Adrenal gland

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Adrenal gland
Lumbar lymph nodes
Identifiers
Latinglandula suprarenalis
MeSHD000311
TA98A11.5.00.001
TA23874
FMA9604
Anatomical terminology]

The adrenal glands (also known as suprarenal glands) are

kidneys. Each gland has an outer cortex which produces steroid hormones and an inner medulla. The adrenal cortex itself is divided into three main zones: the zona glomerulosa, the zona fasciculata and the zona reticularis.[3]

The adrenal cortex produces three main types of

rapid response throughout the body in stress situations.[4]

A number of endocrine diseases involve dysfunctions of the adrenal gland. Overproduction of cortisol leads to Cushing's syndrome, whereas insufficient production is associated with Addison's disease. Congenital adrenal hyperplasia is a genetic disease produced by dysregulation of endocrine control mechanisms.[4][6] A variety of tumors can arise from adrenal tissue and are commonly found in medical imaging when searching for other diseases.[7]

Structure

Adrenal glands, anterior (left) and posterior (right) surface

The adrenal glands are located on both sides of the body in the

retroperitoneum, above and slightly medial to the kidneys. In humans, the right adrenal gland is pyramidal in shape, whereas the left is semilunar or crescent shaped and somewhat larger.[8] The adrenal glands measure approximately 5 cm in length, 3 cm in width, and up to 1 cm in thickness.[9] Their combined weight in an adult human ranges from 7 to 10 grams.[10] The glands are yellowish in colour.[8]

The adrenal glands are surrounded by a

crura of the diaphragm by the renal fascia.[11]

Each adrenal gland has two distinct parts, each with a unique function, the outer adrenal cortex and the inner medulla, both of which produce hormones.[12]

Adrenal cortex

Section of human adrenal gland under the microscope, showing its different layers. From the surface to the center: zona glomerulosa, zona fasciculata, zona reticularis, medulla. In the medulla, the central adrenomedullary vein is visible.

The adrenal cortex is the outer region and also the largest part of an adrenal gland. It is divided into three separate zones: zona glomerulosa, zona fasciculata and zona reticularis. Each zone is responsible for producing specific hormones. The adrenal cortex is the outermost layer of the adrenal gland. Within the cortex are three layers, called "zones". When viewed under a microscope each layer has a distinct appearance, and each has a different function.[13] The adrenal cortex is devoted to production of hormones, namely aldosterone, cortisol, and androgens.[14]

Zona glomerulosa

The outermost zone of the adrenal cortex is the

thin strands of connective tissue from the fibrous capsule of the gland and carry wide capillaries.[15]

This layer is the main site for production of

Zona fasciculata

The zona fasciculata is situated between the zona glomerulosa and zona reticularis. Cells in this layer are responsible for producing glucocorticoids such as cortisol.[19] It is the largest of the three layers, accounting for nearly 80% of the volume of the cortex.[3] In the zona fasciculata, cells are arranged in columns radially oriented towards the medulla. Cells contain numerous lipid droplets, abundant mitochondria and a complex smooth endoplasmic reticulum.[15]

Zona reticularis

The innermost cortical layer, the

DHEA sulfate (DHEA-S), and androstenedione (the precursor to testosterone) in humans.[19] Its small cells form irregular cords and clusters, separated by capillaries and connective tissue. The cells contain relatively small quantities of cytoplasm and lipid droplets, and sometimes display brown lipofuscin pigment.[15]

Medulla

The adrenal medulla is at the centre of each adrenal gland, and is surrounded by the adrenal cortex. The chromaffin cells of the medulla are the body's main source of the catecholamines, such as adrenaline and noradrenaline, released by the medulla. Approximately 20% noradrenaline (norepinephrine) and 80% adrenaline (epinephrine) are secreted here.[19]

The adrenal medulla is driven by the

sympathetic ganglion.[20]
Unlike other sympathetic ganglia, however, the adrenal medulla lacks distinct synapses and releases its secretions directly into the blood.

Blood supply

The adrenal glands have one of the greatest blood supply rates per gram of tissue of any organ: up to 60 small arteries may enter each gland.[21] Three arteries usually supply each adrenal gland:[8]

These blood vessels supply a network of small arteries within the capsule of the adrenal glands. Thin strands of the capsule enter the glands, carrying blood to them.[8]

Venous blood is drained from the glands by the suprarenal veins, usually one for each gland:[8]

The central adrenomedullary vein, in the adrenal medulla, is an unusual type of blood vessel. Its structure is different from the other veins in that the smooth muscle in its tunica media (the middle layer of the vessel) is arranged in conspicuous, longitudinally oriented bundles.[3]

Variability

The adrenal glands may not develop at all, or may be fused in the midline behind the

congenital abnormalities, such as failure of the kidneys to develop, or fused kidneys.[12] The gland may develop with a partial or complete absence of the cortex, or may develop in an unusual location.[12]

Function

Different hormones are produced in different zones of the cortex and medulla of the gland. Light microscopy at magnification × 204.[22]

The adrenal gland secretes a number of different hormones which are metabolised by enzymes either within the gland or in other parts of the body. These hormones are involved in a number of essential biological functions.[23]

Corticosteroids

Corticosteroids are a group of steroid hormones produced from the cortex of the adrenal gland, from which they are named.[24]

Mineralocorticoids

The adrenal gland produces

Angiotensin II and extracellular potassium are the two main regulators of aldosterone production.[19] The amount of sodium present in the body affects the extracellular volume, which in turn influences blood pressure. Therefore, the effects of aldosterone in sodium retention are important for the regulation of blood pressure.[28]

Glucocorticoids

The adrenal gland secretes a basal level of cortisol but can also produce bursts of the hormone in response to

11β-HSD on cortisol. The reaction catalyzed by 11β-HSD is reversible, which means that it can turn administered cortisone into cortisol, the biologically active hormone.[28]

Formation
Steroidogenesis in the adrenal glands – different steps occur in different layers of the gland

All

low density lipoproteins (LDL). LDL enters the cells through receptor-mediated endocytosis. The other source of cholesterol is synthesis in the cell's endoplasmic reticulum. Synthesis can compensate when LDL levels are abnormally low.[4] In the lysosome, cholesterol esters are converted to free cholesterol, which is then used for steroidogenesis or stored in the cell.[29]

The initial part of conversion of cholesterol into steroid hormones involves a number of enzymes of the cytochrome P450 family that are located in the inner membrane of mitochondria. Transport of cholesterol from the outer to the inner membrane is facilitated by steroidogenic acute regulatory protein and is the rate-limiting step of steroid synthesis.[29]

The layers of the adrenal gland differ by function, with each layer having distinct enzymes that produce different hormones from a common precursor.

21-hydroxylase, an enzyme involved in an intermediate step of cortisol production.[30]

Regulation
Negative feedback in the HPA axis

Glucocorticoids are under the regulatory influence of the

inflammatory response.[4]

Mineralocorticoid secretion is regulated mainly by the

Angiotensin receptors in cells of the zona glomerulosa recognize the substance, and upon binding they stimulate the release of aldosterone.[31]

Androgens

Cells in zona reticularis of the adrenal glands produce male sex hormones, or androgens, the most important of which is DHEA. In general, these hormones do not have an overall effect in the male body, and are converted to more potent androgens such as testosterone and DHT or to estrogens (female sex hormones) in the gonads, acting in this way as a metabolic intermediate.[32]

Catecholamines

Primarily referred to in the United States as

fight or flight response, characterised by a quickening of breathing and heart rate, an increase in blood pressure, and constriction of blood vessels in many parts of the body.[33]

Formation

Catecholamines are produced in chromaffin cells in the medulla of the adrenal gland, from tyrosine, a non-essential amino acid derived from food or produced from phenylalanine in the liver. The enzyme tyrosine hydroxylase converts tyrosine to L-DOPA in the first step of catecholamine synthesis. L-DOPA is then converted to dopamine before it can be turned into noradrenaline. In the cytosol, noradrenaline is converted to epinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT) and stored in granules. Glucocorticoids produced in the adrenal cortex stimulate the synthesis of catecholamines by increasing the levels of tyrosine hydroxylase and PNMT.[4][13]

Catecholamine release is stimulated by the activation of the sympathetic nervous system. Splanchnic nerves of the sympathetic nervous system innervate the medulla of the adrenal gland. When activated, it evokes the release of catecholamines from the storage granules by stimulating the opening of calcium channels in the cell membrane.[34]

Gene and protein expression

The 

adrenaline synthesis and expressed in the medulla.[37]

Development

The adrenal glands are composed of two heterogenous types of tissue. In the center is the

embryological precursors and have distinct prenatal development paths. The cortex of the adrenal gland is derived from mesoderm, whereas the medulla is derived from the neural crest, which is of ectodermal origin.[12]

The adrenal glands in a newborn baby are much larger as a proportion of the body size than in an adult.[38] For example, at age three months the glands are four times the size of the kidneys. The size of the glands decreases relatively after birth, mainly because of shrinkage of the cortex. The cortex, which almost completely disappears by age 1, develops again from age 4–5. The glands weigh about 1 gram at birth[12] and develop to an adult weight of about 4 grams each.[28] In a fetus the glands are first detectable after the sixth week of development.[12]

Cortex

Adrenal cortex tissue is derived from the

DHEA-S, an androgen and precursor of both androgens and estrogens (female sex hormones).[41] Adrenal hormones, especially glucocorticoids such as cortisol, are essential for prenatal development of organs, particularly for the maturation of the lungs. The adrenal gland decreases in size after birth because of the rapid disappearance of the fetal zone, with a corresponding decrease in androgen secretion.[39]

