Mhs-1Pituitary Gland Presentation

8/12/2019 Mhs-1Pituitary Gland Presentation

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HYPOTHALAMUS ANDPITUITARY GLAND


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O For many years, the pituitary gland orhypophysis was called the masterendocrine gland because it secretesseveral hormones that control otherendocrine glands.

O We now know that the pituitary gland itselfhas a masterthe hypothalamus.

O This small region of the brain below thethalamus is the major link between thenervous and endocrine systems.


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O Cells in the hypothalamus synthesize at

least nine different hormones, and the

pituitary gland secretes seven.O Together, these hormones play important

roles in the regulation of virtually all

aspects of growth, development,

metabolism, and homeostasis.


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Pituitary GlandO The pituitary gland is a pea-shaped

structure that measures 11.5 cm (0.5 in.)

in diameter and lies in the hypophysealfossa of the sella turcica of the sphenoid

bone.

O It attaches to the hypothalamus by a stalk,

the infundibulum and has two

anatomically and functionally separate

portions: the anterior pituitary and the

posterior pituitary.


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TheAnterior PituitaryO The anterior pituitary (anterior lobe),

also called the adenohypophysis,

accounts for about 75% of the total weightof the gland.

O The anterior pituitary consists of two parts

in an adult: The pars distalis is the larger

portion, and the pars tuberalis forms a

sheath around the infundibulum.


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The Posterior PituitaryO The posterior pituitary (posterior

O lobe), also called the neurohypophysis,

also consists of two parts: the parsnervosa, the larger bulbar portion, and

the infundibulum.

O A third region of the pituitary gland called

thepars intermedia atrophies duringhuman fetal development and ceases to

exist as a separate lobe in adults.


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ANTERIOR PITUITARY


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Anterior Pituitary

The anterior pituitary or adenohypophysissecretes hormones that regulate a wide range of

bodily activities, from growth to reproduction.

Release of anterior pituitary hormones is stimulatedby releasing hormones and suppressed byinhibiting hormones from the hypothalamus.

Thus, the hypothalamic hormones are an important

link between the nervous and endocrine systems.


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Hypophyseal Portal System

Hypothalamic hormones reach the anterior pituitarythrough a portal system.

In aportal system,blood flows from one capillary

network into a portal vein, and then into a secondcapillary network without passing through the heart.

In the hypophyseal portal system, blood flowsfrom capillaries in the hypothalamus into portal

veins that carry blood to capillaries of the anteriorpituitary.


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The superior hypophyseal arteries,branchesof the internal carotid arteries, bring blood into thehypothalamus.

At the junction of the median eminence of thehypothalamus and the infundibulum, these arteriesdivide into a capillary network called the primaryplexus of the hypophyseal portal system.

From the primary plexus, blood drains into thehypophyseal portal veins that pass down theoutside of the infundibulum. In the anteriorpituitary, the hypophyseal portal veins divide again

and form another capillary network called thesecondary plexus of the hypophyseal portalsystem.


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Relation between Putuitary and Hypothalamus

Near the median eminence and above the opticchiasm are clusters of specialized neurons, calledhypothalamic neurosecretory cells.

They synthesize the hypothalamic releasing and

inhibiting hormones in their cell bodies and packagethe hormones inside vesicles, which reach the axonterminals by axonal transport.

Nerve impulses stimulate the vesicles to undergo

exocytosis. The hormones then diffuse into theprimary plexus of the hypophyseal portalsystem.


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Relation between Pituitary and Hypothalamus

hypothalamic hormones flowwith the blood through theportal veins and into thesecondary plexus.

hypothalamic hormones actimmediately on anteriorpituitary cells.

Hormones secreted by anteriorpituitary drain into the anteriorhypophyseal veins and out intothe general circulation


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Types of Anterior Pituitary Cells

4. Lactotrophs secrete prolactin (PRL),whichinitiates milk production in the mammary glands.

5. Corticotrophs secrete adrenocorticotropic

hormone (ACTH) or corticotropin , whichstimulates the adrenal cortex to secreteglucocorticoids such as cortisol. Some corticotrophs,remnants of the pars intermedia, also secrete

melanocyte-stimulating hormone (MSH).


