Articular Cartilage's Struggle with Regeneration

Properties and Structure of Articular Cartilage

The image below illustrates the zonal arrangement of articular cartilage

Middle (Transitional) Zone

The middle zone serves as a transition
between superficial and deep zones. From the
superficial to the deep zone, collagen and water contents gradually decrease and the collagen fiber size increases. Chondrocytes in this zone have synthetic organelles.
Also, the collagen fibers switch from a
tangential orientation to a random orientation

Articular cartilage is a heavily hydrated tissue with 75% of the tissue's weight being water. The solid matrix consists of about 75% of collagen and 25% proteoglycans. The principal collagen type in articular cartilage is type II collagen. Proteoglycans are large sugar molecules that are negatively charged and, therefore, retain a large portion of the tissue's water.

This soft, hydrated tissue exhibits remarkable biomechanical properties. Though articular cartilage is not very stiff, it is very strong in compression and it is remarkably durable. Despite the fact that cartilage cells are very few, very slow, and lack nutrients (and so they do not turn the tissue over), articular cartilage operates as an almost frictionless surface for many decades of life.

When one looks at a side view of cartilage, one can identify four distinct zones: superficial (or tangential), middle (or transitional), deep (or radial), and calcified. Each zone has its unique matrix composition, morphology, cellular, mechanical, and metabolic properties.

Superficial (Tangential) Zone

The superficial zone accounts for approximately 15% of the total thickness; however, it contains the highest collagen density. Collagen, a group of naturally occurring proteins, is abundant in cartilage in the form of fibrils. In this zone, the fibrils are thin, densely packed, and form an oriented lamina covering the joint. Fibroblast-like chondrocytes with few organelles lie along the superficial zone, oriented parallel to the surface, in the direction of sliding where shear stress develops. The way the components of this zone are organized contributes to its shear resistance and tensile strength.

The diagram to the left illustrates the cells found in each articular cartilage zone as well as the collagen fiber orientation in each zone. The right diagram provides another representation of cartilage structure by demonstrating the position of the articular cartilage layer and its underlying subchondral bone.


Deep (Radial) Zone

In the deep zone, collagen fibrils are largest in diameter and are oriented perpendicularly to the articulating surface. They are located across a line visually separating the deep and calcified zones. These fibrils strengthen the bond between cartilage and bone. The cells in this zone show a significantly greater amount of synthetic activities while only having twice as much surface area and volume than cells in the superficial zone.  

Calcified Zone

The calcified zone is the transitional zone that exists in between the articular cartilage and the subchondral bone. It consists of few chondrocytes embedded in a calcified extracellular matrix. This zone contains collagen type X, unlike collagen type II in the other zones, which helps with cartilage mineralization and provides structural integrity 


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