Layers of Articular Cartilage - Everything You Need To Know
Dr. Nabil Ebraheim
https://www.youtube.com/watch?v=9g0TG249BDI
The articular cartilage is made up of many layers. If you look at the joint, this will be the top of the articular cartilage. If you magnify the joint, this is the articular surface, the subchondral bone and the cancellous bone.
How many layers are in the articular cartilage?
There are 4 layers in the articular cartilage.
1. Superficial zone
2. Middle zone
3. Deep zone
4. Calcified zone
In every layer there will be some changes. These layers will differ in the chondrocytes, the morphology, size and in the orientation of the collagen bundles, and in the amount of water and proteoglycan present.
We start with the superficial zone which is about 10 to 15%. Another name for the articular cartilage layer is the tangential layer. The superficial zone is really a thin layer, but it is an important layer. You find that the cartilage fibers are parallel to the surface. The chondrocytes are elongated and parallel to the axis of the articular cartilage. It has the highest concentration of collagen, which means it has the highest tensile strength, the highest concentration of water and the lowest concentration of proteoglycans. It has less cells than the deep zone. You do not want proteoglycans here because it will swell (do not want the cartilage to swell in this area). This is the area connected to the motion of the articular cartilage, so you have to have a high tensile strength. You have to have some collagen and some water, but you do not need proteoglycans because you do not need the cartilage to swell. Both the cells and the collagen fibers are pounded by the pressure, becoming flattened and elongated along the long axis of the joint.
The middle zone is the transition zone (zone between the superficial and the deep zones). The chondrocytes and the collagen fibers are oriented randomly. In the middle cartilage layer, you will find the collagen in an oblique orientation. You can see the disparity between the arrangement of the structures in the superficial zone and the arrangement of the structures in the deep zone. You can imagine the transition zone is really disorganized because they do not know which zone they belong to.
The deep zone is about 30%. The collagen fibers are perpendicular to the surface. There are more cells, they are round and they are arranged in columns perpendicular to the joint. The deep zone has the highest concentration of proteoglycans and the lowest concentration of water. You do not need water inside of the cartilage. In the deep zone, the chondrocytes are arranged in columns and the collagen fibers are oriented vertical to the articular cartilage. The deep layer is strong in compression.