Adrenarche

During early childhood androgen synthesis and secretion remain low, but several years before puberty (from 6–8 years of age) changes occur in both anatomical and functional aspects of cortical androgen production that lead to increased secretion of the steroids DHEA and DHEA-S. These changes are part of a process called adrenarche, which has only been described in humans and some other primates. Adrenarche is independent of ACTH or gonadotropins and correlates with a progressive thickening of the zona reticularis layer of the cortex. Functionally, adrenarche provides a source of androgens for the development of axillary and pubic hair before the beginning of puberty.[42][43]

Medulla

The adrenal medulla is derived from

chromaffin cells because they contain granules that stain with chromium salts, a characteristic not present in all sympathetic organs. Glucocorticoids produced in the adrenal cortex were once thought to be responsible for the differentiation of chromaffin cells. More recent research suggests that BMP-4 secreted in adrenal tissue is the main responsible for this, and that glucocorticoids only play a role in the subsequent development of the cells.[45]

Clinical significance

The normal function of the adrenal gland may be impaired by conditions such as infections, tumors, genetic disorders and autoimmune diseases, or as a side effect of medical therapy. These disorders affect the gland either directly (as with infections or autoimmune diseases) or as a result of the dysregulation of hormone production (as in some types of Cushing's syndrome) leading to an excess or insufficiency of adrenal hormones and the related symptoms.

Corticosteroid overproduction

Cushing's syndrome

stretch marks in the skin, caused by its progressive thinning.[4][6]

Primary aldosteronism

When the zona glomerulosa produces excess

Conn's syndrome). Primary aldosteronism produces hypertension and electrolyte imbalance, increasing potassium depletion sodium retention.[6]

Adrenal insufficiency

Adrenal insufficiency (the deficiency of glucocorticoids) occurs in about 5 in 10,000 in the general population.[6] Diseases classified as primary adrenal insufficiency (including Addison's disease and genetic causes) directly affect the adrenal cortex. If a problem that affects the hypothalamic–pituitary–adrenal axis arises outside the gland, it is a secondary adrenal insufficiency.

Addison's disease

Characteristic skin hyperpigmentation in Addison's disease

Addison's disease refers to primary hypoadrenalism, which is a deficiency in glucocorticoid and mineralocorticoid production by the adrenal gland. In the Western world, Addison's disease is most commonly an autoimmune condition, in which the body produces antibodies against cells of the adrenal cortex. Worldwide, the disease is more frequently caused by infection, especially from tuberculosis. A distinctive feature of Addison's disease is hyperpigmentation of the skin, which presents with other nonspecific symptoms such as fatigue.[4]

A complication seen in untreated Addison's disease and other types of primary adrenal insufficiency is the adrenal crisis, a medical emergency in which low glucocorticoid and mineralocorticoid levels result in hypovolemic shock and symptoms such as vomiting and fever. An adrenal crisis can progressively lead to stupor and coma.[4] The management of adrenal crises includes the application of hydrocortisone injections.[46]

Secondary adrenal insufficiency

In secondary adrenal insufficiency, a dysfunction of the hypothalamic–pituitary–adrenal axis leads to decreased stimulation of the adrenal cortex. Apart from suppression of the axis by glucocorticoid therapy, the most common cause of secondary adrenal insufficiency are tumors that affect the production of adrenocorticotropic hormone (ACTH) by the pituitary gland.[6] This type of adrenal insufficiency usually does not affect the production of mineralocorticoids, which are under regulation of the renin–angiotensin system instead.[4]

Congenital adrenal hyperplasia

Adrenal tumors

Incidences and prognoses of adrenal tumors.[47]

Adrenal tumors are commonly found as

Adrenal carcinomas are very rare, with an incidence of 1 case per million per year.[4]

palpitations. Common signs include hypertension and tachycardia. Surgery, especially adrenal laparoscopy, is the most common treatment for small pheochromocytomas.[49]

History

papal library and did not receive public attention, which was first received with Caspar Bartholin the Elder's illustrations in 1611.[51]

The adrenal glands are named for their location relative to the kidneys. The term "adrenal" comes from Latin ad, "near", and ren, "kidney".[53] Similarly, "suprarenal", as termed by Jean Riolan the Younger in 1629, is derived from the Latin supra, "above", and ren, "kidney", as well. The suprarenal nature of the glands was not truly accepted until the 19th century, as anatomists clarified the ductless nature of the glands and their likely secretory role – prior to this, there was some debate as to whether the glands were indeed suprarenal or part of the kidney.[51]

One of the most recognized works on the adrenal glands came in 1855 with the publication of On the Constitutional and Local Effects of Disease of the Suprarenal Capsule, by the English physician

Philip Hench and Tadeusz Reichstein were then awarded the 1950 Nobel Prize in Physiology or Medicine for their discoveries on the structure and effects of the adrenal hormones.[55]

See also

References

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External links