Endocrine System Pituitary, Thyroid Glands

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Endocrine system

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*consists of ductless glands

*richly vascularized

*produces hormones

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Hormones-

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chemical messengers that are produced by endocrine glands and derived by the bloodstream to the target cells or organs

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Hormones are classified due to their composition:

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*Proteins and polyproteins (mostly water-soluble) e.g. insulin, glucagon, follicle-stimulating-hormone FSH

*Amino-acid derivatives (mostly water-soluble) e.g. thyroxine and epinephrine

*Steroid and fatty acid derivatives (mostly lipid soluble) e.g. progesterone, estradiol, testosterone

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hormone binds to the receptor---> hormone-receptor complex---->

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signal transduction---> cell response

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hormones can bind to:

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*cell-surface receptors located in the cells membrane

*cytoplasmic receptors

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generally thyroid and steroid hormones bind to the...

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cytoplasmic receptors

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hormone-receptor complex can:

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*binds directly to DNA, stimulating gene transcription

*mediated action:

-stimulates adenylate cyclase to synthesize cyclic adenosine monophosphate cAMP, which stimulates protein kinase A in cytosol what leads to phosphorylation of proteins

-cAMP acts as second messenger

*G-protein associated

*use catalytic receptors

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second messengers :

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cGMP, calcium ions, sodium ions, meatbolites of phosphatidylinositol

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these hormones use G protein to activate a second messenger :

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epinephrine

thyroid-stimulating-hormone TSH

serotonin

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these hormones use catalytic receptors that activate protein kinase to phosphorylate target proteins:

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insulin

growth-hormone

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FEEDBACK MECHANISM

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*maintains homeostasis

-when hormone activates target cell an inhibitory signal is generated and returned to endocrine gland (negative feedback)

-when hormone level is to low for sufficient metabolic response in the target, a possitive feedback signal is released travels to endocrine gland- increase in hormone secretion

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PITUITARY GLAND:

(hypophysis)

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*partially derived from oral ectoderm and from neural ectoderm

*produces hormones that regulate growth, metabolism and reproduction

*lies below hypothalamus in hypophyseal fossa, covered by diaphragma sellae

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Adenohypophysis:

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*part of pituitary gland that develops from oral ectoderm (Rathke's pouch)

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Neurohypophysis:

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*part of pituitary gland that develops from neural ectoderm (diencephalon)

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secretion of nearly all of he hormones produced by pituitary gland is controlled by...

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signals from hypothalamus !!!

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the brain center of maintenance of homeostasis:

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hypothalamus

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Adenohypophysis

(anterior pituitary)

regions:

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*Pars distalis (pars anterior)

*Pars intermedia

*Pars tuberalis

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Neurohypophysis

(posterior pituitary)

regions:

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*Median eminence

*Infundibulum

*Pars nervosa

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primary capillary plexus of median eminence --->

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secondary capillary plexus of pars distalis

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atrial supply for the pituitary gland:

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*two pairs of vessels from internal carotid artery:

-superior hypophyseal arteries supply pars tuberalis and infundibulum

-form extensive capillary network- primary capillary plexus in median eminence

-inferior hypophyseal arteries supply the posterior lobe (neurohypophysis) and some part of the anterior lobe

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Hypophyseal portal veins:

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to primary capillary plexus of median eminence---> secondary capillary plexus in paris distalis

(fenestrated)

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hypothalamic neurosecretory hormones:

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*manufactured in hypothalamus

*stored in pituitary gland- median eminence

*releasing or inhibiting hormones

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main releasing-inhibitory hormones :

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*Thyroid-stimulating-hormone-releasing hormone (thyrotropin-releasing hormone) TRH

*Corticotropin-releasing hormone CRH stimulates release of adrenocorticotropin

*Somatotropic-releasing hormone SRH

*Lutenizing hormone-releasing hormone LHRH stimulates release of LH and FSH

*Prolactin- releasing hormone PRH

*Prolactin inhibitory factor PIF

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Adenohypophysis- PARIS DISTALIS composition:

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houses fenestrated secondary capillary plexus

perenchymal cells:

*chromophils:

-acidophils

-basophils

*chromophobes

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ACIDOPHILS:

(orange red with eosin)

-most abounded cells in pars distalis

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*somatotrophs

*mammotrophs

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SOMATOTROPHS

(acidophils)

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*secrete

somatotropin -growth hormone

*stimulated by SRH

*inhibited by somatostatin

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somatotropin action

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*increased cellular metabolic rates

*induces liver cells to produce: somatomedins

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somatomedins:

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= insulin-like growth factors I and II which stimulate mitotic rates of epiphyseal plate, chondrocytes, promote elongation of long bones, hence growth

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hence

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skótkiem tego z tąd

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MAMMOTROPHS:

(acidophils)

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*individual cells rather than clusters

*enlarge during lactation

*fusion of granules that contain prolactin

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during pregnancy circulating estrogen and progesterone ...

