Internal Bone Structure - Introduction
Connective tissue found within bones (osseous tissue) contains bone cells (described in Bone Repair) and a matrix. The cells only account for 2% of a bone's mass. The matrix is dense and solid, due to a large deposition of calcium phosphate as hydroxyapatite crystals. In fact, as these crystals form, calcium carbonate and sodium, magnesium and fluoride ions may also be incorporated into the matrix such that collagen fibres, the other main constituent of bone, only account for one-third of a bone's mass. The matrix crystals withstand compressive forces and the collagen fibres withstand tension (pulling) forces. The matrix composition is the same in the two types of bone, compact and spongy, the structures of which are now summarised.
Compact bone tends to be found towards the outside of bones, as shown in Figure 1.
Figure 1: The structure of a long bone, showing amongst other things the location of compact and spongy bone. Adapted from a diagram courtesy of Wikimedia Commons (http://en.wikipedia.org/wiki/File:Illu_long_bone.jpg)
The following list summarises the features of compact bone:
- It is made up of osteons which consist of a central canal and concentric rings of osteocytes. (Figure 2)
- Central canals carry blood to the bone, parallel to its surface.
- Perforating canals run perpendicular to the bone surface, also carrying blood and penetrating deeper into the bone and its cavities.
- Concentric lamellae surround each central canal and contain collagen fibres, provding extra strength.
- Between the osteons are interstitial lamellae containing recycled matrix.
- Compact bone is very strong because osteons all face the same direction - for example, parallel to the shaft in the femur - this can be likened to a cylinder-shaped scaffolding pole, which is very hard to compress or stretech from its ends, but easier to break with a sideways "kick" to its shaft.
Figure 2: The structure of compact bone. The central canals (deep red) can easily be seen, with the rings of concentric lamellae surrounding them. The interestitial lamellae fill the gaps between the osteons (a central canal and its surrounding concentric lamellae). Courtesy of Wellcome images at http://www.flickr.com/photos/wellcomeimages/4013247483/
This list summarises the main features of spongy bone:
- Unlike compact bone, the matrix in spongy bone forms the main structural "units" of compact bone, called trabeculae. (Figure 3)
- The trabeculae branch to form a network radiating throughout the epiphysis of the bone.
- Between the trabeculae is red bone marrow which contains a rich network of blood vessels to deliver nutrients and remove waste materials from the trabeculae and osteocytes.
- Red bone marrow is the site of blood cell formation (haematopoeisis).
- Yellow bone marrow may also be found in spongy bone and serves as an adipose tissue (fat) store for energy.
- Spongy bone is much lighter than compact bone, reducing the effective weight of the skeleton.
- It is able to withstand stresses from many directions owing to the criss-cross arrangement of trabeculae.
- Spongy bone is the main constituent of flat bones.
Figure 3: The structure of spongy bone. The pink-stained trabeculae form a meshed network between reserves of red bone marrow. Courtesy of Department of Histology, Jagiellonian University Medical College at http://en.wikipedia.org/wiki/File:Spongy_bone_-_trabecules.jpg
Periosteum and Endosteum
The periosteum is a layer of compact bone, which, as labelled in Figure 1, surrounds the shaft of all bones except those in joint cavities. The osteoblasts and osteoclasts within it are active participants in Bone Repair . The periosteum also has the role of providing a strong attachment of bones to the tendons and ligaments, which join bone to muscle and bone to bone respecitvely. This is achieved by virtue of continuitity in structure between the joint capsule and the periosteum. The fibres of the peritosteum interweave with collagen fibres of tendons and ligaments (called perforating fibres) yielding continuous structural flow. Whilst these collagen fibres can of course be snapped, a very sharp pull would more likely break the bone.
The endosteum is found lining the marrow cavity (labelled as medullary cavity in Figure 1) and contains a flattened layer of osteoprogenitor cells (which can give rise to osteoblasts). This layer, like the peritosteum, actively participates in bone repair and remodelling. It is an incomplete layer, and "gaps" expose the matrix, such that osteoclasts can degrade it, or osteoblasts can deposit new matrix onto it.
Fundamentals of Anatomy and Physiology, 7th Edition, Frederic H. Martini, Pearson Education 2006, chapter 6.
Human Anatomy, 5th Edition, J.A. Gosling et al., Elsevier Ltd. 2008, chapters 2, 3 and 6.