Definitions

Endocrine_system

Endocrine system

The endocrine system is an integrated system of small organs that involve the release of extracellular signaling molecules known as hormones. The endocrine system is instrumental in regulating metabolism, growth, development and puberty, tissue function, and also plays a part in determining mood. The field of medicine that deals with disorders of endocrine glands is endocrinology, a branch of the wider field of internal medicine.

Function

The Endocrine system is an information signal system much like the nervous system. However, the nervous system uses nerves to conduct information, whereas the endocrine system mainly uses blood vessels as information channels. Glands located in many regions of the body release into the bloodstream specific chemical messengers called hormones. Hormones regulate the many and varied functions of an organism, e.g., mood, growth and development, tissue function, and metabolism, as well as sending messages and acting on them.

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. Purely neurocrine signaling between neurons, on the other hand, belongs completely to the nervous system.

Endocrine

A number of glands that signal each other in sequence is usually referred to as an axis, for example the Hypothalamic-pituitary-adrenal axis.

Typical endocrine glands are the pituitary, thyroid, and adrenal glands. 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.

Autocrine

Other signaling can target the same cell.

Paracrine

Paracrine signaling is where the target cell is nearby.

Juxtacrine

Juxtacrine signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent.

Role in disease

Diseases of the endocrine system are common, including diseases such as diabetes mellitus, thyroid disease, and obesity. Endocrine disease is characterised by dysregulated hormone release (a productive Pituitary adenoma), inappropriate response to signalling (Hypothyroidism), lack or destruction of a gland (Diabetes mellitus type 1, diminished erythropoiesis in Chronic renal failure), or structural enlargement in a critical site such as the neck (Toxic multinodular goitre). Hypofunction of endocrine glands can occur as result of loss of reserve, hyposecretion, agenesis, atrophy, or active destruction. Hyperfunction can occur as 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. Tertiary endocrine disease is associated with dysfunction of the hypothalamus and its releasing hormones.

Cancer can occur in endocrine glands, such as the thyroid, and hormones have been implicated in signalling distant tissues to proliferate, for example the Estrogen receptor has been shown to be involved in certain breast cancers. Endocrine, Paracrine, and autocrine signalling have all been implicated in proliferation, one of the required steps of oncogenesis.

Table of endocrine glands and secreted hormones

This is a table of the glands of the endocrine system, and their secreted hormones

Hypothalamus

Secreted hormone Abbreviation From cells Effect
Thyrotropin-releasing hormone TRH Parvocellular neurosecretory neurons Release thyroid-stimulating hormone from anterior pituitary (primarily)
Stimulate prolactin release from anterior pituitary.
Gonadotropin-releasing hormone GnRH Neuroendocine cells of the Preoptic area Release of FSH and LH from anterior pituitary.
Growth hormone-releasing hormone GHRH Neuroendocrine neurons of the Arcuate nucleus Release GH from anterior pituitary
Corticotropin-releasing hormone CRH Parvocellular neurosecretory neurons Release ACTH from anterior pituitary
Oxytocin Magnocellular neurosecretory cells Contraction of cervix and vagina Involved in orgasm, trust between people. and circadian homeostasis (body temperature, activity level, wakefulness). release breast milk
Vasopressin Parvocellular neurosecretory neurons Increases permeability of distal convoluted tubule and collecting duct to water in the nephrons of the kidney, thus increasing water reabsorbiton.
Somatostatin, also growth hormone-inhibiting hormone SS or GHIH Neuroendocrince cells of the Periventricular nucleus Inhibit release of GH and TSH from anterior pituitary
Prolactin inhibiting hormone or Dopamine PIH or DA Dopamine neurons of the arcuate nucleus Inhibit release of prolactin and TSH from anterior pituitary
Prolactin-releasing hormone PRH Release prolactin from anterior pituitary

Pineal body (epiphysis)

Secreted hormone From cells Effect
Melatonin (Primarily) Pinealocytes antioxidant and causes drowsiness

Pituitary gland (hypophysis)

Anterior pituitary lobe (adenohypophysis)

