Editing Bone (section)

===Cells===
[[File:604 Bone cells.jpg|thumb|Bone cells]]

Bone is metabolically active tissue composed of several types of cells. These cells include [[osteoblast]]s, which are involved in the creation and [[mineralized tissue|mineralization]] of bone tissue, [[osteocyte]]s, and [[osteoclast]]s, which are involved in the reabsorption of bone tissue. Osteoblasts and osteocytes are derived from [[osteoprogenitor]] cells, but [[osteoclast]]s are derived from the same cells that differentiate to form [[macrophage]]s and [[monocyte]]s.{{sfn|Young|2006|p=189}} Within the marrow of the bone there are also [[hematopoietic stem cell]]s. These cells give rise to other cells, including [[white blood cell]]s, [[red blood cell]]s, and [[platelet]]s.{{sfn|Young|2006|p=58}}

====Osteoblast====
[[File:Active osteoblasts.jpg|thumb|[[Micrograph|Light micrograph]] of [[Bone decalcification|decalcified]] cancellous bone tissue displaying osteoblasts actively synthesizing osteoid, containing two osteocytes.]]

[[Osteoblast]]s are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a [[protein]] mixture known as [[osteoid]], which mineralizes to become bone.{{sfn|Young|2006|pp=189–190}} The osteoid seam is a narrow region of a newly formed organic matrix, not yet mineralized, located on the surface of a bone. Osteoid is primarily composed of Type I [[collagen]]. Osteoblasts also manufacture [[hormone]]s, such as [[prostaglandin]]s, to act on the bone itself. The osteoblast creates and repairs new bone by actually building around itself. First, the osteoblast puts up collagen fibers. These collagen fibers are used as a framework for the osteoblasts' work. The osteoblast then deposits calcium phosphate which is hardened by [[hydroxide]] and [[bicarbonate]] ions. The brand-new bone created by the osteoblast is called [[osteoid]].<ref>{{cite web | url = http://depts.washington.edu/bonebio/bonAbout/bonecells.html | archive-url = https://web.archive.org/web/20110807200120/http://depts.washington.edu/bonebio/bonAbout/bonecells.html | archive-date = 7 August 2011 | title = The O' Cells | work = Bone Cells | publisher = The University of Washington | date = 3 April 2013 }}</ref> Once the osteoblast is finished working it is actually trapped inside the bone once it hardens. When the osteoblast becomes trapped, it becomes known as an osteocyte. Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as bone lining cells.<ref>{{cite journal | vauthors = Wein MN |date=28 April 2017 |title= Bone Lining Cells: Normal Physiology and Role in Response to Anabolic Osteoporosis Treatments |journal=Current Molecular Biology Reports |volume=3 |issue= 2|pages= 79–84 |doi= 10.1007/s40610-017-0062-x|s2cid= 36473110 }}</ref>

====Osteocyte====
[[Osteocyte]]s are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by a bone matrix that they themselves produced.{{sfn|Young|2006|p=192}} The spaces the cell body of osteocytes occupy within the mineralized collagen type I matrix are known as [[lacuna (histology)|lacunae]], while the osteocyte cell processes occupy channels called canaliculi. The many processes of osteocytes reach out to meet osteoblasts, osteoclasts, bone lining cells, and other osteocytes probably for the purposes of communication.<ref>{{cite journal | vauthors = Sims NA, Vrahnas C | title = Regulation of cortical and trabecular bone mass by communication between osteoblasts, osteocytes and osteoclasts | journal = Archives of Biochemistry and Biophysics | volume = 561 | pages = 22–28 | date = November 2014 | pmid = 24875146 | doi = 10.1016/j.abb.2014.05.015 }}</ref> Osteocytes remain in contact with other osteocytes in the bone through gap junctions—coupled cell processes which pass through the canalicular channels.

====Osteoclast====
[[Osteoclast]]s are very large [[multinucleate]] cells that are responsible for the breakdown of bones by the process of [[bone resorption]]. New bone is then formed by the osteoblasts. Bone is constantly [[bone remodeling|remodeled]] by the resorption of osteoclasts and created by osteoblasts.{{sfn|Young|2006|p=189}} Osteoclasts are large cells with multiple [[Cell nucleus|nuclei]] located on bone surfaces in what are called ''Howship's lacunae'' (or ''resorption pits''). These lacunae are the result of surrounding bone tissue that has been reabsorbed.{{sfn|Young|2006|p=190}} Because the osteoclasts are derived from a [[monocyte]] [[stem cell|stem-cell]] lineage, they are equipped with [[Phagocytosis|phagocytic]]-like mechanisms similar to circulating [[macrophage]]s.{{sfn|Young|2006|p=189}} Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as [[tartrate-resistant acid phosphatase]], are [[Secretion|secreted]] against the mineral substrate.{{citation needed|date=September 2013}} The reabsorption of bone by osteoclasts also plays a role in [[calcium]] [[homeostasis]].{{sfn|Young|2006|p=190}}