Parathyroid Gland | Biology for JAMB PDF Download

Parathyroid Gland

• These glands remain embedded in the dorsal surface of thyroid gland .They are two pairs in number . They remain embedded in each lobe of thyroid completely or partially.
• It was discovered by Raynard and its detailed description was given by Sandrom.
• Total weight about 140 mg and its size is 6 × 3 × 2 mm.
• These glands are made by epithelium of third and fourth branchial pouches or pharyngeal pouches slits of embryo i.e. these are also endodermal in origin.
• These glands secrete only one hormone - parathormone. It is also known as Collip's hormone or PTH. Its was obtained by Collip in its pure form.
• This hormone is proteinaceous in nature/Polypeptide hormone.
• Parathormone is essential for survival because it significantly contributes to "homeostatis" by regulating the amount of calcium and phosphate ion in ECF.
• Calcium is key element in many physiological functions like proper permeability of cell membranes, muscular activities, nerve impulse conduction, heart beat, blood coagulation, bone formation, fertilization of ova.
• Calcium is most abundant of all minerals found in the body and about 99% of calcium and phosphorus are contained in the bones. (1% Ca+2 found in ECF).
• Maintenance of proper calcium level under "homeostasis" is in fact, a combined function of parathormone, thyrocalcitonin and vitamin D3 (cholecalciferol).
• Parathormone promotes absorption of calcium from food in the intestine and its reabsorption from nephron in the kidneys.
• Simultaneously, it accelerates elimination of phosphate in urine (phosphaturic action). Thus, calcium level tend to rise in the ECF due to effect of parathormone and phosphate level is decreased in ECF.
• This calcium is then utilized by bone-forming cells, (Osteoblast) in bone formation under the influence of vitamin D₃.
• Parathormone stimulates the osteoclast cells to feed upon bones, these cells remove unnecessary parts of bones by melting, thus change asymmetrical bone into symmetrical bone. The remoulding of bone is done by these cells life long. As a result of this, amount of Ca remains constant in blood in normal conditions. Each 100 ml of blood contains 12 mg of Ca⁺⁺. About 1 kg of calcium is found in an adult man.
• Parathormone maintains the activity of muscles.
• Just opposite to it, thyrocalcitonin hormone works antagonistically to oppose the parathormone and Vitamin D. Thyrocalcitonin reduces the amount of Ca++ in blood by increasing the excretion of Ca++ in urine and by reducing destruction of bone.

1. Hyposecretion

• Due to the deficiency of Ca++ in blood, muscles and nerves get unnecessarily irritated and start convulsion and cramping. Sometimes voluntary muscles remain contracted for a long time, it is known as tetany disease.
• If this tetany happens in intercostal muscles and diaphragm, then animal dies due to Asphyxia.
• Removal of parathyroid gland causes quick death. It is the most fatal state/ lethal condition.

2. Hypersecretion

As a result of this, bones become brittle and weak. This condition is called as Osteoporosis.

• When quantity of Ca++ is increases in ECF and level of PO4--- is reduced, this condition is known as hypercalcaemia and hypophosphatemia respectively.
• Due to excess deposition of Ca++ in kidneys and gall bladder, stones are formed.