Editing Bone (section)

===Composition===
{{Main|Extracellular matrix}}

Bones consist of living cells (osteoblasts and osteocytes) embedded in a mineralized organic matrix. The primary inorganic component of human bone is [[hydroxyapatite]], the dominant [[bone mineral]], having the nominal composition of Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>.<ref>[https://arxiv.org/ftp/arxiv/papers/2001/2001.11808.pdf#page=2 Enhancement of Hydroxyapatite Dissolution] Journal of Materials Science & Technology,38, 148-158</ref> The organic components of this matrix consist mainly of [[Collagen#Types|type I collagen]]—"organic" referring to materials produced as a result of the human body—and inorganic components, which alongside the dominant [[hydroxyapatite]] phase, include other compounds of [[calcium]] and [[phosphate]] including salts. Approximately 30% of the acellular component of bone consists of organic matter, while roughly 70% by mass is attributed to the inorganic phase.{{sfn|Hall|2005|p=981}} The [[collagen]] fibers give bone its [[ultimate tensile strength|tensile strength]], and the interspersed crystals of [[hydroxyapatite]] give bone its [[compressive strength]]. These effects are [[synergy|synergistic]].{{sfn|Hall|2005|p=981}} The exact composition of the matrix may be subject to change over time due to nutrition and [[biomineralization]], with the ratio of [[calcium]] to [[phosphate]] varying between 1.3 and 2.0 (per weight), and trace minerals such as [[magnesium]], [[sodium]], [[potassium]] and [[carbonate]] also be found.{{sfn|Hall|2005|p=981}}

{{anchor|Woven vs. lamellar bone}}
Type I collagen composes 90–95% of the organic matrix, with the remainder of the matrix being a homogenous liquid called [[ground substance]] consisting of [[proteoglycan]]s such as [[hyaluronic acid]] and [[chondroitin sulfate]],{{sfn|Hall|2005|p=981}} as well as non-collagenous proteins such as [[osteocalcin]], [[osteopontin]] or [[bone sialoprotein]]. Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress. The function of ground substance is not fully known.{{sfn|Hall|2005|p=981}} Two types of bone can be identified microscopically according to the arrangement of collagen: woven and lamellar.

* Woven bone (also known as ''fibrous bone''), which is characterized by a haphazard organization of collagen fibers and is mechanically weak.<ref name="Curry2006">Currey, John D. (2002). [http://press.princeton.edu/chapters/s7313.html "The Structure of Bone Tissue"] {{Webarchive|url=https://web.archive.org/web/20170425052316/http://press.princeton.edu/chapters/s7313.html |date=25 April 2017 }}, pp.  12–14 in ''Bones: Structure and Mechanics''. Princeton University Press. Princeton, NJ. {{ISBN|978-1-4008-4950-5}}</ref>
* Lamellar bone, which has a regular parallel alignment of collagen into sheets ("lamellae") and is mechanically strong.<ref name="Buss_2022" /><ref name="Curry2006"/>

[[File:Woven bone matrix.jpg|thumb|right|[[Transmission electron microscopy|Transmission]] [[electron micrograph]] of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers]]

Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all [[fetus|fetal]] bones, but is later replaced by more resilient lamellar bone. In adults, woven bone is created after [[Bone fracture|fractures]] or in [[Paget's disease of bone|Paget's disease]]. Woven bone is weaker, with a smaller number of randomly oriented collagen fibers, but forms quickly; it is for this appearance of the fibrous matrix that the bone is termed ''woven''. It is soon replaced by lamellar bone, which is highly organized in [[concentric]] sheets with a much lower proportion of osteocytes to surrounding tissue. Lamellar bone, which makes its first appearance in humans in the [[fetus]] during the third trimester,<ref name="Salentijn">Salentijn, L. ''Biology of Mineralized Tissues: Cartilage and Bone'', [[Columbia University College of Dental Medicine]] post-graduate dental lecture series, 2007</ref> is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). In [[cross section (geometry)|cross-section]], the fibers run in opposite directions in alternating layers, much like in [[plywood]], assisting in the bone's ability to resist [[torsion (mechanics)|torsion]] forces. After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution". Compared to woven bone, lamellar bone formation takes place more slowly. The orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2&nbsp;[[Micrometre|μm]] per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.<ref>{{Cite book| vauthors = Royce PM, Steinmann B |url=https://books.google.com/books?id=x-Z-cXUGlL8C&q=Lamella+bone+requires+a+relatively+flat+surface&pg=PA70|title=Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects|date=2003-04-14|publisher=John Wiley & Sons|isbn=978-0-471-46117-3|language=en}}</ref>

====Deposition====
The extracellular matrix of bone is laid down by [[osteoblast]]s, which secrete both collagen and ground substance. These cells synthesise collagen alpha polypetpide chains and then secrete collagen molecules. The collagen molecules associate with their neighbors and crosslink via lysyl oxidase to form collagen fibrils. At this stage, they are not yet mineralized, and this zone of unmineralized collagen fibrils is called "osteoid". Around and inside collagen fibrils calcium and phosphate eventually [[Precipitation (chemistry)|precipitate]] within days to weeks becoming then fully mineralized bone with an overall carbonate substituted hydroxyapatite inorganic phase.<ref>{{cite journal | vauthors = Buss DJ, Reznikov N, McKee MD | title = Crossfibrillar mineral tessellation in normal and Hyp mouse bone as revealed by 3D FIB-SEM microscopy | journal = Journal of Structural Biology | volume = 212 | issue = 2 | page = 107603 | date = November 2020 | pmid = 32805412 | doi = 10.1016/j.jsb.2020.107603 | s2cid = 221164596 | url = https://escholarship.mcgill.ca/concern/articles/vq27zt432 }}</ref>{{sfn|Hall|2005|p=981}}

In order to mineralise the bone, the osteoblasts secrete alkaline phosphatase, some of which is carried by [[Vesicle (biology and chemistry)|vesicles]]. This cleaves the inhibitory pyrophosphate and simultaneously generates free phosphate ions for mineralization, acting as the foci for calcium and phosphate deposition. Vesicles may initiate some of the early mineralization events by rupturing and acting as a centre for crystals to grow on. Bone mineral may be formed from globular and plate structures, and via initially amorphous phases.<ref name=r1>{{cite journal| vauthors = Bertazzo S, Bertran CA |year=2006|title=Morphological and dimensional characteristics of bone mineral crystals|journal= Key Engineering Materials|volume=309-311 |pages=3–6 |doi=10.4028/www.scientific.net/kem.309-311.3|s2cid=136883011 }}</ref><ref>{{cite journal|doi=10.4028/www.scientific.net/kem.309-311.11|title=Morphological Characterization of Femur and Parietal Bone Mineral of Rats at Different Ages|year=2006| vauthors = Bertazzo S, Bertran C, Camilli J |journal=Key Engineering Materials|volume=309–311|pages=11–14|s2cid=135813389}}</ref>