||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (June 2008)|
|Endocrine system. (Parathyroid gland not pictured, but are present on surface of thyroid gland, as shown below.)|
|Thyroid and parathyroid.|
|Latin||glandula parathyreoidea inferior, glandula parathyreoidea superior|
|Gray's||subject #273 1271|
|Artery||superior thyroid artery, inferior thyroid artery,|
|Vein||superior thyroid vein, middle thyroid vein, inferior thyroid vein,|
|Nerve||middle cervical ganglion, inferior cervical ganglion|
|Lymph||pretracheal, prelaryngeal, jugulo-diagastric,and lympahtics of thymus|
|Precursor||neural crest mesenchyme and third and fourth pharyngeal pouch endoderm|
The parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone. Humans usually have four parathyroid glands, which are usually located in variable manner on the posterior surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest (mediastinum) or even the thymus. Parathyroid glands control the amount of calcium in the blood and within the bones.
The parathyroid glands are variable in number: three or more small glands, about the size of a grain of rice, can be located on the posterior surface of the thyroid gland. The parathyroid glands usually weigh between 25 mg and 40 mg in humans. There are typically four parathyroid glands. The two parathyroid glands on each side which are positioned higher (closer to the head) are called the superior parathyroid glands, while the lower two are called the inferior parathyroid glands. Occasionally, some individuals may have six, eight, or even more parathyroid glands. These glands are not visible or palpable during extraoral examination of a patient.
The parathyroid glands are named for their proximity to the thyroid but serve a completely different role than the thyroid gland. The parathyroid glands are quite easily recognizable from the thyroid as they have densely packed cells, in contrast with the follicle structure of the thyroid. However, at surgery, they are harder to differentiate from the thyroid or fat.
Because the inferior thyroid arteries provide the primary blood supply to the posterior aspect of the thyroid gland where the parathyroid glands are located, branches of these arteries usually supply the parathyroid glands. However they may also be supplied by the branches of the superior thyroid arteries; the thyroid ima artery; or the laryngeal, tracheal and esophageal artery. Parathyroid veins drain into thyroid plexus of veins of the thyroid gland.
Lymphatic vessels from the parathyroid glands drain into deep cervical lymph nodes and paratracheal lymph nodes.
In the histological sense, they distinguish themselves from the thyroid gland, as they contain two types of cells:
|parathyroid chief cells||darker||many||smaller||manufacture PTH (see below).|
|parathyroid oxyphil cells||lighter||few||larger||function unknown.|
The parathyroid glands were first discovered in the Indian Rhinoceros by Richard Owen in 1850. In his description of the neck anatomy, Owen referred to ‘a small compact yellow glandular body attached to the thyroid at the point where the vein emerged’—now identified as the parathyroid gland.The glands were first discovered in humans by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880 at Uppsala University. In his classic monograph 'On a New Gland in Man and Fellow Animals' he described what he called the "glandulae parathyroidae" in the dog, cat, rabbit, ox, horse and man. Sandström was not aware of Owen’s description, which had been published in a journal with a limited circulation. Sandstrom's report was not well received and the work remained barely noticed for several years. Sandstrom suffered from a hereditary mental illness and unfortunately took his own life aged only 37 years.
It is the last major organ to be recognized in humans so far.
Parathyroid hormone (PTH, also known as parathormone) is a small protein that takes part in the control of calcium and phosphate homeostasis, as well as bone physiology. Parathyroid hormone has effects antagonistic to those of calcitonin.
PTH increases blood calcium levels by stimulating osteoclasts to break down bone and release calcium. PTH also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium conservation (reabsorption) by the kidneys.
PTH is the major regulator of serum phosphate concentrations via actions on the kidney. It is an inhibitor of proximal and also distal tubular reabsorption of phosphorus.
Through activation of Vitamin D the absorption of Phosphate is increased.
Role in disease
Embryology and evolution
The parathyroid glands originate from the interaction of neural crest mesenchyme and third and fourth branchial pouch endoderm. The glands from the fourth pouch become the superior glands and the those from the third pouch become the inferior glands.
Eya-1 (transcripitonal co-activator), Six-1 (a homeobox transcription factor), and Gcm-2 (a transcription factor) have been associated with the development of the parathyroid gland, and alterations in these genes alters parathyroid gland development.
The superior parathyroids arise from the fourth pharyngeal pouch, and the inferior parathyroids arise from the third pharyngeal pouch. They are vertically transposed during embryogenesis. This is significant in function-preserving parathyroidectomy, because both the superior and the inferior parathyroids are supplied by the inferior thyroid artery. If the surgeon is to leave a single functional parathyroid for the patient, he/she must preserve the appropriate blood supply.
In other animals
Parathyroid glands are found in all adult tetrapods, although they vary in their number, and in their exact position. Mammals typically have four parathyroids, while other groups typically have six. The removal of parathyroid glands in animals produces a condition resembling acute poisoning and irregular muscle contractions.
Fish do not possess parathyroid glands, although the ultimobranchial glands, which are found close to the oesophagus, may have a similar function and could even be homologous with the tetrapod parathyroids. Even these glands are absent in the most primitive vertebrates, the jawless fish, but as these species have no bone in their skeletons, only cartilage, it may be that they have less need to regulate calcium metabolism.
The conserved homology of genes and calcium-sensing receptors in fish gills with those in the parathyroid glands of birds and mammals is recognized by evolutionary developmental biology as evolution-using genes and gene networks in novel ways to generate new structures with some similar functions and novel functions.
Parathyroid surgery is usually performed when there is hyperparathyroidism. This condition causes many diseases related with calcium reabsorption, because the principal function of the parathyroid hormone is to regulate it. Parathyroid surgery could be performed in two different ways: first is a complete parathyroidectomy, and second is the auto transplantation of the removed parathyroid glands.
Indications for the surgery
There are various conditions that can indicate the need for the removal or transplant of the parathyroid glands. Hyperparathyroidism is a condition caused by the extra production of the parathyroid glands, which can be divided into three types.
- Primary hyperparathyroidism happens when the normal mechanism of regulation of negative feedback of calcium is interrupted or in other words the amount of blood calcium is less than the production of PTH. Most of the time this is caused by adenomas, hyperplasia or carcinomas.
- Secondary hyperparathyroidism occurs normally in patients that suffer renal disease. The poor kidney function leads to a mineral disequilibrium causing that the organism, synthetize and release more PTH, makes the hypertrophy of the gland trying to compensate, for the disequilibrium.
- Tertiary hyperparathyroidism is developed when the hyperplastic gland constantly releases PTH after the secondary hyperparathyroidism independent to the regulation systems.
Another condition is hypercalcemia, which refers to the rising of calcium level above 10.5 mg/dL one of the consequences of this, are heart rhythm diseases, an extra production of gastrin and peptic ulcers.
Parathyroid transplant is recommended during thyroid surgery if the parathyroid glands are removed accidentally during a thyroidectomy they must be auto transplanted to the sternocleidomastoid muscle because of the small distance or to the forearm that way another intervention would not be so risky. But first is recommended to make an biopsy to be sure that the transplanted tissue is parathyroid and not a lymph node with metastatic disease. During parathyroid surgery if there is an adenoma the transplantation is not recommended in stead it is cryopreserved for research an if there is a recurrent hypoparathyroidism.
The surgery is indicated for all patients that are diagnosed with hyperparathyroidism with or without symptoms especially in younger patients. In some cases the surgery works as therapy for nephrolithiasis, bone changes and neuromuscular symptoms.
Parathyroidectomy, or the removal of the parathyroids, requires general anesthesia. The patient is intubated and placed in a supine position with the chin at fifteen degrees by elevating the shoulders to permit the extension of the neck. Then a transverse cut is made above the sternal notch. The transversal thyroid lobe is reached and is rotated up to discover and ligate the thyroid vein to separate the thyroid artery. Exploration must be done meticulously to search for adenomas. If an adenoma is identified, exploration must be continued because it is common that more than one neoplasia appears. Before the procedure, the glands are marked to make them more visible during the procedure. If one of them cannot be found, the procedure is to remove a complete thyroid lobe on the side where the gland is not found to avoid an intrathyroid parathyroid gland. After exploration, if there is one, two or even three parathyroid glands affected, they are removed and the other one left in situ. If all four glands are affected then three and a half are removed. The remaining half is marked with a suture and the surgeon must be sure that the blood supply will not be compromised. A total parathyroidectomy or auto transplantation to the forearm of the remaining half gland, may also be recommended.
Parathyroid auto transplantation
Parathyroid auto transplantation is part of the treatment when a patient has hyperparathyroidism and three or four parathyroid glands were already removed, but during the surgery one of the glands (in the case of the removal of three) is relocated at another part of the body to make, the procedure less risky another procedure. In the case of complete parathyroidectomy, a half gland is cryopreserved. In case the patient suffers hypoparathyroidism. If this happens the extracted parathyroid is relocated to another place of the body for example the forearm. Parathyroid auto transplantation begins with parathyroid tissue extraction, which must be preserved into a cold isotonic solution until the patient needs it. Research has shown that parathyroid tissue can function at subcutaneous level until the transplantation. If this is not possible, the most common procedure is to create a small pocket of muscle, tissue at least 2 cm deep by separating the muscular fibers. Then the parathyroid tissue is placed into and closed by suturing the area. After the extraction the tissue might be processed at the laboratory, as soon as possible. Once at the laboratory the tissue sample is placed at a frozen petri dish where it is cut into small pieces (approximately 1–2 mm). The small pieces are placed into test tubes and filled with a solution in three parts one at 20% of autologous serum (about 0.6 ml) and the other part of isotonic solution at 20% (about 0.6 ml) then a solution of 2 ml of polypropylene and mixed gently. Then is placed into a container at -70°C for a night then finally the container passes through the phase of liquid or vapor nitrogen immersion and is kept there until needed. When it is needed the sample is taken out of the nitrogen and placed into a bath of water at 37°C until the ice is melted almost completely except for the samples core. Then 0.5 ml of the melted solution is removed and replaced for fresh isotonic solution. 
Scheme showing development of branchial epithelial bodies. I, II, III, IV. Branchial pouches.
Human parathyroid glands
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