The perithyroid glands

Most healthy people have four parathyroid glands - two upper and two lower, located relatively symmetrically on both sides in more than 80% of cases.

Up to 13% of people have more than four parathyroid glands, and the latter may be not only rudimentary remnants of normally formed glands (located in close proximity to the main organ and weighing less than 5 mg), but also true accessory parathyroid glands, located separately from the main glands and weighing an average of 24 mg. Up to 11 parathyroid glands have been described in one subject, although this is a casuistic rarity. These parathyroid glands are most often located near the lower poles of the thyroid gland, in the thyrothymic ligament or the thymus itself, which apparently reflects certain disturbances in the formation of the gland during embryonic migration.

According to some authors, up to 3% of people may have only 3 parathyroid glands, but most researchers are skeptical about such reports, believing that the fourth was simply not found due to its small size or ectopic location.

Macroscopically, the parathyroid glands are yellowish-brown, very soft formations with a flattened ovoid shape, surrounded by a capsule and a layer of adipose tissue, measuring 4-6 mm in length, 2-4 mm in width and 1-2 mm in thickness.

The consistency of the formations is very unclear, practically not perceptible during palpation on the surface of the thyroid gland between the fingers. Normal parathyroid glands weigh no more than 60 mg (38-59 mg, and the total weight is 120 ± 3.5 mg for men and 142 ± 5.2 mg for women. At the same time, the pure parenchymatous weight is only 82 ± 2.6 mg and 89 ± 3.9 mg, respectively.

The superior parathyroid glands are most often located on the posterior surface of the thyroid gland near the intersection of the recurrent laryngeal nerve and the trunk of the inferior thyroid artery. Rarely, they deviate significantly from their usual location and may be located behind the esophagus or pharynx.

The lower parathyroid glands are usually located at the level of the lower third of the thyroid lobes, more often along its posterolateral surface. The variability of their location is associated with close embryonic contact with the simultaneously forming and migrating thymus. Therefore, at least 1/3 of the normal location variants of the lower parathyroid glands correspond to their localization in the thyrothymic tract or upper poles of the thymus.

The upper and lower parathyroid glands are supplied with blood by arterial branches from the upper and lower thyroid arteries, respectively, with the upper ones also receiving collateral supply from the lower thyroid artery. Venous outflow is carried out into the upper and middle thyroid veins for the upper ones, into the lower and middle veins for the lower ones. The vessels have a very small caliber, which requires extremely delicate manipulations by the surgeon in the process of identifying and mobilizing the parathyroid glands during neck operations. Lymphatic drainage is carried out from the subcapsular plexus into the upper deep cervical, para- and pretracheal, retropharyngeal and deep lower cervical lymph nodes.

Despite their small size, the parathyroid glands have all the elements of isolated endocrine structures, their own capsule, vessels, nerves, parenchymatous and stromal components. A characteristic element of the structure is abundant fatty tissue, concentrated mainly at the poles. The presence of a fatty layer between the parenchymatous part and the capsule often allows identifying the parathyroid glands by a specific sign - "swimming" in a delicate fascial membrane when carefully shifting it with an instrument or finger (the so-called gliding sign or symptom of slipping out, encountered in foreign literature).

Histological structure of the parathyroid glands

The parenchymatous portion is formed into cellular cords separated by stromal elements. Although difficult to distinguish visually from the thyroid gland, fat, or lymph nodes, the parathyroid glands are easily recognized microscopically by their characteristic dense packing of cells, in contrast to the follicular structure of the thyroid gland. Histologically, they consist of chief cells and oncocytic oxyphilic cells, which appear with increasing frequency in old age. Chief cells are usually divided into dark, actively parathyroid hormone-secreting cells and water-clear cells, which are apparently variants of "resting" cells in a functional quiescence. Most adenomas consist of dark cells, although water-clear and oncocytic adenomas also occur. The principal cells are present in the predominant quantity, they are smaller than oncocytes, darker, contain a centrally located round nucleus with coarse chromatin and inconspicuous nucleoli. The cytoplasm is eosinophilically stained, sometimes appears clear.

Oncocytic cells are large, have coarse granular cytoplasm, and a larger nucleus than the principal cells. Their function is unknown, and their numbers increase during puberty and with aging, with oncocytic nodules often found in the parenchyma in older people.

The content of stromal fat depends on age and diet. If children and adolescents have almost none, then in adults fat cells make up about 20% of the volume. When losing weight, their number decreases sharply.