Biology 441, Spring 2013: Cartilage and Bone
Cartilage and BoneVertebrate skeletons are made of cartilage and bone.Bone is a composite of type I collagen (protein fibers) and calcium phosphate crystals. Cartilage is a composite of type II collagen and chondroitin sulfate (sulfated polysaccharides) sugar chains with sulfate side groups. Three kinds of cartilage:
* Elastic cartilage: supporting ears, larynx, tip of nose (weak and rubbery)
* Fibrous cartilage: intervertebral disks; very tough and strong: but sometimes it herniates
Cartilage is NOT vascularized; blood vessels can't penetrate into it; there are physical reasons why they can't. The story about sharks not getting cancer seems to be a myth; cartilage won't cure or prevent cancer. Bone is made by different kind of differentiated cell called an osteocyte; some scientists prefer to call them osteoblasts. Osteocytes secrete collagen, but the calcium phosphate crystals are never inside the osteocytes; It is not known how they are caused. Bone is constantly dissolved and digested by acid and enzyme secreted by a large multicellular variant kind of macrophage. These special cells are called osteoclasts. There are some good YouTube time-lapse movies of osteoclasts destroying bone in tissue culture. But don't believe the animations. Nobody seems to have made any good movies of bone formation, although I don't know why not. It occurs in tissue culture. Osteoporosis is a major medical problem; it is caused either by too much breakdown of bone by osteoclasts and/or not enough making of more bone to replace the breakdown. Somebody should develop a reliable criterion to distinguish whether bone destruction is faster or bone making ("deposition", but nobody knows how it works). In principle, bone would also become more brittle if less collagen was secreted; and there may be other ways. Research in this area has not been careful enough, in my opinion. Tons of money was wasted on sloppy tests of whether electric fields stimulate bone formation. A good idea; but not good methods. A class of chemicals called Bisphosphonates was developed to try to prevent or cure osteoporosis. Bisphosphonates all have a carbon atom with two phosphorus atoms covalently bonded to it, and one or two carbon chains covalently bound to that first carbon. The different drugs of this kind differ in these side chains. Researchers hoped, and then believed for years, that these chemicals strengthen bone. Their real effect (the consensus is) that they stimulate programmed cell death ("apoptosis") of osteoclasts. Their many bad side effects mostly (or entirely?) result from inducing apoptosis of other cell types, like in the digestive system. The original use of bisphosphonates was as "water softeners" in agricultural irrigation canals! They were cheap to synthesize, and sold by the ton. But when tried as drugs to treat osteoporosis, the prices went up to $ 50 or $ 100 per pill. Propaganda articles refer to price as cost, please notice. No explanations were made, that I know of. Many interesting web pages are posted on this subject, some of them very bitter (and convincing). In an adult human, the skeleton is about 95% bone, but in the fetus the skeleton is about 95% cartilage. All but some bones in the skull are originally made of cartilage, which is gradually replaced by bone. For example, your ribs are first made entirely made out of cartilage; So are your vertebrae, your humerus, radius, ulna, finger bones, etc. almost everything. Then cartilage is destroyed a little bit at a time, and bone is formed in its shape and location. Sometimes people think and say that cartilage "changes into bone", but replacement is much closer to the truth. (Actually, some of the collagen fibers may continue to exist. That's what it looks like in some of my photographs, but I am not sure.) The reason for this strange replacement is that bone cannot grow by internal swelling, but cartilage can. From the late 1800s until around 1950 or 60, textbooks taught that cartilage was more primitive than bone, and had evolved before bone, and that its replacement was considered a good case of "ontogeny recapitulating phylogeny". Then it was decided that isn't true, and that bone evolved before cartilage, and that this replacement process is a method for the skeleton to grow bigger and longer by swelling. Cartilage and bone are composite materials (made of a mixture of materials, like fiberglass is) In fiberglass, the glass fibers very strongly resist tension, but would bend if compressed along their length. The plastic resin (the pinkish gooey stuff that you have to mix a catalyst with, and that you brush onto the glass fibers) becomes very hard and brittle. So the plastic polymer strongly resists compression forces and bending forces, while the glass fibers resist tension, and the combination of the two is much stronger than either material could be by itself. Concrete reinforced by wire mesh and/or "rebar" iron and steel rods is also a composite material. But iron and steel are very strong alone, so in this case cement is mixed with them to save money, more than to gain strength. Concrete is a composite if cement and gravel. Wood is also considered to be a composite material; and there is a good Wikipedia article on composite materials. Many new composite materials have been developed. Many new materials are used as equivalents to the glass fibers in fiberglass.
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