Next Lesson - Basics of Bone Anatomy
Core
This article describes the structure of bones from a molecular perspective. More information can be found on the anatomical relationships of bones in the rest of our Musculoskeletal Section.
The skeleton in an adult human is made up of 206 bones and facilitates the movement of the body and the protection of important internal organs.
See our article ‘Basics of Bone Anatomy’ which includes information about the types of bones and their functions.
There are three main types of cell that make up bone tissue.
Osteoblasts are derived from mesenchymal stem cells and are responsible for the laying down of bone. This is done through the production of dense collagen and hydroxyapatite (the mineral component of bone). Osteoblasts are closely associated with one another, forming an osteon. Once the osteoblast has surrounded itself with bone matrix, it becomes trapped in its lacuna and becomes an osteocyte.
Osteocytes are star-shaped cells that cannot divide. Osteocytes are formed from osteoclasts that have been trapped in the bony matrix that they have secreted and exist in a space called a lacuna. This restrictive space means that the osteocytes are no longer able to produce osteoid (bone tissue). The processes of an osteocyte instead stretch out, connecting to other osteocytes through canaliculi.
Osteoclasts are the type of bone cell responsible for the break down of bone tissue through the release of acid (to dissolve the hydroxyapatite crystals) and collagenase. They have an important role in remodelling of bone and regulation of calcium levels in the blood, but also play a key part in the pathophysiology of osteoporosis. The osteoclasts originate from haemopoietic stem cells, rather than mesenchymal stem cells which produce osteoblasts.
There are two types of bone tissue namely compact (cortical) bone and spongy (trabecular) bone.
The main building block of bone is an osteon, shown in the image below. It is a collection of osteocytes trapped in the bone matrix that they have secreted. The individual osteocytes communicate with each other through microscopic canals called canaliculi. An osteon has a central canal called a Haversian Canal, which contains blood vessels and nerves to supply the bone.
Image: this diagram shows the structure of an osteon; note the Haversian canal and the lamellae created by osteocytes.
Source: https://en.wikipedia.org/wiki/Osteocyte#/media/File:Transverse_section_of_bone_en.svg
Compact bone consists of closely packed osteons. An osteon is made up of a central canal called the Haversian canal, surrounded by concentric rings of matrix called lamellae. These Haversian canals lie parallel to the axis of the long bone.
Between the lamellae are osteocytes, mature bone cells which have been surrounded by bony tissue and now exist in lacunae. The lacunae are connected by canaliculi.
Smaller canals known as perforating or Volkmann’s canals connect the Haversian canals to provide passageways through the hard matrix. They run at 90 degrees to the Haversian canals, across the width of the long bone.
Image: this diagram shows the structure of a long bone. Key features include the periosteum, the lamellae of the osteons and the trabeculae of the spongy bone centre.
SimpleMed original by Dr. Maddie Swannack
Spongy or cancellous bone is lighter and less dense than compact bone. It consists of plates called trabeculae, bars of compact bone, and irregular cavities that contain red bone marrow (responsible for haematopoiesis). While it may seem that the trabeculae are arranged randomly, in fact they are organised to provide maximum support. They are aligned along the stress points of the bone and can realign if the direction of the stress on the bone changes.
Image: this diagram shows the structure of spongy bone, highlighting the trabeculae and marrow spaces within cancellous bone.
Source: https://commons.wikimedia.org/wiki/File:606_Spongy_Bone_esp.jpg
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