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Endocrine

The endocrine system is referring to the collection of cells, glands, and tissues of an organism that secrete hormones directly into the bloodstream (some of which are transported along nerve tracts) to control the organisms' physiological and behavioral activities. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. The word endocrine derives from the Greek words "endo" meaning inside, within, and "crinis" for secrete. The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. The nervous system sends information very quickly, and responses are generally short lived. Hormones are substances (chemical mediators) released from endocrine tissue into the bloodstream where they travel to target tissue and generate a response. Hormones regulate various human functions, including metabolism, growth and development, tissue function, sleep, and mood. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of internal medicine.

Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.

The endocrine system is made of a series of glands that produce chemicals called hormones. A number of glands that signal each other in sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis.

As opposed to endocrine factors that travel considerably longer distances via the circulatory system, other signaling molecules, such as paracrine factors involved in paracrine signalling diffuse over a relatively short distance.

Endocrine organs and known secreted hormones

Hypothalamus

Secreted hormone Abbreviation Produced by Effect
Thyrotropin-releasing hormone TRH Parvocellular neurosecretory neurons Stimulate thyroid-stimulating hormone (TSH) release from anterior pituitary (primarily)
Dopamine
(Prolactin-inhibiting hormone)
DA or PIH Dopamine neurons of the arcuate nucleus Inhibit prolactin released from anterior pituitary
Growth hormone-releasing hormone GHRH Neuroendocrine neurons of the Arcuate nucleus Stimulate Growth hormone (GH) release from anterior pituitary
Somatostatin
(growth hormone-inhibiting hormone)
SS, GHIH, or SRIF Neuroendocrine cells of the Periventricular nucleus Inhibit Growth hormone (GH) release from anterior pituitary
Inhibit thyroid-stimulating hormone (TSH) release from anterior pituitary
Gonadotropin-releasing hormone GnRH or LHRH Neuroendocrine cells of the Preoptic area Stimulate follicle-stimulating hormone (FSH) release from anterior pituitary
Stimulate luteinizing hormone (LH) release from anterior pituitary
Corticotropin-releasing hormone CRH or CRF Parvocellular neurosecretory neurons of the Paraventricular Nucleus Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary
Oxytocin OT or OXT Magnocellular neurosecretory neurons of the Supraoptic nucleus and Paraventricular nucleus Uterine contraction
Lactation (letdown reflex)
Vasopressin
(antidiuretic hormone)
ADH or AVP or VP Parvocellular neurosecretory neurons, Magnocellular neurosecretory neurons of the Paraventricular nucleus and Supraoptic nucleus Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume

Pineal body (epiphysis)

Secreted hormone From cells Effect
Melatonin Pinealocytes Antioxidant
Monitors the circadian rhythm including induction of drowsiness and lowering of the core body temperature

Pituitary gland (hypophysis)

The pituitary gland (or hypophysis) is an endocrine gland about the size of a pea and weighing 5 grams (0.18 oz) in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae). The pituitary is functionally connected to the hypothalamus by the median eminence via a small tube called the infundibular stem or pituitary stalk. The pituitary fossa, in which the pituitary gland sits, is situated in the sphenoid bone in the middle cranial fossa at the base of the brain. The pituitary gland secretes nine hormones that regulate homeostasis and the secretion of other hormones.

Anterior pituitary lobe (adenohypophysis)

Secreted hormone Abbreviation From cells Effect
Growth hormone
(somatotropin)
GH Somatotrophs Stimulates growth and cell reproduction
Stimulates Insulin-like growth factor 1 release from liver
Thyroid-stimulating hormone
(thyrotropin)
TSH Thyrotrophs Stimulates thyroxine (T4) and triiodothyronine (T3) synthesis and release from thyroid gland
Stimulates iodine absorption by thyroid gland
Adrenocorticotropic hormone
(corticotropin)
ACTH Corticotrophs Stimulates corticosteroid (glucocorticoid and mineralcorticoid) and androgen synthesis and release from adrenocortical cells
Beta-endorphin - Corticotrophs Inhibits perception of pain
Follicle-stimulating hormone FSH Gonadotrophs In females: Stimulates maturation of ovarian follicles in ovary
In males: Stimulates maturation of seminiferous tubules
In males: Stimulates spermatogenesis
In males: Stimulates production of androgen-binding protein from Sertoli cells of the testes
Luteinizing hormone LH Gonadotrophs In females: Stimulates ovulation
In females: Stimulates formation of corpus luteum
In males: Stimulates testosterone synthesis from Leydig cells (interstitial cells)
Prolactin PRL Lactotrophs Stimulates milk synthesis and release from mammary glands
Mediates sexual gratification
Melanocyte-stimulating hormone MSH Melanotropes in the Pars intermedia of the Anterior Pituitary Stimulates melanin synthesis and release from skin/hair melanocytes

