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

Function of the Endocrine System

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 uses blood vessels as information channels. Glands located in many regions of the body release into the bloodstream specific chemical messengers called hormones, which regulate the many and varied functions of an organism. The typical endocrine organ is a ductless gland that secretes chemical mediators directly into local blood vessels which circulate within the body via the bloodstream. These hormones travel to distant organs to regulate the target organ's function. This is classical endocrine signalling. Other signalling can target the same cell (known as Autocrine signalling) or nearby cells (known as Paracrine signalling). Hormones are also instrumental in regulating mood, growth and development, tissue function, and metabolism, as well as sending messages and acting on them. A number of glands which 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 typically 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.

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 , or 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.

Endocrine glands and the hormones secreted

• Hypothalamus produces
  • Thyrotropin-releasing hormone (TRH) Parvocellular neurosecretory neurons
  • Gonadotropin-releasing hormone (GnRH) Neuroendocine cells of the Preoptic area
  • Growth hormone-releasing hormone (GHRH) Neuroendocrine neurons of the Arcuate nucleus
  • Corticotropin-releasing hormone (CRH) Parvocellular neurosecretory neurons
  • Vasopressin Parvocellular neurosecretory neurons
  • Somatostatin (SS; also GHIH, growth hormone-inhibiting hormone) Neuroendocrince cells of the Periventricular nucleus
  • Prolactin inhibiting hormone or PIH or Dopamine (DA) Dopamine neurons of the arcuate nucleus
  • Prolactin releasing hormone

• Pineal body produces
  • Melatonin(Primarily) Pinealocytes

• Pituitary gland (hypophysis) produces
  • Anterior pituitary lobe (adenohypophysis)
    • Growth hormone (GH) Somatotropes
    • Prolactin (PRL) Lactotropes
    • Adrenocorticotropic hormone (ACTH, corticotropin) Corticotropes
    • Lipotropin Corticotropes
    • Thyroid-stimulating hormone (TSH, thyrotropin) Thyrotropes
    • Follicle-stimulating hormone (FSH) Gonadotropes
    • Luteinizing hormone (LH) Gonadotropes
  • Posterior pituitary lobe (neurohypophysis)
    • Oxytocin Magnocellular neurosecretory cells
    • Vasopressin (AVP; also ADH, antidiuretic hormone) Magnocellular neurosecretory cells
  • Intermediate pituitary lobe (pars intermedia)
    • Melanocyte-stimulating hormone (MSH) Melanotroph

• Thyroid produces
  • Triiodothyronine (T3), the potent form of thyroid hormone Thyroid epithelial cell
  • Thyroxine (T4), Also known as tetraiodothyronine, it is a less active form of thyroid hormone (Primarily) Thyroid epithelial cells
  • Calcitonin Parafollicular cells

• Parathyroid produces
  • Parathyroid hormone (PTH) Parathyroid chief cell

• Heart produces
  • Atrial-natriuretic peptide (ANP) Cardiac myocytes
  • Brain natriuretic peptide (BNP) Cardiac myocytes
  • Adenosine Cardiac myocytes

• Striated muscle produces
  • Thrombopoietin Myocytes

• Skin produces
  • Vitamin D₃ (calciferol)

• Adipose tissue
  • Leptin (Primarily) Adipocytes
  • Estrogens (mainly Estrone) Adipocytes

• Stomach produces
  • Gastrin(Primarily) G cells
  • Ghrelin P/D1 cells
  • Neuropeptide Y (NPY)
  • Secretin S cells
  • Somatostatin D cells
  • Histamine ECL cells
  • Endothelin X cells

• Duodenum produces
  • Cholecystokinin I cells

• Liver produces
  • Insulin-like growth factor (IGF) (Primarily) Hepatocytes
  • Angiotensinogen Hepatocytes
  • Thrombopoietin Hepatocytes

• Pancreas produces
  • Insulin (Primarily) β Islet cells
  • Glucagon (Also Primarily) α Islet cells
  • Somatostatin δ Islet cells
  • Pancreatic polypeptide PP cells

• Kidney produces
  • Renin (Primarily) Juxtaglomerular cells
  • Erythropoietin (EPO) Extraglomerular mesangial cells
  • Calcitriol (the active form of vitamin D₃)
  • Thrombopoietin

• Adrenal glands
  • Adrenal cortex produces
    • Glucocorticoids (chiefly cortisol) Zona fasciculata and Zona reticularis cells
    • Mineralocorticoids (chiefly aldosterone) Zona glomerulosa cells
    • Androgens (including DHEA and testosterone) Zona fasciculata and Zona reticularis cells
  • Adrenal medulla produces
    • Adrenaline (epinephrine) (Primarily) Chromaffin cells
    • Noradrenaline (norepinephrine) Chromaffin cells
    • Dopamine Chromaffin cells
    • Enkephalin Chromaffin cells

• Testes
  • Androgens (chiefly testosterone) Leydig cells
  • Estradiol Sertoli cells
  • Inhibin Sertoli cells

• Ovarian follicle/Corpus luteum
  • Progesterone Granulosa cells, Theca cells
  • Androstenedione Theca cells
  • Estrogens (mainly estradiol) Granulosa cells
  • Inhibin Granulosa cells

• Placenta (when pregnant)
  • Progesterone (Primarily)
  • Estrogens (mainly Estriol) (Also Primarily)
  • Human chorionic gonadotropin (HCG) Syncytiotrophoblast
  • Human placental lactogen (HPL) Syncytiotrophoblast
  • Inhibin Fetal Trophoblasts

• Uterus (when pregnant)
  • Prolactin (PRL) Decidual cells
  • Relaxin Decidual cells

See also

• Hormones
• Releasing hormones
• Neuroendocrinology
• Nervous system
• Endocrine disruptor
• Major systems of the human body

Links

• Journals Designed for Clinical Endocrinologists

• Islet cell antibody
• Binding of antibody to pancreas
• Kidshealth.org

References

1. Collier, Judith.; et al. (2006). Oxford Handbook of Clinical Specialties 7th edn. Oxford. pp. 350–351. ISBN 0-19-853085-4. {{cite book}}: Explicit use of et al. in: |first= (help)
2. Kasper; et al. (2005). Harrison's Principles of Internal Medicine. McGraw Hill. p. 2074. ISBN 0-07-139140-1. {{cite book}}: Explicit use of et al. in: |last= (help)
3. Bhowmick NA, Chytil A, Neilson EG, Moses HL (2004). "TGF-beta signaling in fibroblasts modulates the oncogenic potential of adjacent epithelia". Science. Feb 6 303(5659): 848–51.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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• Endocrine system