Editing Bone (section)

==Development==
[[File:Bone growth.png|300px|thumb|left|Endochondral ossification]]
[[File:Bone (1).jpg|thumb|Light micrograph of a section through a juvenile knee joint (rat) showing the cartilagineous growth plates]]
The formation of bone is called [[ossification]]. During the [[prenatal development|fetal stage of development]] this occurs by two processes: [[intramembranous ossification]] and [[endochondral ossification]].<ref>{{cite book | vauthors = Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P | chapter = 6.4 Bone Formation and Development | title = Anatomy and Physiology | date = 25 April 2013 | publisher = OpenStax | access-date = 26 February 2016 | chapter-url = https://openstax.org/books/anatomy-and-physiology/pages/6-4-bone-formation-and-development }}</ref> Intramembranous ossification involves the formation of bone from [[connective tissue]] whereas endochondral ossification involves the formation of bone from [[cartilage]].

'''Intramembranous ossification''' mainly occurs during formation of the flat bones of the [[skull]] but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as [[mesenchyme]] tissue rather than from cartilage. The process includes: the development of the [[ossification center]], [[calcification]], trabeculae formation and the development of the periosteum.<ref>{{Cite web|title=Bone Growth and Development | work = Biology for Majors II|url=https://courses.lumenlearning.com/wm-biology2/chapter/bone-growth-and-development/| publisher = lumenlearning.com |access-date=2020-05-28}}</ref>

'''Endochondral ossification''' occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. This process includes the development of a cartilage model, its growth and development, development of the primary and secondary [[ossification center]]s, and the formation of articular cartilage and the [[epiphyseal plate]]s.<ref>{{Cite book| vauthors = Tortora GJ, Derrickson BH |url=https://books.google.com/books?id=aSaVDwAAQBAJ&q=Endochondral+ossification&pg=PA181|title=Principles of Anatomy and Physiology |year=2018|publisher=John Wiley & Sons|isbn=978-1-119-44445-9|language=en}}</ref>

Endochondral ossification begins with points in the cartilage called "primary ossification centers". They mostly appear during fetal development, though a few short bones begin their primary ossification after [[birth]]. They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth and forms the [[Epiphysis|epiphyses]] of long bones and the extremities of irregular and flat bones. The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the [[epiphyseal plate]]). At skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure).<ref>{{Cite web|title=6.4B: Postnatal Bone Growth|url=https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/6%3A_Skeletal_System/6.4%3A_Bone_Formation/6.4B%3A_Postnatal_Bone_Growth|date=2018-07-19|website=Medicine LibreTexts|language=en|access-date=2020-05-28}}</ref> In the upper limbs, only the diaphyses of the long bones and scapula are ossified. The epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous.<ref>{{cite book| vauthors = Agur A |title=Grant's Atlas of Anatomy|year=2009|publisher=Lippincott Williams & Wilkins|location=Philadelphia|isbn=978-0-7817-7055-2|page=598}}</ref>

The following steps are followed in the conversion of cartilage to bone:
# Zone of reserve cartilage. This region, farthest from the marrow cavity, consists of typical hyaline cartilage that as yet shows no sign of transforming into bone.<ref name="Saladin 2012 217">{{cite book| vauthors = Saladin K |title=Anatomy and Physiology: The Unity of Form and Function|year=2012|publisher=McGraw-Hill|location=New York|isbn=978-0-07-337825-1|page=217}}</ref>
# Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae.<ref name="Saladin 2012 217"/>
# Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy (enlarge), much like they do in the primary ossification center of the fetus. The walls of the matrix between lacunae become very thin.<ref name="Saladin 2012 217"/>
# Zone of calcification. Minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. These are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be weakened by the breakdown of the enlarged lacunae.<ref name="Saladin 2012 217"/>
# Zone of bone deposition. Within each column, the walls between the lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.<ref name="Saladin 2012 217"/>
Bone development in youth is extremely important in preventing future complications of the skeletal system. Regular exercise during childhood and adolescence can help improve bone architecture, making bones more resilient and less prone to fractures in adulthood. Physical activity, specifically resistance training, stimulates growth of bones by increasing both bone density and strength. Studies have shown a positive correlation between the adaptations of resistance training and bone density.<ref name="Layne_1999">{{cite journal | vauthors = Layne JE, Nelson ME | title = The effects of progressive resistance training on bone density: a review | language = en-US | journal = Medicine and Science in Sports and Exercise | volume = 31 | issue = 1 | pages = 25–30 | date = January 1999 | pmid = 9927006 | doi = 10.1097/00005768-199901000-00006 }}</ref> While nutritional and pharmacological approaches may also improve bone health, the strength and balance adaptations from resistance training are a substantial added benefit.<ref name="Layne_1999" /> Weight-bearing exercise may assist in osteoblast (bone-forming cells) formation and help to increase bone mineral content. High-impact sports, which involve quick changes in direction, jumping, and running, are particularly effective with stimulating bone growth in the youth.<ref name="López-García_2019">{{Cite journal | vauthors = López-García R, Cruz-Castruita R, Morales-Corral P, Banda-Sauceda N, Lagunés-Carrasco J |date=2019-12-16 |title=Evaluación del mineral óseo con la dexa en futbolistas juveniles |url=http://cdeporte.rediris.es/revista/revista76/artevaluacion1098.htm  |journal=Revista Internacional de Medicina y Ciencias de la Actividad Física y del Deporte |volume=19 |issue=76 |pages=617–626 |issn=1577-0354|hdl=10486/689625 |hdl-access=free }}</ref> Sports such as soccer, basketball, and tennis have shown to have positive effects on bone mineral density as well as bone mineral content in teenagers.<ref name="López-García_2019" /> Engaging in physical activity during childhood years, particularly in these high-impact osteogenic sports, can help to positively influence bone mineral density in adulthood.<ref name="Van Langendonck_2003">{{cite journal | vauthors = Van Langendonck L, Lefevre J, Claessens AL, Thomis M, Philippaerts R, Delvaux K, Lysens R, Renson R, Vanreusel B, Vanden Eynde B, Dequeker J, Beunen G | title = Influence of participation in high-impact sports during adolescence and adulthood on bone mineral density in middle-aged men: a 27-year follow-up study | journal = American Journal of Epidemiology | volume = 158 | issue = 6 | pages = 525–533 | date = September 2003 | pmid = 12965878 | doi = 10.1093/aje/kwg170 }}</ref> Children and adolescents who participate in regular physical activity will place the groundwork for bone health later in life, reducing the risk of bone-related conditions such as osteoporosis.<ref name="Van Langendonck_2003" />