Posterior pituitary lobe (neurohypophysis)

Secreted hormone Abbreviation From cells Effect
Oxytocin Magnocellular neurosecretory cells Uterine contraction
Lactation (letdown reflex)
Vasopressin
(antidiuretic hormone)
ADH or AVP Parvocellular neurosecretory neurons Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume

Oxytocin and anti-diuretic hormone are not secreted in the posterior lobe, merely stored.

Thyroid

Secreted hormone Abbreviation From cells Effect
Triiodothyronine T3 Thyroid epithelial cell (More potent form of thyroid hormone)
Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rate
Stimulates RNA polymerase I and II, thereby promoting protein synthesis
Thyroxine
(tetraiodothyronine)
T4 Thyroid epithelial cells (Less active form of thyroid hormone)
(Acts as a prohormone to triiodothyronine)
Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rate
Stimulates RNA polymerase I and II, thereby promoting protein synthesis
Calcitonin Parafollicular cells Stimulates osteoblasts and thus bone construction
Inhibits Ca2+ release from bone, thereby reducing blood Ca2+

Alimentary system

Stomach

Secreted hormone Abbreviation From cells Effect
Gastrin (Primarily) G cells Secretion of gastric acid by parietal cells
Ghrelin P/D1 cells Stimulate appetite,

secretion of growth hormone from anterior pituitary gland

Neuropeptide Y NPY increased food intake and decreased physical activity. It can be associated with obesity.
Somatostatin D cells Suppress release of gastrin, cholecystokinin (CCK), secretin, motilin, vasoactive intestinal peptide (VIP), gastric inhibitory polypeptide (GIP), enteroglucagon

Lowers rate of gastric emptying Reduces smooth muscle contractions and blood flow within the intestine.[1]

Histamine ECL cells stimulate gastric acid secretion
Endothelin X cells Smooth muscle contraction of stomach[2]

Duodenum

Secreted hormone From cells Effect
Secretin S cells Secretion of bicarbonate from liver, pancreas and duodenal Brunner's glands

Enhances effects of cholecystokinin, stops production of gastric juice

Cholecystokinin I cells Release of digestive enzymes from pancreas

Release of bile from gallbladder, hunger suppressant

Liver

Secreted hormone Abbreviation From cells Effect
Insulin-like growth factor (or somatomedin) (Primarily) IGF Hepatocytes insulin-like effects

regulate cell growth and development

Angiotensinogen and angiotensin Hepatocytes vasoconstriction

release of aldosterone from adrenal cortex dipsogen.

Thrombopoietin THPO Hepatocytes stimulates megakaryocytes to produce platelets[3]
Hepcidin Hepatocytes inhibits intestinal iron absorption and iron release by macrophages

Pancreas

Pancreas is a mixed endocrine and exocrine gland and it secretes both enzymes and hormones.

Secreted hormone From cells Effect
Insulin (Primarily) β Islet cells Intake of glucose, glycogenesis and glycolysis in liver and muscle from blood

intake of lipids and synthesis of triglycerides in adipocytes Other anabolic effects

Glucagon (Also Primarily) α Islet cells glycogenolysis and gluconeogenesis in liver

increases blood glucose level

Somatostatin δ Islet cells Inhibit release of insulin[4]

Inhibit release of glucagon[4] Suppress the exocrine secretory action of pancreas.

Pancreatic polypeptide PP cells Self regulate the pancreas secretion activities and effect the hepatic glycogen levels.

