Hypothalamus & Pituitary Gland - PowerPoint Presentation

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Hypothalamus & Pituitary Gland

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Hypothalamus and

Pituitary Gland

The hypothalamus and pituitary gland form a unit that exerts control over the function of several endocrine glands (thyroid, adrenals, and gonads), as well as a wide range of physiologic activities

This unit constitutes an example of neuroendocrinology—brain-endocrine interactions

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Morphology

The

pituitary gland (

hypophysis) is located in a small depression in the sphenoid bone, the

sella

turcica

, just

beneath

the

hypothalamusIt is connected to the hypothalamus by a thin stalk called the infundibulum

Sella

turcica

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Histology of the Anterior Lobe

Most of the

cells in the anterior lobe (

adenohypophysis) contain secretory granules, although some are only sparsely granulated

Based on their characteristic staining with standard

histochemical

dyes and

immunofluorescent

stains, it is possible to identify the cells that secrete each of the pituitary hormones

It once was thought that there was a unique cell type for each of the pituitary hormones, but it is now recognized that some cells may produce more than one hormone

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Histology of the Posterior Lobe

The

posterior lobe

(neurohypophysis) consists of two major

portions

:

the

infundibulum, or stalk,

and

the

infundibular process, or neural lobe The posterior lobe is richly endowed with fibers The cell bodies from which these

fibers arise are located in the hypothalamus

Secretory material synthesized in cell bodies in the hypothalamus is transported down the axons and stored in in the posterior lobe

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Physiology of the Anterior Pituitary Gland

There

are six anterior pituitary hormones whose physiological importance is clearly

establishedThey include the hormones that govern the function of the:

thyroid

and adrenal

glands

,

the

gonads,

the mammary glands, and bodily growth

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Hormones

of the Anterior Pituitary Gland

All the anterior pituitary hormones are proteins or

glycoproteinsThey are divided into 3 categories according to structure similarity:Glycoprotein Hormones

Growth hormone and

prolactin

Adrenocorticotropin

family

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Glycoprotein Hormones

Hormone

Target

Major actions in humans

Thyroid-stimulating hormone (TSH), also called

thyrotropin

Thyroid gland

Stimulates synthesis and secretion of thyroid hormones

Follicle-stimulating hormone (FSH)

Ovary

Stimulates growth of follicles and estrogen

secretion

Testis

Acts on

Sertoli

cells to promote maturation of sperm

Luteinizing hormone (

LH)

Ovary

Stimulates ovulation of ripe follicle

and formation

of corpus luteum;

stimulates

estrogen

and progesterone

synthesis by corpus luteum

Testis

Stimulates interstitial cells of

Leydig

to

synthesize

and secrete testosterone

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Growth Hormone and

Prolactin

Somatotropes

(GH producing cells) are by far the most abundant anterior pituitary cells,

and account

for at least half the

cells

Structurally, prolactin (PRL) is closely related to GH

Hormone

Target

Major actions in humans

Growth hormone (GH), also called

somatotropic

hormone

(STH)Most tissues

Promotes growth in stature and mass;

stimulates production of insulin-like growth factor (IGF-I); stimulates protein synthesis; usually inhibits glucose utilization and promotes fat utilization

Prolactin

Mammary glands

Promotes milk secretion and mammary growth

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Adrenocorticotropin Family

Hormone

Target

Major actions in humans

Adrenocorticotropic hormone (ACTH), also known as

adreno-corticotropin

or

corticotropin

Adrenal cortex

Promotes synthesis and secretion of adrenal cortical hormones

β-Lipotropin

Adipose

Tissue

Physiological role not established

The ACTH related peptides constitute a family because:

they contain regions of homologous amino acid sequences, which may have arisen through exon duplication,

and because they all are encoded in the same gene

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Regulation of Anterior Pituitary Function

Secretion of the anterior pituitary hormones is regulated

by:

the central nervous system

provides the primary drive for secretion

and

hormones produced in

peripheral

target

glands

and peripheral input plays a secondary, though vital, role in modulating secretory ratesSecretion of all the anterior pituitary hormones except PRL declines severely in the absence of stimulation from the hypothalamus as can be produced

when the pituitary gland is removed surgically

from its natural location and reimplanted at a site remote from the hypothalamusPRL secretion is normally under tonic inhibitory control by the hypothalamus

