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Adrenal Gland

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Microscopic image of adrenal gland tissue, stained with hematoxylin and eosin. The kidney bean shaped gland is horizontally oriented on a white background. The adrenal cortex, the outer region of the kidney bean shape is pink and grainy and surrounds the dark pink inner medulla of the gland. Several vascular components and connective tissues are visible surrounding the gland's edges.
Figure 1: Pink-stained microscope slide of adrenal gland tissue showing cortex and medulla.

The adrenal glands are paired glands, with each one sitting atop a kidney. Also called the suprarenal glands, these glands are enveloped within the fibrous capsule of the kidney. Only a thin membrane separates the tissue of the adrenal glands from the waste-rich tissue of the kidney.

The adrenal glands are usually depicted as triangular glands in 2D pictures. However, in the reality of 3D, these glands are shaped like pyramids with four sides.

The adrenal glands have a deeply located medulla and a superficial cortex. These two layers have very different morphology and manufacture hormones with different chemical compositions.

Illustration showing the location of the adrenal glands within the human body. The image depicts a simplified outline of a person, with the internal organs visible in the abdominal region. Two kidneys are illustrated, one on each side, with the adrenal glands shown as small yellow structures positioned directly above them. A vertical arrangement of blood vessels, including red arteries and blue veins, runs between the kidneys. A black triangle lines up with each adrenal gland and connects to the words "Adrenal Glands" above them on the chest. The context is educational, showing the relative position of the adrenal glands within the body.
Adrenal gland location in human body. Illustration shows glands positioned above kidneys.
Microscopic view of the adrenal gland layers. The image shows a cross-section starting with the outer Capsule, followed by the Zona Glomerulosa, Zona Fasciculata, Zona Reticularis, and finally the inner Medulla. Each layer is labeled within the image with black dashed lines separating them.
Figure 2: Labeled cross-section of adrenal gland layers: Capsule, Zona Glomerulosa, Zona Fasciculata, Zona Reticularis, and Medulla.
A labeled microscopic image of the adrenal gland's structure. The image presents different layers including the Capsule, Zona Glomerulosa, Zona Fasciculata, Zona Reticularis, and Medulla. Black dashed lines separate and label each zone in the adrenal gland. The tissue is stained pink, indicating cellular structure.
Figure 3: Labeled microscope view of adrenal gland layers: Capsule, Zona Glomerulosa, Zona Fasciculata, Zona Reticularis, and Medulla.

Histology

The adrenal gland has its own capsule, in addition to the kidney’s capsule that surrounds it. The adrenal cortex is located deep to the fibrous connective tissue of the capsule. It is made of three layers. Each layer produces different hormones.

The zona glomerulosa is immediately deep to the adrenal capsule. This zone, or layer, is named for the circular clusters of cells abundant in this region. These cells produce mineralocorticoids, steroid hormones that regulate ions in the fluid compartments. Aldosterone is the main example of a mineralocorticoid. Aldosterone affects the kidney to retain (reabsorb) sodium and flush (secrete) potassium into the urine.

The zone fasciculata is deep to the zona glomerulosa. This zone is named for the ribbon-like arrangement of the cells. The zona fasciculata is the largest zone of the adrenal cortex. Glucocorticoid hormones, such as cortisol, are released by these cells. Cortisol affects many target tissues. These tissues produce responses we commonly associate with long-term stress. Such responses include a depressed immune system, inability to concentrate, and digestive instability.

The zona reticularis is the deepest of the adrenal cortex, just superficial to the adrenal medulla. The zona reticularis creates androgens, which are precursors to the steroid-based reproductive hormones such as testosterone and estrogen.

The adrenal medulla is the deepest of all layers of the adrenal glands. Unlike the adrenal cortex’s steroid hormones with a cholesterol base, the adrenal medulla secretes protein-based hormones called catecholamines. These hormones, epinephrine and norepinephrine, are released into systemic circulation. They have numerous target tissues. This increases the effectiveness of a flight/fight/freeze response.

