Parathyroid gland

Question 1

Anatomy of parathyroid gland

Answer 1

• Partially embedded in the posterior surface of the lateral lobes of the thyroid gland are several small, round masses of tissue called the parathyroid glands
• Usually, one superior and one inferior parathyroid gland are attached to each lateral thyroid lobe for a total of four.

•Microscopically, the parathyroid glands contain two kinds of epithelial cells
The more numerous cells, called chief cells or principal cells, produce parathyroid hormone (PTH), also called parathormone.

•The function of the other kind of cell, called an oxyphil cell, is not known in a normal parathyroid gland. However, its presence clearly helps to identify the parathyroid gland histologically due to its unique staining characteristics.

Question 2

Functions of parathyroid hormone

Answer 2

Parathyroid hormone is the major regulator of the levels of calcium (Ca2􏰐), magnesium (Mg2􏰐), and phosphate (HPO42􏰑) ions in the blood.
• The specific action of PTH is to increase the number and activity of osteoclasts.
The result is elevated bone re- sorption, which releases ionic calcium (Ca2􏰐) and phosphates (HPO42􏰑) into the blood.
• PTH also acts on the kidneys. First, it slows the rate at which Ca2􏰐 and Mg2􏰐 are lost from blood into the urine.
•Second, it increases loss of HPO42􏰑 from blood into the urine. Because more HPO42􏰑 is lost in the urine than is gained from the bones,
•PTH decreases blood HPO42􏰑 level and increases blood Ca2􏰐 and Mg2􏰐 levels.
•A third effect of PTH on the kidneys is to promote formation of the hormone calcitriol , the active form of vitamin D.
Calcitriol, also known as 1,25-dihydroxyvitamin D3, increases the rate of Ca2􏰐, HPO42􏰑, and Mg2􏰐 absorption from the gastrointestinal tract into the blood.

Question 3

Control of secretion of calcitonin and parathyroid hormone

Answer 3

1. A higher-than-normal level of calcium ions (Ca2􏰐) in the blood stimulates parafollicular cells of the thyroid gland to release more calcitonin.
2. Calcitonin inhibits the activity of osteoclasts, thereby de- creasing the blood Ca2􏰐 level.
   3.A lower-than-normal level of Ca2􏰐 in the blood stimulates chief cells of the parathyroid gland to release more PTH
3. PTH promotes resorption of bone extracellular matrix, which releases Ca2􏰐 into the blood and slows loss of Ca2􏰐 in the urine, raising the blood level of Ca2􏰐.
   5 .PTH also stimulates the kidneys to synthesize calcitriol, the active form of vitamin D.
4. Calcitriol stimulates increased absorption of Ca2􏰐 from foods in the gastrointestinal tract, which helps increase the blood level of Ca2􏰐.