrease the solute concentration in their blood because ADH levels are not high enough to trigger water reabsorption in the kidneys. Electrolyte imbalances can occur in severe cases of diabetes insipidus.

    Anterior Pituitary

    The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia is adjacent to the posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the infundibulum.

    Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior pituitary is regulated by two classes of hormones. These hormones—secreted by the hypothalamus—are the releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting hormones that inhibit secretion.

    Hypothalamic hormones are secreted by neurons, but enter the anterior pituitary through blood vessels (Figure \(\PageIndex{3}\)). Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This network, called the hypophyseal portal system, allows hypothalamic hormones to be transported to the anterior pituitary without first entering the systemic circulation. The system originates from the superior hypophyseal artery, which branches off the carotid arteries and transports blood to the hypothalamus. The branches of the superior hypophyseal artery form the hypophyseal portal system (see Figure \(\PageIndex{3}\)). Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary. Hormones produced by the anterior pituitary (in response to releasing hormones) enter a secondary capillary plexus, and from there drain into the circulation.

    Anterior Pituitary Complex and Hypophyseal Portal System
    Figure \(\PageIndex{3}\): Anterior Pituitary. The anterior pituitary manufactures six hormones. Neurosecretory cells in the hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the hypothalamus travel by neuron axons to the primary capillary plexus of the hypophyseal portal system located in the infundibulum. The hormones move through hypophyseal portal veins in the infundibulum to the secondary capillary plexus in the anterior pituitary. Hormones produced in the anterior pituitary are released into the bloodstream via the secondary capillary plexus. (Image credit: "The Anterior Pituitary Complex" by OpenStax is licensed under CC BY 3.0)

    Six hormones are synthesized in the anterior pituitary. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = “turning”) because they turn on or off the function of other endocrine glands.

    Growth Hormone

    The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication. A major hormone involved in this process is growth hormone (GH), also called somatotropin—a protein hormone produced and secreted by the anterior pituitary gland. Its primary function promotes protein synthesis and tissue building through direct and indirect mechanisms involving insulin-like growth factors (IGF) that also increase metabolic rate to support the growth.

    Thyroid-Stimulating Hormone

    The activity of the thyroid gland is regulated by thyroid-stimulating hormone (TSH), also called thyrotropin. TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. As discussed in the next section, it triggers the secretion of thyroid hormones by the thyroid gland. In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then trigger a drop in production of TRH and subsequently TSH.

    Adrenocorticotropic Hormone

    The adrenocorticotropic hormone (ACTH), also called corticotropin, stimulates the adrenal cortex (the more superficial “bark” of the adrenal glands) to secrete corticosteroid hormones (also known as glucocorticoids) such as cortisol. ACTH come from a precursor molecule known as pro-opiomelanotropin (POMC) which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins.

    The release of ACTH is regulated by the corticotropin-releasing hormone (CRH) from the hypothalamus in response to normal physiologic rhythms. CRH also stimulates the release of beta endorphins in response to stress or exercise. A variety of stressors can elicit a complex stress response, and the role of ACTH in the stress response is discussed later in this chapter.

    Follicle-Stimulating Hormone and Luteinizing Hormone

    The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system (these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes and the ovaries). Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in adolescents. Puberty is initiated by gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins—hormones that regulate the function of the gonads. The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in the case of those with ovaries, the onset and cessation of reproductive capacity.

    The gonadotropins include two glycoprotein hormones: follicle-stimulating hormone (FSH) stimulates the production and maturation of sex cells, or gametes, including ova in those with ovaries and sperm in those with testes. FSH also promotes follicular growth; these follicles then release estrogens in the ovaries. Luteinizing hormone (LH) triggers ovulation in those with ovaries, as well as the production of estrogens and progesterone by the ovaries. LH stimulates production of testosterone by the testes, and because testosterone is produced by interstitial cells within the testes, it is also known as interstitial cell stimulating hormone (ICSH) in those with testes.

    Prolactin

    As its name implies, prolactin (PRL) promotes lactation (milk production) in women. During pregnancy, it contributes to development of the mammary glands, and after birth, it stimulates the mammary glands to produce breast milk. However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones. And as noted earlier, the let-down of milk occurs in response to stimulation from oxytocin.

    In a non-pregnant woman, prolactin secretion is inhibited by prolactin-inhibiting hormone (PIH), which is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during pregnancy do prolactin levels rise in response to prolactin-releasing hormone (PRH) from the hypothalamus.

