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Parathyroid Glands

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The parathyroid glands are small endocrine organs located on the posterior surface of the thyroid gland. They play a crucial role in calcium homeostasis by producing and secreting parathyroid hormone (PTH), which regulates calcium levels in the blood.

Location

The parathyroid gland is usually oval or oblong in shape.  The 4 parathyroid glands are located on the posterior side of the thyroid lobes that wrap around the trachea.  

Illustration showing the location of the parathyroid glands in relation to the thyroid. The image depicts the back of a head and neck in a light gray outline. The thyroid is represented in blue, with the trachea below, and the parathyroid glands are shown as textured, yellow-brown ovals nestled on each side of the thyroid. The image is meant to show the anatomical location of the parathyroid glands.
Figure 1: Posterior view of larynx, wrapped with thyroid. The four parathyroid glands are on the posterior lobes of the thyroid.

Chief 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.   Chief cells have a round to polygonal shape with centrally located nuclei.  Their cytoplasm stains darkly. This is called “basophilic” because of the presence of abundant rough endoplasmic reticulum (RER). Secretory granules containing PTH are also present.

Oxyphil Cells

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. The limits of their cytoplasm are easily discerned because they contain numerous mitochondria. Oxyphil cells are often confused with the numerous erythrocytes (red blood cells) that can be seen arranged single-file in capillaries.

Microscopic tissue section of a parathyroid. The tissue is counter-stained pink, revealing a dense cluster of cells oval in shape, centrally located within the frame. The cellular cluster has a mottled appearance due to staining distribution with white appearing between. Tissue surrounding is lighter in color and more translucent.
Figure 2: Parathyroid gland tissue section, stained pink. Oval-shaped cellular area contrasts with surrounding tissue.
A histological slide shows parathyroid cells. The slide is light pink, with both Chief and Oxyphil cells present with white circles drawing attention to them. The Chief cells are distinguished by their smaller size. Oxyphil cells are indicated by their larger size.
Figure 3: Parathyroid histology: Chief and oxyphil cells.

Calcium Homeostasis

Chief cells secrete the parathyroid hormone (PTH) when the calcium level in blood drops.  This phenomenon is known as a humoral trigger. Hormone secretion is induced by chemical levels in the blood.  The histology of the parathyroid gland shows this visually. Numerous capillaries and single-file erythrocytes (red blood cells) run through the organs. PTH acts on target tissues, primarily the bones, kidneys, and intestines. It increases calcium levels in the blood by promoting calcium release from bone by activating osteoclasts to increase osteolysis. It also enhances calcium reabsorption in the kidneys and stimulates the small intestine to absorb more calcium from food.

Fun Fact: The parathyroid glands were discovered through the surgical treatment of hyperthyroidism. Removal of part of the thyroid organ in an attempt to reduce circulating thyroid hormone forced patients into calcium deficiencies. The decreased population of chief cells and their product, PTH, decreased the patients’ ability to combat calcium deficiencies. Surgical removal of the thyroid is still practiced. However, removing thyroid tissue via radioactive iodine guarantees that the chief cell population remains intact. Chief cells, unlike follicular cells of the thyroid organ, don’t take up iodine from the blood.

Figures 2 & 3: Parathyroid, H&E, 40X Slide 220
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.+6

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