Microscopic histological image of the adrenal cortex zona glomerulosa, stained in shades of pink to red with hematoxylin and eosin (H&E). The bottom two-thirds of the frame are filled with spherical clusters of intensely stained cells, representing the characteristic arrangement of the zona glomerulosa. A thin laminar region with flattened cells is present closer to the upper section forming a thin transition band. The upper portion of the image presents a less structured appearance, with fibrous connective tissue stained in gradients of a lighter pink. The image is oriented horizontally, with a consistent focal plane throughout.
Figure 4: Adrenal cortex, zona glomerulosa: Microscopic view showing cell arrangement. Distinct layers of cells are stained pink.
Light microscopy shows stained adrenal cortex zona fasciculata. Cells are arranged in vertical cords or rows. Cytoplasm stains pink, and dark purple/black nuclei are round and centrally located. White spaces separate cell cords, likely representing sinusoids or capillaries. The image suggests a histological sample.
Figure 5: Microscopic view of adrenal cortex zona fasciculata cells in rows. Pink with dark nuclei.
High-magnification light microscopy image of the adrenal cortex zona reticularis. Pink-stained polygonal shaped adrenocortical cells with dark nuclei and lighter cytoplasm are arranged in an irregular network. The arrangement of the cells become visible with the pink staining of the tissue.
Figure 6: Microscopic view of adrenal cortex zona reticularis cells, stained pink.

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University of Michigan’s Collection

Diagram of an adrenal gland depicting the cortex and medulla, and their respective hormone production zones. The cortex is divided into zones - glomerulosa (aldosterone), fasciculata (cortisol), and reticularis (androgens). The medulla produces catecholamines.
Adrenal zones diagram showing hormone production in cortex (glomerulosa, fasciculata, reticularis) and medulla (catecholamines).

Mineralocorticoids

Mineralocorticoids, like all adrenal cortex hormones, are steroid hormones and are thus made from cholesterol. The tissue of the adrenal cortex is lipid-laden, reflecting this nonpolar structure of the adrenal cortex hormones. Aldosterone is made by the zona glomerulosa of the adrenal cortex and is a mineralocorticoid. The changing values of minerals such as sodium and potassium in the body’s fluids trigger the release of aldosterone. It targets the cells of the nephrons in your kidneys to keep sodium and flush potassium. It should be noted that aldosterone has other triggers, such as stimulation of baroreceptors that sense high blood pressure.

Glucocorticoids

Glucocorticoids are cholesterol-based hormones. Cortisol is a great example of the glucocorticoids released by the zona fasciculata of the adrenal cortex. The adrenocorticotropic hormone (ACTH) from the anterior pituitary gland triggers the release of cortisol. It has a wide variety of target tissues. Its effects are associated with low-level, long-term stress. For example, cortisol depresses the synthesis of white blood cells. This leads to an increased ability to become sick during times of stress. Cortisol affects digestive processes. It also impacts neural abilities. This effect leads to increased gastrointestinal distress. Many people suffer from an inability to concentrate during stress.

Androgens

Androgens are cholesterol-based hormones. They play a part in sexual maturity. They are secreted by the zona reticularis of the adrenal cortex. These hormones are active and have their own effects. They mainly affect reproductive organs such as the ovaries and testes. However, most of the androgens are precursors for sex hormones such as testosterone, estrogen, and estradiol. To a lesser degree, they also act as precursors for progesterone.

Catecholamines

Catecholamines are protein-based hormones made by the adrenal medulla. They are in direct contrast to the steroid hormones made by the adrenal cortex. These hormones are polar and able to circulate in the blood without a transport carrier. Epinephrine and norepinephrine are the two catecholamines that are released into systemic circulation upon neural stimulation from the hypothalamus. The hypothalamus detects a threat and induces fear. It sends action potentials to the adrenal medulla. This signals the release of these hormones into systemic circulation. Catecholamines affect many target tissues and organs. Their effect is to increase a sympathetic (fight or flight) response. For example, the smooth muscle of the trachea (windpipe) is inhibited from contracting. This widens the trachea. It increases the amount of oxygen you can intake so that you can run away from the threat.

Figures 1, 2, 4,5,6: Slide 231 Adrenal gland, monkey, H&E, 40X
Figure 3: Slide 230 Human adrenal gland H&E 40x
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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