In this article we will discuss about the Parathyroid Glands:- 1. Origin of Parathyroid Glands 2. Location and Structure of Parathyroid Glands 3. Hormones 4. Disorders.
Origin of Parathyroid Glands:
They develop from the endoderm of the embryo.
Location and Structure of Parathyroid Glands:
The parathyroid glands consist of four separate glands located on the posterior surface of the lobes of the thyroid gland. The cells of parathyroid glands are arranged in a compact mass and are of two types: small chief cells or principal cells and large oxyphil cells (or eosinophil cells).
The cells are enclosed by a delicate connective tissue capsule. The chief cells are much more numerous than the oxyphil cells. The latter are absent in the young and appear a little before the age of puberty.
Hormones of Parathyroid Glands:
The chief cells of the parathyroid secrete a hormone called parathyroid hormone (PTH) or parathormone or also called Collip’s hormone after the name of its discoverer. This hormone regulates the calcium and phosphate balance between the blood and other tissues. PTH inhibits collagen synthesis by osteoblasts and bone resorption by osteoclasts.
It mobilises the release of calcium into the blood from the bones. PTH increases calcium absorption from the intestines. It increases calcium resorption from the nephrons (and inhibits phosphate resorption) of the kidneys. Thus parathormone regulates the metabolism of calcium and phosphate.
Parathyroids are under the feeding back control of blood calcium level. A fall in blood calcium stimulates them to secrete parathormone, a rise in blood calcium inhibits parathormone secretion from them. Thus PTH has an effect that opposes the effect of calcitonin. The functions of oxyphil cells are unknown.
(i) Hypoparathyroidism (deficiency of PTH):
It causes the lowering of blood calcium level. This increases the excitability of nerves and muscles, causing cramps and convulsions. Sustained contractions of the muscles of larynx, face, hands and feet are produced. This disorder is called parathyroid tetany or hypocalcaemic tetany.
(ii) Hyperparathyroidism (excess of PTH):
Excess of PTH draws more calcium from the bones. It causes demineralisation, resulting in softing and bending of the bones. Some of the bone substance is replaced by cavities that are filled with fibrous tissues. This condition leads to osteitis fibrosa cystica or osteoporosis.
Because bones become deformed, they are easily fractured. Osteoporosis is common in women who have reached menopause (cessation of menstruation). An excess of parathormone also causes calcium to be deposited in the kidneys. Analysis of the content of kidney stones sometimes suggests the presence of a parathyroid tumour.