    Intermediate Pituitary: Melanocyte-Stimulating Hormone

    The cells in the intermediate zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone (MSH) that is formed by cleavage of the pro-opiomelanocortin (POMC) precursor protein. Local production of MSH in the skin is responsible for melanin production in response to UV light exposure. The role of MSH made by the pituitary is more complicated. For instance, people with lighter skin generally have the same amount of MSH as people with darker skin. Nevertheless, this hormone is capable of darkening of the skin by inducing melanin production in the melanocytes of the epidermis. Women also show increased MSH production during pregnancy; in combination with estrogens, it can lead to darker skin pigmentation, especially the skin of the areolas and labia minora. Figure \(\PageIndex{4}\) is a summary of the pituitary hormones and their principal effects.

    Summary of Hormones Secreted by the Pituitary Gland and their Effects
    Figure \(\PageIndex{4}\): Major Pituitary Hormones. Major pituitary hormones and their target organs and effects. (Image credit: "Major Pituitary Hormones" by OpenStax is licensed under CC BY 3.0)

    Concept Review

    The hypothalamus–pituitary complex is located in the diencephalon of the brain. The hypothalamus and the pituitary gland are connected by a structure called the infundibulum, which contains vasculature and nerve axons. The pituitary gland is divided into two distinct structures with different embryonic origins. The posterior lobe houses the axon terminals of hypothalamic neurons. It stores and releases into the bloodstream two hypothalamic hormones: oxytocin (OT) and antidiuretic hormone (ADH). The anterior lobe is connected to the hypothalamus by the vasculature of the hypophyseal portal system in the infundibulum and produces and secretes six hormones. Their secretion is regulated, however, by releasing and inhibiting hormones from the hypothalamus. The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).

    Review Questions

    Q. The hypothalamus is functionally and anatomically connected to the posterior pituitary lobe by a bridge of ________.

    A. blood vessels

    B. nerve axons

    C. cartilage

    D. bone

    Answer

    Answer: B

    Q. Which of the following is an anterior pituitary hormone?

    A. ADH

    B. oxytocin

    C. TSH

    D. cortisol

    Answer

    Answer: C

    Q. How many hormones are produced by the posterior pituitary?

    A. 0

    B. 1

    C. 2

    D. 6

    Answer

    Answer: A

    Q. Which of the following hormones contributes to the regulation of the body’s fluid and electrolyte balance?

    A. oxytocin

    B. antidiuretic hormone

    C. luteinizing hormone

    D. all of the above

    Answer

    Answer: B

    Critical Thinking Questions

    Q. Compare and contrast the anatomical relationship of the anterior and posterior lobes of the pituitary gland to the hypothalamus.

    Answer

    A. The anterior lobe of the pituitary gland is connected to the hypothalamus by vasculature, which allows regulating hormones from the hypothalamus to travel to the anterior pituitary. In contrast, the posterior lobe is connected to the hypothalamus by a bridge of nerve axons called the hypothalamic–hypophyseal tract, along which the hypothalamus sends hormones produced by hypothalamic nerve cell bodies to the posterior pituitary for storage and release into the circulation.

    Glossary

    adrenocorticotropic hormone (ACTH)
    anterior pituitary hormone that stimulates the adrenal cortex to secrete corticosteroid hormones to regulate metabolism and the stress response (also called corticotropin)
    antidiuretic hormone (ADH)
    hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to reabsorb water
    follicle-stimulating hormone (FSH)
    anterior pituitary hormone that stimulates the production and maturation of sex cells
    gonadotropins
    hormones that regulate the function of the gonads
    growth hormone (GH)
    anterior pituitary hormone that promotes tissue building and influences nutrient metabolism (also called somatotropin)
    hypophyseal portal system
    network of blood vessels that enables hypothalamic hormones to travel into the anterior lobe of the pituitary without entering the systemic circulation
    hypothalamus
    region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling
    infundibulum
    stalk containing vasculature and neural tissue that connects the pituitary gland to the hypothalamus (also called the pituitary stalk)
    luteinizing hormone (LH)
    anterior pituitary hormone that triggers ovulation and the production of ovarian hormones in those with ovaries, and the production of testosterone in those with testes
    osmoreceptor
    hypothalamic sensory receptor that is stimulated by changes in solute concentration (osmotic pressure) in the blood
    oxytocin
    hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine contractions in labor, milk ejection during breastfeeding, and feelings of attachment (also produced in those with testes)
    pituitary gland
    bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis)
    prolactin (PRL)
    anterior pituitary hormone that promotes development of the mammary glands and the production of breast milk
    thyroid-stimulating hormone (TSH)
    anterior pituitary hormone that triggers secretion of thyroid hormones by the thyroid gland (also called thyrotropin)

    Contributors and Attributions

    OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology