The Ultimate Guide to Endocrine Histology

The adrenal glandPaired endocrine glands above the kidneys producing stress-related hormones. — capsule, three cortex zones, medulla

The adrenal glands are paired glands that sit on top of each kidney. They have their own thin capsule on top of being wrapped inside the kidney’s fibrous capsuleThe outer protective layer of the kidney.. Below that capsule, the gland is built in layers: an outer cortex divided into three zones, and a deeper medulla at the center.

The adrenal glands have a deeply located medulla and a superficialNear the surface of the body. cortex. These two layers have very different morphology and manufacture hormones with different chemical compositions.

ZONA GLOMERULOSA

• First cortex zone (outermost)
• Rounded clusters of cellsThe basic structural and functional units of life., like little balls.
• Just deepAway from the surface of the body. to the capsule. Makes mineralocorticoidsRegulate electrolyte and water balance (e.g., aldosterone). — aldosteroneA hormone that increases sodium and water reabsorption in the kidneys, helping regulate blood pressu is the headline hormone, controlsVariables that remain constant to ensure a fair test. salt and waterThe universal solvent essential for life. balance.

ZONA FASCICULATA

• Middle cortex zone (widest)
• Cells in long ribbon-like cords or rows.
• The widest zone of the cortex. Makes glucocorticoidsSteroid hormones regulating metabolism and immune response. — cortisolA glucocorticoid involved in stress response, metabolism, and immune regulation. is the headline hormone, controls glucoseA simple sugar that is the main source of energy for cells. and stress response.

ZONA RETICULARIS

• Third cortex zone (deepest cortex)
• Net-like pattern of cells, irregular clusters.
• Deepest of the three cortex zones. Makes androgensMale sex hormones, including testosterone and DHEA. — precursors to sex hormones.

MEDULLA

• Deepest layer (own embryologic origin)
• Pale center. Different cell type entirely.
• The medulla is NOT cortex. It comes from the neural crest and makes catecholamines — epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla. and norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response..

The pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon. is special: most of it is exocrine (digestive enzymesProteins that speed up chemical reactions in the body. go to the gut), but scattered through that exocrine tissue are tiny round islands called pancreatic islets that release hormones into the blood. So this slide is two organs in one. The trick is to see the BACKGROUND first, then spot the ISLAND.

The pancreatic islets are dispersed throughout the acinar cells of the pancreas. They are most abundant in the tail and body of the pancreas. Within each islet, the endocrine cells are highly organized. Different cell types are distributed in specific regions. Beta cells are typically found in the center of the islet, surrounded by alpha and delta cells.

PANCREATIC ISLET

• The endocrine ‘island’
• Round, paler region surrounded by darker exocrine tissue.
• 100–200 µm across. Pale because the secretory granules don’t take up H&E as densely as exocrine zymogen granules do.

BETA CELLS

• Center of the islet — most abundant
• Purple-stained core of the islet.
• Make insulin. Most abundant cell of the islet. Cluster in the middle.

ALPHA CELLS

• Rim of the islet
• Reddish stain. Form a thin border around the beta-cell core.
• Make glucagonIncreases blood sugar by promoting glycogen breakdown.. You’ll see them as a single ring of cells around the islet’s edge.

ACINAR CELLS

• Exocrine background
• Tiny rosettes of 5–6 cells around a microscopic lumenThe inside space of a hollow organ or structure..
• Make digestive enzymes (amylase, lipase, etc.) that drain via ducts to the duodenumThe first section of the small intestine where most digestion occurs; it receives chyme, bile, and p.

The pituitary glandEndocrine gland at the brain’s base controlling many hormones. — two halves, two tissue types

This is the only gland in the lab where the histology comes in two completely different flavors on the same slide. The anteriorThe front of the body or toward the front when standing in the anatomical position. pituitary is glandular epithelium full of secretory cells. The posterior pituitary is a piece of nervous tissue that stores hormones made in the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions.. They look NOTHING alike.

The anterior pituitary gland is also known as the adenohypophysis. This area of the pituitary gland is rich is secretory cells that make a variety of hormones and stain darkly. […] There are three main cell types in the anterior pituitary gland: acidophils, basophils, and chromophobesPituitary cells that do not stain well and have unclear function.. These cell names are not specific to the APG, but reflect the staining abilities of the these cells.

Anterior pituitary

ACIDOPHIL

• Pinkish/red cytoplasmThe gel-like substance within a cell that contains organelles and cytosol.
• Acidic proteinsLarge molecules made of amino acids with various functions in the body. in the cytoplasm absorb the eosin (pink/red) stain. Make growth hormone(GH): Stimulates growth and metabolism. (GH) and prolactin(PRL): Promotes milk production. (PRL).