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Types of Anterior Pituitary Cells


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Hormone of Anterior Pituitary

1. Human Growth Hormone and InsulinlikeGrowth Factors

The main function of hGH is to promote synthesis and

secretion of small protein hormones called insulinlikegrowth factors (IGFs) or somatomedins.

In response to human growth hormone, cells in the liver,

skeletal muscles, cartilage, bones, and other tissues secreteIGFs, which may either enter the bloodstream from theliver or act locally in other tissues as autocrines orparacrines.


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2.Follicle-stimulating HormoneIn females, the ovaries are the targets for follicle-stimulating hormone (FSH). Each month FSH initiatesthe development of several ovarian follicles, saclike

arrangements of secretory cells that surround adeveloping oocyte

In males,FSH stimulates sperm production in thetestes.Gonadotropinreleasing hormone (GnRH) from thehypothalamus stimulates FSH release.


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3.Luteinizing HormoneIn females, luteinizing hormone (LH) triggers ovulation,the release of a secondary oocyte (future ovum) by anovary.

Together, FSH and LH also stimulate secretion ofestrogens by ovarianbcells. In males, LH stimulates cells inthe testes to secrete testosterone.

Secretion of LH, like that of FSH, is controlled bygonadotropin-releasing hormone (GnRH).


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4.Prolactin

Prolactin (PRL), together with other hormones,initiates and maintains milk secretion by the

mammary glands.

5. Thyroid-stimulating Hormone

Thyroid-stimulating hormone (TSH) stimulates thesynthesis and secretion of the two thyroid hormones,triiodothyronine (T3) and thyroxine (T4), bothproduced by the thyroid gland


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6.Adrenocorticotropic Hormone

Corticotrophs secrete mainly adrenocorticotropichormone (ACTH). ACTH controls the production and

secretion of cortisol and other glucocorticoids by thecortex (outer portion) of the adrenal glands.

Corticotropin-releasing hormone (CRH) from the

hypothalamus stimulates secretion of ACTH bycorticotrophs.


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7.Melanocyte-stimulating Hormone

Melanocyte-stimulating hormone (MSH) increasesskin pigmentation in amphibians by stimulating the

dispersion of melanin granules in melanocytes.

Its exact role in humans is unknown, but the presenceof MSH receptors in the brain suggests it may

influence brain activity.


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Although the posterior pituitary or neurohypophysis

does not synthesize hormones, it does store and release twohormones.

It consists of axons and axon terminals of more than10,000 hypothalamic neurosecretory cells.

The cell bodies of the neurosecretory cells are in theparaventricular and supraoptic nuclei of thehypothalamus; their axons form thehypothalamohypophyseal tract.

This tract begins in the hypothalamus and ends nearblood capillaries in the posterior pituitary.

Posterior Pituitary


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The paraventricular nucleus synthesizes thehormone oxytocin and the supraoptic nucleus

produces antidiuretic Hormone also calledvasopressin

The axon terminals in the posterior pituitary areassociated with specialized neuroglia called

pituicytes. These cells have a supporting role similarto that of astrocytes.


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After their production in the cell bodies ofneurosecretory cells, oxytocin and antidiuretic

hormone are packaged into secretory vesicles, whichmove by fast axonal transport to the axon terminalsin the posterior pituitary, where they are stored untilnerve impulses trigger exocytosis and release of thehormone.


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During and after delivery of a baby, oxytocin affectstwo target tissues: the mothers uterus and breasts.

During delivery, oxytocin enhances contraction ofsmooth muscle cells in the wall of the uterus;

After delivery, it stimulates milk ejection(letdown) from the mammary glands in response

to the mechanical stimulus provided by a sucklinginfant.

Oxytocin


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an antidiuretic is a substance that decreases urine production.

ADH causes the kidneys to return more water to the blood,

thus decreasing urine volume. In the absence of ADH, urine output increases more than

tenfold, from the normal 1 to 2 liters to about 20 liters a day.

Drinking alcohol often causes frequent and copious urinationbecause alcohol inhibits secretion of ADH. ADH also decreases

the water lost through sweating and causes constriction ofarterioles, which increases blood pressure.

This hormones other name, vasopressin, reflects this effect onblood pressure.

Antidiuretic Hormone


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The amount of ADHsecreted varies withblood osmoticpressure and blood

volume


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Mhs-1Pituitary Gland Presentation