What is cartilage made of?

Do you really want to know? Maybe you should skip this because it is kind of hard to understand. Are you still here? OK, here goes. Cartilage is made of water, “ground substance” and “collagen”. About half of the dry weight of cartilage is collagen and the other half is ground substance. Collagen is a long molecule like rope. It is used to attach cartilage to bone. Ground substance is made of huge molecules called aggrecans (see figure 3).

Notice that it looks kind of like a Christmas tree. An aggrecan is made of about 20 “proteoglycans” attached to a “hyaluronic acid chain”. I told you this was hard. I am impressed that you are still here. Good for you. The proteoglycans are made of a “core protein” to which is an attached chondroitin sulfate ”or“ keratin sulfate chain. Here is where it gets pretty interesting. The chondroitin sulfate and keratin sulfate chains are made of 2 sugars which are repeated over and over again. In the case of chondroitin sulfate the two sugars are “glucuronic acid” and “N-acetyl-glucosamine”. These two sugars are strung together 50 to 70 times. If “GA” stands for “glucuronic acid” and “NAG” stand for “N-acetyl-glucosamine then chondroitin sulfate would look like this:

GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG- GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG- GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG- GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG- GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG-GA-NAG.

Who cares? Why did I drag you through all this? Well the plot thickens. The unusual thing about these particular sugars is that each one has a negative charge. Here is where your common sense comes into play. Do you know what happens when you put the negative poles of two magnets together? That is right, they repel each other. So here is the important thing: there are about 30,000 negative charges per aggrecan. With all those negative charges on an aggrecan repelling each other the molecule remains stiffly extended. If you put weight on it… no problem…those negative charges support the weight. Apply a shock…no problem…the negative charges absorb the shock. If you pass another bone covered with cartilage (agrecan) by it… no problem…the negative charges repel it so there is almost no friction. Thus aggrecans are the perfect weight bearing molecule making cartilage the perfect weight bearing substance.