Adrenal gland histology

Definitions & Key takeaways

Adrenal gland histology is the study of the microscopic structures of tissues of the adrenal gland. The adrenal gland is a small, triangular-shaped organ that sits on top of each kidney. It has two parts: the cortex and the medulla.

The cortex is made up of three layers: the zona glomerulosa, the zona fasciculata, and the zona reticularis. The medulla contains sympathetic nerve cells that release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream.

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Introduction0:00–1:23

The adrenal glands are a pair of endocrine glands that are located just above or superior to the kidneys and are also in the retroperitoneal space of the abdominal cavity.
The left gland has a semilunar shape and the right gland is flattened and more triangular in appearance. The adrenal glands are covered by a capsule made of thick connective tissue.
The parenchymal tissue has two main regions, the adrenal cortex and the adrenal medulla. The cortex is the large outer region of the adrenal gland.
The cortex can be further divided into three zones that each secrete a different class of steroid hormones. Starting with the most superficial zone: the zona glomerulosa secretes mineralocorticoids, the zona fasciculata secretes primarily glucocorticoids, and the zona reticularis secretes primarily gonadocorticoids, or sex hormones.
The adrenal medulla is the region that’s most centrally located and visibly distinct from the cortex. The medulla is responsible for secreting the catecholamines: epinephrine and norepinephrine.
If we zoom in further, we can see some of the blood vessels of the adrenal gland. Just outside of the capsule, there are small afferent blood vessels that are branches of the suprarenal arteries that supply blood to the adrenal gland.
And within the medulla, there are the large medullary veins that drain blood into the suprarenal veins. Now, let’s take a closer look at the outermost layer of the adrenal cortex, the zona glomerulosa.

Zona Glomerulosa1:23–1:58

This layer is just beneath the capsule and comprises about 15 percent of the cortex. The secretory cells are arranged in ovoid or glomerulus-like clusters that are separated by fibrous trabeculae that extends from the capsule.
The individual cells are either columnar or pyramidal with round nuclei, and a lipid-filled cytoplasm that gives the cytoplasm its pale “foamy” appearance.
These are the cells that are responsible for secreting aldosterone into nearby capillaries that can be found throughout this layer.
Moving deeper into the adrenal gland, the zona fasciculata is the largest of the three zones and comprises about 65 to 80 percent of the cortex.

Zona Fasciculata 1:58–2:48

In the bottom right of this high power image, we can also see a portion of the underlying zona reticularis. Instead of clusters of cells, unlike the ones we saw in the zona glomerulosa, the secretory cells in this layer are arranged in long cords or columns of cells, often just one to two cells thick that are aligned perpendicular to the surface of the adrenal gland.
These cells have a larger amount of lipid-filled cytoplasm that results in a lighter stain and a “foamy” appearance. In this image, we can see the sinusoidal blood vessels that run parallel to the columns of cells.
The secretory cells in this zone secrete glucocorticoids, mainly cortisol, but this zone will also secrete a small amount of androgenic sex hormones as well.Continuing even deeper into the adrenal cortex, let’s take a look at the innermost layer of the cortex, the zona reticularis.

Zona Reticularis2:48–3:35

This zone comprises about 10 percent of the cortex and contains an irregular network of cords with wide capillaries throughout.
The capillaries are easily identified in this image since they’re the areas that are filled with red blood cells. The cells in this zone are smaller, with a pink or eosinophilic cytoplasm and round central nuclei.
These cells stain darker than the previous layers because the cells contain fewer lipid droplets. In general, the secretory cells in this layer are primarily responsible for secreting androgens.
Also, iIt’s common to find cells in this layer that have small brown pigment granules called lipofuscin. These granules are composed of lipid-containing residues from lysosomal digestion.

Medullary Hormones3:35–4:05

Let’s move onto the adrenal medulla, which consists of hormone-secreting cells called chromaffin cells. These cells are modified postganglionic sympathetic neurons that secrete epinephrine and norepinephrine.
With H&E, they typically have a granular and basophilic cytoplasm when compared to cells from the cortex. The cells are arranged in clusters with numerous capillaries throughout.
The capillaries drain into venous channels such as the one in this image, before draining into the central medullary vein.##SummaryAlright, as a quick recap.

Review4:05–4:57

The adrenal glands have two main regions, the outer cortex and the inner medulla. The cortex can be further divided into 3 layers: the zona glomerulosa, fasciculata, and reticularis.
The zona glomerulosa is the outer layer that consists of columnar or pyramidal cells arranged in glomerulus-like clusters with a lipid-filled “foamy” cytoplasm.
The zona fasciculata is the middle and largest layer of the cortex. The cells in this layer also have a lipid-filled “foamy” cytoplasm, but the cells are arranged in long columns with sinusoids that run parallel with the columns.
The innermost layer of the cortex is the zona reticularis that consists of an irregular network of cordss. In comparison to the zona fasciculata, the cells are smaller and stain darker.
Next is the adrenal medulla, which contains chromaffin cells. These cells