Adrenal cortexThe adrenal glands also known as suprarenal glands are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. Each gland has an outer cortex which produces steroid hormones and an inner medulla. The adrenal cortex itself is divided into three zones: The adrenal cortex produces three main types of steroid hormones: Mineralocorticoids such as aldosterone produced in the zona glomerulosa corfex in the regulation of blood corticosteroids produced by the adrenal cortex and electrolyte balance.
Adrenal gland - Wikipedia
The adrenal glands also known as suprarenal glands are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. Each gland has an outer cortex which produces steroid hormones and an inner medulla. The adrenal cortex itself is divided into three zones: The adrenal cortex produces three main types of steroid hormones: Mineralocorticoids such as aldosterone produced in the zona glomerulosa help in the regulation of blood pressure and electrolyte balance.
The glucocorticoids cortisol and corticosterone are synthesized in the zona fasciculata; their functions include the regulation of metabolism and immune system suppression. The innermost layer of the cortex, the zona reticularis, produces androgens that are converted to fully functional sex hormones in the gonads and other target organs.
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.
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. The adrenal glands are surrounded by a fatty capsule and lie within the renal fascia , which also surrounds the kidneys. A weak septum wall of connective tissue separates the glands from the kidneys.
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. 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.
The outermost zone of the adrenal cortex is the zona glomerulosa. It lies immediately under the fibrous capsule of the gland. Cells in this layer form oval groups, separated by thin strands of connective tissue from the fibrous capsule of the gland and carry wide capillaries.
This layer is the main site for production of aldosterone , a mineralocorticoid , by the action of the enzyme aldosterone synthase. The zona fasciculata is situated between the zona glomerulosa and zona reticularis. Cells in this layer are responsible for producing glucocorticoids such as cortisol. Cells contain numerous lipid droplets, abundant mitochondria and a complex smooth endoplasmic reticulum.
The innermost cortical layer, the zona reticularis , lies directly adjacent to the medulla. The cells contain relatively small quantities of cytoplasm and lipid droplets, and sometimes display brown lipofuscin pigment. 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 adrenaline and noradrenaline, released by the medulla.
The adrenal medulla is driven by the sympathetic nervous system via preganglionic fibers originating in the thoracic spinal cord , from vertebrae T5—T The adrenal glands have one of the greatest blood supply rates per gram of tissue of any organ: 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. Venous blood is drained from the glands by the suprarenal veins , usually one for each gland: 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.
The adrenal glands may not develop at all, or may be fused in the midline behind the aorta. 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. Corticosteroids are a group of steroid hormones produced from the cortex of the adrenal gland, from which they are named. The adrenal gland produces aldosterone , a mineralocorticoid , which is important in the regulation of salt "mineral" balance and blood volume. In the kidneys, aldosterone acts on the distal convoluted tubules and the collecting ducts by increasing the reabsorption of sodium and the excretion of both potassium and hydrogen ions.
Angiotensin II and extracellular potassium are the two main regulators of aldosterone production. Therefore, the effects of aldosterone in sodium retention are important for the regulation of blood pressure. Cortisol is the main glucocorticoid in humans. In species that do not create cortisol, this role is played by corticosterone instead.
Glucocorticoids have many effects on metabolism. As their name suggests, they increase the circulating level of glucose. This is the result of an increase in the mobilization of amino acids from protein and the stimulation of synthesis of glucose from these amino acids in the liver.
In addition, they increase the levels of free fatty acids , which cells can use as an alternative to glucose to obtain energy. Glucocorticoids also have effects unrelated to the regulation of blood sugar levels, including the suppression of the immune system and a potent anti-inflammatory effect. Cortisol reduces the capacity of osteoblasts to produce new bone tissue and decreases the absorption of calcium in the gastrointestinal tract.
The adrenal gland secretes a basal level of cortisol but can also produce bursts of the hormone in response to adrenocorticotropic hormone ACTH from the anterior pituitary. Cortisol is not evenly released during the day — its concentrations in the blood are highest in the early morning and lowest in the evening as a result of the circadian rhythm of ACTH secretion.
All corticosteroid hormones share cholesterol as a common precursor. Therefore, the first step in steroidogenesis is cholesterol uptake or synthesis. Cells that produce steroid hormones can acquire cholesterol through two paths. The main source is through dietary cholesterol transported via the blood as cholesterol esters within 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. The initial part of conversion of cholesterol into steroid hormones involves a number of enzymes of the cytochrome P 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.
The layers of the adrenal gland differ by function, with each layer having distinct enzymes that produce different hormones from a common precursor.
After the production of pregnenolone, specific enzymes of each cortical layer further modify it. Enzymes involved in this process include both mitochondrial and microsomal Ps and hydroxysteroid dehydrogenases.
Usually a number of intermediate steps in which pregnenolone is modified several times are required to form the functional hormones. For example, the most common form of congenital adrenal hyperplasia develops as a result of deficiency of hydroxylase , an enzyme involved in an intermediate step of cortisol production.
Glucocorticoids are under the regulatory influence of the hypothalamus-pituitary-adrenal HPA axis. Glucocorticoid synthesis is stimulated by adrenocorticotropic hormone ACTH , a hormone released into the bloodstream by the anterior pituitary. In turn, production of ACTH is stimulated by the presence of corticotropin-releasing hormone CRH , which is released by neurons of the hypothalamus. The HPA axis also interacts with the immune system through increased secretion of ACTH at the presence of certain molecules of the inflammatory response.
Mineralocorticoid secretion is regulated mainly by the renin—angiotensin—aldosterone system RAAS , the concentration of potassium , and to a lesser extent the concentration of ACTH. Angiotensin receptors in cells of the zona glomerulosa recognize the substance, and upon binding they stimulate the release of aldosterone.
Primarily referred to in the United States as Epinephrine and norepinephrine , Adrenaline and noradrenaline are catecholamines , water-soluble compounds that have a structure made of a catechol group and an amine group. The adrenal glands are responsible for most of the adrenaline that circulates in the body, but only for a small amount of circulating noradrenaline.
Adrenaline and noradrenaline act at adrenoreceptors throughout the body, with effects that include an increase in blood pressure and heart rate. 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.
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. 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.
The adrenal gland specific genes with highest level of expression include members of the cytochrome P superfamily of enzymes. The adrenal glands are composed of two heterogenous types of tissue. In the center is the adrenal medulla , which produces adrenaline and noradrenaline and releases them into the bloodstream, as part of the sympathetic nervous system.
Surrounding the medulla is the cortex , which produces a variety of steroid hormones. These tissues come from different 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.
The adrenal glands in a newborn baby are much larger as a proportion of the body size than in an adult. 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 g at birth  and develop to an adult weight of about 4 grams each. Adrenal cortex tissue is derived from the intermediate mesoderm.
It first appears 33 days after fertilisation , shows steroid hormone production capabilities by the eighth week and undergoes rapid growth during the first trimester of pregnancy.