The image above is a normal piece of sheep shoulder tendon, not far from where the tendon attaches to the bone of the shoulder blade, magnified about 400 times. The “eyes” you can see are cells with a diameter of about 0.02 millimetres. The blue in the centre of each cell is the DNA in the nucleus of the cell. The white area is where the rest of the cell was (the cytoplasm) before it was lost in the fixing process.
Even though these cells are within the tendon, they are not truly tendon cells; they are cells known as fibrochondrocytes. These cells are a sort of hybrid between the cells in your tendons and the cells in your cartilage. They are also found in other tissues, such as the meniscal cartilage in the knee between the ends of the long bones (tibia and femur). The cartilage covering the ends of the long bones is called articular cartilage. The meniscal fibrocartilage helps congruency of the joint (makes it match up in shape better).
The piece of tendon above was fixed in formalin and then softened to make it cut more easily, as tendons are really fibrous and tough. The fibres are made of collagen and in the above picture are running from the top to the bottom. The purple colour is a stain called Toluidine Blue. It is staining cartilage-like material that is not normally in tensile tendon except in this region near where it attaches to bone. When there is an injury to the tendon, and also when we age, the amount of cartilage-like material increases and the tendon is less able to function properly.
In our laboratories we are researching ways to stop this cartilage-like material accumulating in older and/or injured tendon.