Adrenal gland

The adrenal gland (glandula suprarenalis) is a paired organ located in the retroperitoneal space directly above the upper end of the corresponding kidney. The adrenal gland has the shape of an irregularly shaped cone flattened from front to back. The right adrenal gland, when viewed from the front, looks like a triangle with rounded corners. The apex of the left adrenal gland is smoothed out, its shape resembles a crescent. Each adrenal gland has an anterior surface (facies anterior), a posterior surface (facies posterior), and a lower surface (facies renalis).

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Anatomy of the adrenal glands

The adrenal glands are located at the level of the 11th-12th thoracic vertebrae. The right adrenal gland, like the kidney, lies somewhat lower than the left. Its posterior surface is adjacent to the lumbar part of the diaphragm, its anterior surface is in contact with the visceral surface of the liver and the duodenum, and the lower concave (renal) surface is in contact with the upper end of the right kidney. The medial edge (margo medialis) of the right adrenal gland borders on the inferior vena cava. The medial edge of the left adrenal gland is in contact with the aorta, and its anterior surface is adjacent to the tail of the pancreas and the cardiac part of the stomach. The posterior surface of the left adrenal gland is in contact with the diaphragm, and the lower surface is in contact with the upper end of the left kidney and its medial edge. Each adrenal gland (both right and left) is located in the thickness of the perirenal fat pad. The anterior surfaces of the left and right adrenal glands are partially covered by the renal fascia and parietal peritoneum.

The mass of one adrenal gland in an adult is about 12-13 g. The length of the adrenal gland is 40-60 mm, the height (width) is 20-30 mm, the thickness (anteroposterior dimension) is 2-8 mm. The mass and size of the right adrenal gland are slightly smaller than the left.

Sometimes additional ectopic tissue of the adrenal cortex is found in the body (in the kidneys, spleen, retroperitoneal region below the kidneys, along the aorta, in the pelvis, spermatic cord, broad ligament of the uterus). Congenital absence of one of the adrenal glands is possible. A characteristic feature of their cortex is its ability to regenerate.

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Structure of the adrenal glands

The surface of the adrenal gland is slightly uneven. On the anterior surface, especially of the left adrenal gland, a deep groove is visible - the gate (hilum), through which the central vein exits the organ. On the outside, the adrenal gland is covered with a fibrous capsule, tightly fused with the parenchyma and giving off numerous connective tissue trabeculae into the depths of the organ. Adjacent to the fibrous capsule from the inside is the cortex, which has a fairly complex histological structure and consists of three zones. On the outside, closer to the capsule, is the glomerular zone (zona glomerulosa), behind it is the middle fascicular zone (zona fasciculate), on the border with the medulla is the internal reticular zone (zona reticularis). A morphological feature of the zones is the unique distribution of glandular cells, connective tissue and blood vessels for each zone.

In an adult, the cortex accounts for about 90% of the adrenal tissue. This layer consists of three zones: the outer - glomerular, the middle - fascicular, and the inner (surrounding the medulla) - reticular. Situated directly under the fibrous capsule, the glomerular zone occupies approximately 15% of the volume of the cortex; its cells contain a relatively small amount of cytoplasm and lipids, and produce the hormone aldosterone. The fascicular zone accounts for 75% of the entire cortex; its cells are rich in cholesterol and cholesterol esters, and produce mainly cortisol (hydrocortisone). The cells of the reticular zone also produce this substance; they are relatively poor in lipids and contain many granules. In addition to cortisol, the cells of this zone (like the fascicular zone) produce sex hormones - androgens and estrogens.

The adrenal cortex produces over 50 different steroid compounds. It is the only source of gluco- and mineralocorticoids in the body, the most important source of androgens in women, and plays a minor role in the production of estrogens and progestins. Glucocorticoids, named for their ability to regulate carbohydrate metabolism, are important for maintaining many vital functions and especially for ensuring the body's reactions to stress. They also participate in the regulation of growth and development processes. The main glucocorticoid in humans is cortisol, and an excess or deficiency of this steroid is accompanied by life-threatening changes. Of the mineralocorticoids (named for their ability to regulate salt metabolism), the main one in humans is aldosterone. An excess of mineralocorticoids causes arterial hypertension and hypokalemia, and a deficiency causes hyperkalemia, which may be incompatible with life.

The glomerular zone is formed by small, prismatic cells located in small groups - glomeruli. The endoplasmic reticulum is well developed in these cells, and lipid droplets of about 0.5 μm in size are present in the cytoplasm. The glomeruli are surrounded by convoluted capillaries with fenestrated endothelium.

The zona fasciculata (the widest part of the adrenal cortex) consists of large, light, multifaceted cells. These cells form long cords (bundles) oriented perpendicular to the surface of the adrenal gland. The cells of this zone have a well-developed non-granular endoplasmic reticulum, mitochondria, numerous lipid droplets, ribosomes, glycogen particles, cholesterol, and ascorbic acid. Blood capillaries with fenestrated endothelium are located between the cords of endocrinocytes.

The reticular zone consists of small polyhedral and cubic cells that form small cell clusters. The cells of the reticular zone are rich in elements of the non-granular endoplasmic reticulum and ribosomes.

The listed zones are functionally isolated. The cells of each zone produce hormones that differ from each other not only in chemical composition, but also in physiological action. The hormones of the adrenal cortex are collectively called corticosteroids and can be divided into three groups: mineralocorticoids - aldosterone, secreted by the cells of the glomerular zone of the cortex; glucocorticoids: hydrocortisone, corticosterone, 11-dehydro- and 11-deoxycorticosterone, formed in the fascicular zone; sex hormones - androgens, similar in structure and function to the male sex hormone, estrogen and progesterone, produced by the cells of the reticular zone.

Aldosterone is involved in the regulation of electrolyte and water metabolism, changes the permeability of cell membranes for calcium and sodium, and stimulates collagen formation. Glucocorticoids affect protein metabolism, increase the content of glucose in the blood, and glycogen in the liver, skeletal muscles, and myocardium. Glucocorticoids also accelerate filtration in the glomeruli of the kidney, reduce water reabsorption in the distal convoluted tubules of the nephrons, and inhibit the formation of the main substance of connective tissue and the proliferation of fibroblasts.

In the center of the adrenal gland is the medulla, formed by large cells that are stained yellowish-brown by chromium salts. There are two types of these cells: epinephrocytes make up the bulk of the cells and produce adrenaline, and norepinephrocytes, scattered in the medulla in small groups, produce norepinephrine.

Adrenaline breaks down glycogen, reduces its reserves in muscles and liver, increases the content of carbohydrates in the blood, being a kind of insulin antagonist, strengthens and increases the contraction of the heart muscle, narrows the lumen of blood vessels, thereby increasing arterial pressure. The effect of norepinephrine on the body is similar to the effect of adrenaline, but the effect of these hormones on some functions can be completely opposite. Norepinephrine, in particular, slows the heart rate.

Development of the adrenal glands

The cortex and medulla of the adrenal gland are different in origin. The cortex differentiates from the mesoderm (from the coelomic epithelium) between the root of the dorsal mesentery of the primary intestine and the urogenital fold. The tissue developing from the mesodermal cells and located between the two primary kidneys is called interrenal. It gives rise to the cortex of the adrenal glands, from which the accessory adrenal glands (interrenal bodies, glandulae suprarenales accessoriae) are formed.

The adrenal medulla develops from embryonic nerve cells - sympathoblasts, which migrate from the rudiment of the sympathetic trunk nodes and turn into chromaffinoblasts, and the latter - into chromaffin cells of the medulla. Chromaffinoblasts also serve as material for the formation of paraganglia, which in the form of small clusters of chromaffin cells are located near the abdominal aorta - aortic paraganglia (paraganglion aorticum), as well as in the thickness of the sympathetic trunk nodes - sympathetic paraganglia (paraganglia sympathica).

The introduction of future cells of the medulla into the interrenal adrenal gland begins in the embryo at a length of 16 mm. Simultaneously with the unification of the interrenal and adrenal parts, differentiation of the zones of the cortex and maturation of the medulla occur.

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Vessels and nerves of the adrenal glands

Each adrenal gland receives 25-30 arteries. The largest of these are the superior adrenal arteries (from the inferior phrenic artery), the middle adrenal artery (from the abdominal aorta), and the inferior adrenal artery (from the renal artery). Some of the branches of these arteries supply only the cortex, while others pierce the adrenal cortex and branch out into the medulla. The sinusoidal blood capillaries form tributaries of the central vein, which flows into the inferior vena cava in the right adrenal gland and into the left renal vein in the left adrenal gland. Numerous small veins emerge from the adrenal glands (especially the left) and flow into tributaries of the portal vein.

The lymphatic vessels of the adrenal glands drain into the lumbar lymph nodes. The vagus nerves participate in the innervation of the adrenal glands, as well as nerves originating from the celiac plexus, which contain preganglionic sympathetic fibers for the medulla.

Age-related features of the adrenal glands

In a 5-6 week fetus, a primitive adrenal cortex is formed in the retroperitoneal mesenchyme. It is soon surrounded by a thin layer of more compact cells. In a newborn, the adrenal cortex consists of two zones - fetal and definitive. The first produces mainly precursors of androgens and estrogens, while the function of the second is probably the same as in an adult. The fetal zone accounts for the bulk of the gland in the fetus and newborn. By the 2nd week of postnatal life, its mass decreases by a third due to degeneration of the fetal zone. This process begins in the intrauterine period. The fetal zone completely disappears by the end of the first year of life. The final formation of the three zones of the adrenal cortex is delayed until the age of 3 years. Then the adrenal glands continue to increase in size (especially before and during puberty) and by the end of puberty they reach the size characteristic of an adult.

The mass of one adrenal gland in a newborn is about 8-9 g and significantly exceeds the mass of the adrenal gland of a child in the first year of life. During the neonatal period, the mass of the adrenal gland decreases sharply (to 3.4 g), mainly due to thinning and restructuring of the cortex, and then gradually recovers (by the age of 5) and continues to increase in the future. The final formation of the adrenal cortex is completed during the second childhood (8-12 years). By the age of 20, the mass of each adrenal gland increases and reaches its maximum size (on average 12-13 g). In subsequent age periods, the size and mass of the adrenal glands hardly change. The adrenal glands in women are slightly larger than in men. During pregnancy, the mass of each adrenal gland increases by about 2 g. After 70 years, a slight decrease in the mass and size of the adrenal glands is noted.

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