The Endocrine System
Overview of the Endocrine SystemSystem of ductless glands that secrete hormonesHormones are “messenger molecules”Circulate in the bloodAct on distant target cellsTarget cells respond to the hormones for which they have receptorsThe effects are dependent on the programmed response of the target cellsHormones are just molecular triggersBasic categories of hormonesAmino acid based: modified amino acids (or amines), peptides (short chains of amino acids), and proteins (long chains of amino acids)Steroids: lipid molecules derived from cholesterol
Endocrine Organs
Purely endocrine organs Pituitary gland Pineal gland Thyroid gland Parathyroid glands Adrenal: 2 glands Cortex Medulla Endocrine cells in other organs Pancreas Thymus Gonads HypothalamusMechanisms of hormone release
(a) Humoral: in response to changing levels of ions or nutrients in the blood (b) Neural: stimulation by nerves (c) Hormonal: stimulation received from other hormonesPituitary__________ (hypophysis)
Hypothalamus___________Hypothalamus__
Anterior pituitary__ (adenohypophysis)
_____________Posterior pituitary (neurohypophysis)
Learn the 3 endocrine organs on this slide: Hypothalamus Pituitary (hyophysis) Pineal
Sits in hypophyseal fossa: depression in sella turcica of sphenoid bone
Pituitary secretes 9 hormonesThe Pituitary
1. TSH 2. ACTH 3. FSH 4. LH 5. GH 6. PRL 7. MSH
8. ADH (antidiuretic hormone), or vasopressin 9. Oxytocin
_________________________________________________________________
The first four are “tropic” hormones, they regulate the function of other hormones ________
Two divisions: Anterior pituitary (adenohypophysis)
Posterior pituitary (neurohypophysis)What the letters stand for…
TSH: thyroid-stimulating hormone ACTH: adrenocorticotropic hormone FSH: follicle-stimulating hormone LH: luteinizing hormone GH: growth hormone PRL: prolactin MSH: melanocyte-stimulating hormone ADH: antidiuretic hormone OxytocinHypothalamus controls anterior pituitary hormone release
Releasing hormones (releasing factors) Secreted like neurotransmitters from neuronal axons into capillaries and veins to anterior pituitary (adenohypophysis) TRH-----turns on TSH CRH-----turns on ACTH GnRH (=LHRH)---turns on FSH and LH PRF-----turns on PRL GHRH----turns on GH Inhibiting hormones PIF-----turns off PRL GH inhibiting hormone ---turns off GH*Note: “turns on” means causes to be released
What the letters mean…Releasing hormones (releasing factors) of hypothalamus Secreted like neurotransmitters from neuronal axons into capillaries and veins to anterior pituitary (adenohypophysis) TRH (thyroid releasing hormone) -----turns on* TSH CRH (corticotropin releasing hormone) -----turns on ACTH GnRH (gonadotropin releasing hormone) ---turns on FSH and LH PRF (prolactin releasing hormone) -----turns on PRL GHRH (growth hormone releasing hormone) ----turns on GH Inhibiting hormones of hypothalmus PIF (prolactin inhibiting factor) -----turns off PRL GH (growth hormone) inhibiting hormone ---turns off GH The hypothalamus controls secretion of hormones which in their turn control the secretion of hormones by the thyroid gland, the adrenal cortex and gonads: in this way the brain controls these endocrine glands
The four tropic ones regulate the function of other hormones:
So what do the pituitary hormones do?TSH stimulates the thyroid to produce thyroid hormone ACTH stimulates the adrenal cortex to produce corticosteroids: aldosterone and cortisol FSH stimulates follicle growth and ovarian estrogen production; stimulates sperm production and androgen-binding protein LH has a role in ovulation and the growth of the corpus luteum; stimulates androgen secretion by interstitial cells in testes
The others from the anterior pituitary…
GH (aka somatrotropic hormone) stimulates growth of skeletal epiphyseal plates and body to synthesize protein PRL stimulates mammary glands in breast to make milk MSH stimulates melanocytes; may increase mental alertnessFrom the posterior pituitary (neurohypophysis) structurally part of the brain
ADH (antidiuretic hormone AKA vasopressin) stimulates the kidneys to reclaim more water from the urine, raises blood pressure Oxytocin prompts contraction of smooth muscle in reproductive tracts, in females initiating labor and ejection of milk from breastsTSH: thyroid-stimulating hormone ACTH: adrenocorticotropic hormone FSH: follicle-stimulating hormone LH: luteinizing hormone GH: growth hormone PRL: prolactin MSH: melanocyte-stimulating hormone ADH: antidiuretic hormone Oxytocin
TRH (thyroid releasing hormone) turns on TSH CRH (corticotropin releasing hormone) turns on ACTH GnRH (gonadotropin releasing hormone) turns on FSH and LH PRF (prolactin releasing hormone) turns on PRL GHRH (growth hormone releasing hm) turns on GH
TSH stimulates the thyroid to produce thyroid hormone ACTH stimulates the adrenal cortex to produce corticosteroids: aldosterone and cortisol FSH stimulates follicle growth and ovarian estrogen production; stimulates sperm production and androgen-binding protein LH has a role in ovulation and the growth of the corpus luteum; stimulates androgen secretion by interstitial cells in testes
GH (aka somatrotropic hormone) stimulates growth of skeletal epiphyseal plates and body to synthesize protein PRL stimulates mammary glands in breast to make milk MSH stimulates melanocytes; may increase mental alertness
ADH (antidiuretic hormone or vasopressin) stimulates the kidneys to reclaim more water from the urine, raises blood pressure Oxytocin prompts contraction of smooth muscle in reproductive tracts, in females initiating labor and ejection of milk from breasts
Can we put it all together?
