Parathyroid gland
Encyclopedia
The parathyroid glands are small endocrine
gland
s in the neck that produce parathyroid hormone
. Humans usually have four parathyroid glands, which are usually located on the rear surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest. Parathyroid glands control the amount of calcium in the blood and within the bones.
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.
In the histological sense, they distinguish themselves from the thyroid gland, as they contain two types of cells:
by Richard Owen
in 1850. The glands were first discovered in humans by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880. It is the last major organ to be recognized in humans so far.
level within a very narrow range, so that the nervous
and muscular system
s can function properly.
When blood calcium levels drop below a certain point, calcium-sensing receptor
s in the parathyroid gland are activated to release hormone into the blood.
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 osteoclast
s to break down bone and release calcium. PTH also increases gastrointestinal calcium absorption by activating vitamin D
, and promotes calcium conservation (reabsorption) by the kidney
s. PTH affects the perception of well being and absence of PTH can be associated with feeling of fatigue and anxiety.
, and those causing hypoparathyroidism
.
mesenchyme
and third and fourth branchial pouch
endoderm
.
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.
s, although they vary in their number, and in their exact position. Mammal
s typically have four parathyroids, while other groups typically have six.
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 parathryroid 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.
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...
gland
Gland
A gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
s in the neck that produce parathyroid hormone
Parathyroid hormone
Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids...
. Humans usually have four parathyroid glands, which are usually located on the rear surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest. Parathyroid glands control the amount of calcium in the blood and within the bones.
Anatomy
The parathyroid glands are four or more small glands, about the size of a grain of rice, located on the posterior surface (back side) of the thyroid gland. The parathyroid glands usually weigh between 25mg and 40mg in humans. There are typically two, one above the other, on the left lobe of the thyroid and similarly on the right. 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.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
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
. However, at surgery, they are harder to differentiate from the thyroid or fat.
In the histological sense, they distinguish themselves from the thyroid gland, as they contain two types of cells:
| Name | Staining | Quantity | Size | Function >- | parathyroid chief cell Parathyroid chief cell Parathyroid chief cells are cells in the parathyroid glands which produce parathyroid hormone.... s |
darker | many | smaller | >- | lighter | few | larger | function unknown. |
History
The parathyroid glands were first discovered in the Indian RhinocerosIndian Rhinoceros
The Indian Rhinoceros is also called Greater One-horned Rhinoceros and Asian One-horned Rhinoceros and belongs to the Rhinocerotidae family...
by Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...
in 1850. The glands were first discovered in humans by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880. It is the last major organ to be recognized in humans so far.
Physiology
The major function of the parathyroid glands is to maintain the body's calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
level within a very narrow range, so that the nervous
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
and muscular system
Muscular system
The muscular system is the anatomical system of a species that allows it to move. The muscular system in vertebrates is controlled through the nervous system, although some muscles can be completely autonomous.- Muscles :...
s can function properly.
When blood calcium levels drop below a certain point, calcium-sensing receptor
Calcium-sensing receptor
The calcium-sensing receptor is a Class C G-protein coupled receptor which senses extracellular levels of calcium ion. In the parathyroid gland, the calcium-sensing receptor controls calcium homeostasis by regulating the release of parathyroid hormone .-Signal transduction:The release of PTH is...
s in the parathyroid gland are activated to release hormone into the blood.
Parathyroid hormone
Parathyroid hormone
Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids...
(PTH, also known as parathormone) is a small protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
that takes part in the control of calcium and phosphate
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
, as well as bone physiology. Parathyroid hormone has effects antagonistic to those of calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
. PTH increases blood calcium levels by stimulating osteoclast
Osteoclast
An osteoclast is a type of bone cell that removes bone tissue by removing its mineralized matrix and breaking up the organic bone . This process is known as bone resorption. Osteoclasts were discovered by Kolliker in 1873...
s to break down bone and release calcium. PTH also increases gastrointestinal calcium absorption by activating vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
, and promotes calcium conservation (reabsorption) by the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s. PTH affects the perception of well being and absence of PTH can be associated with feeling of fatigue and anxiety.
Role in disease
Many conditions are associated with disorders of parathyroid function. These can be divided into those causing hyperparathyroidismHyperparathyroidism
Hyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...
, and those causing hypoparathyroidism
Hypoparathyroidism
Hypoparathyroidism is decreased function of the parathyroid glands with under production of parathyroid hormone. This can lead to low levels of calcium in the blood, often causing cramping and twitching of muscles or tetany , and several other symptoms...
.
Embryology and evolution
The parathyroid glands originate from the interaction of neural crestNeural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....
mesenchyme
Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...
and third and fourth branchial pouch
Branchial pouch
In the development of vertebrate animals, pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves form the lateral ectodermal surface of the neck region to separate the arches....
endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
.
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 tetrapodTetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, although they vary in their number, and in their exact position. Mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s typically have four parathyroids, while other groups typically have six.
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
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
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
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
, 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 parathryroid glands of birds and mammals is recognized by evolutionary developmental biology
Evolutionary developmental biology
Evolutionary developmental biology is a field of biology that compares the developmental processes of different organisms to determine the ancestral relationship between them, and to discover how developmental processes evolved...
as evolution-using genes and gene networks in novel ways to generate new structures with some similar functions and novel functions.