Osteoid

Osteoid
Light micrograph of osteoid, containing two osteocytes, being synthesized by osteoblasts.
Identifiers
FMA	66830
Anatomical terminology [ edit on Wikidata]

In histology, osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. ^[1] Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins. The osteoid and its adjacent bone cells have developed into new bone tissue when it becomes mineralized.^{[ citation needed ]}

Osteoid makes up about fifty percent of bone volume and forty percent of bone weight. It is composed of fibers and ground substance. The predominant type of fiber is type I collagen and comprises ninety percent of the osteoid. The ground substance is mostly made up of chondroitin sulfate and osteocalcin.^{[ citation needed ]}

Physiology

To allow bone mineralization to take place, osteoblasts secrete tissue-nonspecific alkaline phosphatase into the osteoid to break down pyrophosphate, an extracellular inhibitor of hydroxyapatite precipitation which otherwise prevents precipitation of hydroxyapetite crystals from extracellular fluid which is supersaturated in Ca²⁺ and PO₄^3- ions. ^[2]

Disorders

When there are insufficient nutrient minerals or osteoblast dysfunction, the osteoid does not mineralize properly and accumulates. The resultant disorder is termed rickets in children and osteomalacia in adults. A deficiency of type I collagen, such as in osteogenesis imperfecta, also leads to defective osteoid and brittle, fracture-prone bones.

In some cases, secondary hyperparathyroidism can cause a disturbance in mineralisation of calcium and phosphate.

Another condition is a disturbance in primitive transformed cells of mesenchymal origin, which exhibit osteoblastic differentiation and produce malignant osteoid. This results in the formation of a malignant primary bone tumor known as osteosarcoma or osteogenic sarcoma. This malignancy most often develops in adolescence during periods of rapid osteoid formation (commonly referred to as growth spurts). ^[3]

References

1. Trammell, Lindsay H.; Kroman, Anne M. (2013-01-01), DiGangi, Elizabeth A.; Moore, Megan K. (eds.), "Chapter 13 - Bone and Dental Histology" , Research Methods in Human Skeletal Biology, Academic Press, pp. 361–395, doi:10.1016/b978-0-12-385189-5.00013-3, ISBN   978-0-12-385189-5 , retrieved 2020-11-18
2. Hall, John E.; Hall, Michael E. (2021). "Chapter 55 - Spinal Cord Motor Functions; the Cord Reflexes". Guyton and Hall Textbook of Medical Physiology (14th ed.). Philadelphia, PA: Elsevier. pp. 994–995. ISBN   978-0-323-59712-8.
3. Ottaviani Giulia; Jaffe Norman (2009). "The Epidemiology of Osteosarcoma". Pediatric and Adolescent Osteosarcoma. Cancer Treatment and Research. Vol. 152. New York: Springer. pp. 3–13. doi:10.1007/978-1-4419-0284-9_1. ISBN   978-1-4419-0283-2. PMID   20213383.

• Netter, Frank H. (1987), Musculoskeletal system: anatomy, physiology, and metabolic disorders, Summit, New Jersey: Ciba-Geigy Corporation
• Jaffe, N.; et al. (2009). Pediatric and Adolescent Osteosarcoma. New York: Springer. ISBN   978-1-4419-0283-2.

External links

• Histology image: 69_03 at the University of Oklahoma Health Sciences Center - "Bone, femur"
• Dr. Susan Ott's website on osteomalacia