What is Bone?

 

What is Bone?

Bones in our body are living tissue. They are made of cells, vitamins, minerals, and proteins.  Bones have a blood supply, and will bleed if broken.  We are born with over 250 soft bones. When we are born, the skeleton is made predominately of cartilage.  As we mature, the cartilage grows and is eventually replaced by hard mature bone. Some of these cartilage bones later fuse together during maturity.  When the skeleton matures, the total becomes 206 bones.

The major functions of bones are to:

·        Provide structural support for the body and a lever system for movement

·        Provide protection of vital organs

·        Provide an environment for marrow for blood production

·        Act as an active storage area for minerals (such as calcium and phosphorous)

 

 

What is bone made of?

1.   Bone matrix

The majority of bone is made of the bone matrix, or osteoid.  Calcified bone contains about 25% organic matrix (of which 2-5% of which are cells, the rest is collagen and proteins), 5% water and 70% inorganic mineral (hydroxyapatite).  Bone is formed by the hardening of this matrix.

2.   Bone cells

Cells in our bones are responsible for bone production, maintenance and modeling.  There are three types of bone cells: osteoblasts, osteocytes, and osteoclasts.

·        Osteoblasts: bone cells which are responsible for creating bone matrix.  Minerals are then deposited into the matrix, making the bone become hardened and strong.  

·        Osteocytes: as osteoblasts create bone matrix, the matrix becomes mineralized, calcified and turns hard.  As a result, these osteoblasts become “trapped” within the matrix.  At this time the osteoblasts mature and turn into osteocytes.  Osteocytes are still connected to another and to osteoblasts via small canals within bone and are able to communicate with each other via hormones. 

·        Osteoclasts: bone cells which dismantle or resorb bone matrix.  These cells use bone-resorbing enzymes to digest and degrade bone.  This releases minerals from the bone matrix back into the blood. 

 

Modified from http://www.ucl.ac.uk/cdb/research/arnett/boneturnovercopy.jpg

 

 

What kinds of bone exist?

Bone is composed of two types of tissue: Cortical bone and Cancellous bone

1. Cortical/Compact/Dense bone:  hard outer layer, which is strong, dense and tough.  It has minimal gaps and spaces. This compact tissue gives bones their smooth, white, and solid appearance.  Eighty percent of the total bone mass of an adult skeleton is made of compact bone.

2. Cancellous/Trabecular bone:  spongy inner layer. Trabecular bone is light and significantly less dense than compact bone.  Trabecular bone has a sponge like appearance.  Within the spaces of this bone are blood vessels which provide nutrients to bone and bone marrow.   Twenty percent of the adult skeleton is made of trabecular bone.  However, as the trabecular bone is very porous, it has nearly ten times the surface area of compact bone. 

Within the two types of bone tissue (cortical and cancellous), two types of bone can be identified on a microscopic level: woven bone and lamellar bone.  These types are distinguished according to the pattern of collagen forming the osteoid:



Matos  et al. Journal of Orthopaedic Surgery and Research 2008 34

·        Woven bone is characterized by a haphazard organization of collagen fibers and is mechanically weak.

·        Lamellar bone is characterized by a regular parallel alignment of collagen into parallel sheets known as lamellae.   Lamellar bone is mechanically strong.

Woven bone is produced when osteoblasts produce osteoid or bone substance rapidly. Woven bone is found in fetal bones.  Later, the woven bone is replaced by remodeling and the replacement with the stronger lamellar bone. In adults, woven bone is formed in fracture repair.  Following a fracture, woven bone is removed and lamellar bone is deposited in its place.  This process is called “bony substitution”.

Does bone change?

Bone has the ability to alter its form over time.  It can increase in size over time (bone growth or bone modelling).  It may also alter its shape after a fracture (bone remodeling).

Bone modeling

Modeling is when bone resorption and bone formation occur on separate surfaces of the bone.   An example of this process is during long bone increases in length and diameter, where activity occurs on two surfaces of the bone. Bone modeling occurs during birth to adulthood and is responsible for gain in skeletal mass to adult form over time.

Bone remodeling

Remodeling is the replacement of old bone by new bone tissue. It is known as bone turnover.  This occurs in the adult skeleton to maintain bone mass or during fracture healing. This process involves the coupling of bone resorption and bone replacement through the function of the osteoclasts and osteoblasts respectively.



Bone remodeling cycle.  modified from http://www.ns.umich.edu/Releases/2005/Feb05/img/bone.jpg



 

How does bone form?

Osteogenesis is the process of bone tissue formation.  Osteogenesis occurs by two processes: intramembranous ossification and enchondral ossification.

·        Intramembranous ossification occurs in the flat bones of the body (e.g. skull, clavicle, maxilla and mandible). It involves the replacement of connective tissue membrane sheets, known as mesenchyme, with bone tissue and results in the formation of flat bones.

 

·        Enchondral ossification occurs in the long bones of the body (e.g. femur, humerus, and tibia).  It involves the replacement of a hyaline cartilage model in infancy with bone tissue to form the adult skeleton.  As the infant cartilage skeleton matures, it grows in length and width.  As the bones increase in size, the cartilage is replaced by bone tissue.  The length increases through activity in the growth plates of the bones at the ends of the long bones.   The width increases by bone formation via the exterior lining of the bone called the periosteum.

 

 

References:

http://www.iofbonehealth.org/what-is-osteoporosis
http://orthoinfo.aaos.org/topic.cfm?topic=a00232
http://www.healthline.com/health/osteoporosis
http://nihseniorhealth.gov/osteoporosis/whatisosteoporosis/01.html
http://nof.org/