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Feedback Control

of

Anterior Pituitary Function

Environmental factors may increase or decrease pituitary activity by

increasing

or decreasing

hormone secretions from hypothalamus

Pituitary secretions

increase the secretion of target gland hormones, which may

inhibit

further secretion by acting at either the hypothalamus or the

pituitary

Pituitary hormones may also inhibit their own secretion by a short

feedback loop13

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Physiology of the Posterior Pituitary

The

posterior pituitary gland secretes two

hormones which are:oxytocin ,

increase uterine contractions during

parturition

Contraction

of mammary

glands to secret milk

and

vasopressin or arginine vasopressin (AVP) (or Antidiuretic Hormone ‘ADH’) contract vascular smooth muscle and thus raise blood pressurepromote reabsorption of water

by renal tubules

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Physiology of the Posterior Pituitary

Oxytocin and AVP are stored in and secreted by the

posterior pituitary gland,

but are synthesized by the hypothalamus

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Regulation of Posterior Pituitary Function

Increased

blood

osmolality or decreased blood volume are sensed in the brain or thorax, respectively

, and increase vasopressin secretion.

Regulation of oxytocin secretion showing a positive

feedback

arrangement.

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Hypophysiotropic

hormones

Hormone

Physiological actions of

the

pituitary

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Pituitary & Hypothalamic Disorders

Hypothalamic-pituitary lesions present with a variety

of signs, including

pituitary hormone: hypersecretion and

hyposecretion

,

sellar

enlargement,

and visual loss

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Pituitary & Hypothalamic Disorders

In adults, the most common cause of

hypothalamic-pituitary

dysfunction is a pituitary adenoma, of which the great majority are hypersecreting

Thus

, the

earliest symptoms

of such tumors are due to

endocrinologic

abnormalities and include:

Early manifestationHypogonadism, the most frequent diminished functional activity of the gonads

Late manifestation in patients with larger tumors or

suprasellar extensionsellar enlargement headache and visual loss,

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Gonadotrophins

Disorders

Hyposecretion

 leads to amenorrhoea,

sterility

and

loss of sexual potency.

In

the young, the sex organs and secondary sexual characteristics fail to develop (delayed puberty

)

Hypersecretion extremely rare, in

children it could lead to sexual precocity (excessive premature development)

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Thyrotrophin

Disorders

Hyposecretion

 produces a clinical picture similar to primary thyroid deficiency

Hypersecretion

 

gives

the symptoms of hyperthyroidism similar to 

Graves’ disease

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Corticotrophin Disorders

Hyposecretion

 

rare causes failure of cortisol secretion,

a

general lack of health and well being,

a

reduced response to stress and skin

depigmentation

Hypersecretion

 due to a pituitary microadenoma, will result in 

Cushing’s syndrome

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Prolactin Disorders

Hyposecretion

 

leads to failure of lactation in womenHypersecretion

 

may

result from a pituitary

tumour

principal symptoms are infertility and menstrual complaintsin men, decreased libido,

inadequate sperm production and impotence, whereas in women, there may be a complete lack of menstruation

inappropriate (non-pregnant) milk production23

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GHRH Disorders

Hyposecretion

caused

by hypothalamic or pituitary dysfunction In childhood this

leads to impairment of growth 

(dwarfism

)

Hypersecretion

This usually results from a benign pituitary

tumour

In young patients, this leads to gigantismIn adults, leads to

acromegaly

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Vasopressin Disorders

Hyposecretion

 

caused by damage or dysfunction of the hypothalamus, can

lead to 

diabetes insipidus,

 

excessively

large amounts of dilute urine (10–15

liters/day

) are produced by the kidneysHypersecretion rare condition of inappropriate AVP production is known as syndrome of inappropriate ADH

(SIADH

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Hypopituitarism

Hypopituitarism is manifested by diminished or

absent secretion

of one or more pituitary hormonesHypopituitarism is either:a primary event

caused

by destruction of the anterior pituitary gland

or

a secondary phenomenon

resulting

from deficiency of hypothalamic stimulatory factors normally

acting on the pituitary26

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Assessment of Target Gland Function

If endocrine

hypofunction

is suspected, pituitary hormone deficiencies must be distinguished from primary failure

of the thyroid, adrenals, or

gonads

Baseline laboratory studies should

include:

thyroid

function

tests (free T4) and determination of serum testosterone levelsTestosterone is a sensitive indicator of hypopituitarism in women as well as in

men

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In primary target gland

hypofunction

, such as autoimmune

polyglandular syndromes types 1 and 2 (APS 1 and 2), TSH, LH, FSH, or ACTH will be elevatedLow or normal values for these pituitary hormones suggest hypothalamic-pituitary dysfunction

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