Parathyroid glands

Superior and inferior parathyroid glands are typically located bilaterally on the posterior surface of the thyroid gland . Multiple in number, these small, oval glands produce parathyroid hormone (PTH), which regulates phosphorus and calcium metabolism in the bloodstream.

1. Vasculature and lymphatics: The main blood supply to the parathyroid glands is by way of the inferior thyroid artery, although surrounding arterial supply can also contribute depending on the exact location. Parathyroid veins drain the glands, and lymphatic drainage communicates with that of the thyroid gland. Also, like the thyroid gland, the parathyroid glands are regulated hormonally. Nerves arising from the cervical sympathetic ganglia carry a vasomotor function, but do not control gland function.
2. Histology: The parathyroid glands are yellow to orange tan in color (depending on the amount of stromal fat). Most individuals have four parathyroid glands, but the number may vary from 1 to 12. They measure about 4 mm wide by 6 mm long by 4 mm thick. The shape of the parathyroid glands varies because the glands are molded by adjacent anatomical structures. Each parathyroid gland is surrounded by a connective tissue capsule that sends connective tissue septae into the parathyroid gland for support. The parathyroid glands contain two types of cells: chief cells and oxyphil cells.
a. Chief cell: Chief cells are polygon-shaped cells with a round, centrally located nucleus (Fig. 1). They contain rough endoplasmic reticulum, polyribosomes, a Golgi complex, mitochondria, secretory granules, lipid, and glycogen. The chief cell secretes PTH that binds to the PTH receptor (a G protein-linked receptor). PTH is involved in calcium homeostasis (i.e., raises blood calcium levels).

Figure 1: Parathyroid histology. A, Mallory trichrome stain. B, Anti-parathyroid hormone/diaminobenzidene stain.

b. Oxyphil cell: Oxyphil cells appear after puberty and have a distinct eosinophilic cytoplasm because of numerous mitochondria. The cell has no known function.
c. Weight: The weight of the parathyroid glands is an important parameter in histopathologic assessment. All parathyroid glands (or parts of parathyroid glands) must be carefully weighed. Each parathyroid gland weighs about 35-40 mg. The total parathyroid weight ranges from 120 to 140 mg.