Secreted hormone Abbreviation From cells Effect
Growth hormone GH Somatotropes stimulates growth and cell reproduction Release Insulin-like growth factor 1 from liver
Prolactin PRL Lactotropes milk production in mammary glands
sexual gratification after sexual acts
Adrenocorticotropic hormone or corticotropin ACTH Corticotropes synthesis of corticosteroids (glucocorticoids and androgens) in adrenocortical cells
Lipotropin Corticotropes lipolysis and steroidogenesis,
stimulates melanocytes to produce melanin
Thyroid-stimulating hormone or thyrotropin TSH Thyrotropes stimulates thyroid gland to secrete thyroxine (T4) and triiodothyronine (T3)
Follicle-stimulating hormone FSH Gonadotropes In female: stimulates maturation of Graafian follicles in ovary. In male: spermatogenesis, enhances production of androgen-binding protein by the Sertoli cells of the testes
Luteinizing hormone LH Gonadotropes In female: ovulation In male: stimulates Leydig cell production of testosterone

Posterior pituitary lobe (neurohypophysis)

Secreted hormone Abbreviation From cells Effect
Oxytocin Magnocellular neurosecretory cells Contraction of cervix and vagina Involved in orgasm, trust between people. and circadian homeostasis (body temperature, activity level, wakefulness). release breast milk
Vasopressin or antidiuretic hormone AVP or ADH Magnocellular neurosecretory cells retention of water in kidneys moderate vasoconstriction

Oxytocin and Anti-Diuretic Hormone are not secreted in the posterior lobe, merely stored.

Intermediate pituitary lobe (pars intermedia)

Secreted hormone Abbreviation From cells Effect
Melanocyte-stimulating hormone MSH Melanotroph melanogenesis by melanocytes in skin and hair.

Thyroid

Secreted hormone Abbreviation From cells Effect
Triiodothyronine T3 Thyroid epithelial cell potent form of thyroid hormone: increase the basal metabolic rate & sensitivity to catecholamines, affect protein synthesis
Thyroxine or tetraiodothyronine T4 Thyroid epithelial cells less active form of thyroid hormone: increase the basal metabolic rate & sensitivity to catecholamines, affect protein synthesis, often functions as a prohormone
Calcitonin Parafollicular cells Construct bone reduce blood Ca2+

Parathyroid

Secreted hormone Abbreviation From cells Effect
Parathyroid hormone PTH Parathyroid chief cell increase blood Ca2+: *indirectly stimulate osteoclasts

(Slightly) decrease blood phosphate:

  • decreased reuptake in kidney but increased uptake from bones
  • activate vitamin D

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

Striated muscle

Secreted hormone From cells Effect
Thrombopoietin Myocytes stimulates megakaryocytes to produce platelets

Skin

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

Adipose tissue

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

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
Secretin S cells Secretion of bicarbonate from liver, pancreas and duodenal Brunner's glands Enhances effects of cholecystokinin Stops production of gastric juice
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
Histamine ECL cells stimulate gastric acid secretion
Endothelin X cells Smooth muscle contraction of stomach

Duodenum

Secreted hormone From cells Effect
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 Hepatocytes stimulates megakaryocytes to produce platelets

Pancreas

Secreted hormone From cells Effect
Insulin (Primarily) ß 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) a Islet cells glycogenolysis and gluconeogenesis in liver increases blood glucose level
Somatostatin d Islet cells Inhibit release of insulin Inhibit release of glucagon Suppress the exocrine secretory action of pancreas.
Pancreatic polypeptide PP cells Unknown

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

Adrenal glands

Adrenal cortex

Secreted hormone From cells Effect
Glucocorticoids (chiefly cortisol) zona fasciculata and zona reticularis cells Stimulation of gluconeogenesis Inhibition of glucose uptake in muscle and adipose tissue Mobilization of amino acids from extrahepatic tissues Stimulation of fat breakdown in adipose tissue anti-inflammatory and immunosuppressive
Mineralocorticoids (chiefly aldosterone) Zona glomerulosa cells Increase blood volume by reabsorption of sodium in kidneys (primarily) Potassium and H+ secretion in kidney.
Androgens (including DHEA and testosterone) Zona fasciculata and Zona reticularis cells Virilization, anabolic

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

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
Inhibin Sertoli cells > Inhibit production of FSH

Ovary

These originate either from the ovarian follicle or the corpus luteum.
Secreted hormone From cells Effect
Progesterone Granulosa cells, theca cells Support pregnancy:

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:

Lung function:

  • Promote lung function by supporting alveoli.

Inhibin Granulosa cells Inhibit production of FSH from anterior pituitary

Placenta (when pregnant)

Secreted hormone Abbreviation From cells Effect
Progesterone (Primarily) Support pregnancy:

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

See also

References

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