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inhibit secretion of prolactin

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release of prolactin from mammothrophs is stimulated by...

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prolactin-releasing hormone PRH and OXYTOCIN

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BASOPHILS

stain blue

chromophils located at periphery of pars distalis

three varieties:

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*Corticotrophs

*Thyrotrophs

*Gonadotrophs

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CORTICOTROPHS:

(basophils)

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*secrete:

- adrenocorticotropic hormone ACTH

- lipotropic hormone LPH

*secretions are stimulated by CRH

*POMC?

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adrenocorticotropic hormone ACTH main action:

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stimulates cells of the suprarenal cortex to release their secretory products

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THYROTROPHS:

(basophils)

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*small secretory granules which contain TSH known as thyrotropin

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secretion of thyrotropin is stimulated by

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THR

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secretion of thyroid-stimulating hormone is inhibited by...

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presence of thyroxine T4 and triiodothyronine T3 (thyroid hormones in blood)

-negative feedback

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GONADOTROPHS:

(basophils)

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*situated near sinuses

*secrete:

- FSH

- LH (sometimes called interstitial cell-stimulating hormone ICSH)

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secretion of FSH or LH is stimulated by...

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LHRH

(lutenizing hormone-releasing hormone)

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CHROMOPHOBES

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*have very little cytoplasm, therefore they do not take up the stain readily

*nonspecific steam cells or partialy degranulated chromophils

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FOLLICULOSTELLATE CELLS

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*nonsensory

*parge population in pars distalis

*function unknown

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PARS INTERMEDIA:

= zona intermedia

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*lies between pars distalis and pars nervosa

*contains many cuboidal colloid-containing cysts that are remnants of Rathke's pouch

* sometimes houses cords of basophils

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basophils of pars intermedia may secrete:

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POMC

pro-opiomelanocortin

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POMC undergoes post-translational cleavage to form...

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alfa-melanocyte-stimulating hormone

alfa-MSH

corticotropin

beta-lipotropin, beta-endorphin

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alfa-MSH is reffered to stimulate...

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release of prolactin in humans-

PROLACTIN RELEASING FACTOR

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PARS TUBERALIS:

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*surrounds hypophyseal stalk

*composed of cuboidal to low columnar basophilic cells

*possibly FSH, LH

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NEUROHYPOPHYSIS:

(posterior pituitary)

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*develops from a downgrowth of the hypothalamus

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Hypothalamohypophyseal Tract-

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unmyelinated axons of neurosecretory cells of suraoptic and paraventricular nuclei that extend into posterior pituitary

axons are supported by glia-like cells- pituicytes

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neurosecretory cells of supraoptic nucleus synthesize:

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VASOPRESSIN (antidiuretic hormone ADH)

neurophysin

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neurosecretory cells of paraventricular nucleus synthesize:

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OXYTOCIN

neurophysin

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PARS NERVOSA of posterior pituitary gland...

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*recives terminals of the neurosecretory hypothalamohypophyseal tract

*technically it is not a endocrine gland

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oxytocin and vasopressin (peptide hormones) travel down the axons of their respective neurons with precursor protein- neurophysin they mature from their precursors....

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called Herring bodies (accumulations of neurosecretory granules) to finally reach pars nervosa

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target for vasopressin ADH are...

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collecting ducts of kidney

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action of vasopressin...

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*increasing resorption of H2O by kidneys

modulates plasma membrane permeability

water absorption

which has the effect of lowering urine volume but increasing it's concentration

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target for oxytocin...

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*myometrium of uterus (late phases of pregnancy)

*stimulates concrations of smooth muscles

*milk ejection from mammary gland

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Pituicytes:

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*occupy about 25% of volume of pars nervosa

*similar to neuro-glial cells- support

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pituitary adenomas

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common tumors of anterior pituitary

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Diabetes insipidus

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*lesions in hypothalamus or pars nervosa

*production on ADH is reduced

*polyuria, dehydration

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adenohypophysis:

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*pars distalis:

chromophils:

a)acidophils:

- somatothropin- growth hormone

- prolactin

b)basophils:

- adrenocorticotropic hormone ACTH

- follicle-stimulating hormone FSH

- lutenizing hormone LH

- thyroid-stimulating hormone TSH (thyrotropin)

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neurohypophysis:

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*pars nervosa:

a)supraoptic nuclei:

- ADH antidiuretic hormone (vasopressin)

b)paraventricular nuclei:

- oxytocin

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Adrenocorticotropic hormone ACTH-

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STIMULATING SYNTHESIS AND RELEASE OF HORMONES (CORTISOL AND CORTICOSTERONE) FROM SUPRARENAL CORTEX

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Follicle-stimulating hormone

