Cartilage and Bone - Key Points

Image from:http://normankpoppenmd.blogspot.ie/

Here are some points I want to emphasize about cartilage and bone.

1. Cartilage is avascular. Therefore it heals poorly and the damaged cartilage is replaced by dys/non-functional scar tissue.

2. Cartilage can grow both by apposition (deposition on surface) and by interstitial growth (expansion from within)

3. In growing cartilage, the perichondrium is the source of new chondroblasts/chondrocytes

4. Both fibrocartilage and articular hyaline cartilage entirely lack perichondrium.

5. Shrinkage of the nucleus pulposus of the IV disk occurs as a normal part of aging and results in a decrease in height of the individual.

6. Bone is highly vascular. Therefore it heals well and damaged bone is (generally) replaced by new fully functional bone.

7. Bone deprived of its normal blood supply will undergo avascular necrosis and die. This has important implications in the management of certain fractures which interrupt the normal blood supply of the bone.

8. Bone deposition and growth is always appositional, never interstitial

9. The highly vascular nature of bone also facilitates its role as a reservoir or sink of important metal ions. These can be rapidly absorbed from blood, or resorbed from bone and secreted into the blood.

10. The internal cavity of bones (medulla) is where bone marrow is found. Bone marrow is the source of stem cells from which all blood cells are derived. Some bone marrow stem cells can also give rise to other tissues. The medullary microenvironment is an important regulatory factor controlling the behaviour of these stem cells.

11. The rates of resorption and bone deposition are highly regulated and interlinked. Normally about 10% of the total bone mass is undergoing remodelling at any given time. When the rates of resorption and deposition become unlinked, or the relative kinetics change, bone disease can occur. Two such (unrelated) diseases are osteoporosis and Paget's disease. In osteomalacia, a disease of defective bone mineralization, the primary cause is lack of Ca due to vitamin D deficiency (rickets).

12. In surgical procedures involving bone, every effort is made to preserve the periosteum as it is an important source of osteoprogenitor cells from which osteoblasts are derived.

Fibrocartilage

As I mentioned in the video, fibrocartilage is interesting because the appearance depends in part on where the fibrocartilage comes from. Although relatively disorganized, it is at its most organized in the annulus fibrosus of the intervertebral disc. Here, it must be particularly good at resisting compression.

The picture above and to the right is a low mag view of an IV disc from a small monkey.
AF = annulus fibrosus NP = nucleus pulposus.
The disc resembles a 'jelly doughnut' where the annulus fibrosus is the doughnut and nucleus pulposus is the jam centre. The force the disc experiences is mainly compression along the axis from top to bottom of the picture.

Microscopically, the fibres of collagen Type I are arranged in layers, with the fibres in adjacent layers oriented at right angles. Chondrocytes are generally found between the layers. This gives the classic 'herringbone' or 'chevron' appearance to the fibrocartilage.

An explanation as to why having the fibres oriented in this way is advantageous in distributing load to reduce compression is shown in the cartoon diagram below.

Elsewhere, fibrocartilage must not only resist compression, but also twisting and shearing (sternoclavicular joints, pubic symphysis). Here the fibrocartilage will appear disorganized, with bundles of fibres interwoven among one another seemingly at random.Although appearing disorganized, the fibrocartilage is organized in such a way so as to maximize strength and this is why dislocations of the pubic symphysis or sternoclavicular joints are relatively rare.

Cartilage

Here are three short clips on the subject of cartilage. The first is a brief general overview, and the next two deal with the appearance of cartilage on slides viewed with the VM.

Cartilage General Overview




Hyaline Cartilage



Elastic and Fibrocartilage

Cartilage and Bone

Image from: http://cloud.med.nyu.edu/virtualmicroscope/v/174/

By now I am assuming you have covered the cartilage and bone chapters in the notes so you should probably begin the practical elements of these also. The attached document contains the instructions you will need and there are various video clips posted here to guide you in your use of the microscope. As always if you have any questions email me.

Ground Bone

A brief explanatory video on what can be seen on one of the ground bone slides.

Bone

Here are three videos covering most aspects of bone that I would expect you to know. In two cases we are looking at developing bone, while in the third we are looking at fully (or nearly fully) developed lamellar bone. You will study decalcified lamellar bone and you should attempt to compare and contrast this with the ground bone specimen you have viewed previously. You will study the epiphyseal plate, the hyaline cartilage structure which drives growth in length of long bones and on this slide you should concentrate on recognizing and understanding the process taking place here. Finally you will look at a developing animal paw with the purpose of gaining an overview of endochondral bone development.

Decalcified bone:



Epiphyseal plate:



Endochondral bone formation:

Bone growth dynamics

Here's my cartoon version of the dynamics of bone growth at the epiphyseal plate which is the mechanism by which long bones grow in length, and an explanation of how bones grow in width. Note that growing in width requires a balance between new bone deposition and bone resorption, and that bone resorption normally occurs only on the internal (marrow cavity) surface of the diaphysis.

It's also important to be aware that in general bone deposition and resorption occur simultaneously on the inner surface of bone and these two activities are very tightly linked and regulated. Paget's disease is a condition in which these activities become unlinked and the result is dysmorphic and inappropriate bone growth in places, accompanied by bone thinning and excessive resorption in other places.


Bone dynamics: