Endocrine gland

Endocrine glands
The major endocrine glands: 1 Pineal gland 2 Pituitary gland 3 Thyroid gland 4 Thymus 5 Adrenal gland 6 Pancreas 7 Ovary 8 Testicle
Details
System	Endocrine system
Identifiers
Latin	glandulae endocrine
MeSH	D004702
TA98	A11.0.00.000
TA2	3852
TH	H2.00.02.0.03072
FMA	9602
Anatomical terminology [edit on Wikidata]

Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testicles, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary glands are neuroendocrine organs.^[1]

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Have you ever thought about the way the different parts of our body communicate? I think we often consider the body to be this one complete thing, this self. But really our body is composed of lots of parts. There are lots of organ systems. And each of those has organs. And all of those organs are made of tissues. And all of those tissues are made of cells. And it's crazy, but there are 100 trillion-- or at least roughly 100 trillion cells in our body. So it's curious then how do those 100 trillion different parts communicate? Well, one way is through the nervous system and through the pre-laid tracks of nerves. But not every part of the body is connected by nerves. I mean how, for example, would part of the brain go about communicating with part of the kidney? Well, to talk about that we're going to have to talk about the endocrine system. And the endocrine system is a system of organs that are called glands. And these glands secrete little chemical messages that are called hormones. And they release those little chemical messages called hormones into the bloodstream so that they can circulate from one part of the body to another part of the body in order to initiate an effect. And there are many parts of the body that use these hormones to communicate. But certain organs are really defined by this method of communication and we call them endocrine glands. And so one of the major endocrine glands is the hypothalamus. And the hypothalamus is located right here. It's a member of the forebrain. And as a member of the brain, it receives a lot of those signals that we talked about from the nervous system. So those nerve signals are funnelling into the brain. And the hypothalamus then, as a kind of dual member of the endocrine system, funnels those signals into the pituitary gland. And so because it plays that dual role between the endocrine system and the nervous system, it often gets taglined as the control center of the endocrine system. In addition to stimulating the pituitary gland, the hypothalamus actually make some hormones itself also. And so it makes ADH and oxytocin. And ADH is antidiuretic hormone. And it's a main regulator of our fluid volume in our body. And then oxytocin is a hormone that stimulates the uterus to contract for females during pregnancy. And so that's the hypothalamus, member of the brain and member of the endocrine system where it all begins, the control center. And then right below the hypothalamus is the pituitary gland. And the pituitary gland is located right here, dangling right below. And so the hypothalamus is about the size of a grape. And the pituitary gland is actually about the size of a green pea. But this little green pea is so important that it's called the master gland. And it's called the master gland because the pituitary gland takes that stimulation from the hypothalamus and it directs it to all of the other endocrine glands, or at least almost all of the other endocrine glands, such that their function is ultimately dependent on the pituitary gland to work well. And so that little green pea is a really important part of the endocrine system. And so one of the endocrine glands that the pituitary directs is the thyroid gland. And the thyroid gland is located right here in your neck. It wraps around your trachea. And your trachea is your windpipe. And so you can feel this thyroid gland on your neck as you swallow. If you hold your hands right around your Adam's apple and swallow, that meaty thing moving up and down, that's your thyroid gland. And one of its main jobs is regulating your body's metabolism. And it does that through the thyroid hormones T3 and T4. And another name for T3 is triiodothyronine. And another name for T4 is thyroxine. But the thyroid uses these hormones, the thyroid hormones, to stimulate the body's metabolism, which is crucial because that's how our body gets energy. And then right behind that thyroid gland are four spots known collectively as the parathyroid. And the main role of the parathyroid is regulating our body's blood calcium level. And the level of calcium in our blood is hugely important because calcium does a lot of stuff in our bodies. It's involved in muscle contraction. It's involved in bone growth. And all of those functions are really sensitive to the level of calcium that's floating around in our blood. And so the parathyroid glands, those four spots on the back side of our thyroid, regulate calcium through the parathyroid hormone, or PTH. And then moving down the torso, we have the adrenal glands. And the adrenal glands are located right on top of the kidneys here. And they're called the adrenal glands because they're adjacent to or right next to the kidney system, which is called the renal system in medical speak. But we really need to further divide the adrenal glands into two parts, the outer part and the inner part. So the outer part is the cortex and the inner part is the medulla. And the reason for the distinction is that the inside and the outside of the adrenal glands have two different functions. And so we'll start with the outside or the cortex. And that's where the steroids, the adrenal corticosteroids, are made. And two major examples of steroids made in the adrenal cortex are cortisol and aldosterone. And cortisol is one of the body's stress hormones. So it functions to increase blood sugar in times of stress so we have energy. And it also has some anti-inflammatory functioning. And then aldosterone is one of the major regulating hormones of our body's blood volume and how much fluid is in our veins and arteries. And so that's the cortex. And then the medulla makes a class of hormones called catecholamines. And two major examples of catecholamines are epinephrine and norepinephrine. And I'm going to shorten those as epi and norepi. And sometimes epinephrine is called adrenaline. And that might be a little bit more familiar to you. But these catecholamines are really involved in our body's fight or flight response, that adrenaline response that we have to a stressful or scary situation. And so the medulla and the cortex make up the adrenal glands. But moving down the list and down the body, we have the gonads. And in females, those are the ovaries, and in males, the testes. And the gonads release the sex hormones. And so in males, the testes produce testosterone. And in females, the ovaries produce estrogen and progesterone. But these sex hormones are mainly involved in the development of our secondary sex characteristics like pubic hair, and larger frames in males, and breasts in women. But they're also involved in progressing us through those life stages that accompany those sex characteristics, like puberty and menopause. And then last, but not least, we have the pancreas. And it's located right here in the upper part of the abdomen. And I saved the pancreas for last because it isn't involved as directly with the pituitary glands as the other endocrine hormones were. But it still uses those hormones to stimulate an effect in a different part of the body. And the effect that the pancreas stimulates is control over the blood sugar. And it does that through the hormones insulin and glucagon. And the pancreas is vitally important because without its hormones insulin and glucagon, we can't regulate how much sugar is in the body's blood versus the cells. And that can lead to major diseases like diabetes. And so with the pancreas, we can conclude our list of major endocrine glands. And so as we look at these glands and at these hormones and we think about all of the different effects that are being stimulated in our body by them, it becomes pretty clear that there aren't just a few of these circulating in our bloodstream. There are literally loads of hormones circulating through our vasculature at any given moment. And so that poses a potential problem. If, say, that you're in the brain and you're trying to tell something to the kidney, you're trying to send him a message, and you put that in the bloodstream and you just float it down to him, how do you know that it's going to get there? I mean, isn't that what every other endocrine gland is trying to do? Well, it turns out that hormones are a lot like radio waves. In your city or in your town, there are many different radio stations and there are many different songs being played at any given time by those radio stations. And even maybe from the next town over, there are radio waves filling the air of your town. But unless you're tuned in specifically to that station, you're not going to pick up on the song that's being transmitted. And in a very similar way, a hormone is not going to be received unless there's a very specific receptor on the target cell. And so the receptor and its location are very important in determining the hormone function. And we have classes that we use to help us identify which hormones fall into which function. And so the first class are autocrine hormones. And the autocrine hormones function at the cell that makes them. An example of this is the T-cell in the immune system. It actually secretes a hormone that it makes called an interleukin, that signals the cell itself to increase its effectiveness and its immune function. And then another class of hormones are paracrine hormones. And paracrine hormones function regionally. And an example of that might be the hormones released by the hypothalamus that direct the pituitary gland. And then last, but not least, kind of the classic class of hormones are the endocrine hormones. And these are the hormones that function at a distance. And an example of this might be the pituitary gland stimulating the gonads, way far away. And so we have autocrine, paracrine, and endocrine classes that help us determine how a hormone functions. And so I know I just told you a whole lot about hormones. But this is your introduction into one of the most important ways that the 100 trillion little tiny individual parts of your body communicate.

Pituitary gland

The pituitary gland hangs from the base of the brain by the pituitary stalk, and is enclosed by bone. It consists of a hormone-producing glandular portion of the anterior pituitary and a neural portion of the posterior pituitary, which is an extension of the hypothalamus. The hypothalamus regulates the hormonal output of the anterior pituitary and creates two hormones that it exports to the posterior pituitary for storage and later release.

Four of the six anterior pituitary hormones are tropic hormones that regulate the function of other endocrine organs. Most anterior pituitary hormones exhibit a diurnal rhythm of release, which is subject to modification by stimuli influencing the hypothalamus.

Somatotropic hormone or growth hormone (GH) is an anabolic hormone that stimulates the growth of all body tissues especially skeletal muscle and bone. It may act directly, or indirectly via insulin-like growth factors (IGFs). GH mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake and metabolism. Secretion is regulated by growth hormone-releasing hormone (GHRH) and growth hormone-inhibiting hormone (GHIH), or somatostatin. Hypersecretion causes gigantism in children and acromegaly in adults; hyposecretion in children causes pituitary dwarfism.

Thyroid-stimulating hormone promotes normal development and activity of the thyroid gland. Thyrotropin-releasing hormone stimulates its release; negative feedback of thyroid hormone inhibits it.

Adrenocorticotropic hormone stimulates the adrenal cortex to release corticosteroids. Adrenocorticotropic hormone release is triggered by corticotropin-releasing hormone and inhibited by rising glucocorticoid levels.

The gonadotropins—follicle-stimulating hormone and luteinizing hormone regulate the functions of the gonads in both sexes. Follicle-stimulating hormone stimulates sex cell production; luteinizing hormone stimulates gonadal hormone production. Gonadotropin levels rise in response to gonadotropin-releasing hormone. Negative feedback of gonadal hormones inhibits gonadotropin release.

Prolactin promotes milk production in human females. Its secretion is prompted by prolactin-releasing hormone and inhibited by prolactin-inhibiting hormone.

The intermediate lobe of the pituitary gland secretes only one enzyme that is melanocyte stimulating hormone. It is linked with the formation of the black pigment in our skin called melanin.

The neurohypophysis stores and releases two hypothalamic hormones:

Oxytocin stimulates powerful uterine contractions, which trigger labour and delivery of an infant, and milk ejection in nursing women. Its release is mediated reflexively by the hypothalamus and represents a positive feedback mechanism.
Antidiuretic hormone stimulates the kidney tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma osmolality. Antidiuretic hormone is released in response to high solute concentrations in the blood and inhibited by low solute concentrations in the blood. Hyposecretion results in diabetes insipidus.

Thyroid gland

The thyroid gland is located in the front of the neck, in front of the thyroid cartilage, and is shaped like a butterfly, with two wings connected by a central isthmus. Thyroid tissue consists of follicles with a stored protein called colloid, containing[thyroglobulin], a precursor to other thyroid hormones, which are manufactured within the colloid.

The thyroid hormones increase the rate of cellular metabolism, and include thyroxine (T4) and triiodothyronine (T3). Secretion is stimulated by the thyroid-stimulating hormone, secreted by the anterior pituitary. When thyroid levels are high, there is negative feedback that decreases the amount of Thyroid-stimulating hormone secreted. Most T4 is converted to T3 (a more active form) in the target tissues.

Calcitonin, produced by the parafollicular cells (C cells) of the thyroid gland in response to rising blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and enhancing calcium deposit in bones. Excessive secretion cause hyperthyroidism and deficiency cause hypothyroidism.

Parathyroid glands

The parathyroid glands, of which there are 4–6, are found on the back of the thyroid glands, and secrete parathyroid hormone,^[2] This causes an increase in blood calcium levels by targeting bone, the intestine, and the kidneys. The parathyroid hormone is the antagonist of calcitonin. Parathyroid hormone release is triggered by falling blood calcium levels and is inhibited by rising blood calcium levels.

Adrenal glands

The adrenal glands are located above the kidneys in humans and in front of the kidneys in other animals. The adrenal glands produce a variety of hormones including adrenaline and the steroids aldosterone cortisol and Dehydroepiandrosterone sulfate (DHEA).^[3] Adrenaline increases blood pressure, heart rate, and metabolism in reaction to stress, the aldosterone controls the body’s salt and water balance , the cortisol plays a role in stress response and the dehydroepiandrosterone sulfate (DHEA) produces aids in production of body odor and growth of body hair during puberty.

Pancreas

The pancreas, located in the abdomen, below and behind the stomach, is both an exocrine and an endocrine gland. The alpha and beta cells are the endocrine cells in the pancreatic islets that release insulin and glucagon and smaller amounts of other hormones into the blood. Insulin and glucagon influence blood sugar levels. Glucagon is released when the blood glucose level is low and stimulates the liver to release glucose into the blood. Insulin increases the rate of glucose uptake and metabolism by most body cells.

Somatostatin is released by delta cells and acts as an inhibitor of GH, insulin, and glucagon.

Gonads

The ovaries of the female, located in the pelvic cavity, release two main hormones. Secretion of estrogens by the ovarian follicles begins at puberty under the influence of follicle-stimulating hormone. Estrogens stimulate the maturation of the female reproductive system and the development of secondary sexual characteristics. Progesterone is released in response to high blood levels of luteinizing hormone. It works with estrogens in establishing the menstrual cycle.

The testes of the male begin to produce testosterone at puberty in response to luteinizing hormone. Testosterone promotes maturation of the male reproductive organs, development of secondary sex characteristics such as increased muscle and bone mass, and the growth of body hair.

Pineal gland

The pineal gland is located in the diencephalon of the brain. It primarily releases melatonin, which influences daily rhythms and may have an antigonadotropic effect in humans.^{[citation needed]} It may also influence the melanotropes and melanocytes located in the skin.^{[citation needed]}

Other hormone-producing structures

Many body organs not normally considered endocrine organs contain isolated cell clusters that secrete hormones. Examples include the heart (atrial natriuretic peptide); gastrointestinal tract organs (gastrin, secretin, and others); the placenta (hormones of pregnancy—estrogen, progesterone, and others); the kidneys (erythropoietin and renin); the thymus; skin (cholecalciferol); and adipose tissue (leptin and resistin).

Development

Endocrine glands derive from all three germ layers.^{[citation needed]}

The natural decrease in function of the female's ovaries during late middle age results in menopause. The efficiency of all endocrine glands seems to decrease gradually as ageing occurs. This leads to a generalized increase in the incidence of diabetes mellitus and a lower metabolic rate.

Functions

Hormones

Local chemical messengers, not generally considered part of the endocrine system, include autocrines, which act on the cells that secrete them, and paracrines, which act on a different cell type nearby.

The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind.

Hormone receptors are dynamic structures. Changes in the number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.

Blood levels of hormones reflect a balance between secretion and degradation/excretion. The liver and kidneys are the major organs that degrade hormones; breakdown products are excreted in urine and faeces.

Hormone half-life and duration of activity are limited and vary from hormone to hormone.

Interaction of hormones at target cells Permissiveness is the situation in which a hormone cannot exert its full effects without the presence of another hormone.

Synergism occurs when two or more hormones produce the same effects in a target cell and their results are amplified.

Antagonism occurs when a hormone opposes or reverses the effect of another hormone.

Control

The endocrine glands belong to the body's control system. The hormones which they produce help to regulate the functions of cells and tissues throughout the body. Endocrine organs are activated to release their hormones by humoral, neural, or hormonal stimuli. Negative feedback is important in regulating hormone levels in the blood.

The nervous system, acting through hypothalamic controls, can in certain cases override or modulate hormonal effects.

Clinical significance

Disease

Diseases of the endocrine glands are common,^[5] including conditions such as diabetes mellitus, thyroid disease, and obesity.

Endocrine disease is characterized by irregulated hormone release (a productive pituitary adenoma), inappropriate response to signalling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminished erythropoiesis in chronic kidney 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 the 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.^{[citation needed]}

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.^[6]

Other common diseases that result from endocrine dysfunction include Addison's disease, Cushing's disease and Grave's disease. Cushing's disease and Addison's disease are pathologies involving the dysfunction of the adrenal gland. Dysfunction in the adrenal gland could be due to primary or secondary factors and can result in hypercortisolism or hypocortisolism. Cushing's disease is characterized by the hypersecretion of the adrenocorticotropic hormone due to a pituitary adenoma that ultimately causes endogenous hypercortisolism by stimulating the adrenal glands.^[7] Some clinical signs of Cushing's disease include obesity, moon face, and hirsutism.^[8] Addison's disease is an endocrine disease that results from hypocortisolism caused by adrenal gland insufficiency. Adrenal insufficiency is significant because it is correlated with decreased ability to maintain blood pressure and blood sugar, a defect that can prove to be fatal.^[9]

Graves' disease involves the hyperactivity of the thyroid gland which produces the T3 and T4 hormones.^[8] Graves' disease effects range from excess sweating, fatigue, heat intolerance and high blood pressure to swelling of the eyes that causes redness, puffiness and in rare cases reduced or double vision.^{[citation needed]}

Graves' disease is the most common cause of hyperthyroidism; hyposecretion causes cretinism in infants and myxoedema in adults.

Hyperparathyroidism results in hypercalcemia and its effects and in extreme bone wasting. Hypoparathyroidism leads to hypocalcemia, evidenced by tetany seizure and respiratory paralysis. Hyposecretion of insulin results in diabetes mellitus; cardinal signs are polyuria, polydipsia, and polyphagia.

References

^ Clarke, I. J. (January 2015). "Hypothalamus as an endocrine organ". Comprehensive Physiology. 5 (1): 217–253. doi:10.1002/cphy.c140019. ISBN 9780470650714. ISSN 2040-4603. PMID 25589270.
^ Endocrinology: Tissue Histology. Archived 2010-02-04 at the Wayback Machine University of Nebraska at Omaha.
^ "Adrenal gland". Medline Plus/Merriam-Webster Dictionary. Retrieved 11 February 2015.
^ "Mortality and Burden of Disease Estimates for WHO Member States in 2002" (xls). World Health Organization. 2002.
^ Kasper (2005). Harrison's Principles of Internal Medicine. McGraw Hill. pp. 2074. ISBN 978-0-07-139140-5.
^ Bhowmick NA, Chytil A, Plieth D, Gorska AE, Dumont N, Shappell S, Washington MK, Neilson EG, Moses HL (2004). "TGF-beta signaling in fibroblasts modulates the oncogenic potential of adjacent epithelia". Science. 303 (5659): 848–51. Bibcode:2004Sci...303..848B. doi:10.1126/science.1090922. PMID 14764882. S2CID 1703215.
^ Buliman A, Tataranu LG, Paun DL, Mirica A, Dumitrache C (2016). "Cushing's disease: a multidisciplinary overview of the clinical features, diagnosis, and treatment". Journal of Medicine & Life. 9 (1): 12–18.
^ ^a ^b Vander, Arthur (2008). Vander's Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 345-347
^ Inder, Warrick J.; Meyer, Caroline; Hunt, Penny J. (2015-06-01). "Management of hypertension and heart failure in patients with Addison's disease". Clinical Endocrinology. 82 (6): 789–792. doi:10.1111/cen.12592. ISSN 1365-2265. PMID 25138826. S2CID 13552007.

Anatomy of the endocrine system

Pituitary gland

Anterior	Pars intermedia Pars tuberalis Pars distalis Acidophil cell Somatotropic cell Prolactin cell Somatomammotrophic cell Basophil cell Corticotropic cell Gonadotropic cell Thyrotropic cell Chromophobe cell
Posterior	Pars nervosa Median eminence Stalk Pituicyte Herring bodies