Blue is from hypothalamus Black is from pituitary
Pituitary__________ (hypophysis)
Hypothalamus___________Now try and remember the anatomy
The Thyroid Gland
Anterior neck on trachea just inferior to larynx Two lateral lobes and an isthmus Produces two hormones Thyroid hormone: tyrosine based with 3 or 4 iodine molecules T4 (thyroxine) and T3 Calcitonin involved with calcium and phosphorus metabolismThyroid is composed of spherical folliclesFollicle cells: produce thyroglobulin, the precursor of thryoid hormone (thyroxin)Colloid lumen is of thyroglobulinParafollicular “C” cells: produce calcitonin
generic
particular example: thyroid hormoneAn example of a feedback loop
A certain item in the blood decreases A certain area of the brain senses this decrease A certain hormone is released This hormone stimulates the release of another hormone This other hormone stimulates the release of the hormone which was sensed to be decreased in the first place, causing it to be increased to desired levelThyroxine (thyroid hormone) Hypothalamus TRF from the hypothalamus TSH from anterior pituitary Thyroxine from the thyroid (TSH has caused cleavage of thryroglobulin into thyroxine)
Some Effects of Thyroid Hormone (Thyroxine)
Increases the basal metabolic rate The rate at which the body uses oxygen to transform nutrients (carbohydrates, fats and proteins) into energy Affects many target cells throughout the body; some effects are Protein synthesis Bone growth Neuronal maturation Cell differentiationThe Effects of Calcitonin
Secreted from thyroid parafollicular (C) cells when blood calcium levels are high Calcitonin lowers Ca++ by slowing the calcium-releasing activity of osteoclasts in bone and increasing calcium secretion by the kidney Acts mostly during childhood
The Parathyroid Glands
Most people have four On posterior surface of thyroid gland (sometimes embedded)Parathyroids (two types of cells)
Rare chief cells Abundant oxyphil cells (unknown function) Chief cells produce PTH Parathyroid hormone, or parathormone A small protein hormoneFunction of PTH (parathyroid hormone or “parathormone”)
Increases blood Ca++ (calcium) concentration when it gets too low Mechanism of raising blood calcium Stimulates osteoclasts to release more Ca++ from bone Decreases secretion of Ca++ by kidney Activates Vitamin D, which stimulates the uptake of Ca++ from the intestine Unwitting removal during thyroidectomy was lethal Has opposite effect on calcium as calcitonin (which lowers Ca++ levels)
Adrenal (suprarenal) glands(“suprarenal” means on top of the kidney)
Each is really two endocrine glands Adrenal cortex (outer) Adrenal medulla (inner) Unrelated chemicals but all help with extreme situationsAdrenal Gland
Adrenal cortexSecretes lipid-based steroid hormones, called “corticosteroids” – “cortico” as in “cortex”MINERALOCORTICOIDSAldosterone is the main oneGLUCOCORTICOIDSCortisol (hydrocortisone) is the main oneAdrenal medullaSecretes epinephrine and norepinephrineAldosterone, the main mineralocorticoid
Secreted by adrenal cortex in response to a decline in either blood volume or blood pressure (e.g. severe hemorrhage) Is terminal hormone in renin-angiotensin mechanism Prompts distal and collecting tubules in kidney to reabsorb more sodium Water passively follows Blood volume thus increasesPeople with adrenal insufficiency: these stresses can cause hypotension, shock and death: must give glucocorticoids, eg for surgery or if have infection, etc.
