Adenylyl cyclase pathway – signalling cascade initiated with ligand binding to GPCR receptor associated with Gαs or Gαi. The former stimulates (while the later inhibits) adenylyl cyclase resulting in production of cyclic adenosine monophosphate (cAMP) and stimulation of protein kinase A (PKA) in result. Downstream of the cascade are present many effectors, including transcriptional modifiers, ion channels, vesicles etc.


CaSR (Calcium sensory receptor) – present on C cells of parathyroid gland. Receptors responsible for sensing plasma [Ca2+] levels in a saturable manner.


Calcitonin – secreted by C cells of thyroid gland, causes inhibition of bone resorption and promotes mineralization of bone. Elevates calcium and phosphate plasma levels by increasing reabsorption of both.


Collagen type I – osteoid protein, composed of a triple helix. Two strands are α1 monomers and one α2 monomer. High glycine content allows for helix formation.


Cortical bone – form of bone tissue. Composed of bone matrix proteins (osteoid) and bone minerals, is dense and compact, giving bone its strength and rigidity.


GPCR – G-Protein-Coupled-Receptor, a seven-pass transmembrane receptor that is associated with a G protein, which is responsible for signalling to its effectors.


Homeostasis – balance of substances that is being adjusted and maintained by the body.


Hydroxyapatite crystal – a salt crystal providing form of storage of phosphate and calcium in bone with the following structure: 3Ca3(PO4)2·Ca(OH)2


Nucear receptors – class of receptors commonly used by steroids, vitamin D and thyroid hormone. Present in the nucleus where it creates a complex with another transcriptional factor upon ligand binding to modify transcription.


Osteoblasts – cells present in the bone. Responsible for mineralization of bone, secreting osteoid and cytokines activating osteoclasts.


Osteoclasts – cells present in the bone. Their main function is resorption of bone.


Osteocytes – cells in the bone. Are descendants of osteoblasts that became trapped by the bone matrix.


Osteoid – complex of bone matrix proteins secreted by osteoblasts.


Parathyroid glands – four small glands located on the posterior side of thyroid gland lobes, two per each lobe. Combined weight of four glands does not exceed 500 mg. Glands are mostly composed of chief cells which are responsible for sensing plasma Ca2+ levels, synthesis and secretion of parathyroid hormone.


PTH (parathyroid hormone) – hormone secreted by parathyroid gland in response to low plasma [Ca2+]. Causes resorption of bone, reabsorption of calcium and synthesis of 1,25-dihydroxyvitamin D. As secondary effect causes decrease in plasma [P] levels.


PLC pathway – pathway may begin with activation of GPCR coupled to Gαq protein. Gαq stimulate phospholipase C (PLC) which generates diacylglycerol (DAG) and 1,4,5-triphosphate (IP3). This in turn stimulates release of internal calcium, which results in activation of Ca2+-dependent kinase (PKC). Intracellular Ca2+ can have other targets and is often utilized as second messenger on its own. Pathway may result in transcriptional modification, ion channel opening, exocytosis or many other functions.


Thyroidectomy – surgical removal of thyroid gland together with parathyroid glands.


Thyroid gland – an endocrine gland situated in front of the neck, attached to larynx and trachea. Composed of two lateral lobes connected by isthmus. Isthmus lies in front of the second, third and frourth tracheal rings. On cellular level gland is made of many follicles filled with colloid and lined with follicular cells. Follicular cells are responsible for secreting thyroglobulin and synthesis of thyroid hormones. Thyroid gland also posseses parafolicular cells, called C cells. C cells are responsible for secretion of calcitonin.


Trabecular bone – also called cancellous or medullary, a form of bone tissue, spongy in appearance. Present inside the bone marrow, near bone marrow.


VDR (Vitamin D receptor) – nuclear receptor for vitamin D present in chief cells in parathyroid gland. Upon ligand binding forms a complex with retinoid acid X receptor and binds to vitamin D responsive element, inhibiting PTH transcription.


Vitamin D – ingested in forms of vitamin D2 or D3, is metabolized first to 25-hydroxyvitamin D in liver and than to 1,25-dihydroxyvitamin D in kidney under tight regulation of PTH. Promotes intestinal absorption of Ca2+ and P, reabsorption of both molecules in kidneys and overall mineralization of bone. Inhibits PTH synthesis and secretion.


Wojciech Stec


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