Kidney

Secreted hormone From cells Effect
Renin (Primarily) Juxtaglomerular cells Activates the renin-angiotensin system by producing angiotensin I of angiotensinogen
Erythropoietin (EPO) Extraglomerular mesangial cells Stimulate erythrocyte production
Calcitriol (1,25-dihydroxyvitamin D3) Active form of vitamin D3

Increase absorption of calcium and phosphate from gastrointestinal tract and kidneys inhibit release of PTH

Thrombopoietin stimulates megakaryocytes to produce platelets[3]

Adrenal glands

Adrenal cortex

Secreted hormone From cells Effect
Glucocorticoids (chiefly cortisol) zona fasciculata and zona reticularis cells Stimulates gluconeogenesis
Stimulates fat breakdown in adipose tissue
Inhibits protein synthesis
Inhibits glucose uptake in muscle and adipose tissue
Inhibits immunological responses (immunosuppressive)
Inhibits inflammatory responses (anti-inflammatory)
Mineralocorticoids (chiefly aldosterone) Zona glomerulosa cells Stimulates active sodium reabsorption in kidneys
Stimulates passive water reabsorption in kidneys, thus increasing blood volume and blood pressure
Stimulates potassium and H+ secretion into nephron of kidney and subsequent excretion
Androgens (including DHEA and testosterone) Zona fasciculata and Zona reticularis cells In males: Relatively small effect compared to androgens from testes
In females: masculinizing effects

Adrenal medulla

Secreted hormone From cells Effect
Adrenaline (epinephrine) (Primarily) Chromaffin cells Fight-or-flight response:
Noradrenaline (norepinephrine) Chromaffin cells Fight-or-flight response:
Dopamine Chromaffin cells Increase heart rate and blood pressure
Enkephalin Chromaffin cells Regulate pain

Reproductive

Testes

Secreted hormone From cells Effect
Androgens (chiefly testosterone) Leydig cells Anabolic: growth of muscle mass and strength, increased bone density, growth and strength,

Virilizing: maturation of sex organs, formation of scrotum, deepening of voice, growth of beard and axillary hair.

Estradiol Sertoli cells Prevent apoptosis of germ cells[5]
Inhibin Sertoli cells Inhibit production of FSH

Ovarian follicle and corpus luteum

Secreted hormone From cells Effect
Progesterone Granulosa cells, theca cells Support pregnancy:[6]

Other:

Anti-inflammatory

Androstenedione Theca cells Substrate for estrogen
Estrogens (mainly estradiol) Granulosa cells Structural:

Protein synthesis:

  • Increase hepatic production of binding proteins

Coagulation:

Fluid balance:

Gastrointestinal tract:

  • Reduce bowel motility
  • Increase cholesterol in bile

Melanin:

Cancer:

  • Support hormone-sensitive breast cancers[9] (Suppression of production in the body of estrogen is a treatment for these cancers.)

Lung function:

Inhibin Granulosa cells Inhibit production of FSH from anterior pituitary

Placenta (when pregnant)

Secreted hormone Abbreviation From cells Effect
Progesterone (Primarily) Support pregnancy:[6]
  • Inhibit immune response, towards the fetus.
  • Decrease uterine smooth muscle contractility[6]
  • Inhibit lactation
  • Inhibit onset of labor.
  • Support fetal production of adrenal mineralo- and glucosteroids.

Other effects on mother similar to ovarian follicle-progesterone

Estrogens (mainly Estriol) (Also Primarily) Effects on mother similar to ovarian follicle estrogen
Human chorionic gonadotropin HCG Syncytiotrophoblast Promote maintenance of corpus luteum during beginning of pregnancy

Inhibit immune response, towards the human embryo.

Human placental lactogen HPL Syncytiotrophoblast Increase production of insulin and IGF-1

Increase insulin resistance and carbohydrate intolerance

Inhibin Fetal Trophoblasts Suppress FSH

Uterus (when pregnant)

Secreted hormone Abbreviation From cells Effect
Prolactin PRL Decidual cells milk production in mammary glands
Relaxin Decidual cells Unclear in humans and animals

Calcium regulation

Further information: Calcium metabolism

Parathyroid

Secreted hormone Abbreviation From cells Effect
Parathyroid hormone PTH Parathyroid chief cell Calcium:
  • Stimulates Ca2+ release from bone, thereby increasing blood Ca2+
  • Stimulates osteoclasts, thus breaking down bone
  • Stimulates Ca2+ reabsorption in kidney
  • Stimulates activated vitamin D production in kidney


Phosphate:

  • Stimulates PO3-4 release from bones, thereby increasing blood PO3-4.
  • Inhibits PO3-4 reabsorption in kidney, so more PO3-4 is excreted
  • Overall, small net drop in serum PO3-4.

