Cartilage and Bone Vertebrate 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: * Hyaline cartilage: stiff and fairly strong: articular cartilages, replacement skeleton, distal ends of ribs. * Elastic cartilage: supporting ears, larynx, tip of nose (weak and rubbery) * Fibrous cartilage: intervertebral disks; very tough and strong: but sometimes it herniates ("slipped discs") Cartilage is secreted by a special kind of differentiated cell, called a chondrocyte. You can grow them in tissue culture. Cartilage is NOT vascularized; blood vessels can't penetrate into it; there are physical reasons why they can't. Nerve growth cones also can't penetrate cartilage. And many people believe there are inhibitory substances, that repel blood vessels. 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. 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 All but some bones in the skull are originally made of cartilage, which is gradually replaced by bone.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.