Organs of the Endocrine System and Their Products

• The following major glands will be covered one at a time with their products:

1.Pituitary gland / Hypothalamus

2.Thyroid gland3.Parathyroid gland4.Adrenal gland5.Pancreas (also has exocrine

function)6. Gonadal hormones

(ovaries and testes)7.Thymus

1 .Pituitary Gland (Hypophysis)• located in sella turcica of sphenoid bone (in cranial

cavity), inferior to hypothalamus• consists of two lobes:

A. neurohypophysis (~ posterior pituitary)• attached to hypothalamus by infundibulum• contains axons and axon terminals of

neurosecretory cells whose cell bodies are in hypothalamic nuclei

B. adenohypophysis (~ anterior pituitary)• consists of glandular epithelium

http://www.usc.edu/hsc/dental/ghisto/end/c_1.html

A. Neurohypophysis (Posterior Pituitary)

• consists of nerve fibers (axons of neurosecretory cells with cell bodies in hypothalamus) and pituicytes (glial cells that support nerve fibers)

• acts primarily as a storage and releasing area for hormones actually made in hypothalamic nuclei

• hormones released in response to impulses from hypothalamus (neural control)

• hormones are short amino acid chains (peptides)– oxytocin– antidiuretic hormone (ADH or “vasopressin”)

A. Neurohypophysis:Oxytocin (OT)

• action, in pregnant or nursing women:

– stimulates contraction of smooth muscle of uterine wall during labor and delivery

– stimulates ejection of milk in lactating mothers

• action, in men and non-pregnant women, may be involved in sexual arousal and orgasm

A. Neurohypophysis:Oxytocin (OT)

• control:

– during labor/delivery, positive feedback: stretching of uterus/cervix --> sensory impulses to hypothalamus --> increased secretion of OT --> increased contraction

– suckling: sucking of infant on breast --> sensory to hypothalamus --> oxytocin release --> release of milk

A. Neurohypophysis: Antidiuretic Hormone (ADH)

• action: antidiuretic hormone (ADH) directly affects blood

pressure - acts as powerful vasoconstrictor --> increases

blood pressure (hence name “vasopressin”)

* action: affects water balance (indirect affect on blood

pressure) - acts on tubules of kidney to increase

reabsorption of water less water lost in urine

A. Neurohypophysis: ADH

• disorders:

– hyposecretion due to damage of hypothalamic nucleus or neurohypophysis--> diabetes insipidus - excessive urine production (polyuria) and thirst

– hypersecretion --> SIADH (syndrome of inappropriate ADH secretion) - water retention, headache, cerebral edema, weight gain, hypoosmolarity

Antidiuretic Hormone (ADH): Control

• neural control: increased electrolyte (NaCl) concentration --> affects (supraoptic) nucleus in hypothalamus --> impulse to neurohypophysis --> release of ADH --> increased water reabsorption --> decrease in electrolyte concentration

• other stimuli: pain, low BP, morphine, barbiturates, nicotine, aldosterone (hormone from adrenal cortex - hormonal control)

• inhibition: alcohol (results in more urine production and, potentially, dehydration)

• diuretic drugs - some act to supress ADH secretion; used to treat hypertension and congestive heart failure

B. Adenohypophysis (Anterior Pituitary)

• linked to hypothalamus via hypophyseal portal system (capillary networks and small veins)

– carries regulatory hormones from hypothalamus to pituitary

• releasing hormones stimulate secretion of pituitary hormones

• inhibitory hormones inhibit secretion

• consists of epithelial cells

• all hormones produced are proteins

* tropic hormones - affect some endocrine glands or provide maintenance oversight for other organs

B. Adenohypophysis:Growth Hormone (GH)

• highest levels during evening and sleep• action: stimulates increased rate of protein synthesis leading

to cell growth and division– bones and skeletal muscle respond more than other body

cells• action: stimulates use of fat as energy source and decreases

rate of glucose uptake and glucose metabolism (diabetogenic effect – “spares” glucose)

• control:– release stimulated by GHRH (growth hormone releasing

hormone) from hypothalamus– inhibited by GHIH (from hypothalamus) and

somatomedins (produced by liver under GH stimulation)

Growth Hormone (GH): DisordersDisorders:

• hypersecretion

– gigantism (in children)

• up to 8’ tall, normal body proportions

– acromegaly (after epiphyseal plates close)

• enlargement of extremities and face, thickening of soft tissue

• hyposecretion

– pituitary dwarfism - in children, up to 4’ tall

– progeria - premature aging, atrophy of body tissues

Growth Hormone Excess

in childhood leads to GIGANTISM

Growth Hormone Excess

in adulthood leads to ACROMEGALY

B. Adenohypophysis: Prolactin (PRL)

• action:

* stimulates milk production in mammary glands;

– helps stimulate development of mammary glands (along with other hormones);

– in males, may help regulate testosterone production

• control:

– stimulation: PRH (prolactin-releasing hormone from hypothalamus), high estrogens, breast-feeding

– inhibition: PIH (hypothalamus), stimulated by rising PRL levels, low estrogen

B. Adenohypophysis:Prolactin (PRL)

Disorders

• hyperprolactinemia = hypersecretion due to adenohypophyseal tumors; results in galactorrhea, lack of menses and infertility in women, impotence in men

B. Adenohypophysis: Thyroid-Stimulating Hormone (TSH)• TSH = thyrotropin

– action:

• stimulates secretion of hormones from thyroid gland (T4 and T3); also stimulates development of thyroid in youth

– control:

• release stimulated by TRH (thyroid releasing hormone from hypothalamus)

• inhibited by rising levels of thyroid hormones and by GHIH

B. Adenohypophysis: Adrenocorticotropic hormone (ACTH)

• ACTH=corticotropin

• action: stimulates release of hormones from adrenal cortex

• control:

– release stimulated by CRH (corticotropin-releasing hormone from hypothalamus)

– release inhibited by rising levels of glucocorticoids from adrenal cortex

B. Adenohypophysis:Gonadotropins

• regulate activity and secretion by gonads (testes in males; ovaries in females)

• control:

– stimulated by GnRH (gonadotropin-releasing hormone from hypothalamus)

– release of GnRH is inhibited by rising levels of estrogens, progestins and androgens (testosterone)

• two important hormones

– FSH

– LH

Gonadotropins:Follicle-Stimulating Hormone (FSH)

• action:

– females (ovaries) - stimulates development of ovarian follicles and estrogen production

– males (testes) - stimulates sperm production and development

• inhibited by inhibin and testosterone from testes (feedback to hypothalamus and anterior pituitary) and estrogen, progesterone and inhibin from ovaries (feedback to anterior pituitary)

Gonadotropins:Luteinizing Hormone (LH)

• LH=lutropin

– action:

• females (ovaries) - induces ovulation; stimulates secretion of estrogens and progestins (e.g., progesterone)

• males (testes) - stimulates production of androgens (e.g., testosterone )

– inhibited by estrogen, progesterone and inhibin form ovaries (feedback to anterior pituitary) and by inhibin and testosterone from testes (feedback to hypothalamus and anterior pituitary)