BASOPHIL

• Darker purple cytoplasm
• Basic proteins in the cytoplasm absorb the hematoxylin (purple) stain. Make TSH, ACTH, FSH, LH.

CHROMOPHOBE

• Pale, almost unstained
• Cytoplasm doesn’t take up much stain. You see a dark nucleusThe control center of the cell that contains DNA and directs cellular activities. with a clear-ish halo around it.

The posterior pituitary gland(PPG): Stores and releases oxytocin and ADH produced by the hypothalamus. has a distinct histological structure. It is characterized by specialized neural tissue. This tissue stains much lighter than the tissues of the anterior pituitary glands. […] As with all nervous tissue, neuroglia are present, as seen by their darkly-stained nucleiClusters of neurons in the CNS responsible for processing information..

Posterior pituitary

PITUICYTE

Modified glia of the posterior pituitary

The dark nuclei you see scattered through the pale wavy fibers ARE the pituicytes. The fibers themselves are axons coming down from neuronsThe functional cells of the nervous system that transmit signals. in the hypothalamus.

The thyroid gland Endocrine gland regulating metabolism through thyroid hormones. — pink rings around colloidA mixture where small particles are dispersed but not dissolved in a liquid.

If you can recognize one endocrine slide on sight, make it the thyroid. Nothing else looks like it. The whole field is filled with folliclesStructures in the ovaries that contain developing oocytes. — little spheres of cells around a colloid-filled center.

The thyroid gland is primarily composed of spherical structures called thyroid follicles. Colloid is a non-cellular, protein-rich substance. It fills the lumen (central cavity) of thyroid follicles. This substance serves as a reservoir for thyroid hormone (TH): Includes T3 and T4, regulating metabolism. storage. Circular and oval areas of colloid can be visualized from low magnifications of the thyroid gland.— from Most Things Biology — Thyroid Gland.

FOLLICLE

• The basicA solution with a pH above 7, having a lower concentration of H⁺ ions. unit of the thyroid
• Sphere of cuboidal cells around a colloid-filled lumen.
• Picture an empty bubble lined with a single layer of cuboidal cells, with the bubble filled with stained pink/orange jelly.

COLLOID

• The ‘jelly’ inside the follicle
• Pink/orange material; stored thyroglobulin (T3/T4 precursor).
• When circulating thyroid hormone is low, the edges look ragged because cells are reabsorbing colloid to release hormone. Smooth round colloid = quiet storage.

FOLLICULAR CELL

• The cuboidal cells lining the follicle
• They TOUCH the colloid — synthesize and secrete thyroglobulin into it.
• When stimulated by TSH, they reabsorb colloid and release T3/T4 into the bloodstream.

C CELLS (PARAFOLLICULAR)

• Sit BETWEEN follicles, never touching colloid
• Larger and paler than follicular cellsCells surrounding the oocyte that support its development.. Often in small clusters.
• Make calcitoninA hormone from the thyroid that lowers blood calcium levels by inhibiting osteoclasts., which lowers blood calcium. ‘Para-‘ = beside.

The parathyroid glandSmall glands on the thyroid that secrete PTH to regulate calcium. — small purples with rare pinks

The parathyroid is the smallest of the five endocrine glands you’ll see, but it has its own classic look: a sea of small purple cells with occasional clusters of larger pink cells.

Chief cells are also known as principle cells. They are the predominant cell type in the parathyroid gland. These cells are arranged in sheets that appear as clusters under the microscope. […] Oxyphil cells, the less common cell type present, are larger than chief cells. Their nuclei are lighter in color than those of the chief cells. […] Oxyphil cells are often confused with the numerous erythrocytes(RBCs): Red blood cells responsible for oxygen transport. (red blood cells) that can be seen arranged single-file in capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues..— from Most Things Biology — Parathyroid Glands.

CHIEF (PRINCIPAL) CELL

• Most abundant — make PTH
• Small, polygonal, purple-staining cytoplasm.
• These dominate the slide. They make parathyroid hormone(PTH): Raises blood calcium by stimulating bone resorption. (PTH), which raises blood calcium. They look basophilic because their RER and PTH-containing granules pick up dark stain.

OXYPHIL CELL

• Larger, sparser, pink-staining
• Cluster in patches. Lots of mitochondria.
• Larger than chiefs. Cytoplasm picks up pink stain (eosinophilic) because of their many mitochondria. Their nuclei are paler than the chief cells’.

RED BLOOD CELL — WATCH OUT

• Looks pink too — but isn’t an oxyphil
• Tiny, oval, single-file in capillaries, NO nucleus.
• RBCs are blank inside. Oxyphils have a visible nucleus. If you see a nucleus inside the pink dot, it’s an oxyphil.