Skin

Secreted hormone From cells Effect
Calcidiol (25-hydroxyvitamin D3) Inactive form of vitamin D3

Targets

Heart

Secreted hormone Abbreviation From cells Effect
Atrial-natriuretic peptide ANP Cardiac myocytes Reduce blood pressure by:

reducing systemic vascular resistance, reducing blood water, sodium and fats

Brain natriuretic peptide BNP Cardiac myocytes (To a lesser degree than ANP) reduce blood pressure by:

reducing systemic vascular resistance, reducing blood water, sodium and fats

Bone marrow

Secreted hormone From cells Effect
Thrombopoietin liver and kidney cells stimulates megakaryocytes to produce platelets[3]

Adipose tissue

Secreted hormone From cells Effect
Leptin (Primarily) Adipocytes decrease of appetite and increase of metabolism.
Estrogens[11] (mainly Estrone) Adipocytes

Diffuse Neuro-Endocrine System (DNES)

The Diffuse Neuro-Endocrine System (DNES) comprises hormone-secreted cells, that have commonalities with neurons and are found in the Epithelium of organs of the body.

Major endocrine systems

The human endocrine system consists of several systems that operate via feedback loops. Several important feedback systems are mediated via the hypothalamus and pituitary.[12]

  • TRH - TSH - T3/T4
  • GnRH - LH/FSH - sex hormones
  • CRH - ACTH - cortisol
  • Renin - angiotensin - aldosterone
  • leptin vs. insulin

Interaction with immune system

Extensive bidirectional interactions exist between the endocrine system and the immune system.[13] Cortisol has major immunosuppressive effects,[14][15] and dopamine has immunomodulatory functions.[16] On the other hand, cytokines produced during inflammation activate the HPA axis at all three levels, sensible to negative feedback.[17] Moreover cytokines stimulate hepcidin release from the liver, which is eventually responsible for the anemia of chronic disease.[18]

In other species

A neuroendocrine system has been observed in all animals with a nervous system and all vertebrates have an hypothalamus-pituitary axis.[19] All vertebrates have a thyroid, which in amphibians is also crucial for transformation of larvae into adult form.[20][21] All vertebrates have adrenal gland tissue, with mammals unique in having it organized into layers.[22] All vertebrates have some form of renin-angiotensin axis, and all tetrapods have aldosterone as primary mineralocorticoid.[23][24]

Diseases

Main article: Endocrine diseases

Diseases of the endocrine system are common,[26] including conditions such as diabetes mellitus, thyroid disease, and obesity. Endocrine disease is characterized by disregulated hormone release (a productive pituitary adenoma), inappropriate response to signaling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminished erythropoiesis in chronic renal failure), or structural enlargement in a critical site such as the thyroid (toxic multinodular goitre). Hypofunction of endocrine glands can occur as a result of loss of reserve, hyposecretion, agenesis, atrophy, or active destruction. Hyperfunction can occur as a result of hypersecretion, loss of suppression, hyperplastic or neoplastic change, or hyperstimulation.

Endocrinopathies are classified as primary, secondary, or tertiary. Primary endocrine disease inhibits the action of downstream glands. Secondary endocrine disease is indicative of a problem with the pituitary gland. Tertiary endocrine disease is associated with dysfunction of the hypothalamus and its releasing hormones.

As the thyroid, and hormones have been implicated in signaling distant tissues to proliferate, for example, the estrogen receptor has been shown to be involved in certain breast cancers. Endocrine, paracrine, and autocrine signaling have all been implicated in proliferation, one of the required steps of oncogenesis.[27]

Other types of signaling

The typical mode of cell signaling in the endocrine system is endocrine signaling. However, there are also other modes, i.e., paracrine, autocrine, and neuroendocrine signaling.[28] Purely neurocrine signaling between neurons, on the other hand, belongs completely to the nervous system.

Autocrine

Main article: Autocrine signalling

Autocrine signaling is a form of signaling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on the same cell, leading to changes in the cells.

Paracrine

Main article: Paracrine signalling

Paracrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell, altering the behavior or differentiation of those competent cells.

Juxtacrine

Main article: Juxtacrine signalling

Juxtacrine signaling is a type of intercellular communication that is transmitted via oligosaccharide, lipid, or protein components of a cell membrane, and may affect either the emitting cell or the immediately adjacent cells.

It occurs between adjacent cells that possess broad patches of closely opposed plasma membrane linked by transmembrane channels known as connexons. The gap between the cells can usually be between only 2 and 4 nm.

Unlike other types of cell signaling (such as paracrine and endocrine), juxtacrine signaling requires physical contact between the two cells involved.

Juxtacrine signaling has been observed for some growth factors, cytokine and chemokine cellular signals.

See also

References

External links

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