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The parathyroid glands
Last reviewed: 20.11.2021
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Most healthy people have four parathyroid glands - the two upper and two lower ones, located relatively symmetrically on both sides in more than 80% of cases.
Up to 13% of people have more than four parathyroid glands, the latter being not only rudimentary remnants of normal glands (located in close proximity to the main organ and weighing less than 5 mg), but also by true additional parathyroid glands located separately from the main glands and having weight an average of 24 mg. Up to 11 parathyroid glands are described in one subject, although this is a casuistic rarity. These parathyroid glands are most often located near the lower poles of the thyroid, in the thyreotomy ligament or thymus itself, which obviously reflects certain violations of the bookmark in the process of embryonic migration.
According to some authors, up to 3% of people may have only 3 parathyroid glands, but most researchers are skeptical of such reports, considering it simply not found fourth due to its small size or ectopic location.
Macroscopically parathyroid glands are yellowish-brownish, very soft formations, having oblate ovoid form surrounded by a capsule and a layer of adipose tissue, 4-6 mm in length, 2-4 mm in width and 1-2 mm in thickness.
The consistency of the formations is very unclear, almost not palpable on the surface of the thyroid gland between the fingers. Normal parathyroid glands weigh not 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.) The net parenchymal weight is only 82 ± 2.6 mg and 89 ± 3.9 mg respectively.
The upper parathyroid glands are most often located on the posterior surface of the thyroid near the point of intersection of the recurrent laryngeal nerve and the trunk of the lower thyroid artery. Rarely, they deviate significantly from their normal location and may be behind the esophagus or pharynx.
Lower parathyroid glands, as a rule, are at the level of the lower third of the thyroid gland, more often along its posterolateral surface. The variability of their location is associated with a close embryonic contact with the forming and migrating thymus. Therefore, no less than 1/3 of the variants of the normal arrangement of the lower parathyroid glands corresponds to their localization in the thyreotymic tract or the upper poles of the thymus.
Blood supply to the upper and lower parathyroid glands with arterial branches from the upper and lower thyroid arteries, while the upper receive collateral nutrition also from the lower thyroid artery. Venous outflow is carried out in the upper and middle thyroid veins for the upper, in the lower and middle veins for the lower ones. Vessels have a very small caliber, which requires extremely delicate manipulations of the surgeon in the process of identification and mobilization of parathyroid glands in operations on the neck. Lymphatic drainage, is carried out from the subcapsular plexus to the upper deep cervical, para- and pre-tracheal, behind the pharyngeal and deep lower cervical lymph nodes.
Despite its small size, the parathyroid glands have all the elements of isolated endocrine structures, their own capsule, vessels, nerves, parenchymal and stromal components. A characteristic element of the structure is an abundantly expressed adipose tissue, concentrated mainly at the poles. The presence of a fat layer between the parenchymal part and the capsule makes it possible to often identify parathyroid glands by a specific feature - "swimming" in a tender fascial envelope with careful displacements by its instrument or finger (the so-called gliding sign or sliding symptom encountered in foreign literature).
Histological structure of the parathyroid glands
The parenchymal part is formed into cellular strands separated by stromal elements. Being difficult to distinguish visually from the thyroid gland, fat or lymph nodes, the parathyroid glands are easily recognized microscopically by the characteristic dense packing of the cells, in contrast to the follicular structure of the thyroid gland. Histologically, they consist of the main cells and oncocytic oxyphilic cells, which appear with greater frequency in the elderly. The main cells are divided into dark, actively secreted parathyroid hormone cells and water-clear cells, which are apparently the variants of "resting" cells that are in functional rest. Most adenomas consist of dark cells, although there are both watery-cell and oncocytic adenomas. The main cells are contained in the predominant quantity, they are smaller than the oncocytes, darker, contain a centrally located round nucleus with coarse chromatin and invisible nucleoli. The cytoplasm stains eosinophilic, sometimes it appears enlightened.
Oncocyte cells are large in size, they have a coarse granular cytoplasm with a larger nucleus than in the main cells. Their function is unknown, and the number increases at pubertal age and with aging, and in elderly people it is often possible to detect oncocytic nodules in the parenchyma.
The content of stromal fat depends on the age and nature of nutrition. If there are almost no children and adolescents, then in adults, fat cells make up about 20% of the volume. At emaciation their number sharply decreases.