Editing Bone (section)

==Remodeling==
{{Main|Bone remodeling}}
Bone is constantly being created and replaced in a process known as [[Bone remodeling|remodeling]]. This ongoing turnover of bone is a process of resorption followed by replacement of bone with little change in shape. This is accomplished through osteoblasts and osteoclasts. Cells are stimulated by a variety of [[paracrine|signals]], and together referred to as a remodeling unit. Approximately 10% of the skeletal mass of an adult is remodelled each year.<ref>{{cite journal | vauthors = Manolagas SC | title = Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis | journal = Endocrine Reviews | volume = 21 | issue = 2 | pages = 115–137 | date = April 2000 | pmid = 10782361 | doi = 10.1210/edrv.21.2.0395 | doi-access = free }}</ref> The purpose of remodeling is to regulate [[calcium homeostasis]], repair [[Microdamage in bone|microdamaged bones]] from everyday stress, and to shape the skeleton during growth.<ref>{{cite journal | vauthors = Hadjidakis DJ, Androulakis II | title = Bone remodeling | journal = Annals of the New York Academy of Sciences | volume = 1092 | pages = 385–396 | date = December 2006 | issue = 1 | pmid = 17308163 | doi = 10.1196/annals.1365.035 | bibcode = 2006NYASA1092..385H | s2cid = 39878618 }}</ref> Repeated stress, such as weight-bearing [[exercise]] or bone healing, results in the bone thickening at the points of maximum stress ([[Wolff's law]]). It has been hypothesized that this is a result of bone's [[piezoelectricity|piezoelectric]] properties, which cause bone to generate small electrical potentials under stress.<ref>{{cite book| veditors = Woodburne RT |title=Anatomy, physiology, and metabolic disorders|date=1999|publisher=Novartis Pharmaceutical Corp.|location=Summit, N.J.|isbn=978-0-914168-88-1|pages=187–189|edition=5th }}</ref>

The action of osteoblasts and osteoclasts are controlled by a number of chemical [[enzyme]]s that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. The cells also use [[paracrine signalling]] to control the activity of each other.<ref name="fogelman" /><ref>{{Cite web|title=Introduction to cell signaling (article)|url=https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell-cycle/cell-communication/a/introduction-to-cell-signaling|access-date=2020-12-24|website=Khan Academy|language=en}}</ref> For example, the rate at which osteoclasts resorb bone is inhibited by [[calcitonin]] and [[osteoprotegerin]]. Calcitonin is produced by [[parafollicular cell]]s in the [[thyroid gland]], and can bind to receptors on osteoclasts to directly inhibit osteoclast activity. Osteoprotegerin is secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation.<ref name=Boron/>

Osteoblasts can also be stimulated to increase bone mass through increased secretion of [[osteoid]] and by [[Enzyme inhibitor|inhibiting]] the ability of osteoclasts to break down [[osseous tissue]].{{citation needed|date=September 2013}} Increased secretion of osteoid is stimulated by the secretion of [[growth hormone]] by the [[pituitary]], [[thyroid hormone]] and the sex hormones ([[estrogen]]s and [[androgen]]s). These hormones also promote increased secretion of osteoprotegerin.<ref name=Boron>{{cite book | vauthors = Boulpaep EL, Boron WF |title=Medical physiology: a cellular and molecular approach |publisher=Saunders |location=Philadelphia |year=2005 |pages=1089–1091 |isbn=978-1-4160-2328-9 }}</ref> Osteoblasts can also be induced to secrete a number of [[cytokine]]s that promote reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells. [[Vitamin D]], [[parathyroid hormone]] and stimulation from osteocytes induce osteoblasts to increase secretion of RANK-[[ligand]] and [[interleukin 6]], which cytokines then stimulate increased reabsorption of bone by osteoclasts. These same compounds also increase secretion of [[macrophage colony-stimulating factor]] by osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion of osteoprotegerin.{{citation needed|date=September 2013}}