Cortisol, the most important glucocorticoid (Glucocorticoid receptors are found in the cells of most vertebrate tissues)
It is essential for life Helps the body deal with stressful situations within minutes Physical: trauma, surgery, exercise Psychological: anxiety, depression, crowding Physiological: fasting, hypoglycemia, fever, infection Regulates or supports a variety of important cardiovascular, metabolic, immunologic, and homeostatic functions including water balance
Cortisol, continued
Keeps blood glucose levels high enough to support brain’s activity Forces other body cells to switch to fats and amino acids as energy sourcesCatabolic: break down proteinRedirects circulating lymphocytes to lymphoid and peripheral tissues where pathogens usually areIn large quantities, depresses immune and inflammatory responseUsed therapeuticallyResponsible for some of its side effectsHormonal stimulation of glucocorticoids HPA axis (hypothalamic/pituitary/adrenal axis)
With stress, hypothalamus sends CRH to anterior pituitary (adenohypophysis) Pituitary secretes ACTH ACTH goes to adrenal cortex where stimulates glucocorticoid secretion Sympathetic nervous system can also stimulate it Adrenal cortex also secretes DHEA (dehydroepiandrosterone) Converted in peripheral tissues to testosterone and estrogen (also steroid hormones) Unclear function in relation to stressIn general:
Steroid-secreting cells have abundant smooth ER As opposed to rough ER in protein-secreting cells Steroids directly diffuse across plasma membrane Not exocytosis Abundant lipid droplets Raw material from which steroids made
Adrenal medulla
Part of autonomic nervous system Spherical chromaffin cells are modified postganglionic sympathetic neurons Secrete epinephrine and norepinephrine Amine hormones Fight, flight, fright Vesicles store the hormonesThe Pineal Gland
At the end of a short stalk on the roof of the diencephalonPinealocytes with dense calcium particles Can be seen on x-ray (because of Ca++)Melatonin helps regulate the circadium rhythmThe biological clock of the diurnal (night/day) rhythmComplicated feedback via retina’s visual inputThe Pancreas
Exocrine and endocrine cells Acinar cells (forming most of the pancreas) Exocrine function Secrete digestive enzymes Islet cells (of Langerhans) Endocrine functionAlpha cells: secrete glucagon raises blood sugar mostly in periphery Beta cells: secrete insulin lowers blood sugar central part (are more abundant) Also rare Delta cells:secrete somatostatin inhibits glucagon
Pancreatic islet endocrine cells
The Gonads (testes and ovaries) main source of the steroid sex hormonesTestes Interstitial cells secrete androgens Primary androgen is testosterone Maintains secondary sex characteristics Helps promote sperm formation Ovaries Androgens secreted by thecal folliculi Directly converted to estrogens by follicular granulosa cells Granulosa cells also produce progesterone Corpus luteum also secretes estrogen and progesterone
Endocrine cells in various organs
The heart: atrial natriuretic peptide (ANP) Stimulates kidney to secrete more salt Thereby decreases excess blood volume, high BP and high blood sodium concentration GI tract & derivatives: Diffuse neuroendocrine system (DNES)Endocrine cells in various organs continued
The heart: atrial natriuretic peptide (ANP) Stimulates kidney to secrete more salt Thereby decreases excess blood volume, high BP and high blood sodium concentration GI tract & derivatives: Diffuse neuroendocrine system (DNES) The placenta secretes steroid and protein hormones Estrogens, progesterone CRH HCG The kidneys Juxtaglomerular cells secrete renin Renin indirectly signals adrenal cortex to secrete aldosterone Erythropoietin: signals bone marrow to increase RBC production The skin Modified cholesterol with uv exposure becomes Vitamin D precursor Vitamin D necessary for calcium metabolism: signals intestine to absorb CA++
Pathology
PituitaryGigantism –too much GH in childhoodAcromegaly – too much GH in adulthoodPituitary dwarfs – too little GH in childhoodDiabetes insipidus - too much ADHPancreasDiabetes mellitus – one type of insulin (not enough)ThyroidHyperthyroidism, commonest is Grave’s disease (autoimmune)HypothyroidismIn childhood leads to cretinismEndemic goiter from insufficient iodine in dietAdult hypothyroidism (myxedema): autoimmuneExophthalmos of Grave’s disease Enlarged thyroid (goiter) from iodine deficiency
Pathology, continuedAdrenal gland Cushing’s syndrome (see next pic)Usually caused by an ACTH-secreting pituitary tumorRarely by tumor of adrenal cortexIatrogenic Addison’s diseaseHyposecretion (under secretion) of adrenal cortexUsually involves cortisol and aldosterone: low blood glucose and sodium, severe dehydration, fatigue, loss of appetetie, abdominal pain(Jane Austin)