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STIMULATES SECONDARY OVARIAN FOLLICLE GROWTH ESTROGEN SECRETION

SERTOLI CELLS TO PRODUCE ANDROGEN BINDING PROTEIN

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Lutenizing hormone

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assists FSH in promoting ovulation

formation of copus luteum and secretion of progesterone and estrogenes

negative feedback to inhibit LHRH in women

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Interstitial cell-stimulating hormone ICSH

LH in men

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stimulates Leydig cells to secrete and release testosteron

formes negative feedback to hypothalamus to inhibit LHRH in men

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thyroid stimulating hormone TSH

thyrotropin

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stimulates synthesis and release of thyroid hormones, increasing metabolic rate

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VASOPRESSIN

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*conserves body water by increasing resorption of water in kidneys, through to be regulated by osmotic pressure

*causes contraction of smooth muscles in arteries thus raising the blood pressure

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THYROID GLAND:

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*localized in the anterior portion of the neck

*secretes hormones:

- thyroxine

- triiodothyronine

- calcitonin

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T3, T4

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*hormones under control of TSH

*regulate metabolic rate

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CALCITONIN

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*decreasing blood calcium level by:

- stimulating osteoblasts activity in bones (storage of Ca2+ in bones)

- inhibits osteoclasts activity in bones

(suppress bone resorption)

- inhibits Ca2+ absorption in intestine

- inhibits renal tubular Ca2+ absorption, more calcium in urine

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suppress

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tłumić, powstrzymywać

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thyroid gland composition:

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RIGHT LOBE and LEFT LOBE connected by ISTHMUS

! in some people additional lobe called PYRAMIDAL lobe is formed at the left

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THYROID FOLLICLE- structural and functional unit of thyroid gland;

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*secretory store place

*composed of simple cuboidal epithelium

*hormone in stored within the colloid

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T3 and T4 sored in colloid are bound to large secretory glycoprotein called...

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Thyroglobuilin

(endocytosed when hormone is to be released)

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cellular organization of the thyroid gland:

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*Follicular cells

*Parafollicular cells

-thin basal lamina

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principal cell of thymus are...

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FOLLICULAR CELLS

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FOLLICULAR CELL:

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*squamous to low-columnar in shape the tallest when stimulated

*basophilic cytoplasm

*numerous apically localized lysosomes

*colloid-follicular cell interferance- place of iodination of thyrosine residues

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this component is essential for synthesis of thyroid hormones T3 and T4...

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IODIDE I-

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what regulates synthesis of thyroid hormones T3 and T4?

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*iodide levels

*TSH binding to TSH receptors of follicular cells

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synthesis of thyroid hormones involves:

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occupation of TSH receptor--> cAMP production-->kinease A activity ---> synthesis of T3 and T4

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Thyroglobulin:

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*synthesized in RER

*glycosylated in RER and Golgi

*transported to apical plasma lemma

*released into colloid and stored in the lumen of the follicle

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IODIDE IN ACTIVELY TRANSPORTED VIA...

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SODIUM/IODIDE symporters

(located in basal plasmalemma of follicular cells)

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when iodide is transfered into colloid-cell membrane occurs the...

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iodide oxidation by enzyme

THYROID PEROXIDASE

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process of iodide oxidation requires :

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*thyroid peroxidase

*presence of hydrogen peroxidase H2O2

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when thyrosine residues of thyroglobulin are ionated they form...

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*monoiodinated tyrosine MIT

*diiodinated tyrosine DIT

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release of thyroid hormones:

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TSH binds to TSH receptors on plasma membrane of follicular cells---> formation of filopodia at the apical call membrane---> endocytosis

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MIT and DIT are stripped of their iodine by enzyme...

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iodotyrosine dehalogenase

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T4 constitutes about 90 % of the released hormone, although...

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it is not as effective as T3

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why T3 is more effective ?

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*T3 binds less avidly to plasma proteins so more of it gains entry into the cytoplasm than T4

*T3 binds with much greater affinity than T4 to NUCLEAR THYROID HORMONE RECEPTOR PROTEINS

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action of T3 and T4 on the body:

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* stimulate transcription

*greater metabolism - carbohydrate!

*growth in young

*decrease synthesis of cholesterol, increase synthesis of fatty acids and uptake of vitamins

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Grave's disease-

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hyperplasia of follicular cells

greater hormone production

(hyperthyroidism)

exophtalmos- protrusion of the eyeballs

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simple goiter

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insufficient dietary

(hypo/hyperthyroidism)

right supplementation

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Cretinism

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extreme form of hypothyroidism occurring in fetal life through childhood that is characterized by failure of growth and mental retardation owning to congenitally missing thyroid gland

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PARAFOLLICULAR CELLS =

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CLEAR CELLS = C cells

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parafollicular (clear) cells:

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* their granules secrete calcitonin

*at the periphery of follicle, do not reach the lumen of follicle

*larger than follicular cells but only 0,1 % epithelium

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calcitonin

THYROCALCITONIN

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peptide hormone that is released when blood level of calcimu is to high