Anatomy Flashcards
(17 cards)
Embryology
- two pairs of parathyroid glands develop from the endoderm of the third and fourth branchial pouches and neural crest mesenchyme.
- inferior parathyroid glands and the thymus develop from the third branchial pouch.
- The paired superior parathyroid glands develop from the fourth branchial pouch along with the lateral lobes of the thyroid gland and the ultimobranchial bodies
Blood Supply
> > inferior thyroid artery is the predominant vascular supply for both the superior and inferior parathyroid glands in 80% of cases.
> > The inferior thyroid artery is a branch of the thyrocervical trunk that originates from the subclavian artery.
> > Alternatively, the parathyroid glands may receive their blood supply from the superior thyroid artery or from small arterial branches directly from the thyroid gland.
parathyroid glands location
> > The parathyroid glands are usually found within 5 mm of the superior and inferior margins of the tubercle of Zuckerkandl along the posterior capsule of the lateral lobes of the thyroid gland.
Location of Superior Parathyroid
> > The superior parathyroid glands are most commonly located on the posterior surface of the upper pole of the thyroid gland, posterior and superior to the recurrent laryngeal nerve
Superior Parathyroid less likely to be ectopic why ?
- shorter embryologic migration compared to the inferior parathyroid glands
- Eighty percent of the superior glands are found in their expected anatomic location, approximately 1 cm superior to the junction of the inferior thyroid artery and the recurrent laryngeal nerve at the level of the cricoid cartilage
- often found within the capsule of the thyroid gland
challenge in identifying the superior parathyroid glands
In such a situation, ligation and division of the superior pole vessels may be helpful in optimizing exposure of an abnormal superior parathyroid gland
Ectopic locations of the superior parathyroid glands include
the tracheoesophageal groove
behind the esophagus or the pharynx
in the posterior mediastinum
intrathyroidal
The inferior parathyroid glands location
> > found 1 cm inferior to the junction of the inferior thyroid artery and the recurrent laryngeal nerve, anterior and medial to the recurrent laryngeal nerve on the posterolateral aspect of the inferior pole of the thyroid lobe
Ectopic locations for Inferior parathyroid
> > the inferior parathyroid glands are more commonly ectopic and can be found anywhere from the angle of the mandible to the pericardium.
> > The most common ectopic location for an inferior gland is within the thymus
> > Other sites
-anterior superior mediastinum
-the aortopulmonary window
-the carotid sheath
-intrathyroidal
-undescended in a submandibular location
Parathyroid color
- In older patients, the parathyroid glands are more often light yellow or tan in color because of a greater percentage of fat
- whereas in young patients, the parathyroid glands are darker with a reddish-brown color because of a lesser percentage of fat
parenchymal cells of the parathyroid gland consist of
- chief cells»_space; responsible for synthesis and secretion of PTH.
- oxyphil cells»_space; function of the oxyphil cells is unknown
- Clear cells are a third cell type»_space; more often present with parathyroid hyperplasia
Physiology
- High calcium also enhances PTH degradation
- 1,25-Dihydroxyvitamin D (1,25[OH]2D), the active form of vitamin D, binds to vitamin D receptors on chief cells and inhibits PTH gene expression and parathyroid cell proliferation.
- PTH is essential in the regulation of calcium and phosphate metabolism
- PTH acts on the ascending limb of the loop of Henle and the distal convoluted tubule of the kidneys to enhance calcium reabsorption, inhibit phosphate reabsorption, and increase phosphate excretion causing phosphaturia
- PTH stimulates the conversion of 25-hydroxyvitamin D (25[OH]D) to the active metabolite 1,25[OH]2D
- PTH binds to the PTHR1 receptor that is found on cells of the osteoblastic lineage, which ultimately leads to the formation of new bone matrix and mineralization.
PTH effect on Bone
- In patients with chronically elevated PTH levels, such as primary and secondary HPT, the net effect is bone resorption leading to osteopenia and osteoporosis.
- However, intermittent administration of recombinant human PTH has been shown to stimulate bone formation more than resorption and is currently in clinical use for treatment of osteoporosis.
primary HPT
It occurs most commonly in postmenopausal women between 50 and 60 years of age.
The incidence of primary HPT increases with age
Unlike primary and tertiary HPT, most other causes of hypercalcemia are associated with
a low serum PTH level
with the exception of prolonged lithium therapy and familial hypocalciuric hypercalcemia (FHH).
the most common causes of hypercalcemia in adults
Primary HPT and malignancy are the most common causes of hypercalcemia in adults
familial hypocalciuric hypercalcemia (FHH)
rare autosomal dominant disorder affecting the renal CaSR that results in a higher set point for renal calcium excretion
hypocalciuria
asymptomatic hypercalcemia
and normal or mildly elevated PTH levels
patients typically develop chronically elevated serum calcium levels before the age of 30
